ON7YD
136 kHz technical pages (homebrew stuff)
last updated on 20 May 2008

About this page

Here you will find a list of homebrew projects for 136 kHz and related topics.
I hope this will be a help to other radio amateurs to build or improve their longwave station.

Do you know any other homebrew projects that are interesting to add to this list ?
Share your knowledge with others and inform me by e-mail.

Note : in the list below you will find a lot of homebrew projects.
A list of articles on 136kHz and related topics can be found here.
A practical overview about 136kHz antennas can be found here.

    I. Antennas

  1. A receiving loop for longwave (by DF3LP)
    One major problem receiving long waves (e.g. 137 kHz ham band) is the presence of local noise sources. Here in North Germany all faint amateur signals on 137 kHz are covered by a curtain of LORAN-C sidebands from the island of SYLT (120 km NW from this site).
    So I decided to erect an antenna which is able to cancel out this unwanted source. One possible answer is to build a magnetic receiving loop. "Magnetic" means that it is only sensible to the magnetic field vector and provides two sharp nulls at the broad sides, rectangular to the plane of the loop. These nulls are almost independent to the mounting height of this antenna. The polarisation of a "standing" loop is vertical.

  2. Vertical antenna with inductive toploading (by ON7YD)
    In an environement with a lot of 'vertical objects' (trees etc.) close to the antenna inductive toploading can significantly increase the performance of a short vertical antenna.

  3. 250 - 400 µH Variometer (by G0MRF)
    Tuning a LF antenna can be tricky. An antenna that's much shorter than a quarter wavelength will have a high Q and consequently a narrow bandwidth. A loading coil with a series of taps can be used, but a constantly adjustable inductor like a variometer will make tuning a lot easier.

  4. Experiments on remote receiving loops (by K0LR)
    Loop antennas offer many advantages for LowFER reception. They're compact, don't need to be high and in the clear, and their directional characteristics can be used to null out local noise or strong interfering signals. Unfortunately a loop doesn't offer a good impedance match to a coaxial transmission line or to the input of most modern receivers.
    Also, unless the loop is balanced and shielded, the so-called antenna effect makes the loop act like a combination of a loop and a short vertical whip. The directional pattern becomes asymmetrical and the nulls off the side may be only a few dB down from the peak of the radiation pattern. An unbalanced, unshielded loop can also pick up conducted interference from the feed line.
    You might find a quiet location in the "back 40", put your loop out there, and then re-introduce the noise from the house when you connect the coax. Shielding adds distributed capacitance to the loop and reduces the Q, which in turn reduces the loop's sensitivity. This article gives some ideas on how to use a simple unbalanced loop with a home-brew transformer to achieve most of the benefits of a shielded, balanced loop.

  5. Sensitivity of Multi Turn Receiving Loops (by N4YWK)
    Multi turn wire loops are often used as low frequency receiving antennas. Applications such as geophysical research, oil exploration and survivable communications require maximum sensitivity of receiving loop antennas. The loop sensitivity decreases as frequency decreases, becoming a formidable problem below 1 Hz. Basic electromagnetic theory is developed here as it relates to electrically small multi-turn loops at low frequencies. Simple algebraic expressions are produced describing the sensitivity of loops in simple geometries. The concept of antenna factor (effective aperture) is introduced, which allows comparison of different loops, and conversion of observations to common magnetic units of measure. It is hoped this work will be a useful reference to geophysical researchers, and to anyone designing loops for low frequencies.

  6. Experimental Vertical Antenna for 136 kHz (by GW4ALG)
    The back garden at GW4ALG is only 15 m long by 11 m wide, so the real challenge presented by operating on 136 kHz is trying to make electrically-short antennas (i.e. antennas that will fit into the garden) actually radiate RF energy.

  7. Transmitting Loop Antenna (by GW4ALG)
    My loop started life as a full-size G5RV antenna in inverted-vee format (basically, a centre-fed inverted-vee dipole, overall length of 34 m, fed with 13 m of feeder). For 136 kHz, I joined the far ends of the dipole together with some thick multi-strand cable to form a sort of top-fed delta loop having an overall perimeter of about 65 m.

  8. Remotely Controller Tuner (by GW4ALG)
    This tuner was devised to provide a method of fine-tuning the experimental vertical antenna. This remotely-controlled tuner has enabled me to change frequency much more quickly than previously, when I only had the larger variometer with which to make such adjustments. (Not easy with an upstairs shack; and the loading coil/variometer positioned in the garden!)

  9. Compact ferrite loading coil for LW antennas (by DK5PT)
    LF antenna matching can be done with a relatively small ferrite pot-core instead of big coupled air coils. The air-gap variometer designed by DK5PT is compact and can be used up to 300W output power.

  10. Constructing a LowFER Antenna (by W9RB)
    Getting on the air with a LowFER station is more complicated than getting on the ham bands since commercial equipment isn't available and construction is involved. The "RB" LowFER antenna is 31 feet tall with a 24 foot diameter top hat. At 186 kHz, the antenna primarily looks capacitive with a small resistive component consisting of the radiation resistance of the antenna in series with the loading coil resistance and the ground resistance. These three resistances are the factors under control of the experimenter and determine how well the antenna works. The radiation resistance of any Part 15 antenna is a small fraction of an ohm, since the 15 meter length limitation means that the antenna will be less than 1 % of the wavelength, as opposed to 25 % which is the generally accepted rule for AM broadcasting and 160 M ham operation.

  11. Superloop receiving antenna for 136kHz (by G3LDO)
    About a year ago I made a receiving loop for 136kHz using computer ribbon cable housed in plastic waste pipe. It wasn't very successful. It would appear that the low Q caused by the construction of the ribbon cable was the problem. I have since made the G3LNP loop although I had difficulties with the amplifier. I finished up using a low impedance pick-up loop without the amplifier - this worked reasonably well but it did lack sensitivity.
    I decided to redesign the plastic waste pipe special because if its inherent weather resistant structure This time I made it twice the size and used Litz wire from the Crawley club source. This antenna turned out to be very good and outperformed all other loops built previously. Furthermore, this design does not need an amplifier. A description of its construction follows.

  12. A remotely tuned MF receiving antenna (by A. J. Cawthorne)
    The author has been interested in long distance m.f. reception for many years from both international and local radio stations. Added to this has been the fascination of using the "null" effect of balanced loop antennas to aid reception where co-channel interference and/or local noise is a problem However, the author has insufficient space in the shack to seriously experiment or use loops of any size. This article describes a solution to this problem by remotely tuning a relatively large m.f. loop which is mounted in the loft space some distance from the operating position.
    The loop design for this application is not critical and can be built in size and style to individual preference. The article in reference I discusses the background, design, construction and use of m.f. loop antennas and is well worth reading.
    Remark : although this antenna is intended for MF it should be easy to adapt it for LF.

  13. Variometer design (by IN3OTD)
    Design your variometer online : enter the length, diameter and number of turns of both coils and the minimum / maximum inductance is calculated

  14. AM box loop antennas (by Bruce Carter)
    The box loop antenna is a (relatively) compact and effective way to receive distant AM stations. Many articles have been written describing the construction of these antennas, but they have been deficient in many respects. This article will describe the mathematics of AM Loop Antennas. Construction projects will be linked from this page, to keep the download time low for those on modems.

  15. Receive loop preamplifier (by G3NYK)
    I have used a receive loop aerial for several years for LF reception. If the loop is big enough, then the signal picked up may be sufficient to overcome the noise in the receiver, and a preamplifier is not really necessary. My loop is marginal at 1.2m and 16 turns. It is mabe from 25 pair telephone cable so the Q is not very high. After some experiments I decided that i could more easily read very weak staions with the pre-amp. At this time I was couling the loop to the reciever with a single turn coupling loop. This is not a very good match to 50 ohms, and I felt that perhaps I was loosing some sensitivity. I decided to use a simplification of the amplifier described by Tony Preedy G3LNP in the Radcom.

  16. Experiments with a Top-loaded Vertical Aerial (by G3NYK & EI0CF)
    Finbar (EI0CF) did some experimenting very early with a novel form of Top-loaded aerial for LF. This was stimulated by a short piece in Pat Hawker’s Technical Topics column in the RSGB Radcom in November 1974, revisited in May 1988. The original write-up carried the source references to an IEEE Transactions paper on LF and VLF, and a Canadian Broadcasting Corporation Engineering Review with some practical details of an MF BC aerial tests. The construction seemed quite simple, and Finbar experimented for a while before the UK gained access to 136Khz. After our sucessful measurements with the simple ground loss bridge, Finbar was keen to see if the aerial could be developed into a form suitable for small gardens as a way of encouraging some more activity.

  17. A Ten Gallon Loading Coil - Variometer for MF/LF (by W5JGV)
    Since I am confined to using a small suburban lot for all of my antenna installations, I have a problem installing anything really effective for use on 500 KHz or lower. Just about anything I plan to use as an antenna will require a serious loading coil. I decided to construct an experimental loading coil and variometer unit suitable for antenna testing and low power low frequency operation.
    Since this unit was to be designed for mostly testing, I decided not to spend a fortune on Litz wire or exotic coil form materials. Instead, I chose the tried and true "Bucket Full of RF" approach and used a couple of 5 gallon paint mixing buckets which are readily available at most hardware supply stores for just a few dollars each. The buckets are manufactured from Polyethylene plastic, and has a very low RF loss factor. The loss of the THWN insulated wire is much higher, but at frequencies below 500 KHz is satisfactory for most amateur power levels. For wire, I chose inexpensive #14 Gauge solid THWN insulated wire, which is also available at most hardware suppliers.

  18. Variometer Construction Notes (by W5JGV)
    After building the 10-gallon variometer depicted in my previous article, I discovered that it was far too large to fit into the tuning cabinet I had managed to obtain for it. (I seem to remember something about measuring twice and cutting once...) Well, it was too late to locate a bigger cabinet, so I decided to do the next best thing, and divide the variometer into two parts, the variometer assembly itself, and a separate loading coil. Each one would be constructed on a 5-gallon Home Depot paint mixing bucket. I reasoned that it would be fairly easy to simply unbolt the buckets and cut the wire between them.

  19. Aktive Antenne für den Empfang des Amateurfunks auf der Langwelle (by DF8ZR)
    Erst durch langwierige Optimierungsversuche ist es mir gelungen, eine kleine Antenne vorzustellen, die den normalen Ansprüchen für den Empfang der schwachen Signale genügt. Dabei kommt diese Antenne schon mit einem senkrechten Empfangssab von 1,5 m aus. Man kann aber durchaus versuchen, mit längeren Drähten, die auch horizontal ausgerichtet sein dürfen, die Empfindlichkeit zu steigern.

  20. A small LF loop antenna (by DL4BBL)
    This paper describes a loop antenna for receiving purposes in the frequency range 10kHz to 150kHz; with reduced performance it can be used up to 600kHz. The antenna is mainly intended for quick direction finding of unidentified utility radio stations. The prototype was built for indoor use. This is possible because it picks up much less noise from the mains than wire antennas, which are most often useless indoors, in particular at frequencies below 50kHz.

  21. The Lowfer Transmitting Loop Antenna – possibly the answer to your tree-related transmitting woes? (by Bill Ashlock)
    This is New England where the average back yard antenna site is densely populated with oaks, maples, and pines. After struggling for three years with standard and not-so-standard top loaded monopole designs and many dollars in ground radials, the total AC resistance of the antenna system was still greater than 100 ohms except on the coldest winter days. This value is about four times worse than the system resistance of a “good” installation and resulted in field strength readings of about ½ of the “good” levels. It was finally concluded after running numerous tests that the absorptive losses of the trees surrounding the antenna site caused a lowering of the antenna Q which made the measurements of system resistance so high. Nothing short of the ‘chain saw approach’ had much chance in helping the cause, so I began looking toward loop antennas that, according to the available literature, had high current and minimal voltage. This equated to a low Z antenna that seemed would not be effected by the 1 to 10 kohm impedance values I was measuring at the trees.

  22. The Lazy Loop (by M0BMU)
    The great advantage of this sytem is that the critical tuning components are nice and dry in the shack, no remote control, no waterproofing, just twiddle and go!

  23. Bandpass receiving loop antennas (by M0BMU)
    The traditional tuned loop antenna is a high Q tuned circuit, buffered with a high impedance preamp. This gives a useful pre-selector action along with directional nulls and small size. It also has drawbacks; even for use over 135.7kHz – 137.8kHz, it is necessary to peak the tuning, since the bandwidth is usually less than 1kHz. This requires some sort of remote tuning. Remote tuning can be avoided by loading the loop to reduce the Q, but this also reduces the output signal level and signal-to noise ratio too, meaning a bigger loop is needed. Also, the out-of-band selectivity is degraded. At my location, the field strength due to the local MF broadcast stations is of the order of 10s of volts per metre, and the selectivity provided by the loop is not good enough to prevent the usual FET based preamps from overloading. So it would be nice to increase the bandwidth without adding resistive loading, and at the same time improve the attenuation outside the passband.

  24. Attempt to estimate antenna efficiency (by I5TGC)
    Although it's very hard to calculate the effective efficiency of my LF antenna, I've tried an indirect evaluation, with the aid of the german station DCF39, transmitting on 139 Kc, and starting with a careful calculation of my single-turn receiving loop.

  25. A 10' receiving loop for low-frequency DX work (by VE7SL)
    My ten foot air-core receiving loop was originally designed and constructed for NDB DXing. More recently, it has been pressed into service for LOWFER DX work. The loop has been instrumental in achieving two notable low-frequency receptions - the first Trans-Pacific reception of ZL (New Zealand) amateur radio signals in North America as well as establishing the present long-distance record for LOWFER reception in North America, the 1-watt signal of "NC" from North Carolina to British Columbia, an overland distance of 2,360 miles. The loop is a good performer!

  26. A four foot loop antenna (by Bruce Carter)
    Step by step construction of a 4 foot loop.
    A smaller (2 foot) version can be found here.

  27. The folding loop antenna (by Bruce Carter)
    In my study of AM Box Loop Antennas, I have quickly realized that no matter how good the reception improvement may be using a loop - the loop antenna itself is large and cumbersome. This limits its usefulness to home installations. The large loops do not lend themselves to portability. How can that be changed ?

  28. A portable 3 foot ribbon cable loop (by Bruce Carter)
    One idea that has been suggested for constructing a loop is to use ribbon cable, with a winding spacing of 0.050 inch, to consturct the loop. I saw a loop antenna article at one time that recommended the use of flat ribbon cable, and showed how to make a loop by using the ribbon cable, offset by one pin, on two "D" style connectors, and plugging together to form the loop. I have never seen that article again, so I suspect it has been removed from the web. So I thought I would resurrect the idea.

  29. The umbrella loop (by Bruce Carter)
    Those of you who have read my previous articles know how I have "learned by doing". I started over 30 years ago salvaging loops out of old radios. About 20 years ago I used a simple formula based on the area and number of turns, and produced a 5 foot planar loop that was good for transatlantic listening - but included no tuning capacitor (it was internal to the radio). I then shelved the project for 20 years.
    I still had my 5 foot planar loop, which was soon adapted to a 4 foot edge wound model with a tuning capacitor. Unfortunately, the wood is actually getting rotted, so it sits in my father's storage shed 300 miles away. It has such a low Q on the high end of the band that I considered the performance unsatisfactory.

  30. Some details of the LF Receiving Loops (by I5TGC)
    Information about the receiving loops, including preamplifiers, at I5TGC.

  31. Some details of the LF Antenna (by I5TGC)
    Description of the 136 kHz antenna at I5TGC.

  32. Loop Antenna Tuning Unit (by GW4ALG)
    The Loop ATU consists of a capacitive matching network (3 capacitors) and a toroidal 4:1 balun. The balun is made using 18 bifilar-wound turns on a 58 mm (overall diameter) Philips toroid of 3C85 material. The high impedance side presents a 50 ohm load to the transmitter/receiver. The capacitors used are from the Philips 378-series and Philips MKP-series of high current capacitors and the SWR is very close to 1:1.

  33. VLF-LF and the Loop Aerial (by VK5BR)
    The article discusses the theory of loop aerials for receiving and how they reduce the level of local noise. A Loop Aerial is described suitable for use on the LF and VLF bands together with a circuit of an interface loop tuner and preamplifier. The discussion extends to the problems of amplifier noise and the advantages of tuning the loop.

  34. A Really Simple LF Receiving Loop (by K0LR)
    While trying to fight a problem with noise from a remote-reading power meter that the electric company had just installed, I decided to try an isolated single-turn loop, as far from the house as my roll of coax would reach. The present remote-tuned, balanced loop is about 70 feet from the house and was picking up a lot of impulse noise from the meter. I assumed that the noise was primarily conducted via the power, tuning and rotator cables leading from the shack to the loop.

  35. "Lightweight" LowFER Verticals (by K0LR)
    As anyone knows who has tried it, erecting a tall, flimsy mast with a large top hat presents quite an engineering challenge. Here is a description of two LowFER vertical antennas that are relatively lightweight and easy to erect, but rugged enough to survive at least a moderate ice or wind storm. Neither of these antennas is in operation today, but they were taken down by the beacon operators, not by Mother Nature. And while they were up, they performed quite well. Both BK (Shell Lake, WI) and MIN (Aitkin, MN) were copied in real-time CW mode (12 WPM) as far away as Illinois and Ohio. In the days prior to ultra-slow-speed CW and all-night computer captures, reception of a LowFER beacon at 600 or 700 miles was rare DX. And BK was not just heard in Ohio; he had a solid two-way CW QSO -- something that is not even considered these days!


    II. Measuring

  36. Field strength meter for the 137 kHz band (by PA0SE)
    Measuring field strengths is essential to be able to determine your ERP. A simple field strength meter for 136kHz was developed by PA0SE. Description includes the calibration procedure.

  37. VSWR meter (by G0MRF)
    A standing wave ratio meter for LF. Good matching is essentail, also on LF. While most commercial VSWR meter (intended for HF) will be very insensitive on LF, this one is designed for 136kHz

  38. A clamp-on RF current probe (by K0LR)
    Measuring HF currents without breaking the wire. With a clamp-on current probe you can measure the RF-current in a wire without beaking the wire (to insert a more traditional amp-meter). This design will work from 100kHz up to 10MHz.

  39. Isolated resistance bridge (by SM6LKM)
    A simple tool to measure the resistance of your LF antenna

  40. Reference signal source (by ON7YD)
    With a few components you can built a reference signal source that can be used to calibrate your receiver. As most receivers are intended to be used on HF the S-meter is ofter very incorrect on 136kHz. Using a reference source you can calibrate your S-meter and give correct reports.

  41. Field strength measurements on 136 kHz (by SM6PXJ)
    In February and March 2000 some measurements were made with a home-brewed field strength meter. The meter setup has been somewhat reconfigured during the test period. First I used an untuned loop as "sensing element". Later this was replaced with a tunable ferrite rod probe with a 11 dB FET preamp. The ferrite probe was calibrated and linearity checked between 0,5 mV/m and 10 mV/m with a pair of Helmholtz coils.

  42. A portable low frequency antenna analyzer (by K0LR)
    For those of us who like to play with antennas on the ham bands, one of the handiest tools to have around the shack is an antenna analyzer. These gadgets combine a signal generator and standing-wave ratio (SWR) sensor in a small battery-powered unit. I wanted an analyzer that would operate in the 100 to 300 kHz range and would measure antenna system resonance, system losses and antenna capacitance, as well as the inductance and Q of loading coils and receiving loops.

  43. MFJ-259 antenna analyzer modifications (by WB6VKH)
    Learn how to adapt the popular MFJ-259 antenna analyzer to tune down to 100kHz.

  44. SWR Bridge (by GW4ALG)
    I found this circuit to perform very well in my 136 kHz transverter and have subsequently used it in my Loop ATU and my 400 W Power Amplifier with great success. It easily copes with 400 W of RF, and probably a lot more besides. Dave, G3YMC has also used this design in his 136 kHz transverter.

  45. Simple impedance bridge for LF (by DF8ZR)
    This simple instrument can be used to use impedances at LF. You will need a signal source with at least 6Vpp, almost any LF generator can be used. The author uses this bridge to match the loadingcoil to 50 Ohm before transmitting.

  46. Ground loss measurement at LF (by G3NYK and EI0CF)
    Whilst there are often problems in winding large high inductance coils to resonate LF aerials, the most important factor which needs to be understood and overcome are the losses. Because the electrical size of an normal amateur aerial at 136kHz is much less than a quarter wavelength, a ‘T’ or ‘L’ aerial can be modelled as a capacitor in series with a resistor. The resistive component is made up essentially of two parts, the radiation resistance, and the loss resistance. At frequencies around 136kHz an normal sized amateur aerial will have a loss resistance component of anything up to 100 times the value of the radiation resistance. In this situation, and for a given available RF power level, if we were to halve the loss resistance, we could double the current flowing into the aerial. This would lead to 4 times the radiated signal strength. The majority of the power will still be dissipated in the loss but there will be a 6dB or 1 S point improvement at the receiving station.

  47. Measuring the earth resistance of an LF-antenna system (by PA0SE)
    In 1988 I made an impedance bridge with a noise source. (See the circuit diagram below.) Most bridges of this type have a transformer between the noise amplifier and the bridge circuit. But I found it impossible to make the bridge frequency independent up to 30MHz. By putting the transformer between the bridge circuit and the detector I managed to make the readings reliable up to 30MHz. When the bridge is used at LF only the matter of frequency compensation does not arise.

  48. LF tuning meter for the 136kHz band (by M0BMU)
    When adjusting the tuning and matching components of an LF antenna, it is extremely useful, if not essential, to have some way of indicating the impedance mismatch at the antenna feed point. One way of doing this is to use an SWR bridge, as is usually done at HF, but this only indicates the magnitude of mismatch, so adjustment is still a matter of trial and error. For a long time I have used a dual-trace 'scope to display the voltage and current waveforms at the feedpoint (see the 'Scopematch' article in the LF Handbook) The antenna loading coil can then be tuned so that voltage and current are in phase (ie. the load is resistive), and then the tapping point of the impedance matching transformer adjusted so that V/I = 50 Ohm. This makes tuning the antenna very quick and straightforward to do, but does need an oscilloscope; OK if you have one in the shack anyway, but not very convenient for /P operation! This LF tuning meter was conceived to perform the same function, but to be a simple, self-contained unit requiring no additional power supply. The tuning meter contains two meters - An RF voltmeter and ammeter, and a phase meter. The voltmeter/ammeter is used to determine the magnitude of the load impedance (V/I), while the phase meter indicates if the load is resonant (voltage and current in phase), inductive (+ve phase; voltage leads current), or capacitive (-ve phase, voltage lags current).


    III. Receivers

  49. A LowFER Receiver Using a "Software" IF (by K0LR)
    This article describes a "software" receiver that uses a simple, low cost hardware downconverter in conjunction with sound-card software such as DL4YHF's "Spectrum Lab" or I2PHD's "SDRadio", which act as a tunable DSP-IF system. Spectrum Lab is a powerful piece of software that can be used for receiving a variety of signal modes, including very slow CW (QRSS) that must be read from a "waterfall" type of spectrogram display rather than being copied by ear. SDRadio does not provide a built-in capability for QRSS, but is very convenient and easy to use for single sideband, CW, AM and ECSS (Exalted Carrier Selectable Sideband) reception.

  50. VLF/LF to 28.5 MHz Receiving Converter (by DF3LP)
    Converts longwave to the 10 meter amateur. If your receiver does not cover 136kHz a converter can be used to upconvert the LF-segment to 10 meter.

  51. "Passive converter" using HCMOS switch (by SM6LKM)
    A LF up-convertor using CMOS switches (74HC4053) as mixer.

  52. Pre-amp / filter for 136kHz (by G3YXM)
    Most modern tranceivers cover LF but the performance is not always very good. All you can hear are strange whistles and burbles which aren't really there! You need this little circuit. It has a gain of about 10dB and a nice sharp band-pass response about 3kHz wide, enough to cover the 136 band.

  53. Loop pre-amp for LF (by G3YXM)
    If you want to use a small multi-turn loop on LF, you will find that the sensitivity is not good enough to receive the weaker signals. A good preamp for a loop needs to have a high input impedance and good signal handling characteristics (to prevent cross-modulation).

  54. A preselector for LF (by DL4YHF)
    Here is a preselector circuit designed by Wolf DL4YHF as part of his transverter. It uses two pot-core filters with FETs and feedback. By adjusting the controls, the bandwith can be set to less than 200Hz. This is the preamp which Wolf uses at the club station DF0WD.

  55. A high-performance low frequency converter (by KF5CQ)
    Many simple designs for low frequency converters have been published over the years. However, most of these designs used basic single-ended mixers with only fair performance in terms of sensitivity and dynamic range, and this has created the perception that a converter is a second-rate method of LF reception. That is unfortunate, because a well-designed converter working into a modern receiver will deliver performance on par with virtually any radio designed for LF.

  56. A universal LF/MF preamplifier (by KØLR)
    Perhaps calling this a universal preamp is stretching things a little, but it works on the LF and MF bands with loop, whip or random-length wire antennas. Regeneration can be used with any type of antenna, although the circuit provides very high gain even without it. Because of its versatility, this preamp is ideal for experimenting with various types of receiving antennas. This article also shows how the preamp can be powered and tuned remotely, with only a single coax line between the remote antenna site and the receiver.

  57. Integrated LF Preamplifier (by KA2QPG)
    Here is a little circuit I put together one evening so I could use a high-impedance E-field probe antenna with my commercial general coverage receiver. I wanted something small and simple which I could use mobile, and which would run on its own batteries so that noise from the car DC supply would not be introduced.

  58. How I gave new ears to my TS-950SDX (by I2PHD)
    After replacing a Kenwood TS-850 by its 'big brother' the Kenwood TS-950 Alberto was rather dissapointed by the sensitivity of the TS-950 on LF. In this article he shows how the source of the insensitivity was located and a cure was found

  59. An electro-mechanical RX for VLF (by M0BMU)
    Historic VLF station "SAQ" was put back on the air on 17.2kHz for a day of special transmissions on 1st July 2001. This inspired Jim Moritz, M0BMU, to construct an appropriate receiver for the event :
    "I was able to successfully receive SAQ with a homebrew electro-mechanical receiver. As far as I know, the Alexanderson alternator at SAQ is currently the only operating radio station with an electro-mechanical transmitter that does not rely on valves or semiconductors. For some time I thought it would be fun to make a VLF receiver based on similar principles, also without any valves or semiconductors, to receive the SAQ broadcasts. At first, I thought this would involve some difficult mechanical engineering, but somewhat suprisingly I was eventually able to make such a receiver using parts from the junk box."

  60. The optimal LF receiver (by DF8ZR)
    Bernd, DF8ZR, has done a lot of tests and measurements in his search for an optimal LF receiver design.
    Although this pages are in the German language the many diagrams, pictures and plots are usefull even for those with only limited (or no) knowledge of the language of Goethe.

  61. A problem with the Datong VLF converter on 73kHz (by G3NYK)
    As a result of a query from Roger G2AJV, we have discovered a potential problem on the Datong VLF Converter. This unit converts frequencies below 500kHz up to the 28MHz band. Roger found he had a high level spurious signal in the 73kHz band on two samples of the unit. Other units may have this spurious but it may well fall outside the band. This problem could occur with any LF converter using 28MHz as the IF. It will not be problem with converters using 10MHz or 4MHz as these units will use a fundamental mode crystal oscillator.

  62. 136 kHz preselector and amplifier (by IK2PII)
    This project was developed to reduce the image frequency response of my direct conversion receiver. It can also e useful as front end for HAM communications receivers and also for selective level meters used as LF receiver. The project focus was selectivity, not gain.

  63. 136 kHz direct conversion receiver (by IK2PII)
    I describe a simple direct conversion receiver, thinked for QRSS and DFCW communications, as companion of ARGO or SPECTRAN programs. I don't pretend to beat the performances of professional or commercial ham radio receiver, the scope is to suggest an easy way for beginner to listen (of course I mean look at) the signals in this poorly populated band. The receiver is useful also as portable receiver, fast turn-on receiver for a quick look an the band or as very low cost (and performances) spectrum analyzer. I currently use this equipment for tests purposes (e.g. visual frequency meter etc.).

  64. Balanced loop preamps (by K0LR)
    I've been meaning to put this balanced preamp schematic and details of the new loop construction on my web page for a long time. Unfortunately, when I put things off too long I forget some of the details of the circuit, like how many turns on what kind of core on the output transformer. The details given here are based on my best recollection of the transformer construction.

  65. Ein einfacher Langwellenempfaenger für den Amateurfunk im Bereich von Visual-Slow-CW (by DF8ZR)
    Für den angehenden Lowfer wird ein Empfaenger vorgestellt, den man mit wenigen Bauelementen schnell aufgebaut hat. Man kann damit die Aktivitaeten im Bereich um 137,700 kHz beobachten. Der Empfangsbereich ist mindestens +/- 80 Hz breit und laesst sich ggf. durch Ziehen des Quarzes nach beiden Seiten erweitern. Da man als Newcomer auf der Langwelle meistens nicht über eine leistungsfaehige Hochantenne verfuegt, kann man mit einer Rahmenantenne(Loop) erste Versuche machen. Diese muß allerdings einen eingebauten Vorverstaerker haben, also eine aktive Loop sein. Der hier beschriebene RX zeigte an einer mittelmaessigen Marconi-Antenne eine Grenzempfindlichkeit von -120 dBm ( = 0,2 uV an 50 Ohm). Dabei war das externe Rauschen ca. -100 dBm.

  66. Active loop converter for the LF band (by VK5BR)
    Localised noise interference is a common problem in receiving signals on the Low Frequency (LF) bands. From my experience, to minimise this noise there should be two essential features incorporated in the LF front end :
    (1) A sharply tuned circuit at the LF frequency. This limits both high level noise and strong signals on other frequencies from cross modulating the desired signal in the following mixer stage. The sharper the tuning, the better this is achieved. Unfortunately many LF converter circuits have broadband front ends.
    (2) Use of a tuned loop antenna. Localised noise predominates in the electric component of the noise field. The small loop picks up the magnetic component of received signal and is insensitive to the electric component of the noise field. Furthermore, because of its directivity, the loop can be rotated to enhance the level of the desired signal relative to other signals or noise which come from a different direction. Also because the loop null is quite sharp, this can be positioned in the direction of the unwanted signal or noise to some advantage. (Reference 1 gives more information on the loop theory).

  67. A notch filter for DCF39 (by G3NYK and M0WYE)
    After discussions with Hugh M0WYE, I sent him the circuit and parameter value for a notch filter that should reduce the strength of the German utility station DCF39 on 138.83 kHz by 30 to 40dB without giving more than a couple of dB loss inside the amateur 136kHz band. He made a quick practical example of the circuit with a small inductor and achieved results very similar to my simulations. This is offered as a possible experimental solution with no guarantees that it will work in every situation.

  68. Remote operated active antenna for 10kHz-40MHz (by LA8AK [SK])
    A wideband active antenna design developed by Jan-Martin Noeding, LA8AK. I received the circuit diagram a a few lines explanation by e-mail and put these on the web.

  69. VLF (136kHz) amateur-radio constructions (by LA8AK [SK])
    An impressive collection of design ideas for longwave converters / receivers.

  70. Receive Pre-selector for 136 kHz (by GW4ALG)
    This pre-selector can be used to increase both the gain and the RF selectivity of an LF receiving system. This circuit is lilely to improve considerably the reception of LF signals when using one of the modern 'general coverage' receivers, as such equipment often performs poorly below 500 kHz.
    A gain of about 14 dB can be expected.

  71. High-Gain Preamp (by K0LR)
    This preamp will work with short whips, parallel-tuned loops, and various forms of coax-fed wire antennas. There are other "bells and whistles" including options for remote tuning and regeneration. All of these features are useful under some circumstances, but frankly I rarely use most of them, and a simpler circuit is usually adequate.


    IV. Transmitters

  72. Class D power amplifiers (by G3YXM)
    Cheap power-FET's can be used to built a low cost PA for 136kHz. Dave Pick, G3YXM, designed some high efficiency PA's with common components. they have been built by many amateurs and proved to be very robust. There are several versions from 300W upto 1kW.

  73. A quarter kilowatt linear amplifier (by G0MRF)
    For several modes (as PSK31) a linear ampifier is needed. David Bowman, G0MRF, developed a 250W linear amplifier. Although efficiency is not as good as with (non linear) class D amplifiers, this one can be used for all modes.

  74. A 100 Watt 13.8 Volt class D amplifier (by G0MRF)
    A small 100 Watt class D amplifier. This 100 Watt class can be used with a traditional 13.8V power supply. 2 power-FET's in class D push-pull operation ensure high efficiency.

  75. A simple 1 Watt lowfer transmitter (by K0LR)
    An easy to built 1 Watt transmitter for LF. This crystal controlled transmitter is intented to be used for the US 'lowfer' band. Although 1 Watt is very little power on the CEPT 136kHz ham band it can be usefull on this band as a signal source for tuning antennas etc...

  76. How to Design a Class-E Transmitter for Your LowFER Beacon (by WD5CVG)
    How to design a very high efficiency amplifier. Class E amplifiers can have efficiencies that are very close to 100%. Read here all you need to know to design your own perfect amplifier.

  77. PIC controlled CW/BPSK beacon transmitter (by SM6LKM)
    This low-power beacon design includes a small transmitter (4 Watt) and PIC-based beacon logic.

  78. VLF / LF baseband transverter (by AA1A)
    This tranverter covers the 0-500kHz range and can be used with any HF tranceiver with a suitable transverter output.

  79. A single board 300W transmitter (by G0MRF)
    This single board transmitter fits on a 178 x 128mm PCB and includes a variable crystal oscillator, low pass filter, tx / rx switching relays, 'optically coupled' over-current protection and VSWR protection.

  80. Keyed PSU for class D transmitters (by G3YXM)
    Keying the drive to a class-D mosfet PA can create key-clicks due to the non-linear characteristic of the PA. A nice clean keying shape can be achieved by keying the supply with appropriate rise and fall times.

  81. 136kHz Transverter (by GW4ALG)
    The 'transverter' (transmit and receive converter) was designed to work with a Yaesu FT707 transceiver, operating at 10 MHz. All of my best DX QSOs were made using this set up. More recently, I have modified the transverter to interface with my IC756PRO.

  82. 400 W Power Amplifier using 572B tubes (by GW4ALG)
    The 400 W power amplifier (PA) uses many of the components from an old KW1000 HF linear amplifier (including the original pair of 572B valves). The amplifier was purchased for under £200 and has been modified from a switch-tuned grounded-grid configuration to an aperiodic (untuned), push-pull amplifier, rather like the QRO Class B Modulators of the 1960s.

  83. The digital amplitude modulator (by W9QQ)
    This PDF document presents a modern approach to digitally synthesized analog amplitude modulation boasting very high power efficiency, and linearity which is only a function of quantization error.

  84. The digital linear amplifier (by W9QQ)
    This PDF document carries the Digital Amplitude Modulator concept to the general purpose Digital Linear Amplifier. There is no restriction on modulation format except that orthogonal carriers may not be used. The output is a distortion free power reproduction of the driving signal with or without carrier and with or without both sidebands since it is virtually impossible to stray away from a straight-line transfer function.

  85. BPSK / Variable Phase Modulators (by M0BMU)
    There are 3 circuits - the Phase Keying Signal Shaper shapes the phase modulating signal before it is applied to either the Linear BPSK modulator, or the Variable Phase modulator. The circuits are designed for 10bits/sec for use with "Wolf" or "Coherent", and use a logic level phase keying signal available from COM port, EPROM keyer etc. Either modulator also requires a 137kHz carrier from an external source. A well-smoothed 12V supply is also required.

  86. A 600W bridged FET PA (by G4JNT)
    This design uses high Voltage FETs straight from the mains with no bulky / heavy transformer.
    Be aware that building this PA requires sufficient experience in handing high voltages / high power.

  87. The 'Marathon-Five' - A Low Power Transmitter for 136 kHz (by GW4ALG)
    As is common on LF, I usually run QRO on 136 kHz (400 watts). But, in May 2001, I started wondering whether the 136 kHz band would be suitable for QRP operation (i.e. operating with a TX power output of just 5 W). It certainly seemed unlikely because of the poor antenna efficiency achievable at a wavelength of 2200 m, when using an antenna just a few tens of metres in length.
    But I decided to build a QRP TX for LF, and give it a try! The 'Marathon-Five' LF TX to be described uses: a FET VFO; FET buffer; 2N2222 amplifier + BC212 keying transistor; 2SC2166 driver; and a pair of 2SC2166 transistors in parallel.
    Within the first week week several stations, at distances up to 122km were worked.

  88. A 200W transmitter for 136 kHz (by IK2PII)
    My first transmission experiments on 136 kHz band where based on a little TX build around surplus components, particularly the output transformer was wound on a TV EAT transformer. In Italy is impossible to find Philips 3C85 cores. For 200W transmitter I decided a more professional approach: buy a surplus switching power supply core and design the output transformer according to ARRL Handbook suggestions.
    The success was assured at the first try, so I publish my experience for all people interested in this band.

  89. A complete 5W transmitter for 136 kHz (by OM2TW)
    This project includes a very stable VXO (variation on the DJ1ZB/DF3LP design) and a 5W power amplifier using a single IC (TDA2030 or A2030). This transmitter can either be used for QRPP tests or to drive an additional high power amplifier (eg. G0MRF design).

  90. Class E power amplifier design (by G3NYK, GW4HXO and EI0CF)
    There have been many different configurations that have attempted to squeeze the best efficiency out of an RF Power Amplifier. Class E is a form of 'switching' amplifier which was patened by Nathan Sokal WA1HQC in around 1976. I first saw it described in Design Electronics in 1977. Being an amateur, Nat is an extremely practical man and the article gave a test circuit that could easily be assembled from laboratory components, and then measured whilst the components were altered. I had little interest at that time but the article was files away until I had attempted to modify an audio amp for 73kHz with very little success. I dug out the old article and read it several times. It did not give any guidance about were to start from in a new design, though it gave a number of optimised equations for calculating the critical component values. I started by putting the equations into a spreadsheet and tried some numbers to see what changed, with power, supply voltage and load. As a result I think I gained a feeling for the circuit and developed a 'recipe' for the process of proceeding through a design.

  91. TX module combiners (by G3NYK)
    The transmitter module combiner circuits shown below were copied from commercial circuits. The "Navtex" combiner was in a 1kW transmitter at 518kHz, but is only really useful for 2, 4, or 8 modules, whereas the other design can be expanded to any number of modules (notice the 'circular' connection of the hybrids). Note that without the output step-up transformer the output impedance is 1/N times the input impedance.

  92. A Tunable LowFER Test Transmitter (by K0LR)
    A variable-frequency transmitter is often useful, especially when checking a new antenna for resonance. The diagram shows a simple oscillator using a hex Schmitt trigger IC to drive the complementary-pair LowFER transmitter final.

  93. A Synthesized LowFER Transmitter With Built-In Keyer (by K0LR)
    Since Bill de Carle, VE2IQ published his multi-mode beacon keyer circuit in the LOWDOWN, I’ve incorporated variations of his design into several homebrew projects. Details of the VE2IQ keyer are found on Bill's web page. This article describes a complete 1-watt LowFER transmitter that can be built on a single 4 by 6 inch board containing a frequency synthesizer, clock reference, keyer and final amplifier. The synthesizer tunes in 100-Hz steps from 130 to 190 kHz, and the built-in identifier provides both Binary Phase-Shift Keying (BPSK) and CW operation.

  94. A Solid-State, High-Power LF Transmitter (by W5JGV)
    This article describes the design and construction of a high power solid-state LF transmitter for use at Part 5 Experimental Station WC2XSR/13. The design concept was to build a high efficiency unit, similar to a conventional switching power supply. It was designed to operate at LF on 166.5 KC, with the ability to modify it for use at MF near 440 KC.
    It was desired to operate the transmitter from a low voltage power supply, as this would minimize the insulation requirements of the components and increase operator safety. Additionally, low voltage rated components generally are less expensive. The design objectives were achieved, and the measured efficiency was in excess of 95%. While operating at a power output of 500 watts, the heat sink gets just barely warm. There is almost no noticeable temperature difference between the case of the PA transistors and the heat sink.

  95. A 400 Watt Low Frequency SSB Linear Amplifier (by W5JGV)
    After obtaining a pair of surplus Motorola Starpoint channel modems to use as SSB exciters at WC2XSR/13 on 168 KC, I wanted to build an amplifier to boost the power to about 400 watts, which is the maximum licensed transmitter output power for WC2XSR/13.


    V. Signal sources (oscillators)

  96. A DDS VFO (by SM6LKM)
    A direct digital synthesis VFO for 0 to 6MHz. For many applications on 136kHz (as slow CW, DFCW and PSK31) a very stable VFO signal is needed.

  97. A Software based DDS for 137kHz (by DK5PT)
    A short description of an experimental DDS. This DDS-development should not be a competitor to the readily available DDS systems and ICs from ANALOG-DEVICES et. al.
    The goal was to develop a cheap, amateur solution. After tests and experience with a DDS, made of discrete HC-MOS circuits which filled a complete lab-pcb. The idea was to replace the glue-logic by software.
    This project includes both theory and practice of DSS design

  98. A simple "discrete" DDS VFO for 136kHz (by OK1DX)
    This DDS uses 74HC series CMOS chips to generate 256 discrete frequencies between 135.68 and 138.23 kHz in 10Hz steps. Because the chips are cheap and easily available the cost of this project is low, about £20.

  99. Mixer VXO circuits (by G3NYK)
    It is not too difficult to design and build a VFO that is adequate at 136kHz for normal hand keyed CW operation. There are several examples on other LF web-sites ( see G3YXM , PA0SE, G3YMC and GW4ALG). Anyone attempting basic operation on QRSS, slow CW, using a soundcard DSP waterfall display for visual decoding, will soon find that they require something a little more stable. Several amateurs have built mixers VXOs around logic gates. These will work but do tend to drive the crystals very hard and can result in less than required stability. Some have tried to employ the standard circuits for Colpits crystal oscillators and found that they will not pull very far so that it is difficult to get full band coverage. The reason for this is that most of the published Colpits crystal oscillator circuits are designed to produce a stable oscillator not one that can easily be pulled off frequency. There is a halfway-house which I stumbled over due to my inate idleness.

  100. 136 kHz transmitter VXO (by IK2PII)
    After some test with my Xtal controlled transmitted I decided that my chances of two ways QSO should be improved by a tunable VFO. Some useful projects where found in the G3LDO (Peter Dodd) "LOW FREQUENCY EXPERIMENTER'S HANDBOOK", RSGB editor, but I'm not able to copy other's projects without some modifications (I call them improvements, HI). My mixture uses easy to find crystals, one transistor tunable oscillator for the higher frequency crystal, one CMOS IC for the other oscillator, for mixer and as output buffer/amplifier. The best from other projects found in the aforementioned book. The output frequency is 272 kHz; the transmitter exciter divides it by two.

  101. Building a Simple LF Exciter (by ZL1BPU)
    Ever wanted an LF Exciter (or a better LF Exciter?) Try this one, which is a state-of-the-art unit designed especially for LF enthusiasts and "Lowfers". It was designed with the advice and assistance of the highly successful ZL6QH team, whose signal on 181kHz has been heard halfway around the world! With good stability and 1W sinewave output, the unit can be used as a low power transmitter, signal generator, or as an Exciter to drive high powered transmitters.

  102. A precision VXO for the WC2XSR/13 transmitter (by W5JGV)
    Transmitters operating on the LF bands have unusual frequency stability requirements because most operation on LF is done using computers running weak-signal detection software. It is essential that the frequency of the transmitter be accurately known, stable, and adjustable by the operator. This page describes an oven stabilized VXO designed for this service.

  103. 136kHz VFO suggestion (by LA8AK [SK])
    Based on some thoughts LA8OJ and I did some time ago I am sort of planning to build an xtal controlled (VXO) source for 136kHz based on the xtal synthesizer principle, but it is unlikely that I need more than one xtal. Have many xtals around 6520kHz and this is the reason for choosing this set frequencies. They are all JAN-crystals bought 35 years ago but I cannot remember which project they were for, probably a 2m xtal synthesizer. Found some xtals with 1090kHz difference and also a par with 2180kHz difference, have many thousands xtals and it was a real task to sort through and calculate the difference frequency of suspect objects.


    VI. Audio filtering & noise reducion

  104. 30Hz audio filter with noiseblanker (by PA0LQ)
    A narrowband audio filter can significantly improve the readibility of weak CW signals. But the major problem the most narrow filters suffer of is ringing. PA0LQ developed a 30Hz filter with minimal ringing. Further the design includes a noiseblanker to reduce powerline induced QRM.
    The same filter is also available on G3YXM's website
    Another version of this filter, modified by GW4ALG can be found here

  105. Synchronous noise blankers revisited (by KA5QEP)
    Noise is always a problem on LF. If the LF DXer lives in a densely populated area, chances are that one kind of noise is worse than all the others. That noise is from many of the electrical appliances that interrupt the power mains due to SCRs at each zero crossing of the 60 Hz signal. It turns out that the predominant frequency of this noise is 120 Hz. If you suspect that this kind of noise is causing your problems, look at the output of an amplified loop or whip antenna with a scope synced to the line frequency. You should see a periodic spike pattern with the spacing between the spikes equal to a 120 Hz repetition rate.

  106. Reducing noise in home LF receiving installations (by WA4GHK)
    It has generally been my experience that the AC line noise that is encountered in most home LF receiving installations comes from two sources. The first and most severe is by conduction through the AC supply line and its safety ground. The second is by either inductive or electrostatic (or both) near-field coupling to nearby AC lines. Fortunately, it is fairly easy to reduce or eliminate both sources of noise.

  107. Noise canceller for 136 kHz (by GW4ALG)
    Operation on LF using small antennas from an urban or sub-urban location is a challenge in itself. But just one local source of noise can make it impossible to hear any amateur signals on the band. It was local noise that prompted me to build my first noise canceller.

  108. A simple signal canceller for 136kHz to combat LORAN or other noise sources (by G3GRO)
    Pulse sidebands from the LORAN navigation system which spread into the 136kHz LF band and often masks weak stations. With this signal canceller the LORAN pulse sidebands can be completely nulled out leaving the DX stations in the clear and making a major improvement to my 136kHz receive capability not to mention the relief from not having to listen through the constant clatter of the LORAN.

  109. Antenna noise and signal cancelling at LF (by VK5BR)
    The fact that the phasing between a wanted signal and an unwanted signal can be different on different antennas is put to advantage. With this condition, the unwanted signal can be cancelled out by controlling amplitude and phase. The heart of the system is some form of phase control circuitry. I described methods for doing this at HF in AR Sept. 1992 (ref. 1) and AR Jan. 1993 (ref. 2). The second system made use of the 180 degree phase shift which was achieved when two lightly coupled tuned circuits were tuned from one side of resonance to the other by a ganged pair of variable capacitors. It is this system which is again used at LF.
    The circuit described provides a phase and amplitude controlled auxiliary signal from a wire antenna. Whilst its output could be used to mix with some other antenna system, it was specifically aimed at mixing in with the signal from the loop antenna in the active loop converter described in AR Jul 2000 (ref. 3).

  110. An interference cancelling system for your receiver or transceiver (by VK5BR)
    The use of interference signal cancellation has been around for some time. The idea is to use an auxiliary antenna (almost any random length of wire) in addition to the main receiving antenna. As the two antennas are physically spaced from each other and also unlikely to have similar field patterns, the amplitude and phase difference between the two antennas for an interfering signal can be expected to be different to that for the desired signal. This particularly applies to a localised interference source where that source is largely coupled into the antenna by induction. This induction field follows a different law of signal attenuation verses distance to that of the radiation field by which the distant desired signal is being received. The two antenna outputs are combined after modifying their relative signal levels and phase such that the interference signal from one antenna is equal but opposite in phase to that from the other antenna. The interference signal is cancelled but as the two desired signals have a different amplitude and phase relationship, a resultant desired signal component is retained. Of course for all this to work. The interference waveform must be continuous and reasonably stable in its shape and amplitude. From my own experience. The system works extremely well for power line noise and frequency dependent noise bars generated by TV line time base and computers.

  111. Narrowband Filters for Weak Signals in Noise (by KA5QEP)
    MedFER beacons broadcast with 100 mw power in the 1600 to 1700 kHz band. DXing these very weak signals is a fascinating way to test receivers, antennas and especially filters, both IF and audio. Filters help greatly because random noise is reduced by 10*LOG(bw2/bw1) in dB, where bw is the filter bandwidth in Hz. For example, in going from a 3000 Hz filter to a 1000 Hz filter, random noise is reduced 4.8 dB, while a CW signal should remain the same amplitude for an ideal filter. Noise in this band is not the pure white variety, however, and has a strong spike component due to thunderstorm static, electrical line noise and atmospheric noise. This demands that filters not add excessive additional length to the noise spikes, while still doing a good job of filtering the signal frequency.


    VII. Other

  112. Assessing unknown ferrite toroids (by G3NYK)
    Ok so you have had a good day at the Rally and you have come back with a carrier bag full of ferrite toroids that "just might be useful for something". How do you find out what they are? Very few seem to be stamped with any indication of their properties, and some are 'painted' garish colours which presumably means something.

  113. Matching low pass filter (by G3NYK)
    This filter was designed from the program that is available on disc with Wes Hayward's book "Introduction to RF design". It is intended to provide good lowpass filtering for a 136kHz transmitter whilst also transforming the impedance from 4 ohms to 50 ohms. It avoids the use of a transformer, and the output capacitor can be made variable (a few switched fixed capacitors) to fine trim the impedance match.

  114. A vectorscope unit for displaying phase modulation (by M0BMU)
    A simple way of displaying phase modulation on an oscilloscope with X and Y inputs. It isn't meant to be a finished design, but certainly works fine as it is. As well as this circuit, you will need an oscilloscope capable of displaying in X/Y mode, and a reference "LO" signal source.

  115. A transmitting low-pass filter for 166.5kHz (by W5JGV)
    Since the transmitter at WC2XSR / 13 is actually a form of a switching power supply with a tuned output tank circuit, the harmonic content of the signal is fairly high. Even though the antenna system used here is a very narrow band system, it cannot be depended upon to suppress harmonics and spurious emissions. A low-pass filter is required to meet FCC regulations. Since various harmonics of the 166.5 KC carrier frequency will fall into the AM broadcast band, harmonic suppression is a real necessity.
    A standard 5-pole filter low bandpass ripple was designed and constructed from available components. The filter is symmetrical, in that either end may be used as the input or output connection. The filter is comprised of two air-core inductors and three fixed mica capacitors, two of which are the same value. The filter is designed to cut off at about 275 KC, and is usable up to about 200 KC. Losses are low, and the filter as shown can handle 600 watts continuous RF power. Losses would be very slightly less if a larger cabinet had been used so as to allow for greater spacing between the inductors and the cabinet walls.

  116. Simple frequency calibration for weak signal work (by W1TAG)
    Accurate calibration of your receiver and sound card is necessary for good results with the weak signal modes in use at LF. Modern receivers with master oscillators that are synthesized to provide all conversion frequencies greatly simplify the process. This article describes a method of making the necessary measurements and adjustments to an ICOM R75 receiver and a sound card, using the ARGO software. No other test equipment is necessary. You will need ARGO beta 1, build 128 or later, as the earlier versions do not have the calibration option. The method is not specific to the ICOM R75, requiring only that you know how to adjust the frequency of the master oscillator in your receiver.

  117. A dual-mode beacon identifier (by K0LR)
    Binary Phase Shift Keying (BPSK) offers a theoretical 6 dB advantage over CW. Experiments have confirmed its effectiveness, not just on 1750 meters but on the MedFER and HF bands as well. However, most of us are reluctant to switch our beacons to BPSK-only operation, because there are a lot more people out there with CW receive capability.
    Here's a way to have your cake and eat it, too. Using a couple of additional ICs with the EPROM-based identifier described by Bill de Carle (VE2IQ) in the April 1996 LOWDOWN, you can set up a beacon that automatically alternates between BPSK and CW operation. Henry Hampel's MedFER beacon STLMO is already using this technique. His BPSK signals have been "printed" at almost 1000 miles, even when the CW signal was completely inaudible. For listeners with only CW receiving capability, the BPSK signal sounds like a long continuous tone that is useful for tuning in the signal. It's like an extended version of the dash after ID (DAID) format preferred by many experimenters.