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 LF Transmitters

 5W Transmitter/Driver for 137kHz

      The 5W transmitter, or drive source for a PA, is shown  opposite. It consists of a TDA2030 IC as QRP PA driven by a driver/keyer consisting of two BC549 transistors. The signal source is either a VFO (2 X 2N3819s) or a crystal oscillator (dividing down from an 8MHz crystal with a 74HCT4060 IC), selectable from the front panel. The mains power supply produces 20V unregulated for the PA, 13.8V regulated for the driver/keyer and RX pre-amp, and 12V regulated for the relays. Each oscillator has a separate 5V regulator. The relay in the back left corner is for aerial switching, next to which is the output low pass filter and aerial current sample circuitry for the front panel meter. The second relay switches the DC power, a 13.8V output is taken to the rear panel from this relay (and is only available during RX) to operate a pepperpot type antenna. A choice of keying socket is provided, either for a 'normal' hand key or computer generated keying. Power output can be controlled from the front panel and a 'carrier test' switch allows a carrier to be output during key up for SWR measurements and ATU adjustments. Front panel LEDs indicate the condition of the unit. The meter current sample board has a small preset for adjusting the meter, this is set so that 5W into 50R is just at the start of the red section of the meter scale. The output waveform is excellent and for brief periods an output in excess of 7W is possible.

 The First PA for 137kHz 

      The first power amplifier at G4AYT is a twin 100W BK Amplifier type MXF200 contained in a 19" rack chassis. This unit consists of two totally separate 100W amplifiers, with independent power supplies and gain controls. Only one amplifier is used, without modification to the original circuitry. The unit comfortably produces 25W of RF at 137kHz and well in excess of 50W without distortion. The above transmitter (backed off) is used as the drive source, coupled in to the PA via a small 3C90 toroid transformer 16:32 turns of 22swg enamelled copper wire, the 32 turn secondary is loaded with 2 X 100R 2W resistors in series and taken to the mono jack input (see Fig. 1). The amplifier output is loaded with a large 3C90 toroid transformer wound with solid insulated copper wire 5:10 turns, the 10 turn secondary driving an inductance fed low pass filter. The inductors in the LPF (L1 and L2) are each 65 turns of 20swg ECW on a T157-2 toroid. The primary is fed from the amplifier output via a 4.7uF 630V 'audio-grade' polypropylene axial capacitor (see Fig. 2). The 4.7uF capacitor can be obtained from Maplin Electronics, part number KR82D.


                         The BK Power Amplifier                                        Output Capacitor and Toroid                                       Transmitter Waveform

 Transmitters for the 472kHz Band

      A 1W CW transmitter for the 472kHz band has been built following the design on GW3UEP's website and works well. G6PKS has followed the same design with equal success, G8NAV is in the process of constructing. The TX uses the 4060 oscillator/divider IC to generate a moderately stable signal at 475kHz from a 7.6MHz crystal. This is then amplified by BC327/BC337 transistors.

      Right: The 1W transmitter, built into a 19" rack tray. Switches are for CARRIER, NET, TX and MAINS (just out of shot on the right). LEDs indicate oscillator and PA supplies 'on'. The two sockets on the left are 1/4 inch jacks for computer control (with a BC547 interface) and straight key. Power is adjustable internally with an LM317T regulator.

      A second transmitter running 8 or 25 watts has also been constructed, again following the GW3UEP design. This time the TX is VFO controlled with relay switching for antenna and PA power. It has an internal PSU providing 24V and two lots of 12V regulated. The VFO stability is adequate for normal CW operation, on the margin of acceptable for QRSS3, but unsuitable for any slower speeds unfortunately. The signal is generated around 3.8MHz using an HCF4049 IC and divided by 8 with a second IC CD4024. This directly drives the power amplifier which is a FET type IRF510. The transmitter has a built in interface for computer keying and an antenna current meter. As is usual at G4AYT, the whole self contained unit is built into a 19" rack tray.


                       The 8/25W 472kHz Band Transmitter Unit                                 IRF510 FET Drain Waveform                    Transmitter RF Output Waveform

 Useful Information

1 10 20 141
2 14.14 28.28 200
3 17.32 34.64 245
4 20.00 40.00 283
5 22.36 44.72 316
6 24.49 48.99 346
7 26.46 52.91 374
8 28.28 56.57 400
9 30.00 60.00 424
10 31.62 63.24 447
15 38.73 77.46 584
20 44.72 89.44 632
25 50.00 100.00 707
50 70.71 141.42 1000
100 100.00 200.00 1414
150 122.47 244.94 1732
200 141.42 282.84 2000

      The table to the left can be used in conjunction with a calibrated oscilloscope connected across a 50R load, which is on the transmitter output, to measure transmitter power. Alternatively, an RF ammeter in the 50R co-ax going to the load can be used. Either method will give an accurate way of measuring how much RF the transmitter is producing. The oscilloscope is perhaps best in that the output waveform can be checked at the same time. This is the procedure adopted at G4AYT.

      Peak voltage(V) = Square root of [100 X Power(W)]

      Current(A) = Square root of [Power(W) divided by 50]

      Effective Radiated Power (ERP) calculations are notoriously unreliable. ERP is best measured, but professional equipment suitable for this is unlikely to be available to most radio amateurs, including G4AYT. A rough approximation of mid band ERP (for the RF travelling in a vacuum) can be obtained from:

For 136kHz band, ERP(W) = [Current(A) squared X Height(m) squared] / 3042

For 472kHz band, ERP(W) = [Current(A) squared X Height(m) squared] / 252

   Interesting note: In air, the denominators become 3040 and 251 respectively.

      Where current is RF current in the antenna and height is the effective height of the antenna (usually between 50 and 100% of the actual height).

 *NOTE: Into a 50R suitably rated dummy load. Also note that the formulas on the right above give ERP (fine for the 1W ERP limit on the 136kHz band), BUT the licence NoV limit on the 472kHz band is 5W EIRP, an important difference. To convert ERP to EIRP multiply by 1.83.

 Testing (1) Wavemeter


      Some sort of RF Field Indicator, such as the wavemeter above, is a handy device for indicating RF around the antenna. The one used at G4AYT is built using a LW aerial coil/coupling coil on a ferrite rod from an old transistor radio. The variable capacitor can be any reasonably high value to hand (> about 150pF) and the meter was salvaged from an FM tuner. The sensitivity pot can be wound down to prevent the meter being damaged in high RF fields. During set up, the VC is set for half mesh and the rod is gradually moved in the coil until a reading is obtained near a known 136/7kHz transmitter or signal generator, then secured in position with a spot of candle wax. The meter at G4AYT also doubles up for the 472kHz band, a double pole two way switch is used to switch in a MW coil/coupling coil with 2,870pf of capacitors across the larger winding (in place of the LW coil/coupling coil and 1nF capacitor) and set up as before using a 472kHz signal source. A small adjustment of fixed capacitor was necessary to compensate for the effect of the metal box being closed.

 Testing (2) RF Current Meter


      The RF Current Meter at G4AYT has two ranges, 1A and 5A FSD. The original version had a 0.5A range but this was found to be too low to be of any practical use. The two ranges are selectable with a single pole two way switch. The 5A range resistor consists of 560k and 68k resistors in series. The inductor is a small 3C85 toroid with a winding of 50 turns of 26swg enamelled copper wire, the wire between the input and output sockets simply passes once straight through the toroid to make a primary 'winding', a complete turn makes no difference to the meter reading but might make the arrangement somewhat more rigid. SO239 sockets increase the versatility of the unit in that standard 4mm plugs may be used as well as PL259s (note the unit does not require an 'earth' to work correctly). Using this meter is an excellent and reliable method for tuning up for maximum RF out from the TX.

 Testing (3) SWR Measurement

      Although the variometer/matching unit has an internal SWR bridge, this was found to be insensitive at very low power levels. The first attempt to construct a low power meter followed the design in issue one of SPRAT, the magazine for the G-QRP Club. Unfortunately this was quite useless at LF (and the diode in the circuit diagram is shown the wrong way round). The second QRP meter design tested was found on the internet and had apparently been published in several radio magazines, including RadCom, PW and SWM. Although this proved sensitive enough it had several 'faults', not least of which was it did not register a 1:1 SWR when operating into a known good 50R dummy load, rendering the instrument useless! The final design used is a combination of ideas from several sources and can be found below. This meter is reliable and effective, as well as showing good sensitivity.

      The QRP SWR bridge above is more than satisfactory for a 1W level on the 472kHz band. It is not frequency sensitive (at HF and LF, but is unsuitable for VHF) and although only been tested at 1W, should be safe at 4W or a little more for intermittent use. Construction is not critical but the components should be bult into a metal box. Capacitors are poly rated at 250V or more, the switch a three pole three way and the meter 1mA or slightly less. The 100R resistors are each rated at 2W. In operation the antenna is connected to the ANT socket and the switch is placed in position 1 'SET' with the potentiometer (which should be linear) turned to minimum (anticlockwise, slider on 'earthy'end). RF is applied to the TX socket and the potentiometer adjusted for FSD on the meter, then switching to position 2 'REF' the meter will indicate any reflected power from the antenna (a zero reading indicates a 1 : 1 SWR). The switch is moved to position 3 'TX' for normal transmitting, where  more power may be used. The unit should, of course, be placed in the 50R co-ax line immediately after the transmitter and before any antenna tuning/matching arrangements.

Note: If the meter used is <1mA FSD, the 2k2 resistor may need to be increased in value.