+++++++++++++++++++ From: "Tony Wells" To: "Elecraft" , "Low Power Amateur Radio Discussion" Date: Sun, 16 Feb 2003 12:50:11 -0000 Subject: [Elecraft] To all Mag Loop fans To all mag loop fans. I've been optimising and testing my 8ft dia mag loop, and whilst trying to make sense of my results, I came across this fascinating recent design note by Texas Instruments. Whilst it is very practical, it also contains some calcs for those that are interested. Preumably it is derived from other material, but nevertheless here is the url: http://www.ti.com/tiris/docs/manuals/appNotes/HFAntennaDesignNotes.pdf Regards, Tony M3CJF G7IGG ++++++++++++++++++ From: "Tony Wells" To: , "Low Power Amateur Radio Discussion" Date: Tue, 18 Feb 2003 16:45:45 -0000 Subject: [Elecraft] To all Mag Loop Fans (& Jackson Capacitor Fans) Folks, This may be already well known, but just in case. Whilst looking for some fittings for my mag loop, I came across the company that took over the manufacture of Jackson caps and spindles. Here is the URL. Plus a very interesting doc on the history of Jackson Bros. http://www.mainlinegroup.co.uk/ Regards, Tony M3CJF G7IGG +++++++++++++++++ From: "Stuart Rohre" To: , Subject: Re: [Elecraft] OT: SGC Coupler / Small Wire Loops Date: Thu, 27 Mar 2003 17:07:11 -0600 Martin, A Google.com search on "Loop Transmitting Antennas" and "Small Loop Antennas" will bring you a number of sites for Loop info. As some have said, one turn loops are the most efficient solution for transmitting. Large diameter conductor, and low loss joints are essential. Multi turn loops then, will have in contrast, higher I squared R losses due to length of several turns of wire. The efficiency of an antenna is measured by the amount of Resistance in the overall Radiation Resistance. Thus, you need to maximize the current flow for efficient transmission, and multi turns of ordinary wire defeat that objective. The single large turn, makes high current flow more easily introduced into the antenna. (If the joints are low loss) The SGC antenna is more of a "gimmick" to demonstrate their coupler will match most anything as a radiator. They HAD to have multi turns most likely to get the coupler to adequately load the thing. Wonder if anyone has tried the SGC on a single turn copper pipe loop? Most of those, that work efficiently, like the MFJ model, have a small coupling loop to the coax, and inductively couple to the large trans- mitting loop. The motor driven capacitor opposite the feedpoint takes care of tuning the loop to various bands, and within a band. Hope that helps, and you will get more detail in the web sites. 72, Stuart K5KVH ++++++++++++++++++ Reply-To: From: "Ron D'Eau Claire" To: Subject: RE: [Elecraft] OT: SGC Coupler / Small Wire Loops Date: Thu, 27 Mar 2003 15:59:19 -0800 A high "Q" resonant circuit, which one had better hope a small "mag = loop" is, will have very high peak voltages followed by very high peak = currents. Remember, the circuit is oscillating! All the energy is stored up as a charge in the capacitor, then it discharges through the coil producing = ... .uh.... I believe the technical term is "HUMUNGOUS" currents flowing in = the wire which transfers the energy to a magnetic field around the antenna. = As the current drops because the capacitor is discharged, the magnetic = field collapses producing a reverse current in the wire that then recharges = the capacitor. And on it goes, like any "tank" or parallel-resonant circuit. Some of the energy gets radiated and some gets lost in the resistance of = the wire and in any leakage across the dielectric of the capacitor. The less = the loss to the wire and dielectric, the more there is to make = electromagnetic waves with...=20 And since these are RF currents, they all crowd in the outer few atoms = of the wire, meaning that you get ... er... here's that word = again...HUMUNGOUS resistive losses unless you have a superconductor handy. Air is a darn = good dielectric - until it ionizes - so the bulk of the losses will be = resistive. The bigger the loop, the lower the Q and the lower the Q, the lower the = peak voltages AND currents. So the bigger the loop, the lower the resistive losses. Ron AC7AC K2 # 1289 -----Original Message----- ...I have to ask - Mag loop users will have noticed the phenomena of extremely high voltages across the loop capacitor - hence the need for high-voltage capacitors when high powers (greater than 20-50 W ) are = poured into the system. How do you reconcile that high-voltage with the "conventional wisdom" of the need for low resistance to achieve high current. Regards, Tony M3CJF G7IGG ++++++++++++++++ From: "Stuart Rohre" To: "Tony Wells" , "elecraft" Subject: Re: [Elecraft] OT: SGC Coupler / Small Wire Loops Date: Thu, 27 Mar 2003 18:06:03 -0600 The low series impedance in the conductor circumferential length is to allow the current to circulate at low loss, but the impedance at the gap across the open spot in the loop goes toward infinity, (open circuit, right?) A cap bridging that spot then will see a high potential difference at the spot opposite the feedpoint location. Some loops are fed by a tap moved around the loop feedpoint side, as in a gamma match. One experimenter has another scheme, which is an insulated conductor wrapped around part of the loop. I have never seen a picture of it, and the description made it sound like it originates on one side at the gap in the loop where the cap for tuning goes, and winds for a portion of the circumference. Our laboratory has been experimenting will various small HF Antennas for over a couple of years. Many copper pipe loops have been built to compare them to our staff member, Bob Rogers's invention of the FLEX Folded Helical Conical vertical. This antenna has much less height than typical single turn HF loops, at greater efficiency when measured in the Wheeler Cap, a large can used to enclose an antenna for efficiency computation by comparing open field S parameters to enclosed field S parameters. Some of the Loops are small enough to be measured in the largest Wheeler Cap we built. (We have two). Perhaps the best description of the advantage of the large conductor element loop, is that it is capable of passing high currents in that region of the loop where the current peaks, and also the large conductor lessens the chance of corona discharge at the gap in the loop. Breakover in the capacitor and losses at that gap can nullify the size of advantage of the loop, by limiting the power that can be transmitted. 73, Stuart K5KVH +++++++++++++++++ Reply-To: From: "Ron D'Eau Claire" To: "'Tony Wells'" , Cc: "'George, W5YR'" Subject: RE: [Elecraft] OT: SGC Coupler / Small Wire Loops Date: Fri, 28 Mar 2003 01:00:31 -0800 OOPS!=20 George, W5YR, kindly pointed out to me that I was ... well, as he put in very gentlemanly, I "gave him heartburn". And with good reason, because = I generalized too much and incorrectly when I wrote: "And, again, you are absolutely right, the current and voltage are 180 degrees out of phase = and MUST be in order for energy to flow efficiently in the circuit. Indeed, = in any resonant circuit (including any antenna) that is essential for power transfer." NOT true. The phase relationship between voltage and current are = critical for power transfer, but to say the phase relationship are 180 degrees is wrong. That is true when describing the operation of a resonant circuit = as I was doing earlier, but that is only one case.=20 James Clerk Maxwell described the behavior of electric and magnetic = fields mathematically in the 1800's and his data still stands. I am not here to refute it, but in answering a question quickly I have demonstrated that = I can scramble it beyond recognition.=20 The phase relationships between the current and voltage in a circuit carrying an alternating current (including those high-frequency = alternating currents we call "RF") are critical to the efficient transfer of power, = and those phase relationships are controlled by the inductive and capacitive reactances in the circuit. An ATU, like virtually any "coupling = circuit", "filter" or other circuit that handles a-c signals uses those phase relationships to do its job helping or impeding the flow of a-c currents from one place to another.=20 So, please let me amend my statement above and say that the "phase relationship of the voltage and current are absolutely critical in order = for the energy to flow efficiently..." And thank you George, for pointing out that I was trying to give Maxwell = get a case of heartburn as well... Ron AC7AC K2 # 1289 ++++++++++++++++ From: "Tony Wells" To: , Cc: "'George, W5YR'" Subject: Re: [Elecraft] OT: SGC Coupler / Small Wire Loops Date: Fri, 28 Mar 2003 10:07:07 -0000 Ron in his previous posting, kindly wrote a lot of stuff about Mag Loops and the voltage and current phase relationships, and the summary was: >So, please let me amend my statement above and say that the "phase >relationship of the voltage and current are absolutely critical in order for >the energy to flow efficiently..." Hi Ron, Well, I ain't JC Maxwell, my education was in quantum mathematics, but I have been a mag loop obsessive for a long time, and I would endorse your caution about stating the exact phase relationship. There is some evidence that the phase relationship varies around the curcumference of a mag loop circle. Stuart K5KVH alluded to this I think in an earlier posting on this off-topic topic. There are certain areas around the circumference of a mag loop where low resistance does not seem important from the point of view of radiating into free space. I suspect, but do not have the tools to prove, this is where voltage and current are more in phase than elsewhere in the circle diameter, and the current is relatively low. Typically the area concerned is approximately 1/8 of the circumference spanning the location of the capacitor. Also, depending on the type of matching network (eg small loop transformer vs gamma match) there is a similar area which seems to vary in size between 1/16 and 1/8 around the location of the matching section. I've also done experiments to extend the unimportant areas of radiation resistance by putting multiple capacitors around the circumference of a loop. I used 5 high-voltage caps and one variable tuning cap and ordinary mains flex connecting the lot. However the results were inconclusive because when I was checking the far field strength I blew all 5 caps on 5W !!!! Lots of smoke and sparks! When I get some more caps I'll report back! There is one other point. Many mag loop users want to get more than one band out of their loop and can get a reasonably good SWR across 2 or more bands when they adjust size and capacitance. However the radiating efficency in the far field is considerably effected by trying to achieve this. This is caused by the "traditional" approach of using fixed ratios in the matching network - eg ratio of diamete of matching loop/gamma match vs main loop. I *think* this is heavily related to the phasing issue, but will probably never prove it. When using an antenna analyser in conjunction with field strength tests, and drawing lots of graphs, it is possible to see that there is only one optimum transformer ratio for a particular frequency for a particular loop diameter. In practice, for frequency, read "band". The way to handle the ratio problem is to use multiple matching ratios - gamma matches are best for this - using relays for example to swich in different gamma match lengths. Anyway.... Regards, Tony M3CJF G7IGG +++++++++++++++++++ Date: Sat, 29 Mar 2003 07:47:04 -0700 From: "James R. Duffey" To: elecraft at mailman.qth.net Cc: rondec at easystreet.com Subject: [Elecraft] Small WIre Loops The radiation resistance and the far field strength of an antenna depends on the current distribution on the antenna, which does not have to be sinusoidal. I believe that George is correct that the voltage and current must be 180 degress out of phase for efficient power transfer. But it is the current distribution that drives the antenna radiation resistance and hence the efficiency, not the phase relationship between the voltage and current. I must confess that I do not understand your statement: "There are certain areas around the circumference of a mag loop where low resistance does not seem important from the point of view of radiating into free space. I suspect, but do not have the tools to prove, this is where voltage and current are more in phase than elsewhere in the circle diameter, and the current is relatively low." For a circular loop, the radiation resistance will be the same no matter where the antenna is fed along he circumference. There is no unique orientation for a loop in free space. For example, in free space hotizontal and vertical have no meaning as there is no reference datum to define these points from. Likewise, the circular loop is a series circuit, and no matter where it is fed the Ohmic resistance appears in series with the feedpoint and will dissipate the same amount of power. When you add the capacitors to the loop you are changing the current distribution to make it more constant, that is less sinusoidal. This makes the radiation resistance and the far field field strength go up. It has nothing to do with the phase between the current and the voltage. You have empirically discovered one of the drawbacks of small loops; even low powers can generate high voltages and hefty components are usually required. - Dr. Megacycle KK6MC/5 +++++++++++++++++++