Dave I read your post with interest, as over the weekend I was looking at an automatic loop tuner described in the November/December isse of ARRL QEX Magazine. This was for a MFJ-1788 Magnetic Loop working with a KX3 and based on an Arduino micro-controller. I don't own the MFJ product but I do have an Alex Loop and with a few simple modifications the circuit should work nicely.
The loop tuner uses a DC motor but I thought a stepper might be more elegant. Thus, I needed to determine the working bandwidth of the Alex Loop to get some idea of the angular resolution I needed from either a direct drive stepper, or a stepper using gearing. I looked at the Alex Loop around 7, 18 and 30MHz. 3:1 VSWR bandwidths were measured using a RigExpert AA-54 antenna analyzer. For 3:1 VSWR the measured operating bandwidths at these three frequencies were determined as 44, 65 and 190kHz respectively. Pretty narrow at the lowest frequency. Thus, to get a good chance of achieving a VSWR approaching minimum, I figured the angular motion should be fine enough at worst case (7MHz)to achieve an angular motion of the tuning capacitor of no more than 1/0th the 3:1 VSWR bandwith = approx. 4kHz. Since one and a half turns of the tuning capacitor tuned the Alex Loop from resonance at 7 through 30MHz (and assuming the frequency response is linear, which it is not) then the required angular resolution is 360 * 1.5 * 4 /(30000-7000) = 0.094 degrees. Since a typical stepper does 200 steps per revolution, then the required gearing ratio would be approximately 20:1. With this type of performance the magnetic loop could be very close to resonance at the chosen operating frequency presenting a VSWR approaching 1:1. 73's Gary K6YOA Message: 3 Date: Wed, 2 Dec 2015 20:52:18 -0000 From: "Dave Lankshear"<d...@lanks.plus.com> To:<elecraft@mailman.qth.net> Subject: [Elecraft] [K2] Tuning a magloop with KAT100 Message-ID: <7B2795A9D35B4139812BF284FD02542B@DaveLLaptop> Content-Type: text/plain; charset="us-ascii" Hello, Brian.
I've read your post and the replies and figure I maybe read what you said incorrectly, but you did say:
Then use the TUNE button on the K2 to tune the loop to resonance, then operate.
You can't use the K2's antenna tuner to tune a loop to resonance. That's not how they work.
The loop is a complete tuned circuit in itself. There's the loop representing inductance and a capacitor in parallel with it. These are resonant at a frequency and the only practical way to move that resonant frequency is to adjust the value of the capacitor that's in parallel with the loop.
You can't adjust that resonant frequency using an auto ATU at the end of a length of coax. It's rather like using a telephone conversation to make a physical change at the other end. You can't use the phone to make the beds back home, when you're away! The auto ATU will see the coax cable and the loop as a lump of L and C and will endeavour to match it to 50 ohms to get a 1:1 SWR. The coax forms part of the antenna and is not behaving like a transmission line and the loop is not behaving like a resonant circuit - you might just as well hook the coax to your automobile's fender and use the auto ATU to tune that!
Please forgive my descriptions if I have misunderstood your question, but re-reading your quoted statement makes me feel like I'm the only one who has understood.
Yes, by all means match the auto ATU into 50 ohms using a dummy load. That way, the PA is looking into a load that matches the impedance of the transmission line, although as Don suggested, you don't really need the auto ATU (and its losses) as the PA should be reasonably well matched by bypassing the auto ATU entirely. Now, at the loop end, there's a gamma match arrangement that ensures the loop, when at resonance, is a decent match to the 50 ohm transmission line you're using. So the rig matches to the transmission line which matches to the loop that's been tuned to resonance with its inbuilt tuning capacitor.
A loop is only a single turn coil, the resonant frequency of which is varied by adjustment of its parallel capacitance. Because it's small and is a low loss inductor operating with a low loss airspaced (or vacuum) capacitor, the Q factor, or "goodness" of the single turn coil at resonance is very high. This means that a small excursion away from resonance, the loop's Q falls very rapidly and renders it pretty useless, thus it is necessary (more so when transmitting through the loop) to retune it for frequency shifts of more than a few kHz. That means that the SWR rises rapidly away from resonance and the coaxial cable is more involved in becoming part of the antenna and less of a transmission line.
The outer surface of a loop (well, outer 6%) needs to be of very low resistance in order to maximise Q at resonance. RF skin effect uses only the outer surface of the conductor, thus the larger the surface area of the conductor, the more its internal resistance is in parallel and thus reduced, so the better performer the loop becomes.
Even a soldered joint on copper piping offers resistance that compromises the loop's performance. Recently a friend gave an old army magnetic loop to a group of collectors/militia enthusiasts. It was in poor condition, but in its prime, its surface area must have been a foot across. This makes a mockery of the little bits of aluminium (aluminium) joined together with bolts and wing nuts. Yes, says the vendor, it is broad banded and only needs retuning every 100kHz or so. What he doesn't say is that its resistance makesthe Q so appalling that its performance is lousy (where lousy is the polite word), but those devices give properly engineered mag loops a bad name by tarring all with the same brush. Also, there are proportionally more crappy mag loops out there simply because they are cheaper than the "real" thing.
The MFJ 1782/86/88 aren't too bad and are just at the crossing point between good and bad, with a bias towards the good, if not too many spiders and other insect life are resident under the black covers. These loops have an airspaced variable capacitor within the black covers and that capacitor is tuned by a small electric motor that's attached to it. DC power for the motor is fed down the coax cable itself, as well as the radio signals. They are not difficult to separate, eliminating the need for a control cable. The DC voltage on the coax is reversed in order to make the motor turn in the opposite direction. The more sophisticated MFJ controller has an inbuilt cross-needle SWR meter and the name of the game is to get the SWR as low as possible on the operating frequency. Their semi-automatic controller drives the motor and detects the lowest SWR point and stops. Of course, to detect the lowest point, it has to begin to increase again, so it always stops at a point that's not quite at resonance and this must be fine-tuned by the operator, hunting to and fro with up/down press-buttons. It takes more effort to describe than to do in real life Hi!
One last caution. Circulating currents are very high in a transmitting mag loop and very high voltages (thousands of 'em) are also present when at resonance, so ensure that the loop can't be touched by anything that matters to you when it's transmitting, even at a few watts. Give it a LOT of respect, indeed, powerline respect.
I hope my efforts haven't missed the mark by a mile, Brian and that instead, the wear and tear on my keyboard has been of some small use to you.
73 and early Season's Greetings. Dave G3TJP ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
