Ron makes a very important point here. Using an ATU in place of properly resonating a Magnetic Loop is a no-no.
If the loop, at resonance, doesn’t provide a good match to 50 ohms, then it is ok to use the ATU to “fix” the mismatch. Note the use of “fix” in quotations. You are really not fixing anything, just making the rig see a resistive 50 ohms or something close to it so that it is happy to deliver full power to the loop. You will incur some additional losses in the tuner, but they are likely minor. Just make sure you properly resonate the loop before kicking in the ATU. One additional point worth mentioning. A Magnetic transmitting loop fed via a coupling loop works like a transformer. The 1 to 5 ratio between the size of the coupling loop as compared to the main loop provides an impedance transformation that should normally give a good match to 50 ohms. If, at resonance, you are consistently seeing an SWR higher than what you would like then it is possible to tweak things by either slightly moving the position of the smaller coupling loop, or by slightly deforming the coupling loop in the vertical plane (i.e. making it rounder or more oval). It is usually easier to just bend the coupling loop. The process is trial and error but usually you can find a configuration that will yield a pretty good match on all bands covered by the loop. I had to do this with my MFJ-1786 Hi-Q Loop as the best match I could get at resonance was around 2:1 SWR. With a bit of tweaking it now typically around 1.5 : 1 SWR on most of the bands. Michael VE3WMB P.S. Manually resonating a loop by ear is an acquired skill. With practice, peaking band noise will get you very close to resonance without transmitting. Then with a second or two of applied RF, aided by feedback from an SWR meter, you should be able to hit the loop resonance point pretty much dead-on. >From: "Ron D'Eau Claire" <r...@cobi.biz <mailto:r...@cobi.biz>> >Date: December 4, 2015 at 4:09:18 PM GMT-5 >To: "'Elecraft Reflector'" <elecraft@mailman.qth.net ><mailto:elecraft@mailman.qth.net>> >Subject: Re: [Elecraft] [K2/KX3] Tuning a magloop >There are two parts to tuning a small transmitting loop antenna: Resonating >the loop at the >operating frequency and matching the loop at resonance to the >impedance of the feed line. >The Alexloop tuner sounds like a great idea, provided it does both at the same >time. >IMX I tune the loop for maximum noise in the receiver. Properly designed small >loops have very >high Q, so the peak is very "sharp". >If I then transmit and find that the SWR is high, I'm sure the coupling >between the loop and the >feed line is not correct. That is, the loop at >resonance is not presenting a 50 ohm impedance to >the feed line. >So the challenge for optimum power transfer is to adjust the coupling and >tuning to get both peak >noise and low SWR at the same time. >If you are not worried about feed line losses, I would expect the loop to work >just fine using the >ATU at the rig to compensate for the odd impedance >presented to it by the feed line, but be sure >to check the loop tuning after >the ATU does its job since it will interact with the loop and shift its >>resonant frequency somewhat. >It's like two people standing up in a rowboat. You both work together or you >both get wet. >73, Ron AC7AC ______________________________________________________________ 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
