It's arrived. Even if Waters and Stanton confused me... I placed the order for an "ICOM" AL-705 loop antenna, and this large box arrived from Alpha antennas USA. No paper work so it made me think a bit. I called W&S they confirmed it was mine. Whoopee.
Now built the antenna
And "borrowed" my wife's camera tripod, got an iPhone clip mounting screwed on top and wired it up.
First trial I found the tuning very sharp, but I found a way to do it. Display the frequency you want on the SDR specturm display (I use an ELAD FDM-DUO TRX). You can then peak the noise. I found I could get an SWR of 2-4. So I put my MAT-125E auto ATU inline and this was able to tune it down to an SWR of 1.4 or better!
Noise levels are much lower (S4-5) than the small 60cm loop (S8-9) I used before, and signals are up 2-3 S points.
I checked it out also with my NanoVNA, and oh boy is the tuning sharp, and the knob very sensitive.
I have now had a few weeks to use the loop. And this is what I have found
1. It does work from 40-10meters (Alpha antenna can supply a 2 extra turn loop to expand to 80m)
2. It is a bit floppy and wuld not stand up in any windy conditions
3. It is sensitive to the shape of the loop and position of the coupling loop
4. It is incredibly sharp tuning as the frequency goes up, and to get a low SWR you must be exactly on tune.
I have used my NanoVNA to look at the SWR, and also to tune the loop This is the SWR on 17m (18.100MHz for FT8)
As you can see the SWR is around 2.3 at the best point. The tuning was very very sharp and difficult to target exactly the right frequency. The problem is that touching the tuning knob affects the resonant frequency, so you have to tune a little bit low so that when not touching it the resonance goes up 5kHz or so. I am starting to think about a motor driven system which I can remotely control back and forwards. Very low RPM (3rpm?) motor or a stepper motor...?
Looking at the Smith chart shows
Here you can see the impedence is not pure resistance at 50R which we want. This is the measured characteristics
An impedance of 61R plus a reactance of -j65 (Capacitive).
I use the loop via my auto0tuner MAT-125e. This brings the SWR to my transceiver down to 1.22 when I add a little inductance.
I use small antenna switch to
change over from Operate to Tune
I have started to be impressed with this loop. I have it indoors and even so it is reasonably sensitive, especially on 30m and above. I have taken up tuning it with my NanoVNA to whatever frequency (+/-20kHz at best) that I want to use. Then connecting it via my MAT-125E auto ATU and optmising the SWR again at the freuency of transmission. Normally I am able to get the SWR from 1.2-1.8.
