I had built SM0VPO's spiral loop antenna. And here it is.
Recently I got myself a NanoVNA to see how finnicky it is to be tuned with that poor man's tuning cap. I brought it from 12 MHz into the 14 MHz 20 m band range by cutting off the extra wire to reduce the capacitance at the end.
But it is still not ideal. Here is the NanoVNA measurements:
I can't get the SWR under the 1.67:1 and essentially the impedance is 80Ω instead of 50Ω.
I also note that this antenna is pretty bad at receiving. I have no comparison to anything better at this point as it's really constraining inside an apartment where I'm still stuck, but I don't trust this baby.
I know that a spiral loop may not be the ideal antenna, but this has got a bit of fame because it is such an easy thing to build from scratch, so I wanted to give it the best chance.
How can I improve the matching to get 50Ω impedance / 1.0:1 SWR?
From the Extra exam I just took, I remember you could put a length of 75Ω coax in series on the feed point to adapt 50Ω to 100Ω.
I also remember the stub match. Especially the very simple technique to just run a parallel open piece of coax from the feed point. But I guess that can only work at precisely one frequency and cannot be tuned, even a little bit. Given that mag-loops in general are such low bandwidth antennas, tuning ability is essential.
There seems to be excess capacitive reactance, what can I do to counteract it and not just reduce the (already bad?) efficiency of the antenna even more? I mean, I could perhaps insert a coil at the feed point, but I was hoping perhaps I could tweak something else?
I twisted and pushed and shoved the wire around to see if I can make an impact, and also moved to different locations, away from that lamp thing, away from the wall, etc. to no decisive avail.
Could I tweak this 1:1 Ruthroff balun, as there is apparently tweak-ability? Mine is according to the instructions done with 7 turns.