I recently build a Loop antenna for CB radio, or at least i tried.
Its made out of a 80cm diameter Loop of RG58 Coax (shield and core connected at the ends), a Coax stub condensator and a unshielded wire primary loop.
When i put my SDR on it, it seams to have way to much of a wide reception (calculator said it would have only like 40-50khz wide reception band).
When i put my analog power/swr meter on it, it claims to have a SWR of 1.2 and takes about 3.5W of power (compared to my dipole taking 4W).
But when i put the NanoVNA on it to get a more accurate reading of SWR, all i see is a flat line that claims a SWR of about 50.
When i pump up the stimulus frequency up to 300+Mhz i get some SWR dips there down to 1.6, but i assume thats just the Primary loop resonating.
Any idea why i get results on my analog SWR meter but not on the NanoVNA? Is the NanoVNA maybe putting to few power into the loop to make it resonate?
My repositioning the primary loop i managed to get the SWR down to 1.5 :D
I just hoped i aint cooking my insides when transmitting while sitting next to the loop antenna, a online calculator said at 4W i should keep at least 13cm distance from the antenna, but that was for a dipole not a loop…i need a longer feedline to get some distance…
I currently at berlin/brandenburg area, i kinda dont think i will be able to make long distance connection till i upgrade my radio to one that supports SSB tx with 12W (AE5900 eventually i hope). Long term i hope to manage to get a setup that allows me to join the europe wide JS8CALL network on 27.245Mhz. JS8CALL supports message forwarding over other nodes, a bit like meshtastic, i heared of people who managed to comunicate to australia via JS8CALL using multiple forwarding nodes, it is very slow tho due to using the same mode as FT8
HF is black magic. Don’t let anyone convince you otherwise.
Ah, damn. I am in central / southern germany, a bit too far for antenna experiments :)
To do some back-of-the-envelope math: Assuming a very much worst-case gain of 6 dB (very optimistic) and 12 W actual output power that all makes it to the antenna (it won’t) and all gets radiated (it won’t):
12 W * 10^0.6 = 48 W
In free space, ignoring near-field weirdness (which you really shouldn’t, especially not with a mag loop!) that would be:
P = E^2 /Z ; E = (P*Z)^0.5 ; (48W * 377R)^0.5 = 135 V/m
The BImSchV (nice name, isn’t it) says a max peak field strength of 5 kV/m and a quadratic average over 6 minutes of 28 V/m is allowed. So, back of the envelope says, running for short periods of time next to your chair is “probably ok”.
Out of curiosity, I asked the official BNetzA WattWächter. It tells me to keep a safety distance of 4m for a mag-loop at 12W, but it only has mag-loops for 7 MHz and below. It has a CB version, but it doesn’t run for me, and I am too lazy to debug some stupid java app.
Note that with mag-loops, the near field is pretty “special” in that it has a pretty high magnetic field created by the large reactive currents between the capacitor and the loop inductance. As a rule of thumb, everything within 1-2 wavelengths of distance (so like 20 m for CB) is near-field weirdness. But don’t quote me on all that, I never looked into this too deeply.
Now, I say about myself that legal limits are boring and that there are several lifestyle decisions to take before I should worry about non-ionising fields, like no more alcohol, more sleep, more exercise… but I am an engineer, not a doctor, so I am not really qualified to talk about this kind of stuff. Make the health decisions you are comfortable with.
Ah yeah sad, southern germany is a bit far away (besides maybe during high solar activity but even they the skips may fly just right over). Thx for doing some calculations, on the safety distance aspect of it, i kinda assume the psychological effect (knowing that u in the RF near field) is probably bigger an the real effect of 4W of RF heating…and i probably just hallucinated the slight metallic taste in my mouth after 1min of keying down while tx testing (my brain do be like that sometimes).