Comment on NanoVNA vs. Loop antenna SWR testing [Question]
LH0ezVT@sh.itjust.works 3 days agoMy repositioning the primary loop i managed to get the SWR down to 1.5 :D
HF is black magic. Don’t let anyone convince you otherwise.
currently at berlin/brandenburg area
Ah, damn. I am in central / southern germany, a bit too far for antenna experiments :)
I just hoped i aint cooking my insides
To do some back-of-the-envelope math: Assuming a very much worst-case gain of 6 dB and 12 W actual output power that all makes it to the antenna 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.
einfach_orangensaft@sh.itjust.works 3 days ago
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).
LH0ezVT@sh.itjust.works 3 days ago
Most likely :)
Keep in mind, I assumed worst case, both with my estimate and with what I’ve input into the program. Plus, the legal limits probably have some rather large safety margin.
To be honest, below 10W and without a highly directive antenna, you’re probably fine.
Again, I am not a doctor, most RF limits assume mostly the heating effect is relevant. I personally don’t believe it causes you brain cancer or makes you want to drink coca-cola. But I might be wrong.