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?
idk how you have done that, maybe i have older version but for me this marker just reads CH0 SWR 1.00/(value), this is some random vhf/uhf dipole that i found
you can pull up a smith chart, this will tell you whether impedance is too low or too high, since it's still not matched at resonance. if matched it'll be much milder for your transmitter but make sure that nanovna is calibrated (with feedline)
All good points, especially calibration. Without, it still gives you a general idea, but it's more a "meh looks OK" than a serious value.
Also, as fullsquare said, take a picture of yhe smith chart / impedance display as a second check, if it shows something like alot + j toomuch, you know your SWR is actually bad, and you know what direction you need to go for (add more capacity, inductance, or some resistive transformation).
Here is the measurement with smith chart:
There is 2m of BNC RG58 coax between the antenna and the NanoVNA. I calibrated the NanoVNA without the coax line because i was under the impression the line to the antenna is considered part of the radiating system and hence should not be calibrated out. But here is the reading with the coax line to the antenna calibrated out:
~~Yeah this lower one looks better but still probably your capacitor value in loop is way off, try to find frequency where impedance is real (purely resistive; green line on smith chart crosses horizontal line in the middle) and work from there, then you'll know whether to increase or decrease it.~~ what LH0ezVT said makes more way sense than that, i forgot how magloops work. but you still might want variable capacitor
resonance is narrow so you might miss it. there's a reason why magloops are made with variable capacitors (sometimes retuning is required due to changes in ex. humidity)
how have you made your capacitor anyway?
you can put some ferrite beads on your coax close to feedpoint in order to eliminate common mode currents. better yet, use a balun. this might help you in getting more reproductible results
e: note how swr gets much higher when off resonance with properly calibrated nanovna. when measuring antenna with cable, you're seeing loss in cable as a degree of lowered swr but only with high swr, because energy is lost in cable when it bounces around and never goes back to nanovna
This is my capacitor (RG58):
I dont have ferrite beads right now sadly, but i will try to make a Air-Core choke from parts of the coax feed line, maybe that helps.
If i would use a balun between feed line and antenna, i need a 1:1 balun i assume?
I will try to find the frequency where the impedance gets real. Also gona try to maybe build a variable capacitor from 2 metal pipes going into each other depth regulated by a screw, but i mostly wana use this antenna on a single frequency so i hope i dont need much adjustment.
Just a remark, remember that everything that you connect to the radio and which is not matched on both sides will have an effect that the radio "sees". So if you get a different result with and without calibrating in the cable, and the cable is used for the radio connection as well, the vna result that doesn't include cable calibration shows what the radio will see.
You don't want the cable to radiate. You want the cable to be a nice 50 Ohms, not radiate, not be influenced by your feet or your cat or your coffee mug near it. A good "sanity check" is touching / gently wigggling the cable / placing metal objects near it, the impedance should not change (because all the field is supposed to be inside the cable).
For you, I'd recommend to first get a half-decent match (at least SWR <3) directly at the antenna, then add a balun of some kind and measure without the cable calibrated in to check if it is still good.
The last picture gives an impedance of 36+j47 Ohm. The imaginary part is >0, which means inductive. I'd add some capacitance to the antenna first, to get rid of that.
BTW, where are you from? I may have a totally licensed (ahem) radio here, or at least listen in and give you a report
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).
Ferrite beads allow you to use old calibration. If you make 1:1 balun just by threading coax through toroid, you can use old calibration as well provided it's the same coax. Keep in mind minimum bending radius of coax. There are other designs, like using twisted pair on toroid, then you have to include balun in calibration as well (it adds some electrical lenght). If you noticed changes after making air core, this suggests that you do have some common mode current, this will make your measurements sensitive to random changes as rf current flows on the outside of cable where it shouldn't
I've seen people using PE-Al-PE pipe for variables, this gives you layer of good dielectric (polyethylene) (but not as good as air) in dimensionally stable form. One connection is aluminum layer inside the pipe, and for the other you'll have to figure it out on your own. Retuning might be required anyway within the band (magloops are narrowband) Common way to make variables is to bolt two of them in series, so that no sliding contact is used, moving part is the same for both. This is good for high voltages also but i'm not sure if you'll need it
I'd argue that your capacitor and loop are OK (otherwise there would not be a dip at 27 MHz).
But my guess is that the match to 50 Ohm is bad. Try playing around with your feeding loop.
You can ignore the cable if 1) it is properly matched to 50 Ohms on both sides and b) there is no field outside of the cable, meaning the cable does not act as an antenna itself.
You are clearly working on the 1) part, and for the 2) part, it depends on how you feed the loop, unbalanced vs balanced. For coax you either need an unbalanced feed loop, or a balun ("balanced-unbalanced" converter) or a ferrite or similar "Mantelwellensperre" (don't know the English word, sorry).
I love the word Mantelwellensperre, peak german language :D
I build a Air-Core choke from a part of feed line right below the antenna now, and got some slight improvements.
I will try to improve the feeding loop position to see if it gets the swr down a bit more. By now i am already quite happy with the antenna, during a test i managed to make a ~15km Thor8 connection while the antenna was still inside the house (with 4W AM Tx and the receiver in SSB/USB).
i'll add that in a way SWR chart is more resistant to misuse, because if nanovna is calibrated with wrong length of 50 ohm feedline, or without feedline at all, then smith chart will be rotated by angle depending on difference in length of that feedline, while SWR chart should look the same. for example, if real part of impedance at resonance is too low (ex. 20 ohm), and feedline is quarter wavelength different from what nanovna was calibrated with, then impedance will be still real but too high (ex. 125 ohm), while SWR chart should look the same (1:2.5 SWR minimum) (barring losses in feedline). (this works the same way as quarterwave long feedline impedance matching scheme). for different feedline length differences (non-multiple quarterwave) impedance will be complex at antenna resonance. this problem is avoided by calibrating nanovna with feedline