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?
Hey, i did this! I made a 2m loop and an HF one before. Both showed a narrow dip in SWR about where I expected it, give or take a bit. With a bit of fiddling, I easily got both down to an SWR of <2.
Oh, and general remark, always consult the bible (Rothammel) or the heretical scriptures (ARRL antenna handbook). Both of them could hypothetically be found on those websites that people warned you against, although not the newest versions.
you can get away with very inefficient antennas on HF reception, so i wouldn’t take SDR reading very seriously. (atmospheric noise dominates all noise, so amplification will get you useful signal with amplifier not introducing significant noise on its own. reverse is true on vhf, and especially on uhf and up)
resonance should happen no matter what power level. do you mean SWR 50 or 50 ohm? i’m not even sure if nanovna can measure SWR that high. it sounds like you have a short or open somewhere it shouldn’t be? you need to calibrate it after changing tested frequency range, have you done that? (calibration can be saved). at the vhf-ish frequencies, it would make sense that your loop becomes full wave or even larger. circular loop has impedance of some 100 ohms, but you have capacitor at the ends of it so it’s gonna be different
with magloops, with set size of loop, tune is via changing capacitance, match is via changing position (closer or further from loop, tilt away from plane of loop), shape or size (cross sectional area) of feed loop, you can match it exactly this way. coax stub can be lossy, if it’s just 4W then probably not a problem but with higher powers check if it’s not overheating
Thank you for your answer!
I think i found the root of my problem, the scale settings on the NanoVNA where wrong, to a point where the super slim SWR dip of the Loop was just not drawn on the graph for lack of data points at this point. I changed the scale and was able to see the dip, then tune the coax stub to the frequency i wanted. Now it claims to have a SWR of 3.8 near the frequency i want to use:
But i am still a bit confused, the yellow number left of the SWR reading, i assume that is the scale? or does it mean a SWR of 6.125:3.8?
Edit: Just tested reception, and its way better than before, also Tx works a lot better now, i would say almost on par with my Dipole just with more directionality
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:
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).
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. 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.
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
Second number should be SWR, first one scale (value change per horizontal box). So that would be 3.8 SWR, which corresponds to about half a box of 6.1 - 1, because the lowest you can get is 1.
