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?
fullsquare@awful.systems 5 days ago
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
einfach_orangensaft@sh.itjust.works 5 days ago
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:
Image
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?
LH0ezVT@sh.itjust.works 5 days ago
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.
fullsquare@awful.systems 5 days ago
idk how you have done that, maybe i have older version but for me this marker just reads CH0 SWR 1.00/(value)
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