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?

  • LH0ezVT@sh.itjust.works
    2·
    20 hours ago

    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).

    • einfach_orangensaft@sh.itjust.worksOP
      1·
      19 hours ago

      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).