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?
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
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