I've build a verical for 40M band using a fishing rod as a mast, 6 radials and an RFI choke (FT240-31 ring, 8 turns of RG58, W1JR winding according to the article by Steve Hunt).
When I measure SWR right in the bottom of the antenna, using only 2-3 meters of RG58 as a feeder, I get this:
This is pretty much what I would expect - a resonance around 7 Mhz + around 21 Mhz as a 3rd harmonic. The SWR meter is not well calibrated, thus it shows SWR 1.3:1 at 7 Mhz when it's in fact 1:1 according to two other SWR meters I have.
If I add about 10 meters of RG58 to the feeder the picture changes, but not much in terms of SWR:
Finally, here is what I see if I measure SWR from the shack (about 30 meters of coax):
This I find quite surprising. I've found a few similar questions, including:
- Will measuring SWR through a feed line mislead me?
- Exactly why do some SWR meters give a changing reading depending on the length of coax used to connect to an antenna?
It looks like usually people name two possible reasons of changes in SWR readings:
- Common mode current, e.g. feeder radiates and works as a part of the antenna.
- Losses in the feeder, thus lower reflected power and lower SWR.
Although both explanations sound plausible 1) I'm quite conviced that common mode current is well suppressed in my case. I used this choke many times before under different conditions and with different power. Nothing indicated that the coax radiated anything. 2) Somehow I doubt that 10 or 30 meters of coax introduce significant losses. Also a 100W transceiver agrees with SWR readings I get.
Are there any other reasons why SWR could depend on the length of the feeder? For instance, can the feeder work as a matching device for the antenna on certain bands? If it's possible then how does it work?