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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:

enter image description here

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:

enter image description here

Finally, here is what I see if I measure SWR from the shack (about 30 meters of coax):

enter image description here

This I find quite surprising. I've found a few similar questions, including:

It looks like usually people name two possible reasons of changes in SWR readings:

  1. Common mode current, e.g. feeder radiates and works as a part of the antenna.
  2. 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?

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  • $\begingroup$ Please, tell us the instruments that you used to make the SWR measurements. And, did you compare the readings obtained by the SWR indicator in your transceiver with the 10m and 30m long transmission lines? $\endgroup$ – Brian K1LI May 7 at 21:46
  • $\begingroup$ The measurement was done with SWR meter Mini60s (a.k.a. SARK100) using it USB-UART interface with a little automation in Python. The readings are consistent with SWR indicator of my Yaesu FT-891 and an arrow SWR/power meter, although Mini60s usually gives an SWR about 0.3 higher then two other devices. $\endgroup$ – Aleksander Alekseev - R2AUK May 7 at 21:59
  • $\begingroup$ For the record - I observe the same behavior with 20M vertical. It's resonates only on 20M with short feeder, but suddenly I see low SWR on 10, 12 and 17M bands + SWR 2 on 15M with a long feeder. The shape of the graph is similar to the shown above. Two other SWR meters confirm this. I also got a confirmation from pskreporter.info that the antenna works on these bands and made a few QSOs in FT8 on 17M. $\endgroup$ – Aleksander Alekseev - R2AUK May 7 at 22:18
  • $\begingroup$ Do you have a matched dummy load you can test in place of the antenna? $\endgroup$ – natevw - AF7TB May 8 at 22:06
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    $\begingroup$ @natevw-AF7TB I have. The scan of the dummy load looks like this b.radikal.ru/b16/1905/d0/fa9f6a6a8116.png However when I do the scan through a long coax cable it starts to look like this b.radikal.ru/b13/1905/0c/ea2c02c6ecb8.png I tried different cables that I bought in different time in different shops and the pattern is always similar, although the longer the cable is the more distorted the image gets. I doubt that all these cables are defective as Brian suggests. It looks more like some sort of miscalibration or artifact of SARK100. $\endgroup$ – Aleksander Alekseev - R2AUK May 9 at 17:33
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The repeating hi-Z measurements seen when SWR is measured with the 30m line suggest that it is defective. One needs to know the velocity factor and loss of the cable to determine the location of the defect. These should be determined from measurement, as actual lines can deviate significantly from published specifications and values can change as a result of electrical and environmental ageing.

As shown on DAE Notes, the impedance measured at the input to an open-circuited transmission line will be high at frequencies at which the line's electrical length is an even multiple of a half-wavelengh:

enter image description here

The fact that you observe similar behavior with a different antenna as the load reinforces this conclusion.

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