25 ft. coax vs. 3 ft. coax
2018-09-11 We all know that the length of the feedline, particularly coax is important, particularly when the antenna is NOT presenting the feedline with its native impedance, typically 50 ohms. What is seen by the radio or tuner on the input end will be quite different from what the antenna end actually sees. I decided to play with that a bit today using my 35ft end fed antenna. The 35 ft radiating element is tied to the high impedance side of a 9:1 unun. The RG174 50 ohm feedline connects to the low impedance side of the 9:1 unun and goes to the radio. In this case the radio, my X5105, has a built in tuner.
I did my testing with the tuner off so the SWR graph would reflect the actual impedance presented by the feedline. I ran three tests: 1) using 25 ft. of RG174, 2) using 3 ft of RG174, 3) connecting the 9:1 unun directly to the rig with no feedline. See the graph below. I'm sure the numbers aren't exact but are at least an indication of how feedline length with a non-resonant antenna affects the SWR at the rig.
On 160 and 80 meters the SWR is so far out at over 10:1 that feedline length doesn't seem to have any useful impact positive nor negative. I'm amazed that the X5105 tuner can make them work at all! 40, 30 and 20 are in a useful range with all feedline approaches. I would guess nearly any tuner can work with that. 17-6 meters are interesting, much lower than I imagined but like 30 and 20 clearly fairly easy for a tuner to handle, not even needing one in some configurations and frequencies.
It would also be interesting to play with the radiating element and the counterpoise to see how that would impact the antenna tuning. Perhaps some day!