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i am considering a "no tune" efhw antenna: https://qrpguys.com/qrpguys-end-fed-wire-antenna

i also saw one with a tuner: https://qrpguys.com/end-fed-half-wave-sota-antenna-tuner

do resonant antennas need to be tuned when moved or over time? if not, why would someone choose the tuner one? random length antennas?

(i will probably use 30m 40m or 80m with my qrp mountain topper.)

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    – rclocher3
    Dec 7, 2020 at 15:15

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The choice between the "no tune" antenna and the use of a "mini tuner" probably concerns the amount of flexibility that any given installation may require to provide an adequate match to a 50-$\Omega$ transmitter. I have not used this antenna, but the description and assembly manual of the "no tune" antenna give us some valuable clues:

Your feed line is essentially the counterpoise, so a separate one is not needed.

There can be some significant changes in (radiator) length due to your environment. Your results can vary depending on local conditions and wire deployment.

The first clue tells us that the feedline - actually, the outside of the coax shield - is part of the antenna system. Since the coax shield is (presumably) connected to the transmitter case and the case may be touched by a person (a big bag of electrolytes!) and/or connected to an electrical ground system, the antenna's "environment" may be quite complex, indeed. The impedance looking into the feedline will, therefore, depend on the lengths and configurations of all directly-connected and nearby conductors that comprise the antenna system and its environment.

What is not stated in the manual may be as important as what is stated:

  • There is no mention of feedline: the feedline is always very short in the drawings and there is no mention of SWR vs. feedline length, so it may be assumed that the stated results correspond to a very short feedline between the transmitter and the matching unit.
  • There is no mention of a grounding system for the transmitter case: in light of the foregoing discussion, it may be surmised that the case is not to be grounded.

A simple model of an EFHW shows that the impedance varies quite rapidly with frequency. If you wanted to use the antenna across a large part of a band, or on multiple bands, an adjustable matching unit will almost certainly be needed.

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