KG4ZSB wrote: ↑Wed Sep 27, 2023 5:54 pm
I want to use my 9:1 unun with my 20m QCX mini.
What length wire recommendations?
9:1 ununs are typically used with random wire antennas, which are intentionally non-resonant. So, you would need a tuner to match the antenna to your QCX. An advantage of the random wire is its ability to be matched with an antenna tuner, for use on multiple bands. Since your QCX is a single band radio, this holds no benefit.
You might be happier with a resonant end fed half-wave (EFHW). The feedpoint (one end of the radiating wire) of a resonant half-wave antenna has very high impedance, so a 49:1 unun is normally used to match the antenna to the 50-ohm feedline and radio. When the antenna is properly constructed, the feedline can be directly connected from antenna to the radio, and no tuner is needed. This simplicity is a big advantage for field operations.
A resonant 20m EFHW has wire length of about 33 ft, but should be cut longer, then trimmed to resonance based on your use conditions (unun, feedline, target frequency, antenna configuration, counterpoise, and soil conductivity). There is some latitude for slight differences in soil conditions and specific setup, so length is normally set once (by careful trimming), as you finish making your antenna. Trimming to resonance is normally performed with an antenna analyzer, a nano-VNA , or lowest power transmissions from your.radio through an in-line SWR meter (with proper identification of your station for all test transmissions).
Hope this helps...
Best 73 de Brian - K3ES