I am a technician interested in building my first antenna. I want to explore HF bands, 160m - 6m. Quick impedance calculations indicate that a 58 ft "random wire" antenna could work for this. But I am unsure which counterpoise length to select.
I read that the counterpoise should be shorter, and should also feature impedance. A 35 ft counterpoise would seem to fit the bill. Would a 58 ft, counterpoised 35 ft speaker wire antenna run well at 10W for the HF + 6m bands?
Honestly, if the antenna is literally a random hunk of copper, so is the counterpoise. Make it anything from 0 to x units long and start experimenting. Getting this to work on such a broad range of bands will be a challenge, to say the least.
However, if we consider what you are building not a "random end-fed" antenna, but rather an offset-fed dipole, then there are many references in the usual places for feedline location which then defines what your "counterpoise" is. Though, of course, it isn't called a counterpoise now -- it's just the short leg of the dipole.
But in many ways that is all a counterpoise is: something for the rest of the antenna to work against, because if we don't, that antenna will find a counterpoise whether we like it or not. And it'll probably be the feedline, or a human body if that is near enough.
A multi-band single-wire antenna depends on how whole number ratio wavelengths distribute the current and voltage across the entire antenna, the compromises being the resulting radiation pattern and feedpoint impedance.