On Wed, Sep 9, 2020 at 10:00 PM, Darrell Carothers wrote:
The antenna is a random wire 9:1, 135¡¯ element, 35¡¯ counterpoise, about
40¡¯ up in the tops of trees
135 feet is very close to 1/2 wavelength for 80 meters, and a full wavelength. A 1/2 wavelength (and multiples of 1/2 wave) antenna has an impedance in the range of 3,000 to 5,000 ohms. As was mentioned earlier, you would need a higher-ratio unun transformer. 49:1 is common and would bring 5,000 ohms down to 100 ohms.
As was also mentioned, your sweep settings may need to be changed. The NanoVNA does not make a "continuous" sweep of frequency, it takes 101 individual measurements across the programmed range. If you set it to scan from 3 to 30 mHz, it scans 27 mHz in 101 samples, so the readings are about 270 kHz apart. That means, if I got my math right, that you'd only get ONE reading in the whole 80 meter band. If you're new to the NanoVNA and left the scan set to 50 kHz to 900 mHz, you're not likely to get any usable, useful reading for an HF antenna.
The point about trouble from leaves (especially when wet with rain) may also contribute to your trouble. A non-resonant antenna, and an end-fed half wave in particular, with an impedance of thousands of ohms requires a much higher voltage (and lower current) than a 50 ohm antenna. Any contact with leaves or branches could cause transmit power to "leak" off into the trees, and would reduce received signal levels too - IF the antenna wire is not insulated.
Do you have anything to tell you your SWR? If you are using an antenna tuner or matching system, can it handle a 100:1 transformation? Not many can. Well, come to think of it, your 9:1 unun would bring the 'thousands' of ohms impedance down into the hundreds .
If you could make your antenna a bit longer or shorter, it might make a big difference, but that's not easy now that it's up in the trees. By the way, the type of lead-in wire or cable also can make a difference. What is your lead-in wire, and how long is it? Is your counterpoise on the ground? How far is the counterpoise from the antenna? If you are using a 2-conductor wire or cable, like coax, from radio to antenna, and your co-ax ends at the counterpoise on the ground, the wire going up to the antenna is also antenna and adds to the antenna length.
And, as was already mentioned, a random wire is not supposed to be resonant, so there's no particular indication on the NanoVNA to look for, other than impedance.
I use a random wire about 270 feet long, strung through/over several trees. It's about 30 feet high, and made of THHN insulated wire. I've made about some qso's with it (and an autotuner), but it's not my main antenna and is used more for just band monitoring, receiving only. It does work well for that. Main antenna is a resonant vertical.
Maybe your main problem is the poor band conditions. Can you -hear- many strong signals?
Doug, K8RFT
