I wished to use a long wire that covered from 80 Meters to 10 Meters, that fitted into my garden. As I had a loading coil that was intended for extending a 40 meter dipole to 80 Meters, into a shortened 40/80 Meter dipole. I decided to use it for a shortened 80 to 10 Meter long wire, with a 49:1 Unun.
The basic diagram is shown below. The measurements are approximate, as I have not measured them, but tuned the wire by cutting it the required frequency. Will update you at a later date with the correct lengths.
You may wonder why a 49:1 Unun. A tuned long wire will have an impedance of about 2000 to 3000 Ohms (2500 antenna impedance / 49 Unun ratio = 51.02 Ohms). As we will be feeding it with 50 Ohm coax we need a matching unit that will give us a reasonable match. Some people use a 62:1 which will give a match close to 3000 Ohms.
Step 1. Cut two length of wire, one to just over 20.14 meters the other to just over 3 Meters of wire.
Step 2. Attach one end of of each wire to the loading coil terminals.
Step 3. Attached the longer wire to the antenna terminal on the Unun and the shorted wire to an insulator.
Step 4. Attach an earth to the other terminal of the Unun. It is possible to use the antenna without an earth, but test have proved that the current in the antenna is better with an earth and also it reduces the static level.
Step 5. Connect a length of coax to the Unun coax terminal.
Sep 6. Hoist the antenna as high as possible.
Step 7. Using an SWR meter or an antenna analyser / network analyser, tune the 7Mhz length to the required frequency on 7Mhz.
Step 8. Now tune the 80 Meter section to the required frequency. Note that the 80 meter section can be very sharp as to the frequency required, so only cut a little at a time.
It is a bit of a chore moving the antenna up and down to tune it but it will pay off in the long run. If you only intend to use 80/40/20/15 and 10, it is possible not have to use an antenna tuning unit (ATU).
The SWR I obtained for 80 and 40 meters according to my NanoVNA, is shown below.
