I needed to make a quick and dirty dipole for 20m for some WSPR experiments and other uses from home. This is for installation in the loft. Lots of stuff in the loft - a water tank, metal shelves and other antennas, so this is not ideal.
It has to resonate at 14150kHz. This is a free space wavelength of 21.2m or 5.3m per leg of the dipole.
I made it from some cheap B&Q speaker/bell wire. The centre insulator is a plastic fixing block from B&Q. And the balun is an LDG RBA-1:1. Cost estimate below. The balun can probably be made for a third or less of the price even if the toroid is bought new. I just happened to have the LDG from use on a previous project.
| 1 fixing block | £0.11 (£11 for a bag of 100) |
| 10.6m of wire | £1.80 (£17 for a roll of 100m) |
| LDG RBA-1:1 balun | £34 (bought mine for much cheaper many years ago) |
| Total | £36 |
You can of course be naughty and just not use a balun for a total of less than £2.00 spent.
It was cut to 5.3m per leg for total length of 10.6m and installed in the loft. This is a bit too long to fit into the loft comfortably, so the last roughly 1m of each leg was left to droop. This gave a centre frequency of around 12900kHz.
Cutting 51cm off each leg in situ, resulted in a nice match at 14150kHz. See below for lengths, results and VF estimate.
| Length per leg [m] | Free space Fcentre [kHz] | Actual Fcentre [kHz] | VF | SWR at 14150kHz | |
|---|---|---|---|---|---|
| First cut | 5.3 | 14150 | 12900 | 0.912 | 4.7 |
| Tuned | 4.79 | 15700 | 14150 | 0.901 | 1.4 |
Now knowing the velocity factor for this kind of construction will result in less pain for future projects. You've got to tell yourself these things!