Something to consider when building loaded dipoles.
the inductance required for loading a “short” dipole depends on the position of the loading inductor along the length of the antenna elements. one inductor is required in each leg.
If the loading is not inserted half way along the antenna, but say 1/4 along its length, the loading inductance required is reduced considerably and with that, the loss resistance is reduced. So with an antenna that already has links at various places along its length there are a number of opportunities to add loading to make it work on 160m. At the link between the “40m” length and the “80m” extension, instead of inserting a link, you could insert an inductor chosen to bring the antenna to resonance on 160m. How you make the inductor is your choice, but the inductors in the QRPguys trap antennas look fairly light and that design would probably be a good choice.
I think that approach may work better than adding a larger, heavier and more lossy inductance and another extension to the ends of the (already long) 80m dipole.
I don’t have an opportunity to try this out at present due to work pressures keeping me away from the shack, but this is an ideal application for the antenna analyser and I’d like to try it. I offer this idea to the experimenter.
That is similar to the approach of John, G4YSS. As I remember it he inserts a loading coil at the 40 metre link. The interesting part is that the coil is wound on a plastic hypodermic syringe and he has glued ferrite slugs to the plungers to allow tuning for best SWR. There is a photograph from an activation somewhere in the reflector but I can’t remember where.
I found John’s description while looking for something else!
"I use a link dipole for 80 thru 20 on a 5m mast with 1m ends. There are break points in each leg for 40m. These are 9.77m from the centre. This is where I insert the loading coils.
Everything is home brewed and the coils weigh 39 grams each and are quite small as follows:
Coil former; 20mm dia x 74mm lg PVC tubing with modified ‘Monoject’ 5ml hypodermic syringe inside. 113 turns of 0.4mm dia enamelled copper wire, close wound over 51mm length of former.
Crush 3/4 of a ferrite toroid into powder, using a hammer. Mix powder with araldite and apply along abraded piston shaft. Paint white so that calibration marks can be added later. Fix on some terminals and fit to dipole. Adjust slug for desired freq. Covers 1.7 to 2.0 MHz, though bandwidth is narrow.
(Ferrite toroid is of unknown specification, 22.2mm dia x 13.3 inner dia x 6.3mm thick. Material is attracted by a magnet. They were just lying about in the garage.)
I wouldn’t say the design is eminently reproducible or even very efficient; just a good starting point for expt and light enough to be summitted easily."
The above dates back to March 2012, which explains my imperfect memory in the previous post. Tempus frangit!
