My portable linked dipole is cut for 20m, 30, and 40m. I also hope to use it on 6m as the fifth harmonic of the 30m section. Unfortunately it does not quite work so is a compromise. It is tuned slightly low on 10m and slightly high on 6m.
Another question is whether I needed the 30m section at all. Could I use it as an off-centre-fed dipole by setting one side to 20m and the other side to 40m? I tested this, as well as a system of short additional stubs on the 30m section to lower the 6m resonant frequency.
I finished by adding an extension to the antenna for use on 80m. The support system uses small plastic carabiners so allowing sections to be slotted in. My analyser and phone battery failed before I could tune this, so I switched to using it with the radio and the radio’s internal tuner. Paul called CQ on 80m slow CW and I jumped in feet first and answered!
The 20-30-40 Linked Dipole on 6m
The linked dipole is cut slightly long giving a resonance at 9.68MHz on the 30m band. Its bandwidth is enough to allow a nice low SWR across the amateur band.
In spite of this the resonance on the 6m band is high, giving performance in the upper part of the band used for FM communication. I want to use 6m for contest and SSB so need good performance in the lower part of the band.
Increase the angle to try to lower the frequency
The Linked Dipole calculator assumes that increasing the angle of the wires causes the frequency to lower. So would bringing the end of the 40m section to ground level be enough?
This increased the resonant frequency slightly, and reduced the SWR reading outside the band. I think this may be attributed to higher loss in the antenna, which is not good.
So increasing the angle, while it would be incredibly convenient, does not help.
The Off-Centre-Fed Dipole
A common recipe for an off centre fed dipole is to connect the feeder 1/3 of the along the length of a normal dipole. If I use one leg at 40m (10m long) and the other leg at 20m (5m long) then I have an antenna of 15m length which should be resonant on the 30m band. It is also connected at 1/3 of its length. It is common to use a 4:1 balun to feed these dipoles. I have a 1:1 choke balun.
Resonances were found at 9.54MHz, 19.31MHz and 48.51MHz. This could imply that the OCFD could be made to work by cutting it a little shorter, though this affects the 20m and 40m tunings of the antenna. A dedicated 30m tuning is the better solution.
Given time it could be interesting to see if this technique could be used to find a resonance on the 60m band with one of the 80m extensions connected.
Small extensions to the 30m section
A small section of wire was added to the 30m section using the crocodile clip fitted to the antenna. The carabiner was tied to the end and the 40m section passed through it to make a slidable toggle.
This antenna resonated at 50.625, lower than no extension but not as low as the vertical extension below.
The same length of wire was allowed to hang down. This worked a lot better and, with tuning, gave a resonance at 50.36MHz. This nicely covers the CW and SSB part of the 6m band.
My preferred solution for 6m use is to use these extensions. I have added small connectors to the end for easy fitting.
The 80m Extension
I cut additional 10m lengths of wire and attached a small carabiner at one end and a SotaBeams centre insulator at the other. This allowed be to unclip the support line from the 40m length, clip the new line to the end of the existing, then clip the support line to the end of the new 80m section. This is a convenient way to provide an 80m dipole but not have to deal with all that length for SOTA use. The additional wire is so light I will expect to store and carry it with the antenna. Walking poles are used to increase the height of the antenna ends.
The phone battery failed as soon as I’d taken the reading. As can be seen, the analyser’s battery was also running low.
I was able to use the antenna knowing that the KX3’s internal tuner would take care of a mismatch. Fortunately I didn’t need to worry as the antenna was close enough to resonance at my operating frequency. I need to shorten this a little to make the useful range cover the CW and SSB parts of the band without needing to cover below 3.5MHz.