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2 antennae, 1 mast


#1

I wondered whether anyone had tried rigging 2 multi-band linked dipoles (inverted V configuration) on the same mast at the same time. I was thinking of trying one a couple of feet lower than the other and probably offsetting the wires. i will only be using one at a time, but wanted to be able to swap bands relatively quickly and easily, without resorting to 2 masts (too much space at the summit and extra weight) or (even worse) having to de-rig and rig the different antennae.

I know the ideal solution would be to make a new linked dipole with the bands I want, but I don’t have the time to do so at the moment.

I was interested in any experience about mutual interference etc.

Thanks


#2

For special event use, I use a fan dipole with dipoles cut for 80 and 40. They use the same feeder with choke balun and have insulated spreaders about 30cm long close to the feedpoint. Because of the spreaders, the dipole wires come towards the feedpoint about 60degrees apart. I splay the aerial tails so that they are about 3m apart at ground level. The literature suggests that fan dipoles with 3 dipoles work without mutual effects but I have not tried them, the potential is great to produce a rat’s nest of wires.

Hope that helps


#3

Look at this design from Carolyn G6WRW

http://www.moosedata.com/g6wrw/hfdipole.pdf


#4

Thank you David and Andy (and also Carolyn for the detailed paper). The solutions you have proposed are both (similar) a bit different from what I was proposing though interesting ideas for the future. I was actually asking about 2 separate link dipoles suspended from the same pole (perhaps with a small amount of vertical and angular separation). Each dipole has its own feeder (one is for 80, 40, 30 & 20m, the other for 17, 15 & 10m from memory).

I’m presuming from the fan dipole, that there shouldn’t be a problem with my proposal.

Thanks again


#5

Yes your proposal is a bit different. I have never done this, my thoughts are:

If the vertical axis of the two dipoles was at 90 degrees there would be little coupling between dipoles.

If they are in the same vertical plane, there is likelyhood of some coupling. Being cut for diffent bands, coupling will be less of an issue but 30 and 10m are almost 3rd harmonically related as are 40 and 15m. The open ended feeder of the aerial that is not in use may also come into play. At some frequencies the aerial feedpoint of the out of service aerial will look like a short, at other frequencies like an open cirtcuit, depending on the length of feeder. Some odd effects such as unexpected SWR may result. If the dipoles are to be in the same vertical plane, perhaps a couple of halliards to raise and lower the dipoles may fit the bill.

Regards
David


#6

Thanks David, I think a back garden test might be in order before risking an activation with it. I had suspected that I would need to orientate them as far as possible apart. The vertical separation is the tricky aspect because there isn’t an awful lot of spare height. I like the idea of a halyard (takes me back a year or 2).


#7

I’ve been thinking about your requirements further.

My link dipole consists of a few components. A dipole T piece that is used to attach the dipole legs and has about 6in of coax ending in a coax socket, a run of coax with coax plugs on each end, 2 dipole legs with the links. I did it like this so it breaks down for easy packing and you don’t end up with tangles etc.

To setup
fix the fishing pole (fence or guys at about 1m AGL)
add the T piece to the top,
add each leg and walk each leg out horizontal laying on the ground,
attach the feede
push up the pole
walk out to the far ends at attach to fence with 1m cord or guy out and use walking pole to support ends

Nothing fundamentally clever or cool there. In your case if you have the two dipoles’ legs laying on the ground you can attach either set (mark them so you attacjh the matching pair!). Changing bands consists of dropping the pole, 10secs say, disconnecting the current dipole legs and attaching the others. With suitable lengths of cord, you can tie off both dipole legs to the same guy. Shouldn’t take more than a minute to swap antennas with a bit of practice.

No need to worry about interactions between antennas.


#8

I quite often deploy 2 antennas on the pole… subject to the weather of course. In my case it is a 2m beam sitting below the apex of the 30 / 17 / 12m dipole. Sometimes the arrangement can be quite challenging if the breeze blows the HF dipole wire onto the beam, but generally it is okay. If I were deploying 2 HF antennas, I would opt for having them at 90 degrees to one another to minimise interaction, but that may make one or the other non-optimal for the best direction for where the chasers are located.