Off-Centre-Fed (OCF) Dipoles

[Bazz4CQJ]

Something I'd never heard of until a few days ago is an OCF dipole, though it seems they been around since the early 1920's. A Google search finds numerous hits e.g http://www.m0zpk.co.uk/40-meter-off-centre-fed-dipole/. One appealing aspect of them is that they can be used for multiband operation without traps, but of course the asymmetric aspect of them also has appeal. I have a fair size garden, but the position of large trees and overhead electricity + telephone cables is restrictive on aerial layout. Anyone been down this route?

B

[David G4EBT]

Countless are in use the world over, as are G5RVs, but I dare say that as with the G5RV, many take the easy option and buy them off the shelf, like the rest of their gear. (See 'Carolina Windom'). A bit of a shame really, but there you go - antenna experimentation is one aspect of amateur radio where there is limitless scope for experimentation for little cost - baluns, be they 1:1 4:1 - can easily and cheaply be made, as can traps. I always made my own traps and dipole baluns when I was active on air, dipping the traps with a home-brew dip meter.

Happy days!

Good luck with your experimentation.

[Bazz4CQJ]

Well, the aerial/balun/atu/GDO will be homemade, but admit it will be my Trio boatanchor that finaly drives it... but at least I have had the covers off that and discovered safely that the full HT (~800V off-load) is present on the valves even on receive .

[Radio Wrangler]

The HT is derived from the same mains transformer as powers the receiver, and I think most valved transceivers used voltage doublers and silicon diodes so there will be full HT available all the time. They just bias the valves far off rather than try to have anything shutting down the HT source.

I use an un-straight inverted Vee over the house. Having bought a new radio a few weeks ago the second contact was a station outside Hiroshima, so I was rather pleased with the antenna... I've used open wire feeder about 5 inch spacing with genuine Collins ceramic spacers straight down the gable end and into the shack to a KW/Decca 109 tuner and the new Icom 7700.

The two halves of the inverted vee are about 30 degrees away from straight in a plan view.

The 7700 has an internal tuner, but it's unbalanced. Trying to do a balun that works over a wide frequency range and over a wide impedance range is a sure disaster, so the Z-match tuner does the job.

It's nice to show what can be done without towers.

David

[Bazz4CQJ]

Originally Posted by Radio Wrangler

The HT is derived from the same mains transformer as powers the receiver, and I think most valved transceivers used voltage doublers and silicon diodes so there will be full HT available all the time. They just bias the valves far off rather than try to have anything shutting down the HT source.

Yes, I guess that it is not surprising really, but in the (stand-alone) Tx's I built as a boy, I switched the HT on and off and it had not occurred to me when I opened the Trio that I should keep one hand so deep in a pocket. The off-load volts get quite close to the rating of the electrolytics in the power supply, so I'm glad that I always bring it up on the Variac when it's not been in use for a while.

I'm planning to sell this house next spring so any antenna which goes up now is really just to get me on 40m over the winter. Definitely no (big) trees at the next house; not only limiting with aerials but demand too much time when they shed leaves over the lawn, as in now the case .

B

[G6Tanuki]

Originally Posted by Radio Wrangler

It's nice to show what can be done without towers.

Quite!

I've experimented in the past with a full-wave dipole coax-fed 1/4-wave in from the end. In theory I should have needed a phasing-section to make it work properly but let's just say that this worked fine in practice.

My current HF antennas are a 100mumble-foot-long end-fed series-tuned with a roller-coaster, and a dipole for 28MHz.

[majoconz]

It started off as a 66ft (short) longwire with a Bryant homebrew 9:1 un-un to 50ohm coax to my NRD515 but it had a big hole right where I wanted to listen, about 8MHz. I read about the OCFD and had a bit of space at the un-un end to add 22ft in a slightly different direction. Presto, problem solved, didn't change the un-un except to lift the ground off the antenna winding of course and now had much better broadband response.