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Author Topic: Centre fed 12m half wave vertical dipole  (Read 1462 times)

Offline G0BVZ

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Centre fed 12m half wave vertical dipole
« on: February 12, 2014, 15:07:48 UTC »
There's more activity on the WARC bands than I remember from years ago so I decided to build a quick vertical centre fed dipole for 12m to see what gives.

I calculated the wavelength for 24.9MHz at 12.04m so 1/4 wavelength panned out at 3.01m. I was going to use plastic insulated wire so I reduced the length by 4%, giving 2.89m.  I wanted the feeder to leave the antenna centre at right angles so it needed to be light, to reduce sag. I chose to use the rare 72ohm lightweight twin which used to be sold in USA as TV receiving feed. It's around 3.5mm wide with stiff plastic insulation and solid conductors which won't take a lot of flexing.  If I'd had RG174 I could have used that but I didn't so I didn't.

It actually does not matter what impedance feeder you choose, so long as it's a multiple of an electrical half wave at the frequency of interest. The nice thing about an electrical half wave of feeder is that, regardless of it's native impedance, the impedance at the dipole centre will magically reappear at the other end (and at the end of every subsequent electrical half wave length).

The real world impedance of a half wave dipole depends on height above ground. Around 50~60 ohms would be fairly typical. I planned to string the feeder horizontally from the 10m fishing pole dipole support across to a home brew 1:1 current balun. The balun would cope with the mild mismatch between the 72ohm twin on one side and the 50 ohm low loss cable on the other. I used the balun to reduce common mode currents on the heavy coax taped to its own supporting glassfibre fishing pole. With the coax being parallel to the lower leg of the dipole there was a possibility of interaction. I was gambling that half wave spacing might be sufficient.

I threw the thing up and connected the feeder to an antenna analyser in the shack: I was amazed at the result. The resonant frequency was 24.9 and the swr was 1.4.  Pure luck, of course, but the weather had been so hostile when I was lashing it up that I was glad I didn't have to go outside again to tweak it.

As I've been writing this I've been listening on the qrp activity centre and it does seem to work. For a while all sorts of US stations kept popping up to work a station inaudible to me. Some of them were quite loud. Then a series of distant Russian stations and a Ukraine station so loud I had to reduce the audio volume -and I'm deaf!!  ;D

I woke up this morning wanting to do this; it worked out and I quite enjoyed this trip down memory lane, applying back to basics principles.  All my antennas with the exception of the 60m NVIS horizontal loop are really /P antennas, quick to erect and quick to change.  I find that photographs of of wire antennas are difficult to interpret so I made a quick, crude drawing to give you an idea of the setup. Click the pic to see a slightly larger version.

Hope you enjoyed this trip down memory lane too.

Vic /no Marconi..... ;D