3/4 wave HF Antenna Budget Omnidirectional

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Posts: 22
Joined: 26 Jan 2016, 16:33
Call Sign: TL24
Location: Lincolnshire

3/4 wave HF Antenna Budget Omnidirectional

Post by TL24 » 27 Jan 2016, 00:28

Many years back on the old 11M days I decided to build myself an omni' 3/4 wave, the idea was to see what I could put together with what was around me. I was more than pleased with the results.

3 x 10' push up poles
2 x jubilee clips
2 x U bolts to clamp the pole
1 x approx. 8' length of plastic pipe to snug fit over one of the poles.
1 x 4x4 wooden fence post concreted into the ground or use an existing fence post.
Length of feed coax (RG8/58).

Formula for length, just incase you don't know it.
300 divided by the centre frequency of your band range, equals the wavelength.
Times the wavelength by 0.75 to get the three quarter wavelength in meters.
This is the length the poles will need to be cut to for a decent standing wave ratio.
The bit you cut off will be your ground plane.

So slide the water pipe over one of the poles leaving enough to clamp a jubilee clip to it end.
Cut to the desired joined up length the poles, for example approx 8.04m for a 28MHz centre frequency.
The bit you cut off now drive that into the ground as close as possible to the base of the fence post, as this will have a jubilee clip on and will be clamping the coax shield.
Now drill suitable holes into the fence post, two near the top and two about 3/4 of the way down, so that the U bolts can pass through the post and clamp the poles with the water pipe acting as an insulator to the post.
Now bare the coax so that the coax core can be connected and clamped to the 8.04m vertical.
The coax shield can then be clamped to the other short bit of pole you cut off and drove into the ground.

That's it!
You could cover over the top of the antenna just to stop rain eroding the inside of the poles, as mine eventually failed on me after about 7 years.
Other than that it worked well. Many were surprised on how well it was working and especially when I said that it was only a few inches off the ground.

I hope it give you ideas or even something to try for yourself.


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