homemade riser/extension

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HomerBB
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homemade riser/extension

Post by HomerBB » Friday 2nd Dec 2011, 15:36

For those who need a riser/extender for whatever purpose, whether to get your mobile antenna coil above your truck cab, or simply to get it higher, one method I've used is

as easy as a trip to the local Home Depot for some brass fittings, a 3/8-24 x 1-1/4" bolt, and a tube that is of 5/8" outside diameter. Cut your tube of any length you

desire and put the parts together. One thing to note is the 1/8" x 1/2" brass bushing must have the 1/8" thread reamed out with a 3/8-24 tap before it will receive an

antenna or the 3/8 x 1-1/4" bolt.

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HomerBB
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Re: homemade riser/extension

Post by HomerBB » Saturday 3rd Dec 2011, 16:06

As an example, here is an antenna load from Radio Shack that requires a 57" whip and is normally bottom loaded. Putting it on the riser required a shorter whip, but not a direct trade off. The over all length of the antenna with the riser under it is 80". According to the MFJ-209, I have a 1.5:1 bandwidth from 27.815 to 26.515 with it up there. And it seems to work well with both local and DX contacts.

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Someone asked me this:
Any thoughts how i can put a whip adjuster on the top of the riser to raise and lower a 102?


Maybe I would if I had some idea of why you'd want to do that. To change the resonance so it can be used on other bands, or just to clear obstacles like a low garage, etc.?

I've been drilling a hole in the top nut on the bushing and threading it with a tap in order to put in a set screw for whip length adjustment.
If that is the idea, you do that and slip the bushing over the whip before you screw it into the top of the extension. Before that is done, a rubber seat for a faucet is placed around the whip in order to keep it centered as it is moved up and down the tube. When you've found your tune, tightened the set screw for the frequency range you are working.

HomerBB
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Re: homemade riser/extension

Post by HomerBB » Tuesday 6th Dec 2011, 15:37

Alright, now about the way an adjustable whip antenna can be devised from this simple extension.

Follow the logic of the photos. If unclear, just ask me. Be aware that the tube length is as needed/desired,
and that because the only 102" SS whip on the premises is up in the air as the top section Of my Sigma4 antenna.
I used the smaller shorter whip simply to illustrate how to do the work. An actual SS whip would be thicker making
it a better fit through the top of the tube setup, and a beter fit for the rubber faucet seal/washer on the bottom
that keeps the whip centered in the tube. The shouldered bottom of a SS 102" whip would prevent losing the washer in the tube, too


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