Newbie Dipole Question (round 2)

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LondonPirate
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Newbie Dipole Question (round 2)

Post by LondonPirate » Tue Apr 02, 2019 3:47 pm

Hey all,
I'm putting together a dipole using copper pipes with a plastic push-fit connector as the "feedpoint". My question is, obviously the pipe sections will be partially covered by the push-fit housing by around an inch or so - does that covered-up inch of copper still count toward the length of each element when it comes to tuning?

Second stupid question: how far should the "arm" be that connects the mast to the antenna? Does it matter if it's 1/4 wavelength away etc?

Albert H
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Re: Newbie Dipole Question (round 2)

Post by Albert H » Tue Apr 02, 2019 5:02 pm

Yes - the covered bits of the copper have to be counted as part of the radiating part.

The boom can be almost any length, but you can make use of the supporting pole, to turn your antenna into an "H" and get some useful forward gain! Look up the dimensions for an "H".

Remember, you're going to have to match your balance dipole antenna to your unbalanced coax feeder. Look up "Pawsey Stub" and "Gamma Match" for a couple of ways to achieve this.
"Why is my rig humming?"
"Because it doesn't know the words!"
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LondonPirate
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Re: Newbie Dipole Question (round 2)

Post by LondonPirate » Tue Apr 02, 2019 8:50 pm

Thanks Albert! That's cleared up a lot - I'll be sure to match my antenna to the feeder. I think I'll have a crack at the pawsey stub since I can implement it while I'm doing the coax to the copper pipe.

Albert H
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Re: Newbie Dipole Question (round 2)

Post by Albert H » Wed Apr 03, 2019 3:04 am

My choice would be the Pawsey stub too. It's easy to make, and not as critical as other approaches.

However....

Do you have to use a dipole? A ¼-wave groundplane gives really good results, is inherently unbalanced, so the coax connects directly, and it minimises RF going downwards into the building you're mounting it on.

Another good choice (and easy to make) is a J-pole. The J tends to work better than a dipole, and it doesn't look like a pirate aerial.

If you're determined to use the dipole, remember that you can paint it with a non-conductive paint (I use acrylic), which will prevent corrosion and can be used to disguise it a bit - pale grey is usually best for British skies!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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thewisepranker
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Re: Newbie Dipole Question (round 2)

Post by thewisepranker » Tue Apr 09, 2019 9:05 am

I've never been a fan of Pawsey stubs because you risk filling your coax and ultimately your rig with water. J-poles are much nicer because you can mount an N connector on the pole - those are IP54 off the shelf if you buy the right ones and fit them properly.

nrgkits.nz
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Re: Newbie Dipole Question (round 2)

Post by nrgkits.nz » Wed Apr 10, 2019 6:49 am

thewisepranker wrote: Tue Apr 09, 2019 9:05 am I've never been a fan of Pawsey stubs because you risk filling your coax and ultimately your rig with water. J-poles are much nicer because you can mount an N connector on the pole - those are IP54 off the shelf if you buy the right ones and fit them properly.
I feed my coax baluns through the boom of the dipole and the whole thing is sealed

Albert H
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Re: Newbie Dipole Question (round 2)

Post by Albert H » Thu Apr 11, 2019 11:42 am

nrgkits.nz wrote: Wed Apr 10, 2019 6:49 am I feed my coax baluns through the boom of the dipole and the whole thing is sealed
That's exactly my approach. For a quick 'n' cheap "pirate" job, you can seal everything up with hot-melt glue. If you want to make a better job of it, epoxy resin is the way to go!

The J-pole is often the best option, though. They're not critical to align, they're quick to build, and they don't look too obvious on top of the communal aerial mast on a tower block!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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