Gamma Match Dipoles Upsidedown??

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Elopid
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Gamma Match Dipoles Upsidedown??

Post by Elopid » Sun Aug 11, 2019 3:08 pm

Hello,

Recently I have seen people using dipoles with gamma match at the bottom and not the top element,

I always presumed that the top (positive) element was the one that required tuning?

Is there a difference or is it easier to tune the Gamma match stub/rod with it on the bottom (negative) element??

And another thing,

Are these Gamma match dipoles I speak of actually upside down and still have the connection from the coax's inner core (positive) to the Gamma stub but with the aerial upside down? (positive bottom element, negative top element)

And one last thing,

Does it make much difference using a dipole upside down? Once I accidentally put a dipole up the wrong way round in the dark and to this day I'm convinced that it got out much better than when I was using it the right way up. (right way up as in - positive top element, negative bottom element)

Thanks.

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thewisepranker
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Re: Gamma Match Dipoles Upsidedown??

Post by thewisepranker » Sun Aug 11, 2019 10:48 pm

In theory it doesn't matter which way up they go because one of the two jobs that the gamma match does is to convert from unbalanced to balanced. This means that the voltages at the tips of the elements are always equal and opposite. This is confirmed by the fact that gamma matched dipoles are DC shorted in the middle.
In other words, the top element spends half of its time positive relative to the bottom element, and the bottom element spends half of its time positive relative to the top element.

Sounds like a worthy experiment though, because I don't know what effect the matching element has on the radiation pattern and I've never tried them with the matching segment pointing downwards. I have always run them pointing upwards, I suppose because that's what the manufacturers' website shows!
There may be a valid reason for running them pointing downwards if you lost the little black waterproofing cap to stop your matching element filling up with water...

I'd be interested to hear from people that have run them the other way up.

Albert H
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Re: Gamma Match Dipoles Upsidedown??

Post by Albert H » Tue Aug 13, 2019 3:55 am

When I put one right at the top of a very high lattice mast, I had the match pointing downwards so that it was easier to reach when tweaking the match. The radiation was just the same as with it the other way up.

I generally don't like gamma matches much - I find a Pawsey stub easier to manage - but the Gamma has the advantage (like a J-pole or a Slim Jim) of being DC short, so making the gear less susceptible to lightning (important in some parts of the world!)

For quick, cheap and efficient, a J-pole beats any kind of dipole every time. The quarter wave bottom matching section effectively feeds an end-fed half-wave above it. It's DC short, it's easy to build and match, it's not too obvious from the ground, has a great radiation pattern, and can be stacked (or made into a Super-J) for greater gain.
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