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Help with balun and matching for halfwave dipole

 
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ccyoung
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PostPosted: Tue May 10, 2016 6:55 pm    Post subject: Help with balun and matching for halfwave dipole Reply with quote

I'm currently trying to build a custom internal antenna for a glider. And I'm looking for some assistance on how to approach constructing a very compact balun.

Some relevant background: My plan is to use reasonably wide and sturdy copper tape to construct a vertically oriented half-wave dipole antenna inside the glider's thin kevlar rudder. The antenna is intended for VHF communication radio (118 MHz and 138 Mhz, with 123.3 MHz being the most important frequency).

Because the rudder is unavoidably close to the all carbon fiber tail section of the glider, my plan was to tune the antenna while mounted to the aircraft and cut the tape down to size to get a suitably low VSWR at the frequency of 123.3 MHz.

As I understand it, at resonance the half-wave dipole will have impedance of around 73 ohm, but it is being fed with 5 watts of power through 50 ohm coaxial cable. The impedance mismatch isn't enormous, and so I could imagine not trying to match the feed. But I do believe I need sort of balun to avoid having the unbalanced coaxial cable become part of the antenna. This is where I'm at a loss. Most of the stuff I can find online about this is relevant for ham radio installations, not for a very thin and compact rudder that needs to stay very lightweight. Some folks have suggested just using ferrite cores, but others have said these won't do anything at the frequencies this antenna is working at.

The best I can tell is that I might do well by using a gamma match for dipole to handle both impedance matching and to act as a balun. But with gamma match I'm just not sure where to get started with sizing and spacing, especially since I'm trying to work with copper tape for size and weight reasons. Perhaps there is another way to construct a small and lightweight balun, but I'm very much not an electrical engineer and am struggling with where to get started. Any help would be much appreciated.
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admin
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PostPosted: Wed May 11, 2016 12:11 am    Post subject: Reply with quote

I like your plan. So you have a network analyzer for measuring the VSWR?

I also agree you are unlikely to experience a problem if your antenna is not exactly 50 Ohms (honestly, if you are at 1.5 or below that should be more than sufficient - this is the equivalent of 100 Ohms to 25 Ohms relative to 50 Ohms).

As for the balun - actually you have no need. It doesn't really matter if the coaxial cable becomes part of the antenna (in fact, you can also use the coaxial cable outer shield as one part of your dipole antenna if you want). The rf current doesn't really care if it is on a specifically designed antenna or the back of a coax, which can be good or bad depending on if you plan for it or not.
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ccyoung
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Joined: 10 May 2016
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PostPosted: Wed May 11, 2016 1:54 am    Post subject: Reply with quote

Yes, I'm borrowing a vector network analyzer from an engineering friend to tune the antenna. I'm a bit surprised that the balun seems unnecessary. I'd like the vast majority of the power to radiate in the vertically polarized orientation with a fairly omni pattern. The coax runs straight out of the antenna, in a perpendicular fashion and then routes horizontally down the inside of a carbon fiber fuselage, so that seems like a bad place to have my antenna radiating rather than from the vertical antenna inside the Kevlar rudder.
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