Topband: Magic 'T' Combiners

[W5UN]

At 06:13 1/14/2005 -0800, you wrote:
>
>--- Pete Smith <n4zr at contesting.com> wrote:
>
>> Maybe I'm misunderstanding something, but why can't
>> two receiving antennas 
>> in phase simply be connected by equal lengths of
>> coax and a tee? 
>
>Pete, the purists might disagree, but there is no good
>reason why you can't do exactly as you said. I have
>done the combining on a couple different phased RX
>antennas with no more than a coax tee and they worked
>exactly as they did when I bothered to use a
>commercial combiner.
>
>I suspect that any problems with phased pennants (or
>any other transformer fed system) is that the
>transformer phasing is incorrect. You MUST pay
>attention to how you hook up the transformer leads or
>you will wind up with an out-of-phase system. Only a
>50-50 chance of guessing it right!
>
>73,
>
>Larry - W7IUV  

Larry is exactly correct here. In fact,  if the feedlines to both antennas
are EXACTLY the same ELECTRICAL length, and the ends of these feedlines
have the same impedance, when you "T" them together, you will get one-half
that impedance at the "T" port. For two antennas that present 50+j0 ohms at
the feedline ends, this would result in 25+j0 ohms. 

We spent years, in the EME community, agonizing over the way individual
antennas should be connected together to preserve correct phasing for the
large arrays. Turns out it is pretty simple in practice, and works fine IF
YOU DO IT CORRECTLY. If you don't, it will simply not be in phase. On 2
meters, quarter wave coaxial line transformers are used to bring the
impedances to the desired value. We call them "splitters" or "power
dividers". This is somewhat impractical on 160 meters due to the long
lengths of transforming coax needed. That is why core wound transformers
are used, instead. And it is mighty easy to screw up the phasing right at
the transformers themselves unless you know exactly what you are doing.


73, Dave - W5UN						w5un at wt.net

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