>"Gareth Bennett" <[EMAIL PROTECTED]> wrote:
> I inherited a TX combiner on a radio site that has
> just grown without any planning and thought involved.
That would be about 80% of the combiners systems I've seen.
Clint Eastwood called it a "Cluster" in one of his
movies.
> There are 5x transmitters from 151.5 through to 153.025
> with the closest TX to TX spacing being 250 KHz.
.... which borders on almost being too close for a Star layout.
Unless the bottles are large and high Q, those two legs prob.
have a lot of loss.
> I have seen star combiners that have been fitted with shorted
> stubs that "Appears" to be in the vicinity of around 1/8
> wavelength... I was hoping that there was some theory or
> maths to get close to this length.
Sure, it is a harmonic trap with the advantage of being a DC
shunt.
> In the past I have maintained TX star combiners where the
> combiner itself has been made for a selective frequency band,
> with capacitive disks inside. (These almost always had shorted
> stubs fitted to a spare star port).
It's nice to have the stub if you have an extra port, but it's
not a requirement.
> I understand the theory of an odd wavelength from the cavity
> to the combiner (Usually 3/4 wavelength)
More often 1/4 wave cable lenghts in the popular 4 port (less
than or equal to) combiners when size and cable lengths permit.
> But would like to know more theory and if possible the maths
> that go into proper transmitter combining.
Well... each combiner is a trade off based on your frequency
list and band of operation. Might be better to go through an
example rather than throw a bunch of technoblather out.
cheers,
skipp
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