--- In [email protected], "skipp025" <[EMAIL PROTECTED]> wrote:
>
> >"Gareth Bennett" <garethb@> 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.
No, 250kHz is fine for a 5 TX star combiner (with isolators and or
more pass cans in series). 10" cans can go down to 60khz.
> > 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.
Sinclair's stubs are near open half waves if my memory serves me and
are adjusted (trimmed) to present a balance return loss across the
span of frequencies used.
>
> > 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.
5 way will require the 3/4 wave version to get to 5 cavities from the
star.
>
> > But would like to know more theory and if possible the maths
> > that go into proper transmitter combining.
>
I was a systems guy and never got into those details.
Look at manufacturer's catalog pictures to see how they do TX star
combiners.
The loop and cable length presented approximately a 1/4 wave or 3/4
wave shorted stub when you look at it through the star with a tracking
generator and spectrum analyser. The resonance I mentioned was a
minimal insertion loss or the best VSWR (best return loss) at the
middle of your frequencies)
> 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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