Hi Gareth,
> "Gareth Bennett" <[EMAIL PROTECTED]> wrote:
> I am appealing to the wisdom of the group for information
> regarding Transmitter combining (Multicoupling).
OK, we'll try to help.
> I am interested in the maths and formula involved in cable
> lengths, coaxial stubs, etc for the "Star" style of TX
> combining.
Yes there is math, but almost every combiner system is an as built
based to the overall system requirements and performance trade
offs. The cable lengths are based no relative odd 1/4 wave lenghts
with lots of things changing the actual length.
> I have inherited a site that has been added to over the years
> with different equipment and filter styles. I need to re-build
> the TX (and RX) side to tidy it all up.
The first think you'll need to do is make a spread sheet of your
tx and rx frequencies.
> Each TX leg is like this... TX, Isolator, Cavity filter,
> 3/4 wavelength coax to star combiner... Nothing flash,
> just want to know the maths behind it and to fine tune
> the system....
There are rules of thumb for the combiner engineering, based
on a number of variables... first the frequecy list, second
the allowed mix problems are checked, third the materials
you have to construct the combiner, fourth the loss or performance
trades vs frequency spacing, fifth the actual equipment you'd
like to use.
> Especially the Transmitter coaxial stub information that
> usually is found on one spare combiner port.
Measure the existing stub for open or shorted end. It's most
often a harmonic trap/stub, 1/2 the length of the in band
quarter wave length.
> Thanks for reading :-)
No problama, it's hard to be specific without more information
from you. If you ask Telewave - Sinclair or the Andrew Decibel
RF Engineers about making a combiner, they'll want your frequency
list first. Sometimes you can make a single antenna do everything,
sometimes you split the antennas. In problematic frequency
combinations, you might need to have/include both additional tx
and rx antennas to avoid system gremlins.
When a multi channel pre-configured tx combiner works right
out of the box, I would consider the owner and mfgr very
good and a bit lucky. Combiners are where I picked up the
"no free lunch" line. In close space combinations, you try
to keep the power loss to less than 60%. Also, your results
will probably vary...
cheers,
skipp
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