Dave,
Some thoughts:
1. your design really should be a full Wilkinson combiner, the output of
each LPF should have a 100R across to the output of the other LPF so
that you have circa 30dB of isolation between the transmitters -
consider what happens if of half fails
2. you "coax 70-75 ohms" needs to be 70.71 ohms of the system is matched
to 50R with a decent return loss
3. at the power levels you're aiming for any loss of very important...
recall that 0.1dB loss is 11% and at your power levels that's a lot of heat
4. if you really do need to use a final combiner made from coax at this
power level then, as pointed out by others, you're looking for 7/8ths or
bigger
5. alternatives for your application that are worth considering are:
a) a six port Gysel combiner (rather than the two three port ones
you have now)
b) a further two port Gysel combiner cascaded after your LPFs
c) a Rat Race combiner which has some advantages:
* doubles up on the coax (you can use a slightly smaller coax)
* provides isolation between the two transmitters
* can be built using 75R coax (lengths 0.25, 0.25, 0.25, 0.75)
or 50R coax (0.153, 0.153, 0.153, 0.653)
using 50R coax you might have success with LDF7-50A
Personally, if I was attempting this project I would tend towards the
KISS approach - you would probably get as much overall transmitter
output from:
5 x 2500W FM pallets -> 5-port Gysel -> single LPF
and avoid a significant amount of combining losses and system complexity.
Mike
On 10/03/2012 16:35, David Shapiro wrote:
> Hi Carl,
>
> Yes, it seems so.
>
> I have just fired off a mail to Allied Cable to see what they suggest.
>
> Thanks.
>
> Dave G4ZOW
>
>
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