[amsat-bb] Re: Diplexer question

[Diane Bruce]

On Sun, Oct 12, 2008 at 10:30:03AM -0400, Tim Tapio wrote:
> I'm installing a diplexer on my downlink side. Since I'm running two
> antennas, do I just leave the 145 MHz alone or is it more better to
> terminate it with a 50 ohm load I suspect the load is a better answer.

A diplexer (in this case) is simply two filters, a low pass filter
for 145mhz coupled to a high pass filter for 430mhz.
Normally a filter "wants" to be terminated properly to work properly,
but in this case, the 145mhz low pass filter is far enough away from the
430mhz high pass filter it won't matter. However, it should be capped to
keep any potential noise from coming in through that port and end up
internally radiating inside the filter. It wouldn't be much noise
but it doesn't hurt, besides it will keep the water out.
A 50 ohm bnc terminator as used to be used with cheapernet would do it,
if you happened to have one in the junkbox, or just solder a piece of
copper/brass/pcb on the end of a connector (pl-259 say) to cap it..

>
> Does the preamp go prior to the diplexer? Or after?

The preamp is going to amplify the noise if it "sees" it. The filter
should be in front of the preamp. Unless you are planning on doing EME
the loss of nf won't bother you.

> Thanks and 73 de Tim, K4SHF FM04

- 73 Diane VA3DB
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