[TowerTalk] Triplexer

[jimlux]

On 4/19/16 7:27 AM, David Robbins wrote:
> on the other hand... if doing this for SO2R operation only one of the transmitters would be keyed at a time.  You
> would only have to worry about combined dissipation for multi-op stations or expeditions sharing multi band
> antennas.
>
> Someone had a misconception on harmonics though a triplexer also.  a radio/amp say on 20m that generates
> a 10m harmonic going into the 20m port of the triplexer would have that harmonic filtered out by the 20m bandpass
> filter on it's port so it would not get to the 10m port at any significant level.  What WOULD be a problem is if the
> antenna or something near it was rectifying and creating a harmonic, that would come back through the triplexer
> to the 10m radio.
>

Depends on the rejection of the 20m bandpass filter. Let's take a 
practical example: putting out 2kW on 20m, and the harmonic is 40 dB 
down (virtually all amps do better than this when new and properly 
adjusted, but...).

So we have +63dBm - 40 dBm = +23dBm at the second harmonic. Now, do you 
want that harmonic "in the dynamic range" or "below the noise floor" in 
your 10m receiver?

The noise floor in 3 kHz BW is going to be around -130 dBm. If you want 
the harmonic below that, you're looking at a rejection of 150dB, which 
is pretty impressive.

If you just want it within, say, 60 dB of the noise floor (knocked down 
to -70dBm (roughly S9)), then you need only 90dB rejection.  That's 
still a lot, but probably practical with multiple stages.

In any case, you're going to be worrying about things like leakage 
through the coax shield.