[amsat-bb] Question about duplexer as a high-pass filter

[Hasan al-Basri]

I use triplexers extensively here and they do a wonderful job of
eliminating desense on Mode B (2m down, 70cm up)
*The key is using good ones, not jun They key is using good ones, not junk.
*If there is no reliable published spec for port to port isolation...it's
junk.

All of mine are Comet CF-514J and isolate 2m/70cm/HF and can handle 500 to
800 watts on 3 segments. The port to port isolation is 60 dB, and I get 0
desense or intermode on my 2m downlink, even running 500w out on HF on the
same coax that has both my 2m and 70cm Eggbeaters. I also use an ARR GASfet
preamp in the 2m side...no issues whatsoever.
============================================
My main sat setup also uses Triplexers (and the unused ports are all
terminated in 50 ohm loads (little bnc terminators).

70cm Tx > Triplexer 70cm Port In  > 1/2" hardline Out to Tower > Dual Band
Interlaced Yagi (common feed point)

HF Port on that Triplexer is 50 ohm terminated in shack.
===========================================
I have even run 2 triplexers back to back for increased isolation to keep
QRO HF out of my 2m Rx. Works just fine, and does not kill sensitivity for
SAT work.

My Noise Floor is calibrated to -95 dBm for a 50 ohm resistor on 2m. The NF
with an antenna is -91 dBm. I see passband noise on all linear sats (Mode
B), with typical peak signal strength of -60 dBm. Typically,. I see 25 dB
SNR when the passband noise is included.

...and not a hint of desense or intermod. I highly recommend the Comet
Triplexers, even if you only need a duplexer, because the port to port
isolation is higher.  I found them as cheap as $28 on line...and as much as
$35 on line. I bought 4 of  them when I found them at $28.

Maybe this will help someone get the most out of their sat setup, and
eliminate what can be a very frustrating problem.

73, N0AN

Hasan


On Sun, Nov 10, 2019 at 5:48 PM Robert Bankston via AMSAT-BB <
amsat-bb at amsat.org> wrote:

> Steve,
> I have had great success using a diplexer on the VFH side to eliminate
> desense on V/u satellites.  I do not use a dummy load/terminator on the UHF
> port. Recently, I switched to a Mini-Circuits BLP-200+ as it has a smaller
> form factor.  Note, the BLP-200+ has an advertised  0.5W power limitation.
> I have used mine with 10W output with no smoke released, yet.  Your mileage
> may vary.
> 73,Robert, KE4AL    On Sunday, November 10, 2019, 05:26:17 PM CST, Bob via
> AMSAT-BB <amsat-bb at amsat.org> wrote:
>
>  Hi Steve,
>
> I asked myself a similar question and decided to measure things both
> terminated and un-terminated.  Bottom line is you want to terminate that
> unused port.
>
> Here is a story on my website:  https://wb4son.com/wpblog/?p=2828
>
> 73, Bob, WB4SON
>
> On Sun, Nov 10, 2019 at 6:18 PM AI9IN via AMSAT-BB <amsat-bb at amsat.org>
> wrote:
>
> > My satellite rig is two HTs running to an Arrow antenna. Trying my best
> > to operate full duplex. When operating V/U I am using the UHF side of
> > the Arrow duplexer as a high pass filter between the UHF antenna feed
> > point and the UHF HT to keep the 2 m transmit signal from desensing the
> > UHF receive.
> >
> > But it only sort of works.
> >
> > This set up leaves the VHF end of the duplexer open and just hanging.
> > Would this setup work better with a 50 ohm load across that empty
> > connection? (Sorry if this is a dumb or elementary question, but I'll
> > never find out if I don't ask.)
> >
> > All helpful insights are welcome.
> >
> > Steve AI9IN
> > _______________________________________________
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> _______________________________________________
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> _______________________________________________
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> to all interested persons worldwide without requiring membership. Opinions
> expressed
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