[amsat-bb] Re: Question re rx/tx relays and diplexer

i8cvs domenico.i8cvs at tin.it
Mon Dec 8 07:30:14 PST 2008

----- Original Message -----
From: "Curt Nixon" <cptcurt at flash.net>
To: <amsat-bb at amsat.org>
Sent: Sunday, December 07, 2008 7:23 PM
Subject: [amsat-bb] Question re rx/tx relays and diplexer

> Good Afternoon All:
> I am working on packaging and interconnections for a mast-mounted unit
> for my 70cm system.
> My plan now is to place the preamp, a diplexer, and t/r relay all inside
> a single aluminum box.
> In and out will be N connectors.
> Internally, I am thinking of SMA links and pigtails.
> My 70 cm rig can produce a maximum of 35watts right now.
> Do commercial coaxial relays for t/r use short the rx line when in tx?
> or do they leave it float?
Hi Curt KU8L

Commercial Tohtsu coaxial relay model CX 520 D shorts to ground the
RX line when in TX and the TX line when in RX with 53 dB of insulation
between ports at 500 MHz but be carefull because if your preamplifier is
not unconditionally stable it can autoscillate when the input is grounded.
There are commercially availabe relays that connect a built in 50 ohm
termination to the RX line when in TX but the insulation is less. 

> Also, should the 2M output on the diplexer for desense be terminated? or
> left open.
It is not strictly necessary but if you have a 50 ohm termination availabe
use it

> Finally, what type small coax would be appropriate for use inside this
> box between modules?
Since you plan to use SMA connectors the RG 188 flexible teflon coax cable
can be used into the box with suitable SMA connectors.

> Main feedline is LMR400.
> Thanks
> Curt
> KU8L

73" de

i8CVS Domenico

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