----- Original Message ----- From: "Curt Nixon" cptcurt@flash.net To: amsat-bb@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