Using 1/2 of mobile duplexer for RX input only?

[MrClean]

I would like to use 1/2 of mobile VHF duplexer as a 'filter' on the RX radio as part of a repeater setup. The radio is RX only and will not be set to transmit at all. I noticed most BP filters only do low db isolation where duplexers do like 50-60db.

Can the unused freq side be just left open/not connected?

 

[M0GVZ]

I would like to use 1/2 of mobile VHF duplexer as a 'filter' on the RX radio as part of a repeater setup. The radio is RX only and will not be set to transmit at all. I noticed most BP filters only do low db isolation where duplexers do like 50-60db.

Can the unused freq side be just left open/not connected?

A bandpass filter should do a hell of a lot more than a few dB. Yes you can use it as you intend if you stick a 50 Ohm dummy load on the other side (even 1/4 watt resistor soldered across a BNC connector) but depending on the spacing of the TX and RX on the VHF repeater you're going to find it has nowhere near enough because of the shape of the filter, they're just not sharp enough for the 2m band.

My friend runs commercial repeaters with a 4.5MHz split and you need to use proper cavities and more than one otherwise you get horrific de-sense. I've tuned them and those ones you can buy off Ebay like the ones below have nothing like enough of a steep skirt on their own to suffice on the 2m band.

At the very least you're going to want to use one on the TX and one on the RX too. You are also going to need a Spectrum Analyser with a Tracking Generator to tune them. Do not rely on the tuning they come with.

There is a lot involved in setting up a repeater so it can hear worth a damn without being de-sensed by its own transmit. Unless you run very wide splits which you're only going to get on 70cm band and above you're not going to manage it with two radios, a cheap isolator and a couple of antennas.

Go do some reading here:
http://www.repeater-builder.com/rbtip/