Duplexer Use Question

[tweiss3]

So, odd question. I understand how a set of duplexer cans create a band pass/reject notch for repeaters so that transmitter doesn't desensitize the receiver. I couldn't seem to find the answer in my searching, but could a duplexer be used to allow a single antenna be used for two fixed simplex 2-way RF uses? Say, I wanted to run an APRS iGate (144.39) and a BBS/Winlink Packet Node (144.01), is it possible to use the duplexer to isolate these two uses from each other instead of getting significant separation? Isn't a set of cans basically just two tuned notch filters? The two above frequencies are more than the typical 0.600 spacing for a VHF repeater, so I would imagine it would work pretty well, but that would be putting RF into the receive side of the cans.

Just the things I think about when I have all these little projects going on, and keep thinking of ways to keep them all running at the same time.

 

[mmckenna]

Sure, that's what a transmit combiner is.

But it's only 380KHz spacing.

 

[mmckenna]

Is it time for Coffee?

Of course it is. It's always time for coffee.

 

[prcguy]

I have not seen a VHF duplexer that will do a 380KHz split, at least not anything recently.

 

[tweiss3]

Sure, that's what a transmit combiner is.

But it's only 380KHz spacing.

So is using a duplexer the most economical way, or should I be looking at something else?

 

[prcguy]

I don't know of a duplexer off hand that will work with that split and if there was one it would probably run way north of $2k new. I can't think of a good economical way to isolate two radios at the same place only 380KHz apart. If you put them on separate antennas I don't think you can get them far enough apart horizontally to work very well. Single cavity filters on separate antennas will not give enough isolation but maybe a dual or triple 1/4 wave cavity on each radio with lots of antenna separation might work. Single 1/4 wave cavity filters start around $75 used and go up.

So is using a duplexer the most economical way, or should I be looking at something else?

 

[tweiss3]

I see where I made a problem, the packet would be on 145.01, not 144.01. Oops. That ends up being 620KHz.

 

[mmckenna]

So is using a duplexer the most economical way, or should I be looking at something else?

Not the most economical way for what you are doing. It's expensive, and really only pays off when you are trying to cram a bunch of stuff onto a tower without putting up separate antennas.

Two separate antennas would be much easier/cheaper.

 

[prcguy]

Ok, a $2k new or over $500 used duplexer will work fine for that. Otherwise two separate antennas spaced very far apart and a couple of band pass cavities on each radio should also work. There is no easy or cheap way out.

I see where I made a problem, the packet would be on 145.01, not 144.01. Oops. That ends up being 620KHz.