Repeater / Antenna question

[jban]

Curious: I always hear everyone using a duplexer on a repeater. Is this based on economics or would seperate Rx/Tx antennas be better?

With that said, we have a VHF setup: 150.xxx & 154.xxx. Are there associated interference issues with seperate antennas?

[Tom in D.C.]

No, the fact is that on VHF repeaters the transmit and receive channels are
pretty close together which results in desensing the system. UHF systems
were designed for a standard 5 mHz split which means you can sometimes
get away without using a duplexer. Your 150/154 repeater is somewhat
unusual; more often the split is about 500 to 600 kHz.

[Bruce1807]

It also decreases the wind load on the tower, provides some lightning isolation, and is generally more cost efffective.

[jban]

Thanks for the information. I was just curious. Tom you stated our repeater pair was unusual. Is this a bad split setup?

[Tom in D.C.]

Years ago you'd have rarely been assigned a 150.XXX channel but since
there has been a lot of frequency refarming by the FCC it may become
more ordinary. The area from 150 to 152 was the general realm of radio
paging and IMTS (the old, operator-assisted, mobile telephone system)
which also is now defunct. There was also some but not much public
service activity as well. In the DC area we had a Parks PD on 151.18 but I think
they've gone over to 800 mHz trunked along with the county.

Assuming your frequency coordinator did a good job then you're pretty
well off with that 4 mHz split. With or without a duplexer it'll probably work
better than a system with a split of less than 1 mHz.

[jban]

Both of our pairs are about the save in spacing. One was an old pair we were able to take over from a defunct EMS service.

Thanks for the information

[kf4sqb]

In a way, you're better off with that split. The closer the split, the more expensive the duplexers become. Shouldn't be too bad (or big) at that split.

[psapengineer]

The short answer is that you need the duplexer. If you're on a site with other VHF users you also need to be sure that your equipment and theirs are protected against each other for transmitter side band noise, recevier desense, and intermodulation. In general wider splits are better but not if your Rx is adjacent to someone elses Tx or vise versa. Good Luck.