Batboard

I'm in the process of setting up a portable vhf repeater system and I have a question about the antenna system. The repeater will TX in the 158MHz range and RX in the 151's, where should the antenna be tuned to for maximum TX and RX performance? Should it tune to the TX freq exactly or in the middle of the split? I'm not really sure if it would matter for the antenna to be tuned to 158 and RX 151.

Any advice appreciated!

Given the duty cycle the transmitter is going to endure I would tune the antenna for the TX frequency. Or better yet you could use a half wave antenna and still tune it to the TX frequency, it should be broad banded enough to be efficient at the RX frequency.

My two bits worth:
Antenna tuning is all about impedance matching.
Receivers are not very particular about the Z match, transmitters are.

The next issue is the effects of VSWR on duplex operation.
reflected energy will lead to isolation problems, causing desese.

Another issue with portable/mobile repeaters is radiation from the antenna entering the receiver chassis causing problems.

Make sure your antenna has an excellent ground plane with as much vertical separation as practical.

An antenna in the center of a vehicle roof is the best bet.
I would use a 1/4 wave antenna, be very careful of the grounding of the antenna mount to the roof. An RF ark will cause noise which will cause all sorts of interference.

One of those cute gutter mount brackets will not do the job.

From my perspective it kind of depends on the application. If you don't need really high gain you could consider using a broad band antenna that covers both frequencies or the entire band. Using antennas with a lot of gain can be an issue when you are off frequency due, not only to VSWR but to up/down tilt of the beam. I'd be temped to use the Telewave 2.5DB co-linear fiberglass stick. It covers the entire band and has modest gain. Good Luck. Bob

Thanks everyone for the replies.

I'm probably going the broadband route. Now, can I use a 1/4 wave antenna with one of those ground plane mounts? I've seen the conversion kits made by antenex. ANyone have any luck with one of those?

601 wrote:Thanks everyone for the replies.

I'm probably going the broadband route. Now, can I use a 1/4 wave antenna with one of those ground plane mounts? I've seen the conversion kits made by antenex. ANyone have any luck with one of those?

Not in duplex operation...

Oh NO, not a quarter wave conversion kit..........

I wouldn't use one; I'd think with all the mechanical connections it would be subject to making noise......

Regards, Bob