When is a duplexer needed for a repeater?

[T23]

When is a duplexer needed when setting up a repeater? I was planning on building a battery powerd mobile repeater from HTs and a battery powerd controller and wanted to know when does one need a duplexer? I understand that it allows you to use the same antenna to receive and transmit at the same time. But how close can two separate antennas be close in receive and transmit frequency without interfering? Also what about if I were to use UHF and VHF input and output, would this be enogh separation not to interfere?

T23

 

[BEJohnson]

Check out this PDF and this link.

 

[Shockwave]

Lots of things will affect whether you need a duplexer or not. Most importantly are frequency separation, receiver selectivity, and transmitter power output. Other variables include separation space available for independent antennas and mode of operation.

AM is not as repeater friendly as FM. I've seen a UHF 45 watt repeater function decent without a duplexer. The frequency split was 5 MHz and two antennas were vertically spaced about 25 feet apart on the same tower. This would fail miserably if it were not for the extreme receiver selectivity the equipment had.

I suspect you would have few problems with a cross band repeater. The UHF and VHF antennas should be vertically spaced apart. This provides several times the isolation that an equal amount of horizontal spacing offers. Use well shielded coax cables to prevent leakage into the receiver channel.

The main issue that the duplexer aims to correct is receiver desensing. This is when the receiver beings to lose sensitivity (like turning the RF gain way down) as a result of the transmitter power entering the receiver front end. This could be a factor that affects how much transmitter power you can run like this.

Other simple things you can do to improve the performance would be to add a appropriate filter on the receiver input. For example if you use a VHF receiver and a UHF transmitter, put a VHF low pass filter on the receiver.

You can make a trap from a coil and a variable cap that is tuned to act like a short at the transmitter frequency and installed at the receiver input. Similarly, a 1/4 wave coaxial stub can be cut for the transmitter frequency and installed at the receiver input. The coax should be the best quality possible to prevent loss and open at the far end.

In the mobile application vertical antenna spacing is not possible. That is an issue. Your best chance is to mount one antenna on the fender or hood and the other on the trunk. I'd use 1/4 wave whips for each to reduce coupling. The passenger compartment may shield much of the radiation then. Where longer 5/8 wave antennas would rise above that point.

 

[W5LZ]

Simplest answer is that a duplexer is needed when you plan to use the same antenna for simultaneous transmitting and receiving. If you can use two antennas and separate them vertically or horizontally far enough, you don't need a duplexer.

Shockwave, think about that again!
- 'Doc

 

[vo1ks]

The transmitter could also make use of a bandpass filter to reduce intermod at a busy site. High level signals arriving at the PA from nearby transmitters can be rebroadcast causing interference to other services. This is sometimes done at FM broadcast sites, even if the antenna services only a single transmitter.

Also check the the third harmonic from 144MHz dropping within the 432MHz band with a crossband repeater. Once it's sent to the antenna it can't be filtered out.

There are different types of duplexer. One built with bandpass filters may provide better out of band response than more simple stopband/passband cavity filters.