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Using a Duplexer

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Bubba8

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Jan 19, 2014
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Can I buy a duplexer and use it to link two commercial radios' antennae that are programmed on the same frequencies but never transmit at the same time? One is at the rear of the vehicle, the other at the front.

I am new to the forums and radios in general so please excuse the stupidity :)

Thank-you for any help you can provide.

Bob
 

kb2ztx

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No. You need 2 antenna's. A duplexer will allow 2 different frequencies to be used on one antenna, generally in a repeater configuration (one transmit frequency and one receive frequency). the closer the frequencies the larger the duplexer.

You can either mount an antenna in the front and rear for the 2 radios, or find a dual head radio.
 

n5ims

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Some info on how a duplexer works to help you understand why it won't be a solution for you. A duplexer is a series of very sharp filters that are designed to, as close as is possible, pass only a single frequency from one connection and to pass only a single frequency to the second connection. These frequencies must be difference by a fair bit more than the bandwidth of the worst filter or it won't work correctly. This is to, as much as can be, prevent the signal from the transmitter from reaching the receiver in a full-duplex system like a repeater.

For example, the example system uses a transmit frequency of 450.000 MHz and a receive frequency of 455.000 MHz. Note, that this would be for the repeater, the other radios would reverse the transmit and receive frequencies to allow them to communicate through the repeater, but this isn't important in my example. My example also will have only the simplest case of a single filter for each input, although often there are several filters needed to work properly, especially if the repeater is in a site with many other transmitters.

The duplexer would have connections for the transmitter, the receiver, and the antenna. On the antenna side of the duplexer, there's most likely a fairly wide bandpass filter to cover the range of 450 - 455 MHz to filter out any strong signals that aren't in that range (basically to filter out any "junk signals". On the receiver side of the duplexer, there will be a very sharp filter that is designed to pass only the 455 MHz signal (let the wanted signals to pass, but not any others) and another very sharp filter to reject (not pass) the 450 MHz signal (to further filter out the transmitter's signal). On the transmit side of the duplexer, there is a very sharp filter to pass only the 450 MHz signal from the transmitter. An additional filter to reject the receiver frequency isn't really necessary since the signal strength there is pretty small).

In theory, you'd end up with signals on 450 MHz (and only on 450 MHz) flowing freely from the antenna to the receiver and signals on 455 MHz (and only on 455 MHz) flowing from the transmitter to the antenna. Everything else would be filtered out in one way or another. Of special interest is the 455 MHz signal from the transmitter is prevented from reaching the receiver, causing all kinds of bad stuff.

One other item of note, is that if you change frequencies (like would be common on a typical user radio moving from channel "A" to channel "B") you'd need to retune all of the filters in the duplexer. This often takes hours, requires some expensive equipment, and must be done by a person with significant skill. There are designs that use some wider bandwidth filters that allow the frequency to change without retuning, but they generally only work with low power transmitters and very widely spaced transmit and receive frequencies (think cell phones here, not two-way radios).
 

Bubba8

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Jan 19, 2014
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ThankYou

I appreciate the information and will be adding a second antenna at the rear of the vehicle.
 

WA0CBW

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When using two antennas if both radios are on a simplex frequency (TX=RX) you might get a squeal on one radio when you key up the other (i.e. feedback). Also if using a repeater the transmit radio can block the receiver of the second radio because of the close proximity of the two antennas.
BB
 
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