transmiter

We seem to have had a spate of questions very similar to this...please look at previous posts.

And at the risk of being pedantic...

A radio signal leaves a transmitter!

Specific threads

Radio mixer 06/04/2007

Transmissions of Radio 06/04/2007

Radio transmission 06/01/2007.

Are you asking what happens when another signal is fed back down the antenna cable into the final stages of a transmitter?

This is a common problem at busy radio sites where transmitting antennas are in close proximity or where several transmitters share one antenna.

The usual result is that a signal from a second source will enter the transmitter's output amplifier, where it will mix with the transmitter's own signal to produce new frequency products. The mixing occurs because most practical transmitter amplifiers use transistors that are non-linear and is often called inter-modulation distortion or IMD.

Although a well designed transmitter has filters to stop other frequencies reaching the output stage, there are practical limits and transmitters with frequencies close to each other can produce some interesting IMD products that can't be filtered out (e.g. if f1=100MHz and f2=101MHz then one product is F=2*f1-f2 => 99MHz). They re-emerge at the antenna to be radiated as interference to other radio services.

Commercial radio operators spend a lot of money on cavity resonators and isolators to minimize such problems!

vk3byy