RX Antenna VS TX Antenna length

[Simmiz]

I've often been told by RF users that TX antenna length was more critical than RX antenna length. Being a wireless microphone users I've noticed that companies can use the same RX antenna for a wider bandwidth than for Tx's... Can anyone point out some theory behind it?
Thanks guys,
Simon

 

[betwixt]

There is a best length for RX and TX which is theoretically the same and depends on the frequency being used. Often, the length is 1/4 wavelength.

For transmitting there is an additional factor through: the transmitter output stage will be most efficient if it drives a matched impedance load. The length of the antenna determines it's impedance so there is an optimum length to get best power out and therefore best range. Note that there are various 'tricks' to electronically match the impedance which give some improvement in output, although not as good as an ideal antenna, while reducing external wire length.

Receiving isn't as important although it still works best if the length is correct. It's easy to make up for variations in signal strength by using an automatic gain control circuit that adapts the receiver to pick up weak signals while not overloading on strong ones. that gives greater flexibility to the receive antenna length.

Brian.

 

[volker@muehlhaus]

Signal loss from wrong antenna length is the same, no difference between transmitter and receiver.
The reason why people are more careful on the transmitter side: antenna mismatch can damage the transmitter's output amplifier.

 

[vfone]

Antenna gain is identical whatever (the same) antenna is used for RX or for TX.
Usually the wireless systems have very stringent TX specifications, but in the same time most of the regulators don't care much if you have a crappy receive system.

 

[FvM]

Good TX antenna matching can be more important than RX antenna matching when external interferences come into play. Often the range of your radio system is limited by noise "in the air" (other radios, RF radiation of digital electronics and switched mode power supplies, brushed DC motors) rather than receiver noise, then raising the transmitted signal over the noise background makes sense.