Friday, November 1, 2013

Common Radio Frequency Questions - RF FAQ

Radio Frequency FAQ’s
I now work for a company called ComProd Communications.  You can find us at www.comprodcom.com    We specialize in Creating RF Solutions.  There are several questions that we get asked a lot.  Here are some of the most common questions.

My antenna came with a ground plane; do I need to use it?
The ground plane is a critical part of any antenna system.  It has both a minimum size as in length and width, as well as a specific distance that it has to be from the antenna.  If you have to replace it for some reason the replacement but be larger in size as far as length and width, and it must be the exact same distance from the antenna!  The distance is critical because the reflected signal has to be in phase with the signal emitted from the antenna.  Changing the distance will cause canceling of the output of the antenna.  Making it smaller will affect the impedance of the antenna as the amount of reflected signal will vary over the length of the antenna.

Why has the cost of Combiners gone up while the cost of transmitters and receivers has gone down?
As transmitters and receivers have gotten cheaper their sensitivity and selectivity has gone down.  If the combiner does not remove unwanted frequencies then the receivers will be overloaded in their front end.  Let’s say we have a transistor RF amplifier running on 12 volts.  If it has an input signal that produces an output that is over 12 volts peak to peak it will turn it into a square wave and produce tons of harmonics.  This will totally hide the frequency that you want to receive.  The combiner must remove the unwanted frequency from both the transmitter and the receiver in order for the receiver to work correctly. 

Are antennas best stacked vertically or horizontally?
Back in the early days of audio it was common to see lots of speakers positioned horizontally across the front of the stage.  The concept was to have a speaker aimed at everyone in the audience.   At some point engineers started pointing out that if you stack the speakers one on top of the other (Vertically) you get much better sound dispersion.  Although this does not look correct to the eyes, time and experience has proven that it works.  Now most bands use vertically stacked speakers.  The same is true with antennas.  Mounting antennas one above the other seems like it would be sending signals out into space, but instead it concentrates the beam horizontally.

Why do we need separate receive and transmit antennas?
Transmitters emit some signals at unwanted frequencies, usually close to their primary broadcast frequency.  These unwanted frequencies are rapidly reduced in strength as you move away from the transmitting antenna.  However these unwanted signals can easily interfere with receivers that are operating on nearby frequencies.  If you have a separate transmit and receive antenna this problem can be drastically reduced.  Otherwise the frequency of the receiver needs to be removed from the transmitters output as well as the frequency of the transmitter must be removed from the receivers input.
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