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Workshop Notes 10/19/04
What Makes an Amateur Radio Repeater System?
Article by Billy Gomban Jr. KH6JPL
What is a 2 meter amateur radio repeater?
Usually installed atop a building or mountain a repeater is like a FM broadcast radio station. The difference being that instead re-broadcasting program information from a DJ (excuse me, I'm with the DJ) the repeated information comes from a 2 meter receiver. So there you are driving around in your car at sea level. You pick up your 2 meter radio and start transmitting on a repeater frequency. The repeater, high atop a mountain, hears your voice and routes it to the transmitter where it is rebroadcast and heard by many more stations. Without repeaters hams would be limited to "line of sight" communications which would limit your communication capabilities. The theory is... the higher your transmitting antenna, the greater your communications coverage. So, if you can't be atop the mountain every time you transmit why not use a repeater system.
A typical amateur radio repeater system consists of the following components. The Transmitter, Receiver, Repeater Controller, antennas, duplexers, and other miscellaneous items.
The transmitter and receiver are the most critical components of a repeater system. Remember that both the transmitter and the receiver will be in close proximity to each other. Your biggest challenge will be to keep the RF power of the transmitter from getting back into the receiver. This would cause a "de-sense" or de-sensitivity of the receiver. Which is often times the cause of a poor repeater system.
Using Mobile Radios to Make a Reapeater System.
Can you use a pair of mobile ham radios to make a repeater system?
Ah.. that was my plan initially. What better way to save money. I can tell you know, don't waste your time. There is not enough RF isolation when using two ham radios. Without this RF isolation you will never get rid of that de-sensing.
What is de-sensing?
De-sensing occurs when the output of the transmitter gets back into the receiver and causes the receiver to be loose its sensitivity to the frequency it is tuned to listen to. Have you ever heard a repeater kerchunking with rhythm when a very week signal is attempting to get through the repeater. Kerchunking occurs when the repeaters on then off again with no intention to pass audio. Like keying a PTT just for the purpose of causing the repeater to key and un-key. I'll cover this in greater detail later.
Commercial analog radios are the best to covert to a ham radio repeater.
Once you have the transmitter and receiver your next step is to find a controller. The controller connects between the transmitter and receiver and provides the following functions. It acts as a volume control for the receiver audio and feeds it to the transmitter. More important, the controller senses when someone is sending a signal to the receiver and in turn keys up the transmitter.
Basic controllers that include a cw station ID unit can cost about $300. More sophisticated controllers have voice ID units, multi-functional switches for controlling devices remotely and even 200 number memory auto dial systems. When connected to a telephone line the controller can be used to turn the repeater on/off. This is a must because when things go wrong your repeater might stick in the on position until you turn it off manually. That's not fun when you have to drive up to the top of a mountain.
Next you will need a duplexer which is a device that allows a receiver and transmitter to share the same antenna. It is a fine tuned instrument that prevents the transmitter's signal from going directly into the receiver even though they are both connected. Duplexers accomplish this by using band pass and band reject radio frequency suppression. You can go without a duplexer if you have vertical separation between the two antennas. I'd place the receive antenna atop a tower and the transmit antenna below the receive antenna with the very least horizontal separation. It's best to place the antenna one above the other because the RF lobe (concentration of RF power) of the transmitting antenna is the very weakest right under the antenna. This will ensure minimal RF feedback to the receiver which will minimize de-sensing.
Finally you will need a good base antenna and could coaxial cable. I'd recommend an omni antenna like the hustler G7. I've been using that for years. Some friends of mine have used commercial station master antennas and have found that internally the solder joints would eventually come apart on the many internal loops of wire.
That's it for repeater design 101... If you have interest in this... drop me a line and I'll hurry to add more info for you... else... more will be coming.
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