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Getting the Most From Your Repeater System By By Kevin Custer W3KKC

First, keep in mind that a repeater's performance is usually limited by its ability to hear the users, not by its power output. Minimizing the attenuation of the received signal by improved receiver performance, minimizing feedline loss, optimal duplexer tuning, etc. is one of the least expensive ways to maximize the useful range of the system. Once the incoming signal is down in the noise, there is nothing you can do to get it back. To reiterate, if you can't hear the users then the repeater is just that much useless hardware sitting on a mountaintop.

Tuning the Receiver and Transmitter -
A good starting place for getting the most out of your repeater, is to verify that your transmitter and receiver is working up to snuff. This may seem a difficult task, especially if you don't have a lot of test equipment. Transmitter alignment is essential for good duplex operation. Follow the alignment procedure for the exciter, then tune the amplifier for maximum on-channel output using a good quality watt meter and dummy load. Use a weak on-frequency signal to tune the receiver. This is normally accomplished with a Service Monitor, however you may not own one of these.

You may want to check around and find a ham that works for a local two-way shop to see if he will assist you in tuning up your receiver. If a monitor is not available, use a signal from someone at a distance from you with a stable operating station. Someone with an all mode rig with variable output power is a big help. Tune the receiver without the transmitter keyed. If your repeater doesn't have a ptt disable switch, add one, a simple toggle switch is fine. The switch will allow you to easily turn the transmitter off and on to test for desense. The most useful configuration is a center-off toggle switch with the wiper hooked to the transmitter's PTT lead, one side hooked to the repeater-controller's PTT output, and the other side grounded. In the center position the transmitter is disabled, in the up position it's keyed full time, and in the down postion it's normal.

Duplexer Tuning -
After verifying proper transmitter and receiver operation, connect the receiver to the duplexer, let the transmitter on the dummy load for now. Place (another) terminating load on the transmitter port of the duplexer. Connect a weak signal to the antenna side of the duplexer, with either a service monitor, or from the antenna with your weak signaled friend. Tune the receive pass section of the duplexer for maximum quieting. You may need to optimize the input tuning stage of the receiver for maximum quieting. Go back and forth between duplexer and receiver, tuning each for maximum quieting until no further improvement.

With the transmitter still connected to the dummy, and receiver hearing a weak signal, you should be able to turn on and off the transmitter with no change in receiver quieting. This step tests for internal repeater desense. If this test fails, the repeater itself needs work to get rid of all internal desense.  If all is ok so far, remove the load from the transmitter port and connect a handheld, tuned to the repeater output frequency, to the duplexer transmit connection. Connect the repeater antenna through a quality watt meter to the duplexer antenna connection. Key the handheld and tune the transmit pass section of the duplexer for maximum output power as indicated on the watt meter. It is important to start with low power, (1 or 2 watts),  if the duplexer tuning is not close to resonance, as you may damage the duplexer, or the handheld.

Disconnect the handheld and replace with the repeater transmitter. Tune the duplexer transmit pass section for maximum output. If you are tuning a tube type transmitter, tune the duplexer for minimum reflected power, (watt meter connected between cans and transmitter), as the tuning of the duplexer will change the reactance of the output section of the transmitter, and upset the tuning and loading of the transmitter.

Now, connect the signal generator to an outside antenna capable of opening the squelch on the receiver.  With the transmitter disabled, turn on and adjust the weak signal source for a good bit of white noise. Turn the transmitter off and on, do you hear any difference? If you do, you have desense, and the notch section of your duplexer need tuning. If the signal is completely wiped out, you have a lot of desense. In order to tune the notch side of the duplexers the receiver must hear a weak signal. Increase the weak signal if necessary to maintain open squelch with some quieting. While listening to local speaker and transmitter keyed, adjust notch section of transmit side of the duplexer for maximum quieting. Reduce weak signal level if necessary to maintain white noise. If the receiver becomes quiet while adjusting duplexer, you won't be able to tell if you are making an improvement. Reducing the signal will allow more critical adjustment. Keep adjusting notch section of transmit side of the duplexer and reducing signal level until no further improvement. Do the same for the receive notch section of the duplexer. You should have eliminated any desense.

Always strive for NO measurable desense if at all possible (and use test equipment, not your ear). The use of a simple toggle switch in the PTT line, and monitoring the noise level of the receiver on a VOM hooked to the local speaker leads is the only good way to test for desense (look in the receiver manual under the heading "20db quieting test", or "SINAD test"). Use a weak signal leaking into the Antenna System, not by inserting it into the receiver, or duplexer line with a "tee" connector or some other unacceptable manner, (everyone else has to go through the antenna, your test signal should also).   I use an outside mounted test antenna - something like a 1/4 wave ground plane or another antenna already mounted on the tower works well. It is necessary that the antenna be fixed in position and away from your body so the received signal level does not change as a result of of you (or anyone else in the building with you) moving around near the test equipment or repeater.

If you still have desense, try connecting a dummy load in place of the antenna because some antennas just don't duplex well. The test antenna should be several wavelengths above the repeater transmitter and receiver.  This vertical separation between antenna and radio set keeps RF from bombarding into the circuits of the repeater. If the repeater and duplexer can be tuned into a dummy load with no desense, your antenna, feedline or lightning aressting equipment may be defective or may not duplex well.