Details of Our Repeaters
From The WB7TJD Wiki
The Superstition Amateur Radio Club owns and operates two repeaters, co-located in northeast Mesa, near the Loop 202 and Brown Road. They perform the magic of listening for and hearing your rubber duck 15 miles away and relaying your voice over two frequencies at over 30 watts each using just one antenna, at the same time.
On this page we will discuss the various components that exist between the repeater radios and the repeater antenna.
The repeaters transmit on 147.12 and 449.60 MHz, and listen on 147.72 and 444.60 MHz, all of this on one antenna, all at the same time.
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Superstition ARC Repeater System as of December 2011
The Superstition ARC owns and maintains the 147.12, a GE MASTR II, operating at 66 watts output, reduced by duplexer and feedline losses by the time it reaches the antenna.{{sa{{ Feedline loss is negligible, as we are using a large-diameter hardline, but typical insertion loss when using duplexers is upwards of 2 or 3 decibels. We can figure we are running about 35 watts at the antenna, ballpark figure.
The club owns and maintains the 449.60 repeater, which is limited to the maximum legal limit of 50 watts output permitted in Arizona.
Duplexer
A duplexer is a set of tuned cavities (think of a metal garbage can with a lid as being a cavity). Cavities can be placed in series with, or across the desired signal path, and tuned by adjusting the height of the cavity's ceiling, the higher the ceiling, the lower the frequency.
The cavity across the signal path shunts the transmitter frequency at the receiver to ground, or the receiver frequency at the transmitter to ground. Sending the transmit frequency to ground at the receiver input allows the receiver to hear incoming signals without being overloaded by the transmitter. Similarly, sending the receive frequency to ground on the transmitter output terminals reduces chance of high-level mixing of strong incoming signals in the transmitter's output stage. such mixing would result in spurious output on other sum and difference frequencies.
Tuned cavities tuned to the receive frequency and placed in series with the signal path will further reduce out-of-band signals at the receiver input and help eliminate unwanted interference, while series cavities tuned to the transmit frequency will work to further reduce the radiation of off-frequency mix results.
Diplexer
The 147.12 and 449.60 duplexer outputs are merged onto the same antenna feed using a diplexer. This unit keeps 440 MHz energy away from the 147 MHz equipment and vice versa.
Isolator
Each repeater further is equipped with an isolator, which is placed at the transmitter output before the duplexer. An isolator has three ports, with signals entering the input port being routed clockwise around the circulator 120 degrees to the output port, where it continues on its journey.
Incoming signals that have managed to get past the other filters, and reflected power from a mismatched antenna will enter the circulator's output port and be routed clockwise 120 degrees to the dummy load port, and dissipated as heat. (Signals coming in on the dummy load would route another 120 degrees clockwise and exit the input port back into the transmitter were there a signal source on the dummy load port instead of the dummy load.)
See this theory and application explanation courtesy of www.saisum.com.
Transmitter, Receiver, Controller
A 100% duty-cycle transmitter with large heat sink and cooling fins for its size is used, in our case a commercial repeater from the 150 MHz and 450 MHz commercial bands, designed and built for heavy-duty repeater service.
The receiver has isolation and shielding to keep it from being bothered by the transmitter at all levels, including feed-through capacitors that allow the desired power supply voltages and audio to enter and leave a shielded compartment while stopping RF voltages and current from entering, bypass capacitors and chokes on power leads, and shielded cables.
Audio leaves the receiver and is routed through the controller and on to the transmitter. The controller generates the Morse code identifier, voiced timeout messages and other messages about the state of the system, together with the courtesy beeps and CTCSS tones heard on the repeater.
Logic also is detected at the receiver to indicate the presence of a signal on the repeater input, and this logic is fed to the controller to trigger such functions as the push-to-talk circuit of the transmitter, together with a timeout timer countdown, another timer for how far out the courtesy beep is delayed after the end of the user's transmission, how long the beep should be, and how long the tail should be after that before the transmitter drops. Another timer keeps tabs on how long it has been since the repeater was last identified, and ensures that one is sent at the end of the next transmission, or sooner if necessary to stay within the law.
The controller also takes audio from the control link receiver and allows a control operator to intervene in shutting down the receiver, and controlling other aspects of the repeater.
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Repeater Coverage (next page)
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