History of the SHARC-2 Repeater
KS3R / R
Revised 5/31/12

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I always wanted to build another repeater. Finding good equipment is one thing, but finding a good site is another. Well, a good friend and co-worker of mine, Jack - K3DMY made me an offer I couldn't refuse! Jack offered to sell me a used RCA community basestation repeater, already housed in a portable rack. It even included the duplexers! All I had to do was recrystal it, retune it, and come up with a simple controller for it. Well, needless to say, I jumped at the deal, and for the next 6 months, my new repeater became another piece of furniture in our living room.

In the meantime, I did quite an amount of research on the 440 band, and submitted my frequency coordination request to the Western Pennsylvania Repeater Council. The council quickly granted my request, saying they had never received a more thorough request in their history of existence! Now that I had secured a frequency, all I needed were the crystals. The repeater was originally tuned to operate around 454 MHz, so it would take a little retuning to bring it down to 442.500 MHz. While I was waiting for the crystals to arrive, I took the duplexers into work and retuned them. I also installed caster wheels on the bottom of the rack, which would make the repeater a lot easier to work on.

While still waiting for those quartz rocks to arrive, I began thinking about a controller. Since it was my intention of eventually linking the new repeater with its sister repeater (145.130), and sharing its controller for higher level functions, I decided to build a rather simpler controller this time around. I borrowed many of the circuits from my first repeater, which proved to be very reliable. This controller would have only (3) plug in cards, and room for (2) expansion cards. The controller was very basic. It only has (8) remote functions, (2) selectable courtesy tones, and a CW identifier. It also has a 8 channel audio mixer board. The tail and timeout features are provided by RCA's repeater control panel, which I would learn to hate later on. I'll get to that in a minute.

SHARC-2 Repeater Antenna

SHARC-2 Repeater Antenna

Well, Jack came back with another great offer which I couldn't refuse....a repeater site! Jack told me about a site that was maintained by a local Emergency Management Agency where he has his MARS Repeater. There was also a 120' tower to boot!

As I stated earlier, the tail and timeout features are controlled by circuits within RCA's repeater control panel. I later discoverd that the tail duration was controlled by a simple RC (resistor/capacitor) circuit, and would change in length depending on how long the user held down the microphone key. On the 145.130 MHz repeater, I use 555/556 timer IC's which produce consistant timing outputs, regardless of input triggering.

The receiver & transmitter audio is also processed in the control panel. It seems RCA takes a sample of the speaker audio, and sends it to the transmitter. The result is very bassy, and muffled reproduction of the original receiver audio. On the 145.130 MHz repeater, I sample the receiver audio directly from the receiver's discriminator. Here, the audio is crisp with more treble than bass.

The problem with discriminator audio is that it is unsquelched; meaning that when no signal is present, you will hear the sound of mixer noise in the receiver. A way to get around this problem is to use a bilateral solid state switch (CD4066), and gate it with the COS (carrier operated squelch) signal.

When a signal is received, the COS line activates the switch, allowing the receiver audio to pass to the transmitter. As soon as the signal goes away, the switch opens, and you hear silence on the tail of the repeater. It really works great, and gives you great sounding audio. So, in the near future, I plan on modifying the repeater's audio and tail circuit to be like that of it's sister repeater.

The 442.500 MHz repeater is very clean, and except for the homebrew controller, its basically stock. Right now, we are only using (1) PL frequency (131.8 Hz), but there are expansion slots for up to (4) different PL modules. Since it's installation in early Spring of 1995, there have been no reported failures.....(crossing my fingers).

Jack - K3DMY & Jim - KA3EBX

Jack - K3DMY (left), Jim - KA3EBX (right)


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