UHF Repeater

The VE6KQ UHF 444.750 + 5 MHz repeater was once part of a remote base configuration tied to 6 meter FM. The 6 meter radio is no longer in operation. See the write up on the remote base operation on this website that describes the UHF and 6 meter equipment in some detail. I will describe here some changes and repairs made to the UHF radio since the 6 meter radio was removed.

Here is the antenna and RF filtering as it exists now. The changes include the addition of an Advanced Receiver Research GaAsFET preamp and the addition of 3 cavity filters. The preamp is located external to the receiver and has a measured gain of 19.3 dB. Two additional cavity filters were added before the preamp to improve selectivity. Initially the repeater used 2 cavities, one for the transmitter and one for the receiver. This was all that was required for the repeater without the preamp. An additional cavity was also added to the transmitter to reduce sideband noise. Without the preamp I was unable to measure any external site noise desens. With the preamp external site noise desens was measured at about 3 dB. There is no desens caused by the transmitter. Calculated system noise figure including the external devices is about 4 dB.

The repeater radio is a GE Mastr Exec II. The GE RF power amplifier became intermittent. Because of the intermittent nature of the problem and non-availability of low cost parts, I decided to replace the amplifier. I had several UHF Motorola Mitrek radios. The plan was to replace the existing GE power amplifier with a Mitrek PA. The main tool required was a hack saw to remove the Mitrek PA from the rest of the radio. A power control board for the Mitrek PA was built using a 3 terminal regulator. To replace the GE PA with the Mitrek PA was not a difficult job and the reliability has been good.

Another problem with the repeater radio that appeared after the removal of the 6 meter radio was a reduction of receiver sensitivity. The problem was tracked down to "whiskering" in the front end. This was a bit of a job to repair, as it required the removal and disassembly of the helical resonator circuitry. If you are unfamiliar with whiskering, a Google search will help.

Other than recently configuring the repeater for tone squelch (PL) no other significant changes or repairs were made to the UHF station.

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