Buildout 800 MHz SMR FCC construction permit
Nextel makes deals with SMR operators with fully built and utilized repeater systems. Licenses were pulled from SMRs that aren’t actually fully built and on-air. To make a multi-channel trunked repeater from salvage parts, consider filtering and connections.
Unlike lower frequency bands where repeaters like Motorola GR300 or the Motorola R1225 repeater that are essentially refitted mobile radios for transmit and receive, 800 MHz subscriber receiver are not trivally retunable by software alone to the 806-821 MHz mobile transmit band. The easier plan is take the 1980s scanners being dumped on eBay as people pickup TrunkTracker scanners. For example, the PRO-2004 has specified 0.5 μV sensitivity. Since we need a receiver distribution amplifier anyway, just leave 6 dB less padding than before–problem solved!
For filtering, that will be part of the distribution amplifier–it will filter out the strong transmit signal on the separate antenna as well as any lower frequency signals (FM/TV/UHF/VHF) and any higher frequencies (AMPS 850 MHz). Given the location there shouldn’t be too many strong in-band signals, though overloading is possible within a km or so of the site. If that became an issue I could put a cavity filter on that channel, again from eBay. But for simplicity let’s stick with the receiver distribution amp 806-821 MHz filter.
The audio connection is made at the discriminator tap since DC-coupled baseband is needed to retrieve LTR data. The volume control is all the way down to avoid bothering other workers in the radio room. Leave the scanner speaker connected for basic diagnostics.
We use the EF Johnson 8600-series radio–get the full feature model with up/down arrows, not the single button model as you need talkaround! Set the radios to 10 Watts transmit power so that they can handle the duty cycle, and to help reduce thermal cycling stresses. Program each radio to a single channel, talkaround on the repeater output frequency. Bypass the microphone jack to get the full non-pre-emphasized DC response to the modulator.
Combiner: any 800 MHz SMR combiner is fine. Receiver distribution amplifier has three parts: 806-821 MHz combline bandpass filter, amplifier, and splitter. These tend to come in multiples of four as it’s easier to cascade that way as per Chip of Angle Linear. I managed to find a used one but in the future I would consider Angle Linear for even higher performance with his 0.7 dB NF amplifiers.
LTR controllers are not particular to the transmitters, but can be particular to each other. EF Johnson and Uniden use distinct backplane signaling. Others are switchable via jumper. Uniden doesn’t need a terminator while EF Johnson does need a 50 ohm terminator.