Building an 800 MHz SMR system using salvage parts to fulfill FCC construction permit.

So this three-year conversion with the SMR guy has finally started to pay off. We see the deals Nextel is making with SMR operators, and the licenses that are being pulled from SMRs that aren’t actually on-air. He is a little non-committal to putting a full spec trunked system on the air, as given his rural location the POP is low and so we want to maximize ROI.

I told him to let me share the risk with him. I will provide the repeater system, a legal system fulfilling the mandate in spirit and letter. What’s the secret? eBay.com

eBay provides a market for people dumping 800/900 MHz LTR gear as their customers grab up Nextel phones (including in our area) as their network expands. The repeaters are still well over $1K each because most SMRs are hanging on. There are late 1980s SMR 800MHz subscriber mobile units that are selling for less than $50. There are also PRO-2004 scanners from the mid-1980s selling for $100 or so.

making an 800MHz/900MHz repeater from scanners and mobile radios

Since we’re making a multi-channel trunked repeater from salvage parts, we have to consider filtering and connections.

receiver

Unlike lower frequency bands where repeaters like GR300 or the new R1225 repeater that are essential refitted mobile radios for transmit and receive, no 800 MHz subscriber receiver is readily retuned to the 806-821 MHz mobile transmit band. I thought about hacking around retuning filters, but going down in frequency is harder than going up. Also I’d have to hack the CPU. 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 distro amp 806-821 MHz filter.

The audio connection is made at the discriminator tap as I need the DC-coupled baseband to retrieve LTR data. The volume control is just left all the way down to avoid bothering other workers in the radio room. I leave the scanner speaker connected for basic diagnostics.

transmitter

I 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! I set the radios to 10 Watts transmit power so that they can handle the duty cycle, and to help reduce thermal cycling stresses. I program each radio to a single channel, talkaround on the repeater output frequency. I bypass the microphone jack so that I get the full non-preemphasized DC response to the modulator. The LTR waveform is a little sloped but test radios seem to decode it OK. I’ll have to test under real-life fluttery conditions to be sure I don’t need further measure to correct the skew of the LTR waveform.

combiner

These are not free but they are on eBay. Any 800 MHz SMR combiner is fine.

receiver distribution amplifier

This has three parts:

  1. 806-821 MHz combline bandpass filter
  2. amplifier
  3. 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. He spun off his company from NASA!

LTR controllers

These 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.

In this case, I got some gently used IDA RLC panels from a system that had downsized.

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