| GB3AM 50MHz Repeater Hardware |
(All about how we built it)
Converted Tait T336. It started life on 72 MHz and has variously been beaten and cajoled into performing superbly on 50.840 MHz with a power output of 6 or 16 Watts, switchable from the control logic. Of course, these levels are arbitrary, in that each of them may be set so as to produce anything between 200 mW and 27 W output.
There is a high power amplifier (pair of BLY90's with integral mains supply) following the transmitter, and this boosts the output power to either 18 or 48 Watts in the current configuration. The gain of the amplifier may be preset anywhere between 0 dB and 10 dB, and it is currently set to run at 4.5 dB gain (nicely towards the middle of it's range, for longevity).
We think that initially we'll want to run the repeater at the 18 Watt level so as we can get a feel for the performace in the intended service area, but of course it may be necessary to go to the higher level to provide good coverage. The answer to this can, we feel, be found only by experimentation.
On soak test - into the obligatory dummy load, naturally - this equipment produced 85 Watts output continuously for 120 hours. I turned it off to save electricity, but I think it would have gone on forever! All harmonics from the converted transmit strip are minimum -80 dB; other suprious emissions are lower than those from my PC (which may not say much ...).
The CTCSS encoder is set to produce a peak deviation of 230 Hz at our allotted tone frequency of 77 Hz, and the transmitter audio level (when fed from the logic) combines with this to produce a maximum peak deviation of 2.3 kHz. Considerable processing of the audio is necessary in order to meet the 14 dB/octave modulation characteristics (curiously) called for by the specification. I think it sounds wierd (anybody remember what the Buggles sounded like when they were killing the radio star?), but others think it's OK. We'll just have to see.
Converted Tait T335. This was an 85 MHz unit and required a little persuasion to work efficiently on 51.340 MHz. I made it work, and I'm tremendously proud of that. The squelch opens at -124 dBm (0.14 uV) and closes at -127 dBm (0.1 uV) giving a 3 dB hysteresys, which isn't called for in the specification, but is a really nice thing to have. The 12 dB SINAD point is at -121 dBm (0.2 uV) - considerably exceeding the specification called for.
The CTCSS decoder will lock on a signal of 150 Hz peak deviation at -121 dBm, and will work down to 85 Hz peak deviation at -113 dBm; it's probably fair to say that on the whole, it's not a bad receiver!
This is a real monster, and it cost an absolute fortune. But you really only get what you pay for, and in trying to produce a technically sound repeater, attempting to save money (if the performance is at risk) is just plain dumb!
We invested our hard earned cash in a 6-cavity Telewave duplexer, which we had custom built to run with a 51.830 MHz transmitter / 51.230 MHz receiver combination. Each cavity is 5 inches diameter and approximately 92" tall (that's right - nearly 8 feet!) If you need to know why we ordered it on those frequencies, go back and read the history page, paying particular attention to the spaces between the lines.
In the break between Christmas and New Year Dave, G8KBV and I trudged up to our repeater site (where the duplexer is neatly installed in readiness for who-knows-when) and, armed with a snazzy HP Network Analyser, we began the tedious task of retuning it to the new (and hopefully final) frequency pair. This was a bit of a battle, but eventually we got what we consider to be satisfactory performance at 2.2 dB per leg insertion loss and 95 dB isolation. We think we can improve the isolation by another 3 to 5 dB - which we'll want to do if we end up running at the maximum 14 dBW ERP (which will imply transmitter output power at the 48 Watt level).
Unsurprisingly simple. We have an end fed vertical (zero gain) omnidirectional mounted at 35 metres above ground. It's on a four legged tower, and stood off 1/4 wavelength from the south eastern leg at an angle of 45 degrees (if that makes sense). This was done deliberately and produces the strange cardoid coverage pattern that we have. We're pleased with this, and hopefully the users will be, too!
The feeder is a single 45 metre length of Andrew LDF4-50, with a measured loss of 0.75 dB at 50.5 MHz for the entire run, including connectors. When added to the duplexer loss, this conveniently gives us a total loss for the filter/feeder system of (as near as makes no difference) to 3 dB. Now you know why I went to such lengths with the receiver performance and the high power amplifier for the transmitter.
Guaranteed to cause a stir at any one of our committee meetings. Thankfully the specification we have been given is simple, and that therefore allows a significant degree of flexibility. I have now produced a modern design for the controller, which is heavily based upon the NHRC-2 (see February 1997 QST), and have now learned sufficient PIC programming techniques to have bludgeoned the software into meeting the specification required for UK use. The current configuration is straightforward, requiring continuous 77 Hz CTCSS tone to hold the receiver squelch open. There's no way around this - no CTCSS = game over. The time out is set at 3 minutes (maximum allowable for a UK 6M repeater is 5 minutes), with the repeater callsign being sent using an 875 Hz audio tone, at 1.1 kHz peak deviation on initial access and every 4 minutes thereafter.
The repeater also identifies every four minutes when it's not in use ("beacon mode") with the same tone and level. Since the 6M specification states that the CTCSS tone "need not" be sent when identifying in beacon mode, no CTCSS tone is transmitted during such idents.
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