Iain Philipps - May 1997
Go to http://www.g8sjp.demon.co.uk/gb3am/ for further details.
The GB3AM
50MHz TeleWave / ProComm Duplexer
Doubtless many readers will be aware that proposals from a number of groups have been ‘in the pipeline’ for a long while now to introduce FM voice repeaters as a facility within the UK It is now expected that the first of these will be licensed and available for use during May 1997.
Hopefully these devices will generate a lot more interest in six metre operating among those who are not fortunate enough to live on a mountain top and sport a vast antenna farm. As you all know, when the conditions are right, a couple of Watts into a dipole (even indoors) can bring remarkable results. As we also know, here at the very trough of the current cycle, it can be exceedingly difficult to get an answer to plaintive ‘CQ’ calls, even running quite high power. And what of the Novice licensee, restricted to a very modest effective radiated power under all circumstances?
Well, the humble FM repeater - ancient technology by today’s standards, I’ll grant you - may just be the answer. There is no doubt about it, a well-designed and operated repeater station provides a focal point on any band. Being new to the UK, it was uncertain for a few years whether the authorities would ever agree to them for six metres. Thankfully (in my humble opinion, at least) common sense has prevailed. And rather than just slipping into the same ‘40 kHz per repeater’ utilisation of bandwidth, the six metre repeater groups now have the opportunity to demonstrate that - at the very least - only 50% of the bandwidth normally occupied by a repeater can work effectively, with the TX and RX channels being compressed to 10kHz each.
So the UK now has an IARU agreed band plan for repeaters on
Six which is incompatible with the rest of the region. Or is it?
Firstly, the allocated frequencies are as follows:
| Channel | Input | Output |
| R50-1 | 51.220 | 50.720 |
| R50-3 | 51.240 | 50.740 |
| R50-5 | 51.260 | 50.760 |
| R50-6 | 51.270 | 50.770 |
| R50-7 | 51.280 | 50.780 |
| R50-8 | 51.290 | 50.790 |
| R50-9 | 51.300 | 50.800 |
| R50-10 | 51.310 | 50.810 |
| R50-11 | 51.320 | 50.820 |
| R50-12 | 51.330 | 50.830 |
| R50-13 | 51.340 | 50.840 |
| R50-14 | 51.350 | 50.850 |
| R50-15 | 51.360 | 50.860 |
| R50-16 | 51.370 | 50.870 |
| R50-17 | 51.380 | 50.880 |
Note that this is overlaid on the Region 1 band plan, which allocates 51.210 MHz - 51.390 MHz as repeater inputs. The "missing" frequencies are designed to avoid those currently allocated to packet radio in the UK
The current applications are for the following systems:
| Call | Chan. | CTCSS | QTH |
| GB3AE | R50-1 | 94.8 | IO71OQ |
| GB3AM | R50-13 | 77.0 | IO91QP |
| GB3EF | R50-1 | 110.9 | JO02PB |
| GB3FX | R50-10 | 82.5 | IO91OF |
| GB3HX | R50-9 | 82.5 | IO93BP |
| GB3UM | R50-3 | 77.0 | IO92IQ |
| GB3PD | R50-14 | 71.9 | IO90KT |
| GB3PX | R50-7 | 77.0 | IO92XA |
| GB3RR | R50-11 | 71.9 | IO93JA |
| GB3SX | R50-8 | 103.5 | IO83WA |
| GB3UK | R50-6 | 82.5 | IO83RO |
| GB3WX | R50-12 | 77.0 | IO80VX |
As a matter of policy, all UK six metre repeaters use omni-directional vertical polarisation, and are accessible only to those with the correct CTCSS (sub audible) tone. Unlike repeaters on other bands, the tone is not used simply as an initial access method; in order to have your input signal relayed, you must transmit the correct CTCSS tone continuously.
It is important to note that with the 10 kHz spacing system employed, you must set your deviation to a maximum of 2.5 kHz, otherwise your transmitted audio will be "chopped" on speech peaks. For IC706 users, a reasonable stab at setting up for 10 kHz repeater operation is to select the "FM Narrow" setting, and adjust the microphone gain to "2".
For those with receivers expecting 20 kHz signals, the recovered audio will be at a lower level than would otherwise be expected. You must either replace the IF filters with narrow ones, or increase the AF gain - nothing else will work.
A few of the listed repeaters are currently built, tested and working, and these are the systems that are expected to be operational during May. They are GB3AM, GB3EF, GB3PX and GB3RR. It is also possible that GB3PD and GB3WX can be brought into service around the beginning of June, with a projected time scale for all of the others being brought into service by the end of August.
Early indications are that under ‘flat band’ conditions, the range for mobile stations is around 10% larger than that for similarly equipped stations on two metres - due mainly to the reduced ‘flutter’. Recent tests between the GB3AM site and G0VDE (JO02PF) have shown that signals are of reasonable quality over that 145 km path. The distant station, however, is sited in a "quiet" RF location, so it is expected that this will be the exception, rather than the norm.
Building and configuring a 50MHz repeater has been (for me) a rather interesting technical challenge, requiring significant investment in both time and money, but it is just possible that this will pay immense dividends in attracting new blood to the band.
Clearly the topic of six metre repeaters is more diverse than can be done justice in a couple of pages; if there is sufficient demand (requests to the Editor, please), then there is a great deal more to be said. Other than that, you may contact the author for further details.
