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24 Nov 2009

The repeaters and beacons of Buxton are all located at Sheffield University's microwave aerial test range at Harpur Hill, about 3 miles south of Buxton.
Most of the equipment is either extensively modified commercial modules, or home designed and built. Some of the hardware design was undertaken as projects for final year students at the University's Department of Electronic and Electrical Engineering. Much of the work was closely scrutinised (and often financed) by Tony Whitaker, G3RKL (of GB3US control logic fame), who is a Senior Experimental Officer in the department.
Nick (G7EKY) is the keeper of the repeaters and beacons, and, along with others from the local club (the Buxton Amateur Radio Club), maintains the facility for all radio amateurs to use. The notes that follow are largely snippets of information I've gleaned from Tony during the past, whether it has been a phone call or a dry-slope skiing session. This is simply an opportunity to collate the information into a readable form.
73 de Nick, G7EKY

GB3BUX Propagation Beacons
These propagation beacons for 4m and 6m are on 70.000.00MHz and 50.000.00MHz respectively.
They transmit the same sequence, and so share the same keyer, which is locked to the 60kHz MSF Rugby time and frequency standard. The callsign is sent on the minute in sympathy to the MSF data. During each second, the leading edge of the carrier break is locked to MSF and the keyer is off for 100ms, then on for 900ms to complete the second.
This allows direct comparison with MSF (providing you can receive MSF) for propagation path surveillance. The callsign, GB3BUX, is sent on the minute, with a long callsign, GB3BUX IO93BF BUXTON, sent on the 5 minute mark.
Both beacons are excited from the same RF source, which is a precision 5MHz frequency standard divided down to 10kHz for comparison and locked to the 60kHz carrier from Rugby. The 5MHz is doubled to 10MHz and distributed to each beacon. The 50MHz beacon uses a x5 multiplier and the 70MHz beacon a x7 multiplier. This affords RF driver accuracy locked to the MSF Rugby standard, and hence you will have noticed that the beacons operate right on, and are licensed for, the band edge. The aerials for both beacons are horizontally polarised turnstiles at about 20ft above ground level. ERP is around 10 watts.

GB3HH 2m Repeater
The repeater operates from the University's microwave aerial test range at Harpur Hill. The surrounding terrain prevents perfect 'all-round' coverage but access should be possible from reasonable locations around:
¨ Manchester
¨ Shrewsbury
¨ Wolverhampton
¨ Leicester
¨ Nottingham
¨ Sheffield
Although this list is by no means exhaustive, under flat band conditions there are sections of the M1 and M6 motorways that are within good coverage. The repeater is fairly reciprocal in the sense that if you can hear it, it can usually hear you. It certainly doesn't fall into the category of an 'alligator' repeater, where the Tx out-performs the Rx, or more simply, all mouth and no ears
The repeater was first switched on at 10:24 GMT on the 9th April 1976. The initial tests were conducted using a 5/8 ground plane aerial at approximately 25ft. The test transmissions were continuous beacon callsign with pips, an Icom IC22A was part of the hardware then and output power was around 12 watts. The Tx and Rx to date are extensively modified commercial modules and the antenna is a co-linear up at approximately 25ft. The single antenna is fed via a six-cavity duplexer. The control logic is a GB3US Mk 2 unit as featured in Oct/Nov '83 RadCom.
For initial access, the repeater requires a 1750Hz tone-burst of 200-1000ms duration followed by at least 5s of well-deviated audio. This criterion is important, since most reports of stations claiming they cannot access the repeater, even though it's 59+ with them, is largely due to the above requirements not been satisfied. Tone duration is checked, too long or too short results in initial access being rejected. Likewise, sufficient audio is checked and, particularly if the station's audio is noticeably quiet, initial access is terminated - audio needs to be strong for the logic to latch into relay mode. The sufficient audio check routine is implemented to reduce spurious triggering, and more so, nuisance triggering, since by its nature, it needs to detect strong audio before the repeater will open to relay. When the repeater changes to 12.5kHz channelisation, stations will require peak deviations around 2.5kHz, as opposed to the former 25kHz channelisation system peak deviation of around 5.0kHz.
The repeater will relay the input signal for up to 120s. If the transmission exceeds 115s, 5s of 'buzz-tone' is superimposed onto the through audio as a pre-timeout warning. This alerts the receiving station as to what point in the QSO the originating station timed-out. Then, if transmission continues, after the pre-timeout finishes at 120s, the repeater times-out (and is non re-accessible) and replaces the through audio with a 1750Hz warble tone (widely known as the 'strangled parrot') for up to 30s. When the timed-out station drops carrier or the 30s is reached, the repeater sends a high-deviation callsign (during which time it is re-accessible by a valid toneburst) and closes down. When the transmission ceases and the input clears, the repeater sends a Morse 'C'. If a user finishes his/her over within the allotted 120s, the next user may either:-
(a) transmit immediately, with or without toneburst, in which case only the remainder of the previous station's 120s is available before timeout. or...;
(b) wait 1s for the repeater to enter the 10s re-access period, indicated by four pips of 1750Hz at 2.5s intervals, during which a continuous carrier of approx. 1.5s, with or without toneburst, will reset the repeater timer for a further 120s of talkthrough.
If there is no re-access attempt within the 10s, the repeater will identify at high deviation, conclude the re-access period and close down. If the mains has failed, repeater operation will be from battery backup, a Morse 'B'; being sent in place of the first 1750Hz re-access pip. Usage should be kept to a minimum to conserve power. The repeater identifies every 5 minutes when not in use, with "IO93BF BUXTON" added every fourth time. A "jammed" input will initiate an automatic Mode 2 operation, which is indicated by a reversal of the usual tones (8751750). In Mode 2, access or re-access is possible at any time with a valid toneburst. Thus, a valid tone, sent either from the operating station or another station being able to transmit "over the top", will reset the timer.

During the spring of the year 2000, it is planned to update the FM 2m repeater, GB3HH. The hardware will be replaced with ex-H.M.Government equipment, of a slightly later vintage. It is worth noting that the current hardware was obsolete equipment surplus back in 1976, but still performs very well. The replacement repeater will utilise CTCSS (continuous tone controlled squelch system) and stations equipped for this are encouraged to use it. Only 'active' sessions on the repeater will be CTCSS encoded on the repeater output. By using CTCSS in your station receiver, you will only monitor actual activity on the repeater. The 5-minute beacon callsigns will not be encoded and so your receiver squelch will not open and the ID's will not be heard. Also, by encoding CTCSS on your transmissions, the repeater will open, providing there is at least 5s of carrier, without the need to send the 1750Hz tone burst for initial access. It is hoped in the future for the CTCSS decoding at the repeater to become "differential", in the sense that the squelch will open at a lower level with CTCSS than without it. This is an attempt to reduce co-channel interference by increasing the squelch threshold to interfering non-CTCSS carrier signals, or input signals with a neighbouring CTCSS tone, yet allowing full sensitivity of the receiver to valid CTCSS encoded signals. In addition, the Tx will be driven from a high stability RF source to yield an overall carrier frequency accuracy to within a few tenths of a Hertz. This is achieved by locking the Tx local oscillator (becoming a VCXO) to a precision 5MHz frequency standard. The whole station will be 12.5kHz channel compliant, which means that the IF filters in the receiver will be narrower, and so any ‘wide’ signal on the input will distort, more so if it's particularly weak. Finally, the transmitter peak deviation will be set to 2.5kHz.

73 de Nick, G7EKY ©


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