|
Notice To UsersGB3BS has now moved frequency (RU68).Output - 430.850 MHz, Input - 438.450 MHz. 1750Hz Tone or CTCSS 118.8Hz access.
For more information please go to the Latest News section or to the GB3BS Forum. |
Location: Cossham Memorial Hospital, Kingswood, Bristol.NGR: ST 643,746
|
|
Welcome to the Bristol 70cms Repeater Group page. The repeater GB3BS, has been operational from it's site in Kingswood which is located on the North East side of Bristol since 1972. The coverage of the repeater ranges from West along the M4 motorway to the outskirts of Cardiff, to around Junction 18 of the M4 going East. The outskirts of Gloucester to the North, down to Weston-Super-Mare are also covered. However, attention to providing good coverage for hand-portable & mobile users in the built-up areas including the city centre of Bristol has always been paramount. |
|
|
 |
 |
 |
 |
 |
 |
 |
 |
TRANSMITTER
Modified Pye T462, producing an Output Power of 18 Watts MAX. Effective Radiated Power set at 20 Watts. Frequency Reference, Standard High Stability Crystal. 5 KHz peak deviation. CTCSS deviation 350Hz max. Modified for external reference source unit, HS-400, providing less than a 8Hz drift per-year @ 433 MHz. (Currently not active)
RECEIVER
Modified Pye R460. Upgraded front-end, giving a 12db SINAD reading at approximately 0.2uV/-121dbm. Squelch hysterisis 3db. IF's 1st 10.7MHz, 2nd 455KHz. Frequency reference, Standard High Stability Crystal. CTCSS Tone Detector Sensitivity -40db rel. CTCSS Filter, Active <250Hz, Tone Attenuation >50db.
AERIAL SYSTEM
The aerial system consists of two commercial grade antennas of the SA-400 series. Each antenna is made up from a phased array of 4 half-wave dipoles which are mounted at 90° to each other along a 2.5 metre pole.
Both poles are then stacked one above each other, giving a total of 8 dipoles, 4 for the TX and 4 for the RX, the top set is connected to the Receiver for maximum advantage. Each antenna has a ±8° "E"-plane beamwidth.
One of the Half-Wave Dipoles.
This configuration gives a good even 360° "E"-plane pattern and good TX to RX isolation of around 45db. Each of the 8 dipoles can be individually moved (if required) to produce a custom "E"-plane pattern. The antenna system has a gain of around 5.5db for both the TX and the RX. The dipoles have been adjusted to give a -5° tilt (downwards), so as to reduce the amount of energy being propagated towards the horizon. This enforces the downward lobes so as to improve coverage down in the city centre. Both sets of antennas are fed with low-loss Heliax LDF-450 Co-Ax.
CAVITY FILTERS
Both the TX and the RX are fitted with tuneable cavity filters. The receiver cavity is tuned to reject at the TX frequency (433.250MHz), likewise the transmitter cavity is tuned to reject at the RX frequency (434.850MHz). Each cavity provides a notch of around 20db, thus between TX and RX ports there exists a rejection of some 40db. When added to the antennas 45db of isolation this provides a total TX port to RX port isolation figure of some 85db thus making receiver de-sense non-existent.
CONTROL LOGIC
The Control Logic was designed by the University of Sheffield (GB3US) and uses the RCA CDP1802 microprocessor. Built and tested by GB3BS repeater group. Installed on GB3BS in 1984 after further modifications to software parameters. This replaced the MK1 version, also designed by the University of Sheffield and remains on site as a STANDBY should there be a problem or failure.
OPERATING CHARACTERISTICS
Beacon callsign at 875Hz every 10min when not in use, sent as "DE GB3BS J" (short bcs), with every third one sent as "DE GB3BS J BRISTOL IO81RL" (long bcs). Valid tone plus 4sec of audio for full initial access, success indicated by a low deviation callsign at 1750Hz. The carrier must not be dropped between tone and the 4sec of audio. 4min talkthrough time. 1sec forbidden gap before reaccess, to allow other stations to break in. Carrier reaccess during a 10sec reaccess period, indicated by two high-level 1750Hz pips spaced 5sec apart. Identification callsign sent when the timer is reset if a period greater than 4min has elapsed since the last one. Pulsed pre-time-out warning, under the through audio during the last 5sec of relay. Non-reaccessible time-out indicated by a trimfone type tone with 30sec maximum default time. Tone reaccessible, high-level callsign (1750Hz) at the end of time-out or first end-of-reaccess. Automatic Mode-2 during a "jammed" input, indicated by a reversal of the usual keyed tone frequencies. This allows the timer to be reset at any time by a valid tone. Automatic return to Mode-1 when input clears. Letter "C" sent (1750Hz) to indicate that the input has cleared after time-out and the transmitter has closed down.
CTCSS FACILITY
CTCSS stands for Continuous Tone Controlled Squelch System. Its is also known as Tone Squelch or Tone Lock. GB3BS uses a CTCSS frequency of 118.8Hz, tone letter "J", hence the callsign "GB3BS J". Users using CTCSS can access the repeater in the normal way except that the 118.8Hz tone can remain ON during normal overs. The sub-audible tone from the receiver is first detected and then filtered out from the normal through audio. The repeater will re-generate the tone and pass it to the transmitter. During NORMAL operation, while the repeater is being used, the sub-audible tone is constantly being sent along with the relayed receiver audio. Deviation of the tone is currently set at 350Hz. When the repeater is NOT being used and the short or long beacon callsigns are transmitted (every 10min) the sub-audible tone will NOT be sent. This allows users equipped with a tone-squelch facility the ability to filter out the regular 10min beacons.
CTCSS FREQUENCIES
| Tone Freq Area | Freq (Hz) |
| A | 61.8 |
| B | 71.9 |
| C | 77.0 |
| D | 82.5 |
| E | 88.5 |
| F | 94.8 |
| G | 103.5 |
| H | 110.9 |
| J | 118.8 |
For anyone who's rig does not support a CTCSS encoder then perhaps there is room inside to build & fit one yourself. A simple circuit, using a FX315 by CML, that works well and allows switching to all the above tones is available, click here for the circuit diagram.
ACCESSING
Accessing GB3BS can be achieved by the normal 1750Hz Tone-Burst or by using a Sub-Audible tone of 118.8Hz, tone slot "J".
Accessing using 1750Hz.
To Access GB3BS the 1750Hz (± 25Hz) tone should be at half system-deviation (2.5KHz) and have a duration of between 200mS and 400mS. Once the tone has been sent the user must then provide 4 seconds of audio. It is important not to drop carrier between the tone burst and the 4 seconds of audio. Dropping carrier between the end of the tone burst and the 4 seconds of audio will be treated as a non-valid access. A valid access is acknowledged by the repeater giving it's Callsign.
Accessing using Sub-Audible tone.
Users equipped with the CTCSS facility can access GB3BS by simply pressing their PTT and provide 4 seconds of audio. The Sub-Audible tone should be 118.8Hz (± 3Hz), it's deviation should preferably not exceed 400Hz. A valid access is acknowledged by the repeater giving it's Callsign.
PIPS AND TIMERS
Once the repeater has been correctly accessed normal conversations or overs can take place. When the squelch on the repeater closes (after an over) the logic will wait for one second, this period is known as the forbidden gap, after which it will send a "PIP". This first pip indicates to the users that the input is clear and the through-audio timer is reset to zero. The forbidden gap is there so that other stations have an opportunity to break in on a QSO etc. If a station uses the forbidden gap period to break in on a QSO the process of waiting a further one second to the first pip is re-started. If the repeaters input remains clear for a further five seconds beyond the first pip being sent the second pip will follow. The second pip does not reset any timers it just indicates that the repeater is about to close down. If the input still remains clear for another five seconds after the second pip, the repeater will send it's callsign and close down.
TALKTHROUGH TIMER
The talkthrough timer is set at 4 minutes. This timer starts incrementing when the repeaters input becomes busy and the receivers audio is passed to the transmitter. If the input remains busy for a time greater than 3min 55sec then the logic will give a pre-time-out warning for the final 5 sec, this is indicated by a engaged type tone being superimposed over the top of the through audio. If the input remains busy beyond the 4 min timer the through audio will be cut off and a Trim-Phone type tone is transmitted. This will remain for a maximum of 30 seconds, after which the repeater will send it's callsign and close down. After the repeater has closed down and the input becomes clear again the repeater will send the letter "C" in Morse to indicate that the input has finally Cleared and can be re-accessed as normal.
TIME-OUT & MODE 2
If the input to the repeater remains busy or jammed for a period exceeding 4 minutes, pass the original talkthrough time, then the repeater will automatically switch into MODE2. This will be indicated to users by a reversal of the keyed tones, i.e. a "pip" will be at 875Hz instead of the normal 1750Hz. Should GB3BS be in MODE2 and the input of the repeater becomes clear users can continue to use the repeater as normal. However, users are asked to let the repeater return to it's normal mode, MODE1. To do this the repeater must be allowed to close down in the normal way, after which it can be re-accessed in the normal way, the repeater will then be in it's normal mode.
NEW USER to GB3BS ? - Please Read.
Please be aware that due to car locking & car alarm devices, using 433.92MHz, on site there is a Local Closedown facility which can be operated by staff at the Repeater Site to shut GB3BS down for short periods. For a detailed explanation please follow this link!
Accessing.
GB3BS must have a tone & 4sec of audio. Don't drop your carrier between sending the tone and speaking. Make sure your access tones (1750Hz or 118.8Hz) are within proper limits !. Over deviation of access tones is a common cause of accessing difficulty.
Pip's.
No, you don't have to wait for the second pip. The talkthrough timer is reset on the first pip.
Time-out.
After 4 minutes the through audio is switched off and a trim-phone type tone tells others that you have done so !
Jammed.
If the input of the repeater becomes jammed by accident or deliberately by stations unknown, please let the repeater time-out and close down, wait for the repeater to send the letter "C" to indicate that the problem has gone away. Don't be tempted to re-access over the top of the interference.
Mode2.
Indicated by reversal of the keyed tones, usually after a jammed input has cleared. Please allow the repeater to close down and reset to normal.
Abuse.
Over the years GB3BS has seen it's share of abusers, pirates & jammers, call them what you like. If this should ever affect you while you are using the repeater then we ask you to totally ignore them, they usually get bored and go away.
FAULTS & QUERIES
If you suspect a problem or possible fault with the operation of GB3BS, would like to comment on it's service then we will be glad to receive some feed back. Please contact the repeater keeper G4SDR who is QTHR or any committee member (see email option below).
Should you wish to send reports, queries or comments on anything to do with GB3BS then Email can be sent to the addresse below.
When using GB3BS, or any Repeater come to that, please remember that Local Amateurs have given their spare and valuble time to providing the service. Repeaters don't just come out of thin air, they require dedication and support. If a Repeater is not given that support it will die.
We hope that you will want to become part of GB3BS's future by becoming a member, which costs just £6.00p per-year.
PLEASE REMEMBER - Repeaters cost money to run & maintain. There is usually Site rent, Electricity, & Insurance bills to be paid. Some work like aerial maintenance & rigging must be done by professionals, their fees do not come cheap. So your support by being a member of the Repeater Group is essential in the day to day running & up-keep of GB3BS.
If you wish to Email the Repeater Group, Please send to:-
i n f o @ g b 3 b s . c o m
|
Page Hits: |
© Copyright Notice: Written & Produced by the Bristol 70cms Repeater Group. |
Last Updated June 2008.
