Location: Stoke on Trent
Location Ref: |
IO93BA |
Latitude: |
53.036446 |
Long: |
-1.913464 |
NGR: |
SK059488 |
Freq Band: |
6 metres |
Channel: |
R50-8 |
Mode: |
Analogue |
|
|
Input: |
51.290 MHz |
Output: |
50.790 MHz |
CSTSS: |
G 103.5 Hz |
|
|
N.O.V. Holder |
Geoff Booth |
Callsign: |
G8DZJ |
| _________________________________________________________________ |
|
GB3SX, 50.79 MHz, FM Analogue Repeater
GB3SX is a 6 metre Repeater
(co-sited with GB3SM)
At QRA locator IO93BA, 351 metres, 1142 feet above sea level
at Ipstones Edge, Staffordshire Moorlands near Stoke-on-Trent.
United Kingdom
The GB3SX repeater became operational in 1997, it Transmits on 50.79 MHz and Receives on 51.29 MHz, UK channel number R50-08. Both the repeater Receiver and Transmitter use full Continuous Tone Carrier Squelch System (CTCSS) at a frequency of 103.5 Hz. It should be noted that the repeater will not respond to a 1750 Hz tone burst or allow any talk through function without the use of the correct CTCSS frequency and this is common with all currently licensed UK six metre repeaters.
At 50 MHz electrical interference can be a real problem and the original repeater site suffered from electrical interference that progressively worsened, due to this and during 2006 the repeater was re-sited to its present location, however and whilst not as severe or constant, even from the location of the repeaters new site electrical interference can still occasionally be somewhat problematic.
A local reading of GB2RS the RSGB news service can be heard on most Sunday evenings at 1830 Hrs local time, at the end of the news reading the reader always welcomes on-air reception reports.
The GB3SX Transmitter, Receiver, Six Cavity Duplexer, Repeater Control Logic the Final PA and PA Power Supply:
All the Repeater equipment has been very carefully and purposely designed and it is all completely home made.
The equipment includes a number of special design considerations and two such examples involve;
(a) The receiver uses a phase lock loop type audio discriminator.
(b) The transmitter makes use of two different types of audio modulator, the primary modulator makes full use of a "voice-operated gain adjusting device" (VOGAD) (Mode A), whilst a second modulator without the VOGAD facility, is automatically enabled when the measured received ambient noise level on the input of the repeater receiver is greater than that of a pre-determined set maximum level (Mode B).
The Transmitter:
1. Equipment cover removed showing bottom view of the Transmitter.
2. Equipment cover and Power Amplifier screening cover / fan assembly removed showing bottom view of the Transmitter and Power Amplifier.
3. Driver Stage to the Transmitter Power Amplifier with screening cover removed.
4. Transmitter Power Amplifier with cover and Fan Assembly removed.
5. Transmitter Ambient Temperature Sensor Circuit Board complete with Fan Switching Assembly.
Voice-Operated Gain Adjusting Device (VOGAD):
Under normal circumstances the modulator makes use of the VOGAD (voice-operated gain adjusting device) facility.
Due to the 10 kHz channel spacing of all UK 6 metre repeaters, the maximum permitted deviation level is only plus and minus 2.2 kHz, therefor any extra small amount of through audio always proves to be an added bonus. However, and on a down side, any received electrical / impulse interference tends to confuse the operational characteristics of the VOGAD circuitry.
To help counteract this undesirable effect, the receiver samples the residual noise floor during a continually re-setting period of ten seconds, in turn this then produces an average level of received noise. If the average measured amount of noise is found to be above a pre determined (and adjustable) level, then the VOGAD facility is removed and the transmitter uses a second modulator, this is known as mode B.
As an indication that the process of switching from the normal mode A to the auxiliary mode B has taken place, the normal between-overs identification changes from the usual Ä (di-dah-di-dah) to that of a letter T (dah).
Indications appear to confirm the noise monitoring system to be functioning satisfactorily and the audio indication is proving very useful.
Function Control and Monitoring Circuits:
1. Quad Line Monitoring Circuit Board: This facilitates a visual indication of possible fault conditions that fall outside the normal operating parameters of the repeaters equipment.
2. RF Switching Module: Should a mains supply failure occur, then the repeater automatically switches to an Uninterruptible Power Supply (UPS). The RF Switching Module bypasses the external final Power Amplifier reducing the repeaters normal 25 watts effective radiated power (ERP) to that of only 2 watts and in turn this proportionally reduces the equipment’s total current consumption, thus extending the total operational running time available whilst the repeater continues to operate from the UPS.
3. Voltage Monitoring Watchdog and Over-Voltage Crowbar Circuit Board: This constantly monitors the Regulated DC Operating Voltage of the 20 Amp Power Supply. Should it be that the operating voltage happens to exceed that of an already predetermined maximum voltage, then the Crowbar Circuit shuts down the power supply, in turn this automatically switches the repeater to a low power setting of 2 watts effective radiated power.
4. Temperature Sensing and Dual Fan Controller Circuit Board: This constantly measures the temperature of the heat sink within the linear 20 Amp power supply, operating and cycling the dual fans as required.
The Final Power Amplifier:
The GB3SX Repeater Transmitter produces sufficient output power, after taking into consideration all the losses involved in such as the homebrew Duplexer and feeder loss, to enable the repeater to run the maximum as licensed 25 watts effective radiated power (ERP).
The Receiver:
1. Equipment cover removed showing top view with all screening covers in place.
2. Equipment cover removed showing top view with all screening covers removed.
3. Received ambient noise measuring circuit board, enabling Modulator A or B as required.
The Receiver:
1. Intermediate Frequency Amplifier at 455 kHz and Phase Lock Loop Audio Discriminator.
2. Equipment cover removed showing bottom view of PSU and Control Logic Circuit Board.
3. Dual stage Intermediate Frequency Amplifier at 10.7 MHz.
4. Various circuits, involving Audio Switching / Mixing, Loudspeaker Audio Amplifier, plus the CTCSS circuit board.
5. Equipment cover removed showing bottom view of the GB3SX Receiver Mains Operated Power Supply.
The Repeater Control Logic:
The repeater control logic uses CMOS technology and is again both in-house designed and constructed and the timeout is set at 6 minutes.
The Six Cavity Duplexer:
The Duplexer consists of six cavities, all specially designed and constructed in the home workshop.
It took a total of three months to design, manufacture and fully test the first cavity, then a further seven months to make the remaining five cavities, all six cavities from start to finish taking a total of ten months to construct and fully test.
Each cavity is more than six feet long and 18 Kg in weight and making the Duplexer was both a technical and very physical challenge.
The Duplexer Housing:
The complete Duplexer is housed in its own well insulated and temperature controlled cabinet, the temperature within the cabinet is maintained to an accurate 20 degrees Centigrade.
The Repeater Aerial:
The Aerial is a unity gain End Fed Dipole located at 15 metres above ground level.
A local reading of GB2RS the RSGB news service can be heard on most Sunday evenings at 1830 Hrs local time, at the end of the news reading the reader always welcomes on-air reception reports.
This repeater is part of the UKFM Group Western and is operated on a completely voluntary basis, the receipt of any financial help is always most gratefully acknowledged.
Please also take a look at GB3VT, GB3ST, GB3SE, GB3SM.
73, Geoff, G8DZJ.
