Location: Stoke on Trent
Location Ref: |
IO83WA |
Latitude: |
53.03464219 |
Long: |
-2.098365731 |
NGR: |
SJ934484 |
Freq Band: |
23 cms |
Channel: |
RM3 |
Mode: |
Analogue |
|
|
Input: |
1291.075 MHz |
Output: |
1297.075 MHz |
CSTSS: |
G 103.5 Hz |
|
|
N.O.V. Holder |
Geoff Booth |
Callsign: |
G8DZJ |
___________________________________________________________________
GB3SE, 1297.075 MHz, FM Analogue Repeater and Beacon
GB3SE is a 23 cm Microwave Repeater and Beacon
(co-sited with GB3ST and GB3VT)
At QRA locator IO83WA, 273 metres, 887 feet above sea level
in the City of Stoke-on-Trent. Staffordshire.
United Kingdom
GB3SE first became operational from its original site in September 1985 and then from its present location in 1996 and shares its location with GB3VT on 2 metres and GB3ST on 70 cm. The Repeater and Beacon Transmits on 1297.075 MHz and Receives on 1291.075 MHz, UK channel number RM03.
Both the Transmitter and Receiver use full encode and decode Continuous Tone Coded Squelch System (CTCSS) at a frequency of 103.5 Hz. To access the repeater facility and obtain full talk through function, either a tone burst of 1750 Hz or CTCSS at 103.5 Hz can be used. The repeater / beacon will not transmit its CTCSS of 103.5 Hz unless full repeater mode has been activated. Thus if using a transceiver that has a tone activated squelch function, it is possible to monitor the repeater part of GB3SE without having to listen to the continuous transmission of carrier whilst the equipment is in Beacon Mode function.
PLEASE NOTE: Due to a lack of available digital imagery, most of the images as displayed originate from scanned prints, this being the main reason for the slightly poor quality. However, on the basis that something is better than nothing and with that in mind, I hope you will find the content interesting.
The GB3SE Equipment:
GB3SE is in good old Amateur Radio fashion totally home made, designed and built.
The photographs are of the various pieces or modules of equipment used to make up the complete Repeater and Beacon and the equipment in use today is the same as was constructed back in 1985 when GB3SE became licensed and operational.
The Power Supply Unit:
The Power Supply Unit is fully stabilised and over voltage protected and complete with
adequate heat sinking to allow for continuous operational use.
The Transmit Mixer Module:
The function of the Transmit Mixer is to mix two very low level signals to produce the one desired 23 cm output frequency. This signal is then amplified and fed through a two stage Bandpass Filter.
The output drive signal from the module is externally connected to the input of the Power Amplifier.
The Master Oscillator Module:
The Oscillator Unit is and in many ways the Heart of the Repeater / Beacon Station.
The unit produces all the required output frequencies at the necessary drive levels to correctly drive and operate all the other modules. All the various signals are derived from an internal Crystal Controlled Source housed in an Electronically Temperature Controlled Oven.
The Master Oscillator Unit also contains the circuitry for the Audio Line Amplifier and the Phase Modulator to produce the transmitted audio.
The VHF Receiver Module:
The VHF Receiver contains all the required circuitry to fully demodulate the received signals.
It uses three stage frequency conversion and is complete with all the necessary circuitry for such as Noise Squelch Control, CTCSS Decode, Line Amplification and a local Loud Speaker facility.
Whilst incoming Audio Demodulation is achieved with a Phase Lock Loop (PLL) Type of Discriminator.
Several control voltages are also derived for use by the Logic Controller.
The 23 cm Receiver RF Head "front end" Module:
The aerial input is connected to the 23 cm Receiver via a two stage Bandpass filter and the receiver consists of a two stage Bipolar RF Amplifier followed by a Hybrid Ring Mixer.
The derived output signal is externally connected to the input of the VHF Receiver Module.
The Repeater / Beacon Control Logic:
The control logic is both fully in-house designed and built using C-Mos technology.
When GB3SE is in Beacon mode, the control logic allows the transmitter to identify using Frequency Shift Keying (FSK). In Beacon mode the repetition rate is one callsign every 35 seconds with each eighth callsign sent by MCW. When GB3SE is in Repeater mode all callsigns / identification signals are sent using MCW.
If received signals are greater than plus or minus 5 kHz from the repeaters nominal input frequency, an acknowledgement in Morse code consisting of either a letter H or L is sent, reverting to the letter T (or K) for all signals that are correctly on frequency.
The time out is currently set to 5 minutes.
The Crystal Oven and Associated Control Circuit Board:
The frequency accuracy and stability involving any Transmitter or Receiver is always of great importance.
The repeater equipment is not located in a temperature controlled air conditioned building and during winter the air temperature within the building is often below zero degrees centigrade, whilst during the summer months temperatures exceeding plus 40 degrees are common place, such excessive swings of temperature could cause problems involving frequency drift of the transmitter and receiver.
To help reduce such undesirable effects both the transmitter and receiver oscillator crystals are housed in a dedicated enclosure that is electronically controlled at a constant temperature. In the past the most commonly used type of crystal ovens were those of the Bi-Metal Thermostatically controlled variety. However, apart from the regular problem involving the Bi-metal thermostat sticking on causing the crystal to over heat and cook, another problem and particularly with such as beacons, was the constant drifting of the carrier frequency of plus or minus that of the nominal frequency due to the cycling effect of the temperature change as the heating element warms up and then cools down.
Whilst such crystal ovens were efficient at controlling the temperature of the crystal housed within the enclosure, their reliability and method of achieving the end result was somewhat questionable and with this in mind I decided to think of a different method of controlling the heating element voltage.
After giving the situation some thought, I decided to design and build a method of powering the heating element with a proportionally controlled voltage. The type of crystal oven in use was ideal as a temperature enclosure so it made perfect sense to continue with their use, by fitting a small Bead-type Thermistor located directly next to the crystal to measure the internal temperature of the enclosure. The thermistor along with a suitably designed and built circuit involving a high power Voltage Comparator supplies a proportionally controlled voltage (the total amount of voltage is purely dependant upon the measured temperature within the housing) to power the heating element of the oven.
I have now duplicated this design idea with several other repeaters and beacons where good frequency stability has proven essential.
1. Internal view of sealed oven.
2. Internal view of the Crystal Oven and fitted Bead Thermistor.
3. The Proportional Voltage Control circuit board.
4. View of complete Crystal Oven with Octal Base.
GB3SE Cabinet Temperature, Fan Cooling System:
Because of the 100% duty cycle requirement of the GB3SE beacon, an initial problem with the equipment was due to excessive cabinet / equipment temperatures.
During the cold winter months the cabinet and associated equipment keeps warm, however, during that of the hot summer months the situation is somewhat different. The equipment room has no means of air conditioning and during the summer months the repeater and beacon equipment was regularly subjected to critically high temperatures.
To help overcome this problem a system of controlling the air temperature within the cabinet was installed, this consisted of a home made temperature sensor that measured the ambient air temperature within the equipment room, when the air temperature exceeds that of a pre-determined level the controller automatically powers up a suitable cooling fan attached to the side of the equipment cabinet.
The home made temperature sensing device also includes a digital display allowing full visual indication of the measured ambient air temperature.
Although only a relatively simple modification, the operating temperature extremes of the equipment within the cabinet is now controlled within that of a more acceptable limit.
The Repeater Aerials and Cavities:
A total of two cavities are used, one each in the transmitter and receiver coaxial feeders.
The repeater uses a two aerial system that consists of two close spaced colinear type aerials, the transmit aerial is used solely by the GB3SE transmitter, whilst the receive aerial coaxial feeder is terminated with a Triplexer to supply a total of three separate outputs:
Output 1: The GB3SE receiver.
Output 2: The GB3ST 70 cm repeater receiver.
Output 3: A dedicated receiver used for remote control purposes.
The aerial installation is located at 10 metres above ground level and the repeater outputs its maximum licensed power of 25 watts ERP
During the summer of 1985 and after a waiting period lasting almost two years, the Department of Trade and Industry (DTI) eventually issued the GB3SE license, by then all the required equipment had already been fully tested and ready for immediate installation.
The Repeater and Beacon equipment was installed in an outside gents toilet block, at a garage located in the City of Stoke-on-Trent, Staffordshire.
At that time GB3SE was the third analogue voice repeater to be licensed in the Stoke-on-Trent area, the first was in 1976 with GB3ST and then in 1980 followed by GB3VT. All three repeaters continued to share the same location until 1995, when due to pending building work that was to take place at the garage, all the equipment was removed and the site vacated.
The photograph shows the equipment installed in the room of the gent's toilet, with the two Alford Slot Aerials mounted on the roof of the garage building and because more than three metres of physical spacing existed between the two aerials, the use of cavity filters proved totally unnecessary.
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, GB3SM, GB3SX.
73, Geoff, G8DZJ.