GB3ST - RB09

Location: Stoke on Trent


Location Ref:     


IO83WA


Latitude:


53.036446              


Long:


-1.913464    

 

NGR:


SJ934484


Freq Band:


70 cms


Channel:


RB09


Mode:


Analogue




Input:   


434.825 MHz


Output:


433.225 MHz  


CSTSS:


G 103.5 Hz




N.O.V. Holder


Geoff Booth         


Callsign:


G8DZJ      

___________________________________________________________________

 

GB3ST, 433.225 MHz, FM Analogue Repeater

GB3ST is a 70 cm Repeater
(co-sited with GB3SE and GB3VT)
At QRA locator IO83WA, 273 metres, 887 feet above sea level
in the City of Stoke-on-Trent. Staffordshire.
United Kingdom

The GB3ST repeater was licensed in 1976 and became operational in August of the same year. It was in 1976 when the UK Home Office agreed to issue the license for the first major batch of 70 cm UHF repeaters and GB3ST was included in that initial batch. In 1995 due to a loss of site, it was necessary to vacate the repeaters original location and from 1996 onwards, GB3ST now shares the location of its current site with two other repeaters, GB3VT on 2 metres and GB3SE on 23 cm.
 
GB3ST transmits on 433.225 MHz and receives on 434.825 MHz, UK channel number RB09 (originally RB02). Both the transmitter and receiver use full encode and decode Continuous Tone Coded Squelch System (CTCSS) at a frequency of 103.5 Hz and to access the repeater to obtain the full talk through facility, either CTCSS of 103.5 Hz or a 1750 Hz tone burst is required followed by three seconds of good audio.
 
~~~~~ From 1976 and until the end of 2008 the GB3ST repeater was operational on its original channel of RB02, but due to increasing levels of interference to the repeater’s receiver from the use of "Licence Exempt" Low Power Devices (LPD) and Short Range Devices (SRD) that have officially been assigned part of the 70 cm band as their frequency allocation, the need to vacate RB02 and apply for a frequency change proved necessary. From the beginning of 2009 the repeater now operates on its new channel of RB09. This regrettable but necessary change of frequency now takes the GB3ST repeaters receiving frequency outside that of the current SRD and LPD frequency allocation. Hopefully this will stop any future occurrences of what is currently proving to be an increasing problem to many of the lower frequency allocations of the UK "RB channel" 70 cm analogue repeaters, along with commonly used designated simplex frequencies. ~~~~~

 

 

The Transmitter and Receiver:

The Transmitter and Receiver are both purpose designed and homebuilt, the equipment includes several special features.

The Transmitter in use today, is basically the original transmitter as built back in 1976, however, over the years it has experienced several partial re-builds and modifications, including when in the early 1980's the repeater was struck by lightning. On that occasion the transmitter was severely damaged and the "burn / weld" marks on the metal case are still visible today.

The Receiver now in use is the mark 3.
The original receiver used valve technology and apart from having to replace the RF Amplifier and Mixer valves on what was a near regular basis, it actually functioned remarkably well. The mark 2 was a modified commercial and in some aspects was not as good as the valve mark 1.
The current mark 3 receiver has been in use since 1996, it is specifically designed and totally home built.

 

A selection of various circuit boards used in the Receiver:

1. Dual Stage 10.7 MHz Intermediate Frequency Amplifier, 10.245 MHz Oscillator, and Mixer to 455 kHz circuits.

2. Noise Squelch, Audio Processing and Audio Switching circuits and circuitry to produce the correct operational voltage levels for use by the Repeater Logic Controller.

3. Phase Lock Loop Audio Discriminator.

4. Loudspeaker Audio Output Stage.

5. 455 kHz Intermediate Frequency Amplifier.

 

The Receiver RF Head (front end):

"There really is no point in trying to re-invent the wheel".

During the late 1970’s early 80’s PYE Telecommunications produced a Receiver that had a first class RF Head (front end). The item was manufactured from solid brass, complete with tuned lines and fully silver plated throughout, it was designed with two RF amplifier stages and a mixer stage and by today’s standards the production costs alone would most likely have made it unthinkable.
As far as Amateur Radio purposes are concerned the only down side to it was the type of semiconductor devices used, although in all fairness they were Field Effect Transistors (FET's). Whilst I am aware of several repeater groups that did make use of this RF Head, it was always by firstly adding a stage of RF pre-amplification, however I much prefer my own method that involved the replacement of both RF amplifier devices with Dual Gate Mos FET's and the type of device that I decided to use was the NEC 3SK88.
To successfully carry out this change a small amount of circuit modifications are required, but the resulting performance makes all the rework involved very worthwhile. With the two RF amplifier devices changed, the RF Head now has a very useful increase in gain along with a reduced noise figure and thanks to the tuned lines the overhaul selectivity is truly excellent.

A word of caution:
Because of the tuned lines realignment of the RF Head does require a high degree of patience, however the end result is most certainly worth all the effort and most importantly, without attempting to "re-invent the wheel".

1. View of the complete Receiver RF Head (front end) with the screening cover removed.
 
2. Close up view of the first RF Amplifier replaced with a 3SK88 Dual Gate Mos FET device.
 
3. Replacement 10.7 MHz Coupling and Crystal Filter Matching Circuit Board.

 

 

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. The Proportional Voltage Control circuit board.

2. Internal view of the Crystal Oven and fitted Bead Thermistor.

3. Internal view of sealed oven.

4. View of complete Crystal Oven with Octal Base.

 

 

The Repeater Logic Controller.

The Repeater Control Logic uses C-MOS technology and is also in-house designed and built, and the time out is currently set at 4 minutes.

 

Early 2008, the Repeater Equipment Service: 

To enable a full service of all the equipment, at the beginning of 2008 I decided to remove from site the transmitter, receiver, control logic and all four cavities to my workshop. This was the first time for 12 years that any serious attention had been given to the equipment since its installation back in 1996.
 
> The cavities- were dismantled, internally cleaned and realigned as required.
> The receiver- was purposely designed and newly built in 1996, upon investigation it had collected a small amount of surface dust and apart from that it checked out satisfactorily.
> The control logic- was tested and much to my surprise I found that it was no longer producing periodic callsigns. The problem was diagnosed down to a faulty tantalum capacitor that was replaced as required.
> The transmitter- at the time this was some 32 years old, it contained numerous "tired" original semiconductors and components and as such it went on to prove a very different story. Following a few minutes of investigation work, I decided that it was time to either replace the transmitter, or justify spending a great deal of time in rebuilding the existing one. It took me the best part of a week to completely finalise my decision and the said decision was to completely redesign and rebuild the existing equipment, firstly I decided to completely rebuild the power supply section, then generally replace and upgrade all the other components and semiconductors as necessary.
The rebuild of the power supply unit involved the fitting of a new aluminium back plate of 5 mm thickness, the original had a thickness of only 1.5 mm and tended to be rather flimsy, the total amount of heat sinking was also to be increased. All the PSU smoothing capacitors were replaced and because the original type or equivalents were no longer available, a certain amount of rework proved necessary.
Many semiconductors and aged components were replaced in the transmitter oscillator, multiplier, PA and modulator stages.
Finally, due to the passing of many years along with the ageing process and the effects of heat, much of the internal wiring had become brittle, cracking when worked with and as such it was necessary to replace most of the existing wiring with new.
 
Two weeks into the rebuild and I was beginning to wonder if my end decision had been my wisest.
However, following that of successfully overcoming numerous problems and completing near enough two hundred hours of hands-on engineering work, the repeater equipment was finally ready for reinstallation.   

 

 

The Repeater Aerials and Cavities.

A total of four cavities are used, two 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 GB3ST transmitter, whilst the receive aerial coaxial feeder is terminated with a Triplexer to supply a total of three separate outputs:
Output 1: The GB3ST receiver.
Output 2: The GB3SE 23 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.

 

 

 

When back in 1976 the GB3ST Repeater first became licensed and due to the very difficult topography of the surrounding area within the City of Stoke-on-Trent, I believe its perfectly true to say that apart from maybe two or perhaps three Radio Amateurs operating Base Station's, the amount of local activity on 70cm was almost non-existent and certainly no serious mobile activity at all.
Back then even the local Police were experiencing difficulty with radio communications at a distance of only 1 or 2 miles from the Police Station and to be honest, at times they still experience problems now !
So taking all this in, it was obvious from the very start that we were in for a very difficult time, particularly if we were to make the repeater function and produce the required mobile coverage.
Several tests took place involving different types of aerials and it has to be said that at the time not a great deal was readily available "over the counter" intended for the Amateur market. That was also true when it came to the general availability of surplus equipment involving both Base Stations and Mobile use, with no readily available "Black Box Rigs", thus it became absolutely necessary to get out the soldering iron.
Since then and during the past years, 70cm's has moved forward by massive leaps and bounds and today's situation shows no comparison to that of back in 1976.

The following photographs represent just a brief glimpse into the past, indicating a few of the trials and tribulations that has helped to make GB3ST what it is today.

 

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, GB3SE, GB3SM, GB3SX.

73, Geoff, G8DZJ

 

 


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