50.770MHz Tx 25Watts ERP. 51.270MHz Rx
A continuous 77.0Hz tone is required for access - no tone-burst access!
The base station consists of Tait T300 series modules. Specifically a T335 rx, and a T336 Tx. These are already on a nearby PMR allocation and tuned easily to our required frequency. I obtained the crystals for the Tx and Rx from a local company Euroquartz, in Ilminster, Somerset.
You might think that crystal technology is a bit old hat but its just what you need for a repeater as the signals generated from crystals are much cleaner and purer spectrally than even the best synthesizers. This is important as noise sidebands on the transmit signal and the receive local oscillator contribute to receiver de-sensing or blocking and when the transmit signal is only 500KHz away you need all the help you can get!
The receiver is fitted with a Garex preamp, without the preamp the Rx has about 10dB less sensitivity. I was a bit worried as to how this would affect the de-sense but initial tests show things to be fine. Time will tell.
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The Transmitter, although capable of running 25w on its own, this is flat out! When I was a PMR system engineer at Zycomm in Bristol, we had a number of these Tait 300 series base stations in service. On some of these, I had trouble with the RF power output devices getting so hot the solder joints went dry on them! With this in mind, and also the need to run 25w to the aerial the losses in the duplexer need to be overcome. Thanks to Mike, EI2DJ, I got hold of a pair of 75W 86Mhz power amps, which have been extensively modified by Chris, G3VEH and made clean and stable at 50MHz. Quite an achievement and absolutely necessary when you consider our second harmonic could be right on top of Classic FM! So we should be able to leave the Tait running just a few watts into the amplifier which should give us the full 25Watts easily :-)
The aerials are Jaybeam type 7050. For a detailed spec on the aerial, check out the spec sheet PDF file. HERE
The aerial is 34m above ground level on the South Western leg of the tower. This means that the tower is in the way for stations to the North East , East and due North but it is hoped to have another 6m repeater in the affected area and link it and FH together.
It is hoped that ultimately two dipoles will be mounted on the tower, fed with a power splitter which has been expertly constructed and tested by Mike EI2DJ. Mike has made this power splitter with silver plated connectors and "T" piece and sent it all over to me from Dublin, complete with a donation to the groups funds! Truly a demonstration of the Amateur Spirit! Thanks Mike!
If you want to know what a power splitter is and how it works, please look at the UK Six Metre Group's web site HERE
A duplexer is a device which allows simultaneous transmission and reception through a single antenna. Repeaters need a duplexer OR separate aerials sufficiently far apart so there is enough separation between the transmit signal and the receiver to stop the receiver being blocked or desensed. We don't have the ability to separate our aerials by the several miles it would take to do this so we need to use a duplexer.
There are two different types of duplexer - band pass and band stop (or notch) - the one are using is a notch type. It has to be, because our transmit and receive frequencies are so close together - just 500KHz apart. Commercial systems have splits in the order of 10MHz or even 13.5MHz. However, the close spacing of our frequencies does work in our favour in some other ways.
Our duplexer consists of eight resonant circuits - four tuned to notch out the RX frequency but pass the TX frequency and four tuned to notch out the TX frequency and pass the RX frequency. They are joined together by critical lengths of coax cable and a "T" piece in the middle, where the aerial will connect. The clever design of this duplexer allows each circuit to remove about 20dB of the unwanted signal but only remove about 0.5dB of the wanted signal, 500KHz away. Nice!
Do not confuse a DUplexer with a DIplexer. A diplexer is a device which splits out different bands to different ports. If you had a dual band aerial and you wanted to use two separate radios, one on each band at the same time, you'd need a diplexer.
Brian, G4UTM is making the duplexer for us. He's doing a great job of it too! See the pictures below for some examples of Brian's fine work.
The duplexer is constructed from off-cuts of Andrew LDF7-50 Heliax that would have otherwise been wasted. This is 1 5/8" in diameter with a hollow tube centre conductor. It's big, heavy and very stiff cable and a bit of a challenge to work with! Brian says he's not making another one, I don't blame him either.
The original design for the duplexer was by WB5WPA. See his website detailing the construction details of the duplexer HERE.
Click on the thumbnail for a closer look.
The pictures above show Brian G4UTM, holding the completed duplexer. Yes, it is BIG!
Brian has spent many hours making this for us and he is now late starting the decorating! I'm sure you will all join me in apologising to June, his XYL!
As you can see from the size, this duplexer is a real beast but, thanks to Brian's hard work and fine tuning, performs really well.
Brian's initial measurements indicate a notch of over 90dB with an insertion loss of just 1.5dB! That's a real achievement and means that our receiver should not be affected by the transmitter, which means greater coverage overall.
The other picture shows the detail of an innovative idea Brian had to enable easier adjustment of the stub . This method has never (as far as we know) been used before and will make the whole assembly much easier to adjust and once finally tuned, more likely to stay in tune. Well done Brian!
The Tx power amp is a modified ASCOM lowband PA. Chris, G3VEH did all the work in getting these from 86MHz down to 50MHz and making it stable and have a level of harmonic output to allow the people of Somerset to still listen to Classic FM!
Pictures and technical description of the PA, the circulator associated with it and the mods Chris did will be put on here s time permits.
CTCSS stands for Continuous Tone Controlled Squelch System. It means that simply a low level, low frequency tone is transmitted by the mobile the whole time the user presses the Tx key. This tone is used to control the squelch circuit in the receiver, rather than simply just the noise level, or carrier level. This gives a high degree of immunity to interference.
This tone is filtered out at the repeater, after it has been detected and used to open the receiver's audio path. A tone of the same frequency is re-applied to the repeater's transmitter to activate tone squelch facility in the mobile. If this seems a bit of a waste of effort, filtering out the tone and then re-applying a tone of the same frequency to the transmitter, it is done because you could never guarantee that the two tones are exactly the same frequency and so they would slowly go in and out of phase causing the mobiles tone decoder to mute and un-mute all the time - not good!
The sub-audible tone frequency allocated to GB3FH is 77.0Hz. RSGB tone "C"
Note that unlike other amateur bands, it is a requirement that CTCSS is used on 6m repeaters to control the receiver squelch.
No other form of access is allowed i.e. 1750Hz toneburst
GB3FH uses an ARCOM RC210 controller system.
This advanced design has many facilities for controlling external equipment such as switching attenuators in or out of the Rx path, alarm inputs from external sources like a mains fail relay and analogue inputs that can be programmed to read pretty much anything!
At GB3FH, we plan to use two temperature sensors (inside and outside) and - when I can find one - wind speed from an anemometer.
The controller is a very complex piece of engineering and really high quality PCBs and components are used. Ours was a kit and I can recommend that option. Everything was included and the instructions are first class!
For a full description of he Arcom RC210 and its facilites, check out the Arcom web site.
The repeater aerial is sited on a shared site, which is located on the Western end of the Mendip Hills, above the town of Axbridge in Somerset, South-West England. For details of the location of the repeater, click here.