Two metre repeater.
Worthing, West Sussex.
Antenna feeder: RG214.
Thanks to Roy and Andy.
Many thanks to Roy G4WTV and Andy G3UEQ for all their hard work, time, expertise and financial input to get GB3RW up and running. To show appreciation, it would be nice to contribute something to the setting up and running of the repeater. £10 sounds good to me. That’s barely the price of a couple pints these days! Use the button below to contribute via PayPal…
Thank you! Your contribution is much appreciated!
GB3RW: Predicted coverage map.
After the demise of our local two metre repeater some time ago, we now have a new repeater up and running. This has been welcomed by local radio amateurs as the once underused band has been brought back to life. The predicted coverage map for GB3RW is shown below. Signal reports received so far have been most favourable, which is encouraging considering the height of the antenna is only 10 metres above ground.
Click on the photos for larger images.
GB3RW is housed in a 19 inch rack. The photos below show the Tait T300 series VHF base station, front and rear.
Above left are the bandpass cavity filters. Receive lowpass on the left. The battered dark blue one on the right is the highpass transmitter filter.
As seen in the above photo, all cavity interconnects are now complete. The lengths had to be millimetre precise. The difference between 145.000 and 145.600 is 2mm in length, otherwise it would be 10dB worse.
The two photos above show spectrum analyser response curves. The photo on the left shows the RX side at 145.000MHz, showing over 70dB of rejection of 145.600MHz from the TX. The photo on the right shows the TX side at 145.600MHz showing over 70dB of rejection of transmission at 145.000MHz.
The three port circulator, shown above, is now in situ. Another cavity had been installed on the TX side so isolation is now minus 70dB per side, TX and RX. Including the circulator, the total isolation between TX & RX is about 100dB.
Above is the spectrum analyser which was used to set it all up.
The repeater is working on a singe aerial for transmit and receive. This is a Slim Jim at 10 metres above ground. See photo below.
How the repeater works.
Repeaters were originally intended to enable mobile operators to communicate with each other over distances not possible from one vehicle to another. The repeater would normally be located on a hill or tall building. A mobile operator transmits his message to the repeater’s receiver. The repeater transmits his message to the other vehicle.
The repeater consists of a receiver which operates on one frequency and a transmitter which operates on another frequency. The receiver picks up the signal from the first mobile station and transfers the audio to the transmitter. The transmitter sends the signal to the second mobile station. This happens simultaneously, there’s no delay, and it’s known as talk-through. Only one station at a time can speak through the repeater. One speaks, the other listens. This is not full duplex.
Imagine that the repeater is on a hilltop. A mobile, or fixed, station on one side of the hill can’t possibly communicate directly with a station on the far side of the hill. However, the repeater can hear, and transmit to, both stations. Problem solved.
Repeaters use transmitters and receivers which operate on different frequencies. Obviously, you can’t transmit and receive on the same frequency at the same time. GB3RW transmits on 145.600MHz and receives on 145.00MHz. When programming a dual band radio, you may need to change the shift. This must be set to minus 600kH. Unlike 70cms repeaters, the shift on two metre repeaters is minus as the repeater receive frequency is always lower than the transmit frequency. In this case, 145.600 minus 600. Which is 145.00. To work through GB3RW, your radio must transmit on 145.000 and receive on 145.600.
The repeater also uses CTCSS. This stands for – continuous tone controlled squelch system. The repeater’s receiver won’t transfer your audio to the transmitter unless it hears a tone. This is a continuous tone transmitted by your radio. The tone frequency is important. Different repeaters use different tones. GB3RW uses a tone of 88.5Hz. You must set you radio’s transmit CTCSS to this frequency. If you don’t, you won’t be able to access the repeater. CTCSS tones are important because, under lift conditions, you might open two repeaters which, although miles apart, are on the same frequency. Each repeater will only respond to its unique CTCSS tone, which is transmitted continuously by your radio when you are talking.
When programming your radio for the repeater, set the bandwidth to narrow.
Extensive filtering is used between the repeater’s transmitter and receiver. This is necessary because, very often, the transmitter and receiver use the same aerial, or aerials situated very close together. With only 600kHz difference between the transmit and receive frequencies, the transmitter will desensitise the receiver. This also true if separate aerials are used in close proximity. Repeater designers go to great lengths to make the filtering as best as possible so there’s minimal interaction between transmitter and receiver.
Most repeaters have a time-out. This is a setting where the repeater will cut you off after a predetermined time, usually three minutes. The idea is to stop people hogging the repeater. It also prevents accidental opening of the repeater, for example, by sitting on the PTT button on your microphone.
Between overs, the repeater will transmit the letter K in Morse code. This is an invitation for a station to transmit. Wait for the K before transmitting. This will reset the repeater’s timeout.
Be polite and considerate when using a repeater. Those situated close to a repeater, or running high power, might be able to access the repeater and block out weaker stations. Bear this in mind and leave a pause now and then for other stations who might wish to access the repeater. Priority should always be given to mobile stations. Always wait for the K before transmitting.
If you encounter an idiot on the repeater, ignore him. Idiots thrive on response to their idiocy. Don’t respond. If you have uncontrollable idiotic tendencies, don’t use the repeater.