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| SWITCHERY |
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| Adding more signal sources and more CRT monitors such as "Preview" and "On Air" does bring with it more signal switching. This is where the layout of each station will differ as each operator has his own ideas on the subject. |
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| Video signals are normally 1 volt peak to peak as |
| standard. The video input to the Tx can be switched from the camcorder 1 volt output to a VHS recorder 1 volt output just by using a simple 2 way toggle switch. The change over is instant and smooth. It is quite easy to use a double pole toggle switch to cross two signals over between "On-Air"to the Tx and into a small "Preview" monitor. This allows you to centre and pre-focus a picture before inflicting it on the viewer. |
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| TX/RX CHANGEOVER SWITCHING |
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| If your station uses an antenna changeover relay, then this subject needs careful attention. You do not want to apply power to the Tx whilst the antenna relay is still in the process of changing over. Some switches 'make before break' and others 'make after break', both can cause problems. The preferred method here is to use a three position switch system. Easily done with a Post Office type key switch. |
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| POSITION 1 Receiving: Applies voltage up to the Pre-amp and lights an green bulb on the switch box to confirm this. The Rx itself is switched on at the mains and is not controlled by this switch. |
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| POSITION 2 Relay change: Removes the voltage from the Pre-amp and the green bulb, energizes the antenna Relay to the transmit position, and lights a white bulb to confirm this. ( When not energized the relay sits in the Rx position). |
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| POSITION 3 Transmit: Switches 12v to the Tx, whilst |
| retaining the position 2 voltages, i.e. relay to transmit and white bulb still on. Position 3 also turns on a red bulb to show that station is in transmit mode. This is the method used by EI2HR for a few years now, and reduces the changeover to only one mechanical action. Too many switch actions cause too many problems. |
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| With the Rx left on at the mains but the pre-amp switched off, the transmitted picture can be monited by spill radiation when tuned to the transmit frequency. |
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| ATV REPEATERS |
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| Irish repeaters are dealt with elsewhere, but another repeater of interest is the Welsh repeater on the Isle of Anglesey, GB3TM. This can be received slightly outside our band but inside the UK band allocation on 1316 MHz. As the input to this repeater is 1249 MHz we can quite legally work through this repeater. GB3TM uses 10 watts and an omni directional antenna, and can only be worked from EI when conditions are favorable for enhanced UHF propagation. At such times stations can be worked all along the North Wales coast and onto the Wirrel peninsular. At such times also direct contacts have been made as far afield as Bolton in Lancashire. From Drogheda to a repeater on Dartmoor in Devon, And on 3cm from Dublin to the Isle Of Man. |
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