Receivers:Many amateur rigs these days have receive capability down to LF. However with a few exceptions these are relatively insensitive on these frequencies and unsuitable for amateur use. Most operators therefore use some form of receive converter to a higher frequency, allowing use of the existing HF station receiver. This also allows some gain to be included in the receive convertor.At G3YMC a receive converter (included in the transverter) converts to the 10MHz band. This was selected primarily because 10MHz computer crystals are readily available. A 3N201 dual gate mosfet mixer is used, preceded by a J310 JFET preamp. Two tuned stages are included in the front end - good front end selectivity is essential on 136 to eliminate intermodulation products from broadcast stations. Note that 198kHz (BBC Radio 4) - 60kHz (Rugby) = 138kHz. If you hear a time signal on 138 you have not got enough selectivity. |
TransmittersWhereas receivers all seem to use similar techniques, there is great variety in transmitter techniques used on the band. All the following techniques are in active use:
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Relative merits of different transmit techniques:Crystal derived transmitter sources offer clear advantages for the more specialised techniques used on the band, in particular very slow CW where the transmitter must remain stable for an hour or more. In these digital processed modes drift of 1Hz is disastrous.However transverters and similar techniques offer a fairly simple way to get on the band and provide adequate frequency stability for most purposes. There is strong feeling in the current band users as to which method is best, however it must be remembered that all the techniques currently being used allow excellent results and each amateur must be left to decide which one is best for him. |
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