AM on 40m – old is new, again!
For those not "in the know", amplitude modulation – abbreviated AM – the first voice modulation mode for transmitting speech, never disappeared from the airwaves of the HF bands with the march of new technology in the guise of single sideband. Enthusiasts of AM just kept at it in “hidden corners” of the bands.
Roger Harrison - VK2ZRH
Now, AM is back out in the open with visible recognition in the Band Plans for the Australian amateur bands below 30 MHz.
AM is not a recommended mode on the HF bands because of its bandwidth. However, it is a fully legal mode and there is AM activity on several bands. They are:
on 160 metres, the upper portion of the band is recommended for AM use.
on 40 metres, there is crystal controlled AM operation around 7125 kHz.
on 10 metres, the recommended segment for AM is 29.0 – 29.1 MHz.
Technical Advisory Committee Co-ordinator, John Martin VK3KM, has updated the Band Plans recently, which can be found on the WIA website at Link
The advisory accompanying the Band Plans, Note 1, has been updated to add the frequency of 7125 kHz, which is being used for crystal controlled as well as VFO-rig AM operation.
Note 1 to the HF Band Plans also advises: the SSB segment can also be used for digital voice modes and image transmission modes, such as SSTV or Fax, using bandwidths up to 4 kHz, or for AM.
WIA President, Phil Wait VK2ASD, can be heard around 7125 kHz using his homebrew AM transmitter, which employs a Class E RF power final modulated by a Class D high level pulse-width modulation audio amplifier. Quite a non-traditional AM rig configuration.
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