AM and AM Modulation

Someone suggested I put this on my blog, so here goes with a brief introduction.

My favorite method is series modulation using a transistor as the "modulation" stage. The modulated RF stage is set so the stage voltage is close to half rail. With modulation the voltage swings to close to full rail (4 times the power). This stage is followed by a linear amplifier to bring the power up to that required.  I used this in my Fredbox and Sixbox rigs without a linear. If I ever do a simple Tenbox AM rig I'd add a linear RF stage.

See Fredbox .
See Sixbox .

Don't forget that if you click on a picture or schematic on my website a larger version is usually available.

AM rigs are simple to build. I am truly surprised that these have gone out of favour in recent times. Surplus PMR AM rigs can often be found for almost nothing.  Back in the 1950s and 1960s AM was the predominant mode on HF and VHF.

There is plenty of space for AM on 10m, 6m, 4m and 2m. A decent AM rig should only occupy 6kHz or less of bandwidth.

Tenbox 10m AM RX video

It is hard to contain my enthusiasm for simple transceivers and super-regen receivers! This video shows just how well the RX breadboard for the little Tenbox 10m AM transceiver works. 1uV (-107dBm) is a very good signal and it will still detect a well modulated AM signal at -120dBm. Selectivity is not good with this type of receiver, which is the main drawback. However for its intended application - a simple transceiver for local nattering across town - it should be fine. Some people use super-regens as the IF stage of AM receivers. If a filter is added ahead of the super-regen IF this overcomes the selectivity issue but makes the RX design more complicated.

AM Day Sept 19th (info via João Costa, CT1FBF)

The ‘Associação de Radioamadores do Litoral Alentejano’ invites everyone to participate in a special Amplitude Modulation (AM) transmission commemoration, using the 80, 40, 20, 10 and 2m bands, on Sunday, Sept 19, 2010.
Frequencies and periods:

3.685 MHz - from 0600 to 0800 UTC
7.1 MHz - from 0800 to 1100 UTC *
14.286 MHz - from 1300 to 1400 UTC
29.075 MHz - from 1400 to 1500 UTC
144.575.0 MHz - from 1500 to 1600 UTC

*alternative frequency is 7.143 MHz