Monday, March 26, 2012

Home Brew Group challenge project

At the meeting on Sunday at Dural, I proposed a construction challenge for the Home Brew Group: To build an AM transmitter for 40m, specifically 7.125MHz.

As in past years, these challenges are not a competition, although we do get together to compare what everyone came up with.

My reasons for suggesting this project are:
  • 7.125 is very active with AM activity here in NSW
  • AM sounds lovely on air
  • There are quite a few ways to generate AM
Crystals are available for 7.122 from Expanded Spectrum Systems for $2.55 ($2.15 in lots of 10). I used a technique suggested by Peter, VK3YE, to pull the crystal to 7.124 - which is near enough for AM.

Note that I couldn't get Peter's design to oscillate (I had substituted a 2N2222a I had at hand) but values from this page worked fine.

Ways to make AM

  • Series modulation, either with a modulation transformer or heising modulation which passes the audio through an RF choke.
  • Make DSB and re-insert the carrier - there's a bit on this in EMRFD.
  • Pulse Width Modulation
  • Software defined radio
The idea is that we'll build our transmitters, have a few contacts, and get together for a show and tell before the end of this year.

There seemed to be a good amount of interest at Sunday's meeting, I hope you'll join me.


panda said...

Hello Peter,
Great ideas on AM on 7.125 I can see the Panda Cub up on AM

Peter Marks said...

Ha! Thanks Ian,

My Panda cub was modified before I got it to only do 160m, I need to look into the possibility of changing it back.

Best wishes,