N2CQR 20 METER PORTABLE DSB
With sunspot numbers declining I reluctantly decided to shift my portable one watt DSB HB transceiver from 17 meters to 20 meters. This rig is a very simple DC RX with a corresponding transmitter side. VXO is the only common stage. On TX, the VXO feeds an NE602 balanced modulator. I used an LM386 as the mic amp (that's what I had available). For the microphone I use an old SONY walkman mic.
On RX, I have a bandpass filter followed by an NE602 functioning as
an RF amp. Balanced output to another NE602, this one the product
detector. Then ANOTHER NE602 as an AF amp (again, balanced input)
followed by an LM386. I get the NE602s to work as amps by putting five
volts on pin 6. Nicest DC RX I've ever used. No common mode hum, even
with AC supply.
Shifting from 17 to 20 was easy. Some 50 pf caps across the few
tuned circuits brought them into the band. I got two new crystals from
JAN (very nice folks) and rigged up a panel switch to go from one to
the other. With the two rocks I could tune 14.185 --14.211.
But that wasn't enough frequency space. So I came up with an
alternative. It is what I call the Variable Ceramic Oscillator.
It will appear in the Fall 2006 issue of SPRAT:
I had some trouble with the carrier suppression, but I found that I could improve it by reducing the VXO energy going into the 602. Instead of the recommended 200 millivolts, I went to around 20 by using a "gimmick" cap (just two wires twisted together) between pin 6 and the VXO. By varying the amount of twist I could reduce the carrier to minimal levels. This gimmick was fun.
Later, I got even better carrier suppresion by using a version of the balanced modulator circuit from the Elecraft K2 transceiver.
The rig is working very well. I'm getting 1-2 watts PEP out. In two days I've worked Spain, Italy, and the U.K. Soon after I built it, I had a nice ragchew with K3LP in Maryland. It is a real hoot to tell people that they can switch to LSB and still copy just fine.
I went on to build a little outboard linear amplifier using a single switching MOSFET. Brings me up to around 5 watts.
I don't think QRPers should be scared away by the big guns on 20 phone. And I think the simplicity of DSB makes it an ideal choice for homebrewers of intermediate skill level who want to break into the world of phone.
If you hear me, please give me a shout. USB or LSB!