2 Oct 2012

The OXO QRP transmitter

OXO schematic on the G3PTO website

One of the most simple and popular HF transmitters is the OXO, originally design by GM3OXX. The circuit appeared in the GQRP club's SPRAT magazine about 30 years ago. It is essentially a 2 transistor QRP transmitter (plus another for keying) capable of working as a fundamental crystal controlled or VXO controlled transmitter on an HF band. I used this design as the TX part of my Pipit 800mW transceiver for 15m and later the Tenner transceiver for 10m. On the higher bands there is more chance of a little chirp, but perfectly usable. On the lower HF bands the OXO is capable of over 1W. It is a very easy transmitter to build, is almost guaranteed to work first time, and is great fun to use.

As it is some time since I've built one, I might just knock one up this afternoon and see how I get on, perhaps on 80m or 40m CW.

4 comments:

Anonymous said...

Hello Roger

Classic QRP design and great fun to build and play with
I have even built one to use as a Crystal tester. Can be used with a simple D.C and Regen RX. Built one in the late
1980' and it gave me the QRP bug

Kind Regards

Tony

Petr, OK1RP said...

Hi Roger,

this OXO is one of the famous rig I ever seen for me...
I built it at my 15yrs and we worked (together with my dady) all the globe.
On 40m we worked 100DXCC and the best DX for us in 80's was YV7QP > Margarita Isl. DX-pedition. We worked them in big pileup early morning using our OXO TX giving 700mW out.

GL 73 - Petr, OK1RP

mark w said...

Hi, I'm looking for a simple QRP that I can dial the output power back at will. In this design, would increasing the value of Q2's emitter resistor accomplish that? Say, instead of a 39 ohm resistor, we increase the value to 100 ohms? I'd heard the emitter resistor should only be 1/10th the value of the collector resistor, but I'm uncertain how L6 would behave ~7 MHz 40M?

Roger G3XBM said...

Yes increasing the emitter resistor of Q2 should reduce power. Don't use a pot as the rating is unlikely to be good enough. Maybe a switch to select the power you want? The OXO works well. I have used this design to 10m. You should get 1W, or more, maximum on 40m. You should work plenty.