The FETer 80m CW “micro transceiver”
1 FET and 14 parts             Best DX 18kms so far in daylight
Page created 24.10.08

Introduction

Some years ago a beautifully simple circuit for a 40m CW transceiver by W2UW was published in GQRP SPRAT.  This inspired me to have a go at an even simpler version for 80m using just 14 parts apart from the key and crystal earpiece. You'd be hard pressed to get the parts count lower and achieve solid and reliable local (and DX?) communications.

Basically the circuit shares a single MPF102 FET between a very simple regenerative RX and a QRP crystal oscillator.  RX and TX switching consists of a 4 pole 2 way switch which changes over the antenna and all 3 connections to the shared FET. If you want a simpler TX-RX switch you could use two FETs instead.  Although a little PCB would be neater, I just used “ugly” construction on a piece of single sided copper clad board. The FET is mounted on the back of the switch. The small aluminium box was obtained from Maplins.

 

Receiver

Feedback is provided by the tap on the main toroid winding. You may need to optimise this but start with the tap point as shown. A single turn antenna winding is over-wound on the main coil. Regeneration is set by C3. Once regeneration is set to just oscillate it won't need readjustment unless you change the power supply volts or antenna. The band tuned is set by choosing values of C4 so that the variable capacitor comfortably tunes across the 80m CW band. In my case the RX tunes 3.48 to 3.62MHz with a few hundred pFs of fixed capacitance at C4. The receiver is quite remarkable for something this simple: mine measures around -100dBm sensitivity for a usable signal level in the earpiece and picks up plenty of CW and SSB stations.

NOTE: Please note that this simple receiver will radiate a small carrier on the frequency to which it is tuned (because it is set to be just in oscillation), so avoid using this when operating in close physical proximity to other 80m CW stations.

Transmitter

On TX, I adjusted and fixed the value of C1 so that the oscillator started reliably with a 50 ohm load. In my case 68pF was optimum. Observing on a scope, R3 was adjusted from zero until maximum RF output was obtained, in my case about 18mW. Most people will use an ATU between the output and antenna, but a low pass filter is recommended if not. By the way, my antenna is just a 15m random wire with a central heating radiator as ground.

Results

It is early days yet but two solid local two-way QSOs at 599 and one two-way QSO at 18kms with M1KTA suggest very many more miles are possible with a quiet band and a clear frequency. 

What amazes me is how well this little circuit works: the RX is stable enough and not at all fiddly. It is, all things considered,  remarkably sensitive and picks up plenty of European SSB and CW signals at night.  On TX the signal reports have been good.  Even buying all parts new it will not cost you much more than a fish and chip supper.

For me, this sort of circuit is the ultimate QRP challenge. Making and using it has been real fun. Next I want to try the same idea on 28MHz.  Check my website and blog pages for latest developments and news of further contacts.

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