A VXO controlled transmitter for the UK 500kHz Experiment

This VXO transmitter was built fairly quickly once I had got my NOV for operation on 500kHz. It is largely based on the one I built for the UK 5MHz experimental allocation, in fact the VXO and PA are virtually identical.

VXO and Mixer stages

The 500kHz signal is derived by mixing two crystal oscillators, at 5.9MHz and 6.4MHz. The choice of these frequencies was solely because I had a lot of crystals in the 5.5 to 6.4MHz range at 100kHz intervals, and also because I could use my 5MHz VXO design almost unchanged. I have some spare crystals available if others wish to use this technique. The VXO circuits are fairly standard and identical. The 5.9MHz one is fixed in frequency and pulled slightly lower than its nominal frequency by the series 10uH inductor. The 6.4MHz one is tuned with a 50pF variable capacitor which pulls it hf in frequency. The two oscillators are then mixed with a 3N201 dual gate MOSFET to produce an output at 500kHz. T1 and T2 are wound on 1/4inch formers, T3 is a 470kHz IF transformer from a scrap radio which needed just a small tweak to pull it to frequency. With the values shown the full 501-504kHz allocation can be covered with a little overlap.The oscillators are powered from an 8V stabilised supply using a 7808 regulator, and the mixer direct from 12V. Power to these stages is present at all times, transmit and receive.

Dual VXO oscillator

Keying and Driver Stages

The keying stage consists of a ZTX550 transistor which switches the collector supply of the driver stage.   The components in the base and collector of the ZTX550 are chosen to give a rise and fall time of the keyed rf of around 4ms and reduce keyclicks - on air use indicates these values are ideal.

The ZTX450 driver produces sufficient output for the pa transistor. Its output is coupled by a toroidal transformer wound on a T50-61 ferrite ring.

The keying and driver stages are also powered at all times. For netting purposes operation of the key produces a low level signal which can be tuned to the listening frequency.

Driver and Keying

PA and Low Pass Filter

The PA uses a Motorola MRF315A 144MHz turnstile transistor - no particular reason for this choice, except that I had a few in the junk box and I had also used it in the 5MHz transmitter.  It is configured as a conventional class C stage. The output transformer T2 is wound on a 26mm diameter 3C85 core.

The low pass filter is from a design by David G0MRF designed to give good rejection of the second harmonic (which is in the broadcast MW band). I used T50-2 toroids rather than the larger T130-2 ones David used. These are adequate for 10W but the wire is fairly tight as more turns are needed. Do NOT use T50-61 toroids, they get VERY warm!

A ferrite bead is required on the collector connection - a single wire through the bead.  Without it the PA is unstable, as being a VHF transistor it has considerable power gain at these frequencies.

The Transmit/Receive switch SW1 connects the 12V to the PA on transmit. A second pole is used for switching the antenna. The PA produces around 6W from a 12V supply.

PA stage
SWR Bridge Circuit

A simple SWR meter circuit is connected in the transmitter output and is useful as an antenna tuning aid and rf current meter. The value of the AOT resistors depends on the sensitivity of the meter, and 680 ohms was found suitable at G3YMC - adjust for full scale deflection in the forward direction. T1 and T2 are wound on the same T50-61 torroids as used in the transmitter. The diodes should be point contact or germanium ones with low forward voltage drop (eg OA91).


The transmitter is built in a suitable diecast box.  A PL259 connector is used for the antenna, phono sockets for the keyer and receiver connections, and a double pole toggle switch for the receive/transmit switching. Current consumption is around one amp and any convenient 12V power supply can be used.