The Sixbox, an ultra-simple 6m QRP AM transceiver which I'd had in my mind for some time. This is a derivative of the Fredbox transceiver built many years ago. Bear in mind the receiver is a simple one: it is very sensitive, but lacks decent selectivity. Nonetheless, it should hold its own on 6m AM most of the time. The transmitter is simple and is certainly capable of improvement. Use the ideas here as a springboard for your own version - don't be afraid to experiment.

The Circuit

The SixBox produces around 50mW AM (200mW peak) from the 2N3904 series modulated PA when run from a 13.8V supply, slightly less from a 12V gell cell battery. From an internal 9V battery it produces around 20mW. The receiver is a super-regen using MPF102s with an isolating RF amplifier loosely coupled to the super-regen detector. The TX consists of a 50MHz 3rd overtone xtal oscillator and a series modulated buffer and PA. Ubiquitous 2N3904 transistors are used throughout apart from the super-regen stages. These work well from AF to VHF and cost very little (a few pence in the UK).

The design would readily scale for 4m (70MHz) although the T37-6 toroids would be a bit marginal this high in frequency. I was surprised how well they worked at 50MHz. A small polyvaricon tuning capacitor works best to tune the super-regen stage. It covers a 5MHz range with a 15pF in series. The whole rig is now in a small die-cast box and is working nicely.


You don't have to spend much money or work very far to have fun. In fact a recent 16km contact with this rig gave me as much buzz as working sporadic-E DX with a 5W SSB rig. This QSO was to a station using a V2000 triband vertical, which appears to have less than unity gain. With a small beam at each end considerably further would be possible. Another QSO, this time with G3PTQ at 6kms, was achieved yesterday, despite Terry only having a low horizontal dipole. These QSOs and some respectable reports (up to 59) have proved the rig is a viable one for cross-town/inter-village nattering.

The receiver works well for something this simple and copies 0.5mW AM signals from Andrew G6ALB located 3 kms away across the fields. It also makes a useful band monitor: 49MHz equipment, 6m SSB signals (envelope detected) and TV video signals are copyable when tuning around.

Versions for 4m and 10m could be easily produced and make a very useful natter-box for these bands when there is no DX activity about. On 10m this is almost all the time at night and at most times around sunspot minimum. Working around 29MHz there would be no issues with using this simple receiver and transmitter.

Clearly increasing the power with a small linear would help, but the low power does reduce battery requirements and allows QSOs to be made without too much effort, locally at least. I have no doubt that 50mW AM would allow some DX to be worked in a decent sporadic-E band opening when signals can be very stong, but then the receiver selectivity could be limiting. Also, I don't think there are many Europeans on 6m AM. In the USA there is some AM centred on 50.4MHz.

Further Development

A DSB version of this would be a better bet for more serious 6m band use. I would use a direct conversion receiver and a balanced mixer for the DSB generation. 200mW pep of DSB (equivalent to 100mW SSB) would be quite a useful power and would certainly work some European DX in the sporadic-E season. A future project is to complete the DSB version, either for 6m or 10m.

Sixbox schematic