10m AM Tenbox

Currently under development, the Tenbox is a higher power version of the Fredbox and Sixbox, but this time for 10m AM between 29-29.1MHz. The intended purpose is for local QSOs, within a few miles, rather than for DXing. I have in mind using the rig to talk to locals on 10m when the band is not open for DX, such as in the evenings or during the quieter sunspot cycle years in the daytime when 10m is dead.

So far, I have breadboarded the RX, a simple super-regen with remarkable sensitivity, and the TX which puts out around 250mW carrier (1W peak). I have range tested the breadboard over a few miles locally.

Heathkit "Benton Harbour Lunchbox" styling?

As a fan of old Heathkit products, I quite fancy packaging the Tenbox a bit like a tiny version of the HW19, the 10m version if the Twoer and Sixer. The small black plastic boxes used for my 472/500kHz transverters would be ideal if mounted vertically with the top facing the front with a loudspeaker, RX tuning, mic socket and TX/RX switching.

Schematic (Rev A) - please click to get full sized image.

The above schematic does not yet include a simple loudspeaker amplifier. I may, or may not, add a linear amplifier to lift the carrier power to around 1W carrier.

The Tenbox Transmitter Breadboard

This image shows the breadboarded version of the Tenbox TX feeding an improvised vertical antenna via the Elecraft T1 auto-ATU. A recorded MP3 message is being fed from the PC into the Tenbox microphone. I then went out in the car to see where and how well the signal could be copied around the local area. With a mag-mount on the car (actually a 15m whip not fully extended) I could copy the signal out to about 3km. With better antennas at both ends (a 1/2wave vertical and a proper CB style whip on the car) I would expect about 3-6dB improvement, so greater range or better S/N over the range originally tested.

Video of the Tenbox Receiver to show sensitivity

Tenbox RX breadboard

The receiver breadboard is very roughly made, but the circuit is simple and heavily based on a design that I have used many times in the past. This particular super-regen circuit based on 2 x MPF102 FETs just works. It is not fiddly, very stable in use, extremely sensitive and has good noise immunity and automatic AGC action. Its one limitation is selectivity, which is unlikely to be problematic when the band is quiet, which is when the Tenbox would be used locally.  I plan to replace the air-spaced variable with either a varicap tuned circuit or a polyvaricon variable capacitor.

More complex receiver circuits are possible, but these negate the ultra-simple approach used here.