Tenbox 10m AM RX video

It is hard to contain my enthusiasm for simple transceivers and super-regen receivers! This video shows just how well the RX breadboard for the little Tenbox 10m AM transceiver works. 1uV (-107dBm) is a very good signal and it will still detect a well modulated AM signal at -120dBm. Selectivity is not good with this type of receiver, which is the main drawback. However for its intended application - a simple transceiver for local nattering across town - it should be fine. Some people use super-regens as the IF stage of AM receivers. If a filter is added ahead of the super-regen IF this overcomes the selectivity issue but makes the RX design more complicated.

Tenbox 10m AM rig - a bit more

Although I had little time today, I experimented with adding varicap tuning to the Tenbox AM transceiver's RX section. Only problem was I had no varicaps, so tried using a few different diodes and a transistor as a varicap. 1N4148 and a Germanium diode reverse biased hardly produced any change in capacitance going from 2-12V. I know a 1N4007 works quite well but could not find one. Then I used a 2N3904's base-collector junction and got a few pF of change. In the end I just did as I did in the Sixbox and used a fixed capacitor in series with a 365pF polyvaricon tuning capacitor. This moves the RX frequency as much as I need at around 2MHz maximum.

Tenbox progress (10m QRP AM transceiver)

Tenbox breadboard RX -115dBm MDS

Today I breadboarded a simple, but sensitive receiver for 10m AM in the Fredbox tradition. The standard Fredbox and Sixbox circuit was simply scaled down to 29MHz with great results using the breadboard with the Marconi 2022 generator. MDS for a well modulated AM signal is better than -115dBm (around about 0.5uV) which is actually better than I was achieving on 6m and 2m. Not bad for just 2 MPF102 FETs and 1 2N3904. For the tuned circuits (RF amp output and detector) I used a couple of T37-6 (yellow) toroids tuned with 15pF trimmers. There is a 10pF coupling capactor between them.  Of course this is a super-regen with a grounded gate RF amp to isolate the detector from the antenna.

The intended use of the Tenbox is local "across town" nattering when the band is quiet. The selectivity would NOT be suitable for DX use, although I'm sure it would pick up stateside 10m AM stations OK.  I am always amazed by how well a simple super-regen RX works: they just fly for me and are always incredibly sensitive and non-critical. People who have not played with these do not know what they are missing. I highly recommend the RX in the Fredbox

Next stage is to breadboard up a simple AM modulator and TX strip. Again, I shall copy the basic Sixbox design but this time add a linear amp (a few 2N3904s in parallel as in the WISPY beacon) to get the carrier power to a useful level.

In summary, an interesting and productive hour or so after tea.