Tenbox 10m AM QRP

Way back, one of my (many!!) projects was this simple AM transceiver for local natters on 10m, such as at sunspot minimum or at night. The RX is very sensitive, but quite broadband, so not really suitable for DX.  

Because of my 2013 stroke, it never got finished. It was based on the 2m Fredbox. I just wish someone would do a PCB and finish it. It could make a nice club project.

See https://sites.google.com/view/g3xbm4/home/hf-mf-and-lf/homebrew/10m-am-tenbox .

Tenbox

Many years ago, before my stroke, I intended to build a simple 10m phone transceiver aimed at local contacts only. What I had in mind was a 10m version of my 6m Sixbox and 2m Fredbox. 

It used a super-regen RX with isolating pre-amp to avoid radiation, so would not be suitable for DX work. Having said that, it had no problems receiving USA AM stations in the car with just a base loaded whip. 

My idea was to build a rig that could be used when 10m is dead, such as at night or in poor solar years. 10m is a great band and very little used when not "open". 

It is ideal for local contacts as antennas are small especially when compared with 160m. CB antennas work well. 

In my mind was something like the old Heathkit Benton Harbour Lunchboxes (remember those?), but far smaller and on a single simple PCB.

Sadly my stroke got in the way and the project was not completed.  It would be great if someone else completed this project. There may well be better solutions these days.

See https://sites.google.com/view/g3xbm4/home/hf-mf-and-lf/homebrew/10m-am-tenbox .

10m Tenbox

With 10m again becoming a good DX band, it does not seem right talking about a simple cross-town natterer for 10m!  Mind you 10m is often a daylight DX band and there are plenty of times when it is dark when the band is totally unused.

Derivatives of this simple radio might make an ideal 10m cross-town natterer. Just above 29MHz would be an ideal spot with plenty of space and very low chances of causing anyone else problems. This receiver has heard USA AM stations with just a base loaded whip on the car, although I would not recommend it for serious DX work.

In my mind I see a rig a bit like a tiny Heathkit Twoer (remember these?) with a tunable RX and crystal controlled TX. On a suitable PCB it could make a nice club project. Improvements would include an AF amplifier to drive a loudspeaker and maybe a very simple linear to bring the power up to a few watts of RF.

I cannot imagine the rig costing much more than £10-20. Put it another way, how many cups of coffee or beers will that be?

See https://sites.google.com/view/g3xbm4/home/hf-mf-and-lf/homebrew/10m-am-tenbox .

10m Tenbox QRP transceiver

Because of my poor health, progress on this project stopped. It would make an ideal winter project. It is designed as a local natter box, ideal for local nets. It is not intended for DX.

See https://sites.google.com/view/g3xbm4/home/hf-mf-and-lf/homebrew/10m-am-tenbox  .

10m Tenbox transceiver

Like many of my projects, this came to an abrupt halt after my 2013 stroke. I just hope someone, or some group, can take it further. It would not be suitable for DX working, but could make a nice, simple, club project if finished and a PCB made.

If anyone has finished this, please let me know as I am very happy for this to happen. All the data is freely available on the website below. Copy and feel free to change!

See https://sites.google.com/site/g3xbmqrp3/hf/tenbox

Tenbox - simple 10m AM transceiver

Years ago, before my stroke, I started work on the Tenbox, a very simple AM transceiver for 10m. As this used a super-regen receiver, this would really only work on AM and not be suitable for DX. It was to have been a "cross town" natterer. Sadly, my stroke got in the way and the rig was never completed.

I had intentions of making this in the style of the old Heathkit lunchbox rigs of old. It would make a nice club project on a simple PCB. When I left it, it needed a loudspeaker amplifier and another 10dB power on TX, maybe a simple linear amplifier.

See https://sites.google.com/site/g3xbmqrp3/hf/tenbox .

Tenbox 10m AM transceiver (again)

10m (28MHz) will probably be in decent shape for another 12 months, possibly more. After that, apart from summertime Es and the occasional N-S F2, the band will behave more like a VHF band again.  Even now, it makes a decent band for local natters, when the band is "closed" for DX, especially in the dark evenings. If you are interested in local QSOs on 10m, you might care to start with my Tenbox transceiver (29-29.1MHz is where most 10m AM is concentrated), which is a 250mW AM design. I started this before my stroke. It is really unfinished and it could do with a TX linear amp and a LS amp for the RX. It is a very simple design, based on low cost parts, which can be used as inspiration for your own project. This uses a very simple RX and it is not designed for AM DXing, although it has copied Ws on 10m AM. The RX, although simple, is very sensitive on AM. There is a video on the link below.
 
See https://sites.google.com/site/g3xbmqrp3/hf/tenbox .

Project "on hold" - AM Tenbox

Like many of my projects, this transceiver is "on hold" due to my stroke. I am still too unwell to do much in the way of designing and building. One day, not too far away, I hope to complete this project. If anyone else wants to do this, please use my schematic as a starting point. With a small PCB it would make a nice club project.

It will make an ideal simple rig for nattering across town, especially as 10m loses its fizz and starts to act more like a vhf band again. Such conditions may be with us for years. I fancy putting this in a box like the old Heathkit Lunchboxes with a proper LS amp and LS.

See https://sites.google.com/site/g3xbmqrp3/hf/tenbox .

First on-air testing of the Tenbox TX

This evening, with little sign of F2 propagation, I ventured out to test the range of the 10m Tenbox TX as it is currently. For the TX antenna I used the coax up to my 10m halo as a vertical tuned with the Elecraft T1 ATU. At the RX end, in the car, I used the FT817 with a small base loaded mag-mount.

Tenbox AM TXing with MP3 callsign message on the PC

What I should have thought about was the noise on AM from street lights! When the signal is strong in the village this is no issue at all, but traveling to the next village 3km away there were several spots where the buzz from sodium street lights just killed the little signal. Both antennas are compromised, so another 3-6dB of signal level should be possible with a proper 1/2 wave antenna at each end or with a better car antenna.

Maybe this project, although great simple fun, isn't going to quite meet my needs, whereas a DSB transceiver would?  Receiving the DSB signal as SSB and the improvement over AM will be around 9dB with better noise immunity. However a simple direct conversion RX would not be suitable to use (easily) with a DSB TX, whereas the AM Tenbox would be fully 2-way compatible with another identical AM unit.

Conclusions so far
(a) 6-10dB more may be needed to meet the design objectives with AM.
(b) It is worth repeating the very same test now using a DSB TX with the same power to compare.

The first Tenbox Schematic

The breadboarding of the Tenbox 10m AM transceiver for local cross-town night-time nattering is almost complete now, with just a receiver audio amplifier to drive a loudspeaker to be added. Power output is a conservative 250mW AM carrier (peak power around 1W).  The first schematic draft is shown here, but as always I warn that I may have forgotten to correctly label some parts and the values may change as the circuit is further developed. I hope to road test this over the weekend going out in the car with a mag-mount antenna whilst AM beaconing with the rig from home. As with WISPY, I have paralleled up 4 2N3904 transistors for the PA. This is a very inexpensive way of making a 10m PA stage! The alternative, such as a 2N3866 would be about 5-10 times more expensive.

Tenbox TX progress

Good progress today with the transmit side of my Tenbox 10m AM transceiver.

The series  modulator was breadboarded a few days ago, so today I built the 3rd overtone oscillator, the buffer and the modulated PA stage. Results were as expected with around 50-60mW of clean well modulated AM available at the output. With a single linear amplifier with around 10-13dB gain (easy at 29MHz) this will take me into the 500mW-1W carrier level, which is my design aim. As I did not have a suitable 29.05MHz crystal (anyone know where these can be obtained cheaply?) I used a 28.500MHz one on a dummy load. With this frequency, any range tests will have to be done at night to avoid QRM to SSB stations.

When the linear is added , maybe even before then, I'll do a local "drive around" test with the TX on "speech beaconing" mode (using an MP3 file on the PC) and with the RX attached to a mag-mount antenna on the car. This will establish the useable local coverage.  Assuming this power level is OK, I'll then complete the full design on a tidy breadboard and publish the initial schematics on my blog and website. After that, a PCB may be in order and a neat enclosure.

This project is great fun: a simple, straightforward and easily reproducible design and a project that will be useful at the end. Much more fun than working DX with 400W and a 5 el beam in a contest with a £5k commercial HF transceiver.  Honestly, I'm having a real ball with this.

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.

Testing the Tenbox on 10m AM today

Today I wanted check how good the Tenbox 10m AM receiver (part of the intended transceiver) was in real use as opposed to measuring it on a signal generator.

So, I set up my FT817 on 10m AM TX on the lowest power setting, which is around 150mW carrier, and fed this into a makeshift vertical tuned with the T1 ATU. Using my PC, I made a short message "G3XBM testing on 10m AM" that I played in auto-repeat.

I then went for a drive locally with the little super-regen RX in the car being fed from a vertical mag-mount antenna on the car roof. The only antenna I had was an ATX15 base loaded whip for 15m, so I pushed a few sections in to make it resonate on 29MHz.

Results were encouraging. Out in the car my 150mW AM TX beacon message could be copied out to about 2 miles away, although this was limited by QRM from W2VW (!) and VE3OWV (!) who were coming through with 59 signals on the RX on 29MHz AM. When they dropped carriers copy was fine. It's fascinating to think my 150mW AM signal was having QRM from 5000km away!

A super-regen is not really suitable for use when 10m is wide open, like it was this afternoon. Nonetheless I was able to copy stateside stations on 10m AM on a short base loaded mag-mount pretty well. For its intended purpose as a local across town natter box, I think the Tenbox is going to be fun. With 6dB more power out (my aim for it) it should be certainly possible to work G6ALB 3km away in the next village.

The next stage is now to build the TX part and test this.

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.

The Tenbox - name chosen

Thanks for the suggested names for my 10m DSB project. The one that wins is the Tenbox, which is the logical name following on from my earlier AM transceiver projects called the Fredbox (2m) and the Sixbox (6m). The Tenbox is just right - a small box with a complete, yet simple, 10m sideband transceiver inside that should be completed in the next 4-6 weeks.

There will be no progress now for a week as my little 3 year old grandson is here to stay and he sleeps in the shack bedroom. Maybe I will teach him a little more CW.