Fredbox 10mW 2m AM Transceiver


Revised Nov 2011 - layout changes

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 This is the story of the Fredbox, a rig that first saw the light of day in 1974 in Cambridge. In the last few years it has been restored to full working order to enjoy the renewed interest in AM operation on 2m in the UK.

For several years I'd worked local stations with a simple very low power 10mW AM transmitter. This was coupled with a super-regen receiver that first appeared in Practical Wireless in the late 1960s. Incidentally, this receiver design has just been republished in a recent copy. The simple combination was used on the bench with just a toggle switch change-over. For some time the antenna changeover consisted of unplugging the antenna from the receiver then plugging it into the TX and vice versa.

The antenna at that time was a small dipole or indoor yagi rotated by hand.

Combining these two circuits into one small handheld took only a couple of weeks. A small PCB was etched after a suitable box was found and the circuit worked first time. People working along side me were so impressed by its small size that very soon 3 other copies were made. The first Fredbox to Fredbox QSO was over about 0.3kms.

The odd shaped board was to allow a PP3 battery to be put inside the box as well as the microphone and TX-RX switch. A crystal earpiece was used on RX. Current drain was under 1mA on RX and only about 15mA on TX so the battery would run for days.

 
Local contacts were frequent around the Cambridge city area and the most regular QSOs were on 145MHz with a local disabled amateur, Fred, G8BWI. Because of this, the little box became know as the FREDBOX.  I dedicate the circuit and the memories of those fun times to dear old Fred.

How Fred could talk! Sometimes you'd start a QSO, then hand over to Fred, have your tea, and he'd still be talking away. Such good fondly remembered times indeed.

 

The most exciting results took place away from Cambridge in Yorkshire and in South Devon. In Yorkshire the Fredbox was regularly used to make QSOs from my wife's parent's house in Barnsley up to Leeds about 35km to the north. In Devon, it was used to make several QSOs from Start Point to Portland Bill in Dorset at 90km to the east, all with just the rig handheld with a whip antenna. Then, on one occasion the best result of all - a 160km QSO from Bolberry Down across to Brittany in France. I was so amazed that this happened, but it most certainly did one fine summer morning.

I was so impressed by these results that I submitted an article on the Fredbox to the RSGB for publication in RadCom. The fact that several had been made with good results was testimony to its reproducability. Sadly the committee of the day thought it was "not suitable for its readers", so the article was never published. It is reproduced in its exact form here (follow the link). 

 
 One reason cited was the amount of re-radiation from the super-regen oscillator on RX. This was very small and I do not believe it would have been audible beyond a few metres. Gradually the 2m band became busier and people moved over to FM and SSB. AM all but died out on the band so the Fredbox was consigned to the cupboard and rarely saw the light of day again until this year.
The picture shows me in mid-QSO holding the Fredbox in my hand. You can just make out the small crystal earpiece in my ear. 
 With a small revival in AM operation on 2m the Fredbox was rebuilt into the very same box as I still had the box and the built PCB - see photo. It has again been on the air in the Cambridge area and was heard at 76kms away by G1HDQ (using a whip antenna too) when down in Devon, so its STILL works. So, if you hear a weak AM signal calling it may be me.

Update:

July 23rd 2009 I gave the Fredbox an outing and went /P to a local hilltop not far from home. Using a 3/4 wave whip on the rig handheld (quite long!) I had a solid QSO with G6ALB some 16kms to the south of me. He was just using a triband colinear antenna to receive me. I also had QSOs with a 1/4 wave whip and a failed QSO with a small helical antenna. At the same range the 2m AM 10mW rig was received stronger than a 40mW AM SixBox 6m transceiver one, although this may be because the V2000 vertical used to receive both rigs at G6ALB's end is less efficient on 6m than on 2m.


Also on this site is the SixBox, a derivative AM transceiver for 6m.




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