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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.
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.
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
on 145MHz with a local disabled amateur, Fred, G8BWI.
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
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).
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.
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.