Hi All,
Vic G8IK, located in Solihull, Birmingham, has been kind enough
to write a personal account describing his first 2 years on LF.
Vic writes:
Initial 73 kHz tests
--------------------
Seeking fresh fields, and pastures new, I thought I'd have a go
at 73 - not very optimistic but I thought maybe someone else in
this region might be similarly motivated. Luckily I had just
purchased some coils at a rally and had sufficient for a simple
receiver. I used FETs in three RF stages followed by a voltage
doubling detector with regeneration to improve selectivity, then
the usual audio stages. The coils were single windings with no
coupling coils, so I used resistance capacity coupling between
stages with very small coupling capacities to minimise damping of
the tuned circuits. I have a number of accurate oscillators
which, with Rugby, helped me to find the band. The receiver
worked well, with plenty of gain and selectivity.
For the rig, I used a small ferrite rod antenna, with a BC108 as
oscillator tuned to the band with two 2000 pF compression
trimmers, which drove two TIP126s in parallel. Using an 80 ohm
power meter as load, these delivered 30 watts or more at 350 mA
80 v. In spite of a reasonable heatsink they overheated after a
few minutes, so I made a multi-vibrator with an on-off ratio of
1:1, driving an on-off relay which enabled the rig to run for
long periods. I wound a 2.5 litre drum full of 22 SWG wire (with
taps) and, with the present antenna extended to about 170 ft
(twice its present length), this resonated somewhere near the
band. I had a variometer (a museum piece) which proved excellent
for fine tuning. With everything in order, I switched the gear
on and made my way, with the receiver, to a colleague who has a
substantial antenna and who lives about a mile away, hoping that
the signal would travel at least this distance, but no - not a
trace!
Success on 136 kHz
------------------
136, I thought, offered more promise as the antenna would be
effectively twice as high and twice as long. I retuned the gear
to the new band and, to my delight, had a reply to my first CQ
call: only seven miles away (G3YXM) but it was a start, and, to
my amazement, this was followed by reports from G4GVC and GW4ALG.
Thus encouraged, I decided to build a 200 watt rig - I had the
main ingredients: two high voltage transformers, a 50 watt
tetrode and two high voltage variable capacitors (45 pF
Eddystone, ca 1935). I assembled all this on a 3 ft x 1 ft
board with a hardboard panel of similar size, using the existing
rig (down-powered) as the driver. My main problem was getting
the right L/C ratio for the anode coil and, at the same time,
allowing for the two variable capacitors (in parallel) to give
some degree of tuning. I partly solved this by tapping the anode
half-way down the coil (a 2.5 litre drum two thirds full of 22
SWG) and using rather less than the optimum tuning capacity (700
pF instead of 800-1000 pF). I wound a coupling coil at the cold
end of the coil, and was ready to go. I'd arranged the power
supplies to give me 500, 1000, or with the two in series, 1500
volts, and with full power almost burnt out two one-hundred watt
lamps! Before going on the air, I was concerned about that L/C
ratio, so I had a look at the waveform - it was dreadful - I
visualised strings of harmonics every 136 kHz well into the HF
region. I amended matters by reducing the grid drive from about
8 to 2 mA (with only a slight loss of efficiency), and included a
low pass filter in the output.
I was now up and running, but at this stage the lack of
'operateability' of the receiver became a problem - I had to
jiggle two RF tuning controls; regeneration; and BFO, so decided
to build a superhet. I used 2 MHz for the IF (for which I could
wind coils) with a simple bridge circuit for the crystal filter
in which the capacity of the crystal (plus holder) is balanced by
a variable 'phasing' capacitor - which gives wide control of
selectivity: at exact balance, it was extremely sharp.
The receiver works well and I have no trouble 'winkling' out the
DX, but have been disappointed in not being able to work outside
the British Isles. I have hopes, next autumn, of making
improvements to the antenna, at present only 80 ft long and
barely 20 ft high. I have some tall trees at the bottom of the
garden which either must go, or else serve as supports for the
end of the antenna (I am thinking an archer might help!).
Vic Morse, G8IK
|
