[HCDX] MW Loop improvements

["Sean \(G4UCJ\)" <g4ucj@xxxxxxxxxxxx>]

Hi all, some of you are building my "Hula Loop" MW antenna and I thought I
would share an improvement I have made. The improvement is to the design of
the FET preamplifier. Initially the design had a high/low gain switch but I
found this to be limited in use so I looked for a way to vary the gain
rather than to have it switched. My initial design incorporated a variable
resistance in the supply line which worked well, but even though the preamp
only draws just over 8mA it still generated a fair amount of heat. After 2
years of use I found that this heat dissipation in the variable resistor had
caused it to become very sticky and stiff to turn (the usual methods of
cleaning failed to restore proper operation). After many experiments, I have
found that removing the high/low switch and putting a 10k variable resistor
in parallel with the 330 ohm resistor and connecting  the wiper to the
common point of the switch gives excellent gain control (i.e. a signal can
be increased from zero on the meter up to s9.). The other advantage is there
is very little heat dissipation through this resistor. Sorry for the long
winded explanation. I will update the circuit diagram shortly to reflect the
changes.

73 Sean, G4UCJ
RECEIVER: ICOM IC756; GRUNDIG SATELLIT 600, 3000
ANTENNA: Low Band vertical with 32 x 10m ground radials
                   Indoor dipoles for 14-30MHz and 50MHz/Band I TV
                   1.3m diameter MW loop + FET Preamp.


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