Dear Dennis, Roger, LF Group,
Thanks for the additional reports for M0BMU/P. Here are some details of the
station - see the attachment for pictures:
The TX antenna was a 15.3m fibreglass mast with 4 x 10m long top loading
wires sloping down to just over 9m. The ground system was 4 x 1m ground rods
spaced on a 2m circle round the base of the mast. The antenna was tuned with
approx 300uH fixed inductance near the base of the mast, and connected to
the 18 - 70 uH fine tuning variometer in the shack about 15m away by a
transmission line wire about 1.5m above the ground. The antenna ground was
also connected to the tuner via a return wire about 300mm off the ground.
The tuner contained the variometer, ferrite cored matching transformer, and
a common mode choke to isolate the antenna ground return from the shack
ground. This arrangement puts the tuning and matching controls in the shack,
and I found it was easy to adjust, with no problems due to unwanted
resonances or ground loops. The total antenna resistance was only about
11ohms, including 2 or 3 ohms due to the loading coil and variometer, so
antenna current was approx. 3A with 100W PA output. I estimate the antenna
effective height was 11m, giving Rrad of 0.53ohm, and ERP of 8.6W.
The receive antenna was a single K9AY terminated loop, oriented to reject
wideband noise coming from the Brookmans Park BC site to the west, and give
good reception of Eu stations from the east. I found a terminating resistor
of about 200ohms gave a good null - this is rather lower than the values
quoted for the HF bands. The RX antenna was positioned about 50m from the TX
antenna, to minimise interaction, and give the RX preamplifier an easier
life!
I used my 200W EER transverter rig, although just for CW on this occasion,
driven by an IC-718 as the IF. The minimalist shack ("hutch" might be a
better name for it :-)) was supplied with 130m of mains cable going to an
outbuilding and into a slightly dodgy mains socket, but much less hassle
than a generator or batteries. It was also quite handy to be able to follow
the cable and find the gate in the dark!
Cheers, Jim Moritz
73 de M0BMU
M0BMU_P_II.jpg
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