LF, Robin, Chris, Alan,
Last Tuesday morning (17 August between about 08 30 and 1030 UTC) Robin, Chris
and I carried out some tests on 137.000 kHz from Mare le Bas (JN17vg). I
apologise for not having posted earlier. Many thanks to those who responded to
my previous posting.
Transmitter: 30 W output (subsequently falling to 20 W) to an 8 m vertical with
two 20 m horizontal wires as a top hat. Many thanks to Alan for advice on
this. Feed current initially 330 milliamps. Manual ID every 15 minutes
interspersed with long dashes.
Receiver: a 1.6 m mag mount whip in the centre of the roof of a Renault
kangaroo, feeding a Lowe HF 250 receiver with a DVM connected to the AGC line.
Mode: CW. Bandwidth 2.2 kHz.
Environment: the area is extensively wooded, and the underlying rock appears to
be on the boundary between limestone and granite. To the north-east there is
a fairly clear view out to several kilometres, with a small belt of trees
cutting across it a few hundred metres from my location. To the south-east
there is a belt of trees nearby.
Results: rather to our surprise, we were only able to make measurements to a
distance of a little under 2 km to the north-east. The limitation appeared to
be man-made interference. We suspect that this may be from the car electronic
system (Cam Bus?) which seems to be doing something even when the ignition and
all accessories are turned off. On the return from the south-east signals were
noticeably weaker.
Lessons we have learned:
- the cause of the falloff in transmitter output power needs to be investigated
- the calibration of the antenna current meter needs to be checked
- the bandwidth of the HF 250 is really too great for good measurements.
Something more appropriate is needed
- making measurements with a small loop a little way away from the vehicle
should enable us to deal with weaker signals
- moving to 137.700 would be a good move as it should place us within the
capture range of one or more grabbers
Robin and Chris have had to return to the UK but I hope to continue with the
work in the meantime.
We would welcome comments and advice from those who are more experienced in
this corner of the field.
73
John F5VLF
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