Hi all.
Firstly, thank you to Jim and Chris for the QSOs yesterday and for the
many useful reports and comments on the relector.
Some detail.
THE OBJECTIVE: To test the very good theory that horizontal
antennas don't work at LF. The idea being that by operating on very poor ground
you are actually many 10s of metres above 'radio ground' and therefore the
antenna is much higher electrically than you may first imagine. This technique
is used by the US military in underground nuclear command centres to avoid
EMP.
The location in East Sussex is at the top of a hill 170m above sea level.
The ground goes down rapidly in two directions which I hoped would also aid the
propagation of the 500k signals.
We started off with a reference antenna - A 9m vertical with a 5m top
section and intended to use reports and the grabber at G3YXM to evaluate the
radiated signal by comparison to the 100mW ERP 137.7 beacon. This was a big
mistake as the poor ground made it very inefficient, with 100 W from the
transmitter generating an unimpressive 125mA of antenna current. However, it did
allow a couple of QSOs.
We then added a large amount of wire to the top section to make the
antenna self resonant. To my surprise, and I'm not sure if this has been done
before, it worked. Total wire used 128 metres. Series tuning
inductance....zero. Compared to the wavelength of 596m this
represents 85% of a quarterwave, which was rather more than I had anticipated. I
expected the 'shortening effect' to be much greater. Perhaps this was a good
indication that as I suspected the 'ground' under the antenna was either a
considerable distance from the surface or, operating from a hilltop had an
advantage. Antenna current was now 1A. Still not very good but this was due to
us using the quaterwave against a few radials and a single ground rod. I
notice that Rik reports a signal increase between the two different antenna
types, but this could be explained by higher current in the vertical rather than
any radiation from the horizontal section.
Just when things were looking good, it all went, to use a local term, 'pear
shaped'
It began to get dark. The static level wiped out the grabber so we
could not see our own signal and our final plan to turn the whole thing into a
single horizontal dipole mounted along the ridge of a hilltop had to be
abandoned as one of the 2 fields suddenly acquired a large flock of
sheep.
So, unfortunately a nice day in the countryside but only a
little knowledge gained and the transmitter never turned up beyond
100W.
Next time...I'll take a VNA and start with a dipole! At least the
local manager at Homebase wont have to worry where all his 2 m bamboo canes are
going anymore. My garage is full of them!
Finally, to answer Dave's question about the callsign. I have
probably misread my NOV but I was under the impression that just like the old
73k licence, /P was not allowed. Hence a letter to OFCOM a week in
advance changed my 'station address' to East Sussex for the weekend. - So
although I was working from a field....they knew where to find me.
As for 10 W ERP. - We probably made about 2% of that. Sorry
Mal. Oh well.
To be continued..?
David G0MRF
On 24
May 2009 at 18:24, James Moritz wrote:
> Dear LF Group,
>
> Just worked G0MRF at 1705utc. Signal was 549 - apparently they are
still
> working on the antenna, so might improve yet... Got a good
report from
> them, so obviously receiving OK. Good luck for the rest of
the
> operation, David and Garo.
>
> Cheers, Jim
Moritz
> 73 de M0BMU
>
>
I listened on and off to
David's signal yesterday evening between other
jobs.
Some
observations....
G0MRF (/P but not sending /P, which I thought was a
bit confusing) was
569 here in Bracknell, later peaking 579 but the static
was intolerable
from around 9pm making copy very hard.
I did give
David a couple of calls after he finished his QSO with Chris
G3XIZ but
obviously he didn't hear, hardly surprising with my setup and
in any case
I found the tuning had gone off and I had a bad SWR, a
problem with
sharply tuned short verticals.
