Hi All,
A couple of weeks back Tom, G3OLB, brought his catapult round and we managed to
get a line up into the crown of one of the tall trees overlooking my small
piece of real estate. Subsequently I used this to pull up a wire but couldn't
find the right tap on the loading coil to tune it. Tuning using a G0MRF Tx can
be troublesome, because of the inbuilt Current Limiter, which operates unless
you are already quite close to tune.
At this point I decided it was time to build Alan's (G3NYK) Antenna bridge
design as described on his web site. Once the bridge was completed it
immediately proved its worth, by showing me the capacitance of the new wire was
higher than I had expected, but also that the ground resistance had
considerably increased. With this information I soon managed to pick the right
tap and tune the loading coil to get an SWR at the Tx of 1:1. This confirmed
the increased ground resistance as having increased from 93 ohms to 130 ohms.
The helical vertical that I have previously described was disconnected in the
new arrangement and just the original inverted L type of vertical consisting of
a total of 80 feet (24m) at a height of about 20 feet (7m) (ground resistance
93 ohm) connected to 60 feet (18m) of new wire at an angle of 40 degrees (44
grades) or so out across the neighbouring wood up to a total height of 55 feet
(17m) (now 130 ohm).
My thoughts on the increased resistance are:
a) the extension is above new ground outside the limits of the earth mat which
covers most of my property.
b) the wood is rather dense and there are many smaller trees filling the
spaces, so it is very difficult to maintain the 2 metre separation recommended
by Laurie and others, especially when the wind blows!
Any thoughts?
On Saturday I managed 2 QSOs with Dave G3YXM and Laurie G3AQC. Both gave me much
better reports than before 7-8 from Dave and 8 from Laurie. About 1 S point better.
Then it happened, the weekend storm got going and by Sunday morning, the nylon cord
that I had been assured could stand several hundred weight (lots of kilos) had
snapped. Conclusion 6H round braid nylon cord is not man enough for the job. I wish
I had used my polypropylene rope now, but everyone tells me it goes brittle,
although this has not happened to any of the mast guys that I used it for. Just as
I was going to try calling those continentals who can never hear me&. Never
mind, try try again as they say.
Another interesting result of using the Antenna bridge is that the capacitance
it showed for just the antenna wire without the loading coil was about 60 pF
less than the value obtained after loading up the Tx and tuning the loading
coil. This must therefore be the capacitance of the loading coil to ground.
Quite high.
The figures are a bit tentative at the moment, but when an Antenna of some sort
has been re-constructed, I shall use the bridge to measure the capacitance with
and without the loading coil attached (but disconnected from the Tx) in an
attempt to get more accurate results.
73, John, G4CNN
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