Topband: Linear Loading

[Bob Marshall-Read]

Topbanders,

With a few days off, the vertical transmit antenna got some attention (after
the XYL's garden list).  A new machine is being sold here in the UK that
cuts the turf into strips about 60 cm wide.  The local hire shop had one so
after a convincing discussion about airing of the grass roots, mole proofing
the back garden, and other black magic that makes my XYL (and a lot of
others as well) think that this black magic of RF has fried my brains, I
hired it for a day and promptly cut the turf into nice little strips that
roll up and are out of the way.

A chicken wire mat was then put down on top of the already wired earth (a
few radials got tangled in the turfer machine but that was easily solved
with industrial sized dikes).  Bonded together, wire wrapped, soldered, and
pegged down with ground rods at the perimeter, no moles will ever appear in
my back patch!

Then the turf was rolled back down and with a lot of marching up and down
the strips, all looked like before.

After reading all about linear loading in ON4UN's book, I attacked the
antenna next (neglecting my project axiom of never more than two variables
in a new study).  The base of the vertical is a NATO surplus 12 metre pump
up that keeps the council and the neighbours pacified if not occasionally
happy when it comes down at sunrise.  Then there is a 3 metre fibreglass
tube, followed by a ten metre telescoping fishing rod for a total of a very
whippy 25 metres when fully extended on a calm night.

Loading mid-way up with a coil of about 80 micro Henrys was the best I could
do until now, and the coil has been replaced a few times after various
configurations failed the smoke test.

A one metre fibre rod attached outward from the top of the 12m pump up and
linear loading up and down looked pretty good, the base matching network
handled the rest, after it resonated at 2.10 MHz.  (A feedpoint of 19 ohms
at resonance without the matching network on a very dry day). No coils, no
heating, no flashover in the middle of the night in a pile up, this is
great!

Running the FT1000 barefoot has gotten across the pond the past few
mornings, the noise on RX is terrible (must be working) and it has a 1.5/1
bandwidth of about 20 kHz.  Now, off to 4X and hope conditions are better on
the next leave flight home for a real test.

73 and will be QRV Sunday from Tel Aviv.

Bob   G4VGO






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