A bit more about the Decca mast:
It's basically a 325 ft (100m) self-supporting tapering tower (see Mike
Dennisons'
and my own pix).
When it was built in 1946 it was hoped one Decca chain might cover the whole
of the UK so maximum radiating efficiency was the key. Top-loading is best
(yes, they
knew that even in 1946!) so the mast carried four horizontal booms at the
250ft level
with wires from the top of the mast to the ends of the booms and then down
to the top
of the base insulators. This gave a capacitance of 3750 pf and a radiating
efficiency of
around 16% at 85 kHz.
Unfortunately it was found a few years later that wind loading on the booms
was
twisting and fracturing the mast so two booms were taken off and
relocated lower down at the 100 ft level.
By then it had been found that skywave was limiting usable range to about
240 n.m.
and max. efficiency was no longer needed so to reduce maintenance costs the
four
booms were taken off altogether and the 3750 pf capacitance restored by
running eight
wires from the top down to a new set of insulators spaced 50 ft frm the mast.
Capacitance had to be kept at 3750 pf because the loading coils had been
designed
for this and couldn't be altered - too expensive! Efficiency went down to
around 12%
but didn't matter.
Grounding system was 36 wires at 10 deg intervals out to 500 ft from base
of mast,
buried 2 ft.
Decca's "model" aerial for artifical loads was 20 uH in series with 3750
pf and 2 ohms.
Transmitter produced 1200 w which put 20 amps "up the spout" in normal pattern
transmssion and peaked at 60 amps when lane idents went out.
Incidentally, the UB5 (KN58 - 2225 kms) report of 42 in daylight over a
totally
land path (see Dereks report) is equivalent to 2800 kms over a sea path if the
CCIR curves are to be believed. Newfoundland is only another 900 kms
further away but
the curve is falling off so fast at this range it would need another 20 db
to make it on pure groundwave from Puckeridge. Be nice if we still had the
use of it next winter...............
Walter G3JKV
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