Re Jim's aerial tests, I had a word with my pal the (retired) Decca
aerial guru and showed him some of the emails. He said :
(a) If both aerials are vertically polarised only then there shouldn't be
any difference between their radiated fields if they're radiating the same
power because both are electrically very small. The only difference would
be in their conversion efficiency (RF in to RF out). If there is a genuine
difference in observed field strength then it simply means there is
something wrong in the calculation of ERP - ae efficiency perhaps.
Decca obtained their ae. efficiency figures by feeding with a known
amount of power and making absolute field strength observations
at multiple distant points allowing for different path conductivities.
(b) If the small one has an assymetric horizontal component (flat-top)
this will complicate matters considerably since it will radiate both
vertical and
horizontally-polarised and there will be interaction between them that
will affect the radiation pattern. So it would be unlikely to have a circular
polar pattern and this might account for some of the observed variations.
Said it would be better from this viewpoint to make a "T" rather than
a "L", then the horizontal would cancel out.
(c) Jim's precautions against interaction between aerials were correct
and with the big one grounded there would have been very little, if any.
between the aerials themselves. However, the small one would have been
sharing the earth mat of the big one since it was well within the induction
field so it was probably more efficient than thought.
(d) The dropped wires were put in simply to increase capacitance back
up to the standard Decca 3750 pf. All their coils etc were designed for
this figure and it was much cheaper to do this than to have to design and
build new coil systems. Don't forget Decca had to transmit five frequencies
simultaneously every 20 seconds (lane idents) and getting the same power
at the correct phases without any interaction was quite a problem. Hence
the complicated ATU coil setup.
(e) At one time Decca had a transportable Decca system that used a 100ft
mast with a 100ft radius ground plane (wires run out on the surface). Using
a 500W Tx they used to get ranges of up to 300 miles over water.
(f) Finally he said the main problem with small aerials was the greatly
increased
voltages and insulation problems. They always had problems on the transportable
system to the extent they had a man whose job it was to clean all the
insulators
every morning and they couldn't be located on coasts because the salt water
shorted out everything "in hours". The 100m aerials were only installed at
a few sites in the early days in the hope of reducing the number of chains
required
but it turned out skywave was the limiting range factor (screwed up phasing
on which the whole system depended). This meant Decca couldn't be used more
than about 200 miles at night, so later transmitters had smaller and much
cheaper
aerials (150ft "T") which covered up to 200 miles quite adequately.
Walter G3JKV.
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