From Dave G3YMC
My loop has been in use for well over 3 years now, so the following may clarify
recent discussion and surprise some used to 'conventional' vertical antennas.
1. Loops are largely unaffected by ground resistance. There is a small earth
coupling loss which reflects to a series loss resistance. The actual mechanics
of
this I have never understood, but the effective resistance of this is no more
than
0.5 ohms. There is no connection whatsoever between my loop and real ground
(the tx
is grounded to mains earth), and the lower leg of my loop is just one foot above
ground. This is the big advantages of loops - they are effectively independant
of
earth losses.
On this basis there is a limit to how low you can sensibly make the wire
losses. My
loop has a dc resistance of 0.1 ohms - it is constructed of 30A speaker cable,
two
conductors 2.5mm diameter paralleled. Because of the reflected earth loss (0.5
ohms)
and skin effect losses increasing the wire size or using Litz would be a law of
diminishing returns.
2. Matching is via a capacitive network, there is no lossy loading inductance
in the
system at all. Q is high, and bandwidth is about 100Hz. Contrary to what has
been
suggested, the resonance is stable and has stayed within a few hundred Hz all
the
time it has been up. Match at resonance is always 1:1 (and an swr bridge is a
far
more relevant tool for loops than an rf current meter). Sometimes the wire sags
after
wind and resonance shifts by 300Hz or so - retensioning the loop quickly puts it
back. In heavy wind there is some evidence of match change but never anything
dramatic. The only effect seen is temperature coefficient change in the
matching
network capacitors - between a cold winter morning and midday summer the
resonance
moves up by a few hundred Hz. All these changes can be accomodated by changing
the
switch settings on my match box. In a word, my loop is 'rock stable' and has
needed
no attention for 3 years. It is always available for transmit with no messing
about.
Not so my vertical. Bearing in mind it is not helped by a 300 ohm earth loss,
and
the fact that the tuning is very broad anyway, in the short time it has been in
use
it has gone off resonance/match after any small change in conditions - rain,
frost,
wind etc. It is infinitely more effected by the elements than the loop, and
needs
constant attention to keep it in a useable state.
Hope this clarifies a few things.
Dave G3YMC
[email protected]
[email protected]
http://www.dsergeant.btinternet.co.uk
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