From G3XDV
I contend that, although I may have my theory incorrect, using
elevated inductance can be worthwhile (even with the increased
resistive losses caused by the total inductance having to increase)
in circumstances where the top section is relatively small and
therefore the effective height is low, or where the lower part of the
vertical section is obstructed. The theory I defend is that the
effective height of the antenna is being increased by this method.
I look forward to further discussion on exactly what theory fits this
practical result.
Quite correct and the theory is very simple, the height is being
increased quite a lot :-)
No top loading at all results in a tapered current distribution from max
at the bottom to zero at the top of the antenna. For a short antenna,
<< 0.1 lambda, the taper is linear. Averaging (integrating) over the
total length gives an effective height exactly equal to half the
physical height.
Adding an INFINITELY big top loading capacitance to a perfectly
conducting ground (infinite radial system) means the effective height is
equal to the actual height of the antenna.
Everything else is in between and not easily modelable (is that a valid
word ?)
If a sufficiently large L is raised to the top of the vertical to self
resonate a practical top load, this behaves as an infinite load. Think
about a series tuned circuit, at just below resonance the impedance
drops quickly and this looks like a rapidly increasing capacitor, (until
actual resonance which then appears as a short circuit or small
resistance). Hence a fully resonant top load equals an infinite top
load ! And current in the vertical section stays constant. Since the
system is now resonant no loading coil is needed at the bottom. And
the effective height has increased and ERP gone up by the square.
In practice the loss resistance of the top loading coil - which now has
to be bigger as the capacitance of the top section, which which it
resonates, is less than top plus vertical, so gives an increased series
resistance over that of a bottom coil alone. Also some bottom L is
still needed to allow tuning over frequency
I must admit, Mike, to being concerned as to why you got a serious
reduction in current, but now you've explained about the change to a
lossier plain wire coil from a Litz wound one that explains everything.
When first on 73kHz I tried putting about 1mH at the top - just a loose
coil of enamelled wire - not really expecting much improvement with a
22mH loading coil. It gave me about 5% reduction in antenna system
resistance over no top loading so had to be more efficient, but no
field strength measurements were ever made. The coil was too big to
hang permanently from the centre of the Tee so got put away and
forgotton until now. In fact the wire formed part of the 137 loading
coil in use now.
May be time to revisit the idea. And I bet my back garden at 10m x
4.8m is smaller than yours Mike :-(
Andy 'JNT
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