Dear Richard, LF Group,
1) T antenna. Am I right in thinking that in the classical T
configuration, the horizontal part of the aerial is only there to add
capacity - i.e. it makes no contribution to the groundwave? If so, I
assume that I can use very thin wire for the T?
More or less - The idea with all the top loaded vertical structures is to
maximise the averaged current component flowing in a vertical direction. If
you have a plain vertical element, the current tapers to zero at the top,
but if you add substantial top loading, the current at the top of the
vertical section is only slightly less than at the bottom, since most of the
current returns to ground via the distributed capacitance of the top loading
wires, rather than that of the vertical section. Thus a top loaded vertical
has greater "effective height" than a plain vertical. The maximum increase
in effective height is theoretically a factor of two. When the length of
wire in the top load is more than about 5 times the vertical section, you
are getting quite close to this - so there is not much benefit to very long
top loading wires in this context.
Because current is flowing in all the wire, resistive losses in the wire
will contribute to the overall loss resistance of the antenna. But in
practice, wire strong enough to support itself will only add a few ohms to
the loss resistance, probably insignificant compared to other losses (of
course, if the wire is iron or nichrome or something, this might not be
true...).
2) In the more likely configuration with "horizontal" wires actually
acting as an inverted V can I still use very thin wire without reducing
performance?
The resistance of the wire will again be insignificant. If the top loading
wires slope downwards, this gives rise to a downwards vertical component of
the current that partly cancels out the upwards component in the vertical
element, which is counterproductive. So for sloping top-loading wires, there
is a trade-off between increasing the length and capacitance, and reducing
the effective height. Hence the "average height" of the wires is important.
Most of the time, this will mean if the lower ends of the sloping wires are
at least half the height of the higest point of the antenna, they will be
beneficial. But wires coming close to the ground will be counter-productive.
3) Is there any benefit to be gained by having an inductor at the top of
the vertical section of the T? If so, will the loss of the inductor be
relevant if the part of the antenna above it (the T wires) don't
contribute to useful radiation?
In principle, the elevated loading coil will reduce the voltage on the
vertical section, reducing the current returning to ground through the
distributed capacitance of the vertical section, so increasing the current
at the top as with top loading. Also, the lower voltage means reduction in
loss produced by objects like trees and buildings close to the vertical
section. In practice, this is offset by the difficulty in supporting and
tuning the elevated coil. In your situation, where quite a lot of top
loading is feasible, and the antenna is out in the open, the advantantages
will be small. Most of the antenna current flows through the elevated coil,
and the TX still has to supply the resulting power dissipated, so the
inductor loss is certainly relevant. However, it can be made quite small.
Cheers, Jim Moritz
73 de M0BMU
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