Dear Richard, LF Group,
Not a great success. Tried a 200ft inverted L with 7m vertical section fed
against 3 radials each between 50 and 100ft long. The final 100ft was very
low - within 1 metre of the ground.
(Mal seems to have started something here - I have always thought of an
inverted L as having a roughly horizontal span, rather than being a sloping
wire!) To get the maximum effective height, radiation resistance and
therefore efficiency, the important dimension of the antenna is height -
essentially the average height of all the wire making up the antenna. The
long section at very low height is therefore counterproductive - it will
reduce the effective height, and also increase the loss resistance. So it
would be better to eliminate the last 100ft or more, and increase the
inductance of the loading coil. If it is necessary to have that overall
length to reach an available fixing point or something, use non-conducting
string. Also, with a single span of wire and a single mast, you will
probably get greater average height if you support the wire in the middle in
an inverted-V shape.
I am not sure about the radials - there is a similar length of wire in the
radials as there is in the antenna itself, so you could expect a significant
RF voltage between the radials and the actual ground, which would not help
matching or losses. Longer radials, more of them, or ground rods would
probably be a good idea.
Did not succeeded in tuning aerial up in link configuration. Tried tapping
the feeder at various points on the main coil but this reduced the Q
dramatically
The tapped loading coil type of antenna tuner with the TX connected to a
low-impedance tap is one of those devices that looks appealingly simple, but
isn't! The impedance match depends on the inductance of the complete loading
coil, the inductance of the tapped section and the mutual inductance between
the tapped section and the rest of the coil. These in turn depend on the
shape, size and winding pitch of the coil as well as the number of turns. I
made a spreadsheet that would calculate the impedance match for a tapped
loading coil (downloadable from http://www.wireless.org.uk/features.htm),
but suffice it to say that there are limits to the range of resistance that
can be matched with a particular coil geometry. By "link configuration" I
assume you mean the loading coil has a low impedance link winding over it,
connected to the TX. This is similar to the tapped coil, but adds further
variables into the mix, due to the different size and pitch of the link
coil. I have not done the calculations for the link case, which will be
similar to the tapped coil, but generally the mutual inductance will be
less, which will reduce the maximum resistance that can be matched. What all
this means is that it is quite possible that your coil has the correct
inductance, but at the same time it is impossible to match the antenna
resistance using it. This is likely if the antenna loss resistance is high;
you need a "short, fat" loading coil (large diameter with fine winding
pitch) to match a high resistance. I prefer to use a seperate loading coil
and matching transformer; resistance and reactance are then almost
independent of each other, and you only have to worry about the inductance
of the coil and the turns ratio of the transformer.
Band was still noisy even on relatively remote hilltop with horrible
"line-timebase" type burble drifting across band.
This is a problem most of us have had - several portable expeditions have
put out big signals, but not been able to receive much. I have spent much
more time trying to get good reception than setting up TX antennas, and
often it is not possible to get good transmission and reception with the
same antenna. There is always a temptation to try to get QRV as soon as
possible, but a bit of reconnaisance with a receiver first is always a good
idea.
Hope to hear you /P soon,
Cheers, Jim Moritz
73 de M0BMU
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