> Everything I read told me I should be about 30 Ohms or so
> at the base
> and have a relatively small bandwidth (maybe 30 kHz or
> so). Not
> complaining about having a flat match or the wide
> bandwidth, but I
> have to wonder why I'm so out of spec with this, and if in
> fact I'm
> heating up the ground more than I am pumping a signal into
> the ether.
Peter,
Some of what you are reading is not correct.
First, the bandwidth of a perfect Inverted L at reasonable
height is twice as wide as a dipole made from the same wire
size. This is because the length to diameter is effectively
doubled. You have HALF the antenna length. 30kHz is
unrealistic unless we are doing something foolish with the
ground system like making it small and resonant.
Second, the base resistance in the real world can be all
over the map without any correlation to efficiency. Roy
Lewallen, W7EL, was here several months ago and we measured
a 40 meter vertical under various ground system
configurations. With some smallish buried radial systems we
had over 50 ohms of base impedance, but the efficiency was
the same as with elevated resonant systems having 30 ohms of
base impedance. There was very little correlation between
base impedance and efficiency.
We have a lot of articles floating around that initiate new
theories or reach profound conclusions, but they never
actually measure what we really need to know! There can be
several pages of official looking data with charts and
graphs....but not a single measurement that actually looks
at what the article claims.
It almost seems silly to have to say this, but if we want to
know field strength change we have to measure the field
strength change. We can't do it with antenna or radial
currents, we can't do it with base resistances alone. They
simply don't measure what we need to know.
If we want to know propagation loss we have to know the
transmitter ERP at the desired angle or range of angles and
the overall receiving antenna gain and pattern. We can make
a thousand measurements and none will prove anything unless
we know that. The same is true for transmitter field
strength, we have to measure what we want to know....not
something we guess or assume tells us what we want to know.
Not measuring what we should measure often makes a great
sounding article, but it wastes a tremendous amount of time
for the experimenter and the rest of the world trying to
learn something useful. Unfortunately even our best
publications are full of this type of mistake.
73,
Tom
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