Greg Fuchs wrote:
> Of course Frans, you are completely right. Bending an antenna will change
> its impedance. Looking at the impedance curve diagram for a half-wave
> antenna, it shows that the impedance is very high out at the tips, and very
> low at the feedline. So I would expect that if most of the antenna is
> straight, and only the tips are bent slightly, the impedance change will be
> minimal.
We are talking about impedance, not resistance. A bend tip will interact
with the rest of the antenna, and the impedance change may be much more.
> Then the VSWR is ( 1 + .18699) / (1 - .18699) = 1.46, or about 1.5
Ok, I see what you mean. This may be right. Keep in mind though that in
a Europa, a dipole will be anything except the theoretical impedance.
There is too much stuff around it. Most of the stuff around it will
lower the impedance, so 50 ohms will be quite close in most cases. In my
ship, it is close to 50 ohms.
> Well, according to this book you can:
>
> In MODERN ELECTRONIC COMMUNICATION by Gary M. Miller, circa 1978,
> Go to page 422, and it shows a tank circuit in between the two dipoles, at
> the feedline.
Well, it is worth some investigation. Never heard about it.
> The point I was trying to make here, but did not quite describe it well
> enough, is that the overall height of the Marconi antenna is only half the
> equivalent overall height of the Hertz antenna. That might be where an
> advantage is.
>
> Since 1/2 wavelength is better than 1/4 wavelength, of course 1 wavelength
> is better than 1/2 wavelength, for antenna heights.
No, this is not true. (Assuming that you mean antenna length, and not
antenna height). A dipole (whether that be two 1/4 elements or 1/4
elements and a ground plane) is the best omnidirectional radiator. 100%
of the energy is radiated. A longer antenna has no benefits. Sure, you
can bundle the signal to certain directions, but that will be at the
cost of the radiation of other directions. Compare it with a light bulb.
Want to get more light somewhere? Can be done, but at the cost of light
at other places.
Now, to explain again: a dipole (as used in the Europa's) is exactly the
same as a 1/4 antenna on a ground plane. In a dipole, one of the 1/4
elements IS the ground plane. It is the same, and the radiation pattern
and efficiency is therefor similar.
I get the impression that you are reading books about short wave
communications. A lot of what you write would be correct for short wave.
But in airplanes, we are talking VHF. And we want an omnidirectional
radiation pattern. A standard dipole is perfect for this.
Further: what is the purpose anyway? The limit is the line-of-sight, and
we can reach that with a standard dipole. Nobody is going to send a
QSL-card or giving awards if you manage to crank out a stronger signal
anyway. The rest is airplane logic: keep it as light weight as possible,
and as simple (reliable) as possible.
Frans
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