Posted by
Don Wilhelm-4 on
Jan 05, 2008; 1:12am
URL: http://elecraft.85.s1.nabble.com/dipole-antenna-efficiency-tp458071p458076.html
Folks,
Not to dispute the info that Ron has provided, but to add a bit to it ---
Let me throw in my $.02 worth in this. Any antenna will radiate if you
can feed power to it, and it will radiate all the power that is fed to
it, but there is loss to consider too. If the radiator and the
transmission line have zero loss, then all your transmitter power will
go into the antenna and be radiated - period, but the practical world is
not quite so simple, real components have real loss - the challenge is
to make the loss a minimum given the circumstances that exist for your
installation.
OK, so zero loss is the ideal condition. In practice, we have resistive
and other losses to contend with. When the load impedance of an antenna
falls below 20 ohms or so, I begin to worry about losses because the
radiation resistance is a significant fraction of the total resistance.
A loss of 10 ohms is insignificant with a radiation resistance of 100
ohms, but a loss of 10 ohms with a radiation resistance of 5 ohms is
indeed significant. - that is especially true of verticals where ground
loss is a significant factor. A higher feedpoint impedance can be dealt
with by using a matching network of some sort, but low feedpoint
impednces need to be given special attention so that the line resistance
(and the ground resistance in the case of verticals) is minimized.
Remember that low impedances mean high currents, and given a fixed
resistance, the higher currents mean greater losses - it is all in the
physics - you cannot fool Mother Nature.
As a side note - window line performs well and with low loss when dry,
but when it is wet, the losses in window line can become severe. If you
can arrange true open wire feeders, the loss should be low, dry or wet.
73,
Don W3FPR
.
Ron D'Eau Claire wrote:
> That's a "linearly loaded" doublet. The zig-zag provides a little more
> inductive reactance to help offset the capacitive reactance caused by the
> overall length being short. Studies suggest that the resistive losses in the
> wire is somewhat less using the "zig zag" than using loading coils each side
> of center. So the linear loaded antenna is at least somewhat more efficient.
>
>
> Since that antenna is fed with a ladder line, the length is unimportant
> anyway, except to choose a combination of radiator and feeder length that
> provides a reasonable impedance on each band. If you have a wide-range
> tuner, that's not important either.
>
> You can skip the linear loading and simply put up a center fed wire fed with
> high-impedance (400 to 600 ohm), low loss line. There will be almost no loss
> in "gain" over a 1/2 wave radiator as long as the radiator segment is at
> least 1/4 wave long, end to end, plus another 1/8 wavelength in the feeders.
> That adds up to a total of 1/2 wave of wire counting the two wires in the
> feeder and the radiator.
>
>
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