Darrell makes some real good points regarding efficiency.
Many "wideband" or "multiband" antennas claim low SWR over wide
frequency ranges or multiple bands, and in fact they often live up to
that claim. Often the reason they have low SWR over a wide frequency
range is because of high losses. Remember that low reflected power does
not necessarily mean that the power is radiated as RF, it just means
that not much power is being reflected back to your transmitter. Often
the power is being radiated at completely different wavelengths than the
ones you intend to communicate on, such as infrared wavelengths,
otherwise known as heat.
All too often hams seem to be stuck on the idea that low SWR is the
most important thing an antenna system can have. If that were true we
could all just buy quality terminators (dummy loads) and forget about
all that outdoor stuff completely. Some good terminators have very low
SWR from DC to 1 GHz or even higher. How nice it would be to just hook
one of them up to your rig and save all the trouble of putting up antennas.
A good rule of thumb for antennas is "simpler is often more efficient".
(but no so simple as a dummy load) A plain straight piece of wire with
the simplest feed line is probably a more efficient radiator than
anything that includes coils, capacitors, ferrite material, conductors
with current running opposed to the main radiating current (linear
loading). Any of those features are likely to reduce radiation
efficiency, even if they do reduce SWR and make some of you feel better.
Yes, you have to do something make the impedance at the shack end of the
feed line acceptable to your transmitter. Whatever methods are used, if
there are no adjustments necessary to change bands or to cover all of 75
and 80 meters, you should be suspicious that there are high losses
somewhere.
There are some special cases where a simple antenna and feed line may
have the right dimensions to work on two or three bands with no
switching or adjustments to change bands, and it can be efficient too,
at least in some directions on each band. And there are log periodics,
and discone or other "fat" antennas like cage dipoles that can really be
efficient and have a very flat impedance curve over a wide frequency
range. If you have the space and the resources for one of them, go for it!
There are always trade offs to be made. If an antenna system's frequency
or band coverage seems to good to be true, it may really be true,
usually at the expense of poor efficiency on some or all frequencies.
DE N6KB
>
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