[Elecraft] dipole antenna efficiency

[Ron D'Eau Claire]

Absolutely pattern is of significance, Alan, but unless you have a lot of
space, there's not much one can do about the pattern of HF antennas and most
wire antennas will be omni directional at heights most Hams can achieve on
typical lots. 

Even a full-sized 1/2 wave horizontal antenna is essentially omni
directional unless it's at least 1/2 wavelength high in the air! So you need
a 66 foot run in the correct orientation with a 66 foot high supports to
plan a specific pattern with a simple 40 meter dipole. And, even then, it's
very, very broad with a maximum front/side ratio of perhaps one S-unit. Put
that dipole up at a more typical height of 30 feet and its maximum radiation
is absolutely unidirectional and straight up! 

You can squeeze some significant directivity out of lower, smaller antennas
with Yagi Ueda or cubical quad "beam" designs, but it's fairly minimal
unless you can get it up near 1/2 wavelength off of the ground. That's why
most Hams don't bother with "beams" - Yagis or Quads - except on 14 MHz and
above where they can get them up high enough to be effective. 

Perhaps the biggest pattern issue most Hams working at HF can reasonably
tackle is the question of vertical vs. horizontal polarization, and even
then the differences aren't all that great unless the horizontal antenna is
at least 1/3 to 1/2 wavelength above ground or the vertical is over an
excellent ground (e.g. over salt water) that extends for many wavelengths in
at least the important directions. 

The end result is the vertical vs. horizontal question is usually settled
based on the space limitations. That's why few horizontal antennas are found
in use on 160 and 80 meters compared to the other bands.

On the other hand, efficiency drops like a stone as an antenna is made
physically small. That's something we can address, at least in a small way,
by getting clever about making the antenna as large as possible, minimizing
resistive losses in the conductors and minimizing ground losses. 

Sometimes Hams are confused when they see patterns published for very small
antennas such as mobile whips mounted on a car. An azimuth plot will show a
significant pattern suggesting the signal is much stronger in certain
directions. That's true, but the important issue is that directivity is a
measure of inefficiency, not gain. That is, those patterns show the
directions where the losses are slightly lower than in other directions, so
the signal appears somewhat stronger in those directions! Those are exactly
the things we can address: how to make some small improvement in efficiency
that improves the signal. Sometimes we can make our small, low antennas less
INefficient in the most desirable direction. For example, a mobile at the
beach might discover it's easier to work DX across the ocean in that
direction than it is to work someone a few hundred miles away across land in
another direction. That's because the losses over the salt water are less
than over the land, but it's still not a very efficient antenna. 

Bottom line is that it comes down to tackling issues we can do something
about and not fretting over those we can't. We can often do something about
efficiency where we often cannot do something about the pattern. 

Ron AC7AC

 

-----Original Message-----
Interesting discussion. I have a question  regarding this topic I hope the 
more learned in the group cans help clarify.  Although efficiency is
important 
is determining an antenna selection, is antenna  lobe pattern just as 
important? If the lobe pattern did not allow you to say  work DX or to work
a 
particular direction then is efficiency all that important?  Should we look
at the 
desired pattern for a particular operating goal then  consider the antenna 
efficiency?

Alan KB7MBI
Woodinville, WA
FISTS:  5702   CC: 1885   ARS: 582
SKCC: 1988   NAQCC:  058   ARCI: 12141