On 21 Sep 2006 19:19:52 -0700, "art" wrote:


Tom Ring wrote:
art wrote:

When one looks at a.radiating array pattern one can see that the yagi
is very inefficient. Does anybody know of the relative volume
Art


Art

Yagis, when made of almost anything but something like nichrome wire,
are very efficient. Aluminum element yagis run in the high 90's of
percent efficiency when properly designed.

Hmmmmm I would question your logic on that figure. since I am not aware
of the normal ratio between actual resistance versus radiation
resistance which would point to
the manufacture of radiation energy relative to the total energy input
however my question relates to the efficient radiation to the
requirement at hand
Art


Art, from Wikipedia and me, some definition that might help you
express your problem in conventional terms:

Directivity is a property of the radiation pattern produced by an
antenna. It is defined as the ratio of the power radiated in a given
direction to the average of the power radiated in all directions.

Gain is the product of the efficiency of the antenna and the
directivity.

Efficiency is the ratio of total power radiated to power into the
antenna.

Efficiency of practical Yagis is very high as Tom has told you. Loaded
/ trapped Yagis are not so efficient due partly to losses in the
loading coils / traps. This is the same issue that commonly arises
with shortened antennas.

It is quite wrong to say in general "that the yagi is very
inefficient".

You seem to be talking HF, the azimuth beamwidth of most practical HF
Yagis is so large that you are unlikely to notice much difference in
gain within 3 deg of boresight, so on a fixed heading you would expect
to cover the 10 deg to 4 deg target area comfortably with little
variation in gain.

If you want to maximise the transmitted signal for that specific path,
you should minimise losses (eg avoid lossy traps and coils, feed
system losses etc), increase directivity (more elements, better
design), pay attention to the desired path elevation (eg mounting
height of the antenna).

Owen
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