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June 6th 04, 05:28 AM

On 5 Jun 2004 19:02:56 -0700, (Art Unwin KB9MZ)
wrote:

That says that narrow bandwidth contributes to higher efficiency
which is how I always understood it.(Though many do not accept that)

Hi Art,

When a low bandwidth antenna has very high efficiency (in the range of
95-98 percent), "more" efficient is not very remarkable.

The problem you are encountering is trying to equate bandwidth/Q with
efficiency. There is an old adage: correlation is not causation. A
successful antenna design is by definition lossy = radiation
resistance. You can reduce this loss to zero, boost the Q, make for a
very small bandwidth and come up with a very inefficient antenna.
Same thing goes for Tube finals' tuning. You don't want narrow
bandwidth/high Q because that means no power will leave the
transmitter. Q for the Finals falls between 10 and 15 because the
load must be a loss.

The coil/capacitor that makes up the finals tuning has an "unloaded" Q
that is high, which means that the coil/capacitor in and of itself
does not contribute to the loss, but as a system, Q is relatively low
(by at least a power of 10 if not 20). It would be impossible to
measure the unloaded Q of an antenna because it is necessarily MUCH
larger than a conventional coil/capacitor lumped circuit. Being large
means that it encompasses that loss of radiation resistance.

There is, of course, a hazy area where antennas get very small, or
finals get very big. The 1M loops sold as "magnetic" loops certainly
approach that, and yet the ratio of radiation resistance and loss
resistance in the low bands is clearly a loser proportion for this
breed. If you have no choice, even 5% efficiency is gold bond stuff.
However, simply because it has high Q does not elevate its poor
performance.

This returns us to that crude triad of pick any two characteristics
and leave the third for the vultures.

73's
Richard Clark, KB7QHC

73's
Richard Clark, KB7QHC