That says that narrow bandwidth contributes to higher efficiency
which is how I always understood it.(Though many do not accept that)
But as you say it is a shorthand type statement.
My antenna on 160 is very narrow banded at any particular setting or
frequency
( two loops and a short dipole coupled in tight cluster form),when
modelled ,shows part of the current curve breaking out into a sino
soidal oscillation (no phase change) for portions of the antenna.
Such modelling, ofcourse, requires a large amount of points per unit
length for high accuracy and the occillation would probably not show
up visually if calculation points were reduced.
It does not seem to affect things in practice on the air but I have
often wondered what the consequences would be if the bandwidth was
narrowed even more and the current oscillation possibly propagate over
all the of the antenna !
With the current taking on an occillation it would suggest changes in
radiation

Art



Richard Clark wrote in message . ..
On 4 Jun 2004 10:44:01 -0700, (Art Unwin KB9MZ)
wrote:
"According to current
theory, you have to give up one of the three-size, efficiency, or
bandwidth-to achieve any one of the other two."

snip
Is the above statement correct ?

Hi Art,

In a crude and shorthand way, yes. This is why your small 160M
vertical dipole is up to 15 to 17dB below performance in comparison to
a full size one. All common legacy for CFAs, EHs, fractals, and the
rest of this ilk that come down the pike.

73's
Richard Clark, KB7QHC

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