On Mon, 23 Feb 2004 16:51:43 GMT, "aunwin"
wrote:
You can't ever discard the factor Q in any discusion with respect to antenna
efficiency or any calculation for that matter.
Q is intrinsic in any calculation that determines efficiency especialy when
considering what the object of an antenna is.
Hi Art,
Q is NOT the arbiter of all that is efficient. In fact high Q can
lead to very poor communication links.
The most efficient and simplest antenna, the dipole, exhibits a very
low Q for the very obvious reason: it is built to lose power by
design. The loss to radiation resistance, Rr, is indistinguishable to
Ohmic loss when computing Q. This in itself directly states that
maximizing Q is inimical to transmitting power if you do not separate
out the two losses.
Terman treats this inferentially in his discussion of Power Amplifiers
and their Plate Tank's Q. To select one that exhibits too high a
value is to risk very poor operation. He suggests that a Q of 8 to 15
is a reasonable value. This confounds many who seek to peak their
designs and fail to come to terms with unloaded and loaded Q
valuations.
It is the Q's relation to power loss to heat that makes the
difference, not to the power curve in isolation of this loss. Small
antennas suffer from high Q for this very reason - too little thought
is given to the radiation resistance's correlation to the Ohmic loss
of the system. As Richard has pointed out, one Ohm loss within the
structure is hardly a loss leader for an antenna with 73 Ohms Rr. To
achieve 50% efficiency requires your antenna to exhibit less than this
same value of Ohmic loss (however, let's be generous in comparisons to
1/1000th that value). The rage of "High Q" antennas is in various
loops of small diameters.
Let's look at small loops' Rr for various sizes in tabulated form:
Fo 1M diameter Efficiency with 1 mOhm loss
160M 29 µOhms 2.8%
80M 500 µOhms 33%
60M 1.5 mOhms 60%
40M 7.5 mOhms 88%
30M 24 mOhms 96%
20M 120 mOhms 99%
Let's examine the validity of that generous assignment of 1 mOhm loss
and see if it is reasonably warranted. Skin effect is the single
largest contributor to this loss as a source (aside from poor
construction techniques). Using the 1M diameter loop as being a
practically sized construction, and if were using 2.54cM diameter
copper wire/tubing we find:
Fo skin effect loss
160M 13.8 mOhms
80M 20 mOhms
60M 23 mOhms
40M 28 mOhms
30M 33 mOhms
20M 39 mOhms
Well, 1 mOhm was too generous and if we look at those loops' Rr once
again against a robust, thick loop element:
Fo 1M diameter Efficiency with skin effect loss
160M 29 µOhms 0.2%
80M 500 µOhms 2.4%
60M 1.5 mOhms 6%
40M 7.5 mOhms 21%
30M 24 mOhms 42%
20M 120 mOhms 75%
These "High Q" loops are NOT efficient, they are convenient. The two
terms are not the same at all and yet in common discussion they are
confused to mean the same thing.
I would point out further, that commercial vendors do not use 1 inch
tubing (as the numbers above would force to even bigger conductors).
Instead they use much larger tubing; but even here, one vendor uses a
flat strap which is a very poor substitute as the skin resistance is
defined at the edges and the face of the flat strap is far less
conductive. The physics of conduction forces current to seek the
smallest radius (the edge) to the exclusion of the broad surface (this
is why we use tubular conductors and not flat ones).
I will leave it to the student to reverse-engineer the required
conductor size to obtain the same 1 mOhm results of the first table
above. Even then, it will be seen that the common dipole still reigns
supreme in efficiency.
73's
Richard Clark, KB7QHC
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