On Wed, 16 Jul 2003 18:20:22 -0700, Roy Lewallen
wrote:

An antenna can reasonably be viewed as a transducer. It converts the
electrical energy entering it into electromagnetic energy -- fields. As
is the case for any transducer, the stuff coming out is different than
the stuff going in. Think in terms of an audio speaker, which converts
electrical energy into sound waves, and you'll be on the right track.

Roy:


Great analogy!

The characteristic acoustic impedance of air (standard temp &
pressure) is about 413 Rayleighs (or Pascal-Seconds/cubic meter).

Do we worry about matching 8 ohms of electrical speaker impedance to
413 Rayleighs? C.f. Paul Klipsch and the Horn speaker.

I wonder if much of the antenna radiation resitance/Tline
impedance/reflection/intrisnic impedance of free space confusion stems
from use of the same words to describe things that may be modeled
mathmetically identically, but have different physical modalities?

In heat sink calculations, for example, we use "thermal resistance"
and an Ohm's law model but few would confuse ohms of resistance with
degrees C/watt.


Jack K8ZOA

On Thu, 17 Jul 2003 07:49:40 -0400, Jack Smith
wrote:
Do we worry about matching 8 ohms of electrical speaker impedance to
413 Rayleighs? C.f. Paul Klipsch and the Horn speaker.


Hi Jack,

Someone must, or we would see more 600 Ohm speakers.

It is facile by half to simply accept the end product of design and
anoint it as an example of a general solution. There are many
antennas that are NOT 50 Ohms. What they ARE is actually of no
consequence except in the sense of efficiency and mission (the antenna
cares not a whit about either).

I have many examples of 2 Ohm antennas (and lower) and of 600 Ohm
antennas (and easily higher) and ALL can be induced to radiate all of
the power applied to them. The distinguishing factor across the board
is that each hi-Z antenna presents similar, physical characteristics
to all other hi-Z antennas; and each low-Z antenna presents similar,
physical characteristics to all other low-Z antennas. All couple
power to the same load of the æther. Clearly impedance and size are
correlated and it is up to the designer to accommodate losses to
achieve similar performance. The same statement is equally applicable
to speakers of any impedance.

Is the antenna transforming its Z to that of the æther? Of course it
is just as the speaker is. Are they both transducers? Of course they
are when transducer is applied loosely (but strictly speaking - no).
Injecting this notion that transducers are a class distinct from
transformers is simply myopic to force an argument. No sooner is the
notion introduced than we find the correlative transducer of the
receive antenna introduced to recover the power - now transformed (and
very inefficiently one might add). The remainder of that power
becomes part of the background noise of the cosmos (far more of it
than is ever recovered for actual use).

Transducers, as a class, are far more prone to the loss through
resistance than transformers - by definition. The speaker is feeding
a lossy medium of air, and the sonar is feeding the less lossy medium
of water. The difference is in the compression characteristics that
turns power into heat. Core loss of the transformer is not due to
compression, but is a direct analog (and electrons bumping into each
other and atoms does constitute a form of compressive loss). There is
no loss in space/æther but neither are there any phonons, the classic
transport of transducer emission and coupling. If an antenna is to
qualify as transducer, it must be with the proviso that it is
distinctly different from every other transducer in lacking the common
transport mechanism of phonons. This is like say walking is a form of
mass transportation if you simply ignore the word mass. To support
these specious forms requires enormous exaggerations.

Another transducer available as a common example (or perhaps not for
less well-heeled equipment) is found in the Collins mechanical filter
for interstage coupling. It has both input and output transducers
that couple the mechanical (and thus heat-prone) energy into
nickel-steel resonant disks. Nickel-steel is obviously less
compressive than either air or water, and exhibits far higher Q (which
is a factor of both antennas and transducers - in their medium) to the
advantage of the circuit. To any bench tech working on receivers,
they would unhesitatingly call these IF Transformers.

Does an antenna "transform" any Z to another Z? The process is
obviously performed with concomitant and equivalent issues of
efficiency regardless of the term inserted between quotes. Does the
term substitution bring any change, or does it correct any error? No.
It is a tautology to suggest that "transducer" is appropriate when
every presumption finds a corresponding "transducer" necessitated by
the force of discussing fields (how does one know these fields exist
without the absolute necessity of completing the transformer action?).
One may "know" in the purely abstract sense, but such knowledge
through the centuries has rarely preceded the actuality of observation
in the real transformed world.

The distinction between transduction and transformation does not
preclude the sense of an antenna serving as a bridge between two
system impedances. Neither hi-Z nor low-Z structures have a
stranglehold on design, except through economy. We commonly employ
very low-Z sources (transistors) to feed modest-Z loads (a common
quarterwave antenna). The economic factor of that load (a quarterwave
at 160M) is sometimes unsupportable and yet we find very few short,
low-Z antennas designed with direct feed from the same low-Z
transistor. Economy again forces some form of transformation (I would
hesitate to call a Tuner a transducer) in that the commercial market
sees very little sense in building low-Z sources for an incredibly
small niche who would refuse to pay the price. Instead, commercial
design accommodates to one Z and expects the user to transform it
along the way. The same logic extends to, and through the antenna.

73's
Richard Clark, KB7QHC