Wayne Watson wrote:
I hardly know where to start with this topic. If one picks up some of
the fairly popular (available?) books on the matter, the authors
invariably start throwing different types of antennas at the reader,
yagi, helical, dipole, folded dipole, parabolic, loop, dish,
microwave, quads, etc. For example, I'm looking at an older book on
the topic I bought some 20 years ago, The Radio Amateur Handbook by
Orr and Cowan. The book is basically for builders. Many such books
are. What about the underlying methodology behind this? More
generally, here's my question.

I would guess that in the beginning (late 1800s) the simple dipole
was it. As years passed, the complexity of antennas has increased.
What was the driving force for these changes?


Since the beginning of radio, antenna design has been driven mostly by
specific needs, to name just a very few examples:

-- By broadcasters to increase their coverage areas
-- By military users:
to increase portability and range
to decrease detectable emissions in some directions
to allow broadband performance for frequency hopping
-- By satellite system designers to concentrate limited energy in
particular regions.
-- By cell phone companies to provide coverage to well-defined regions

The list is endless.

Take a trip to your local library and get an index to the I.R.E. (now
the IEEE) Transactions on Antennas and Propagation. You'll see that many
advances were made in the '20s and '30s in AM broadcast antennas, in the
'40s and '50s in aircraft antennas, in the '60s and '70s in satellite
antennas. Much of today's development work involves compact antennas for
the wireless networks now proliferating.

Antennas provide a limitless pool of tradeoffs involving size, cost,
ruggedness, and a large handful of performance characteristics such as
directionality, gain, and bandwidth. This pretty much guarantees that
new designs will continue to be created.

For example, how did the inventor of the Yagi (Yagi-Uda) ever dream
up the idea for the antenna? Was it the application of theory or did
he just get lucky?

How is any creative design accomplished? How did Armstrong come up with
the idea for FM, Watt for the steam engine? I've spent most of my career
doing circuit design, and it requires a deep knowledge of theory, but
also involves a creative synthesis not unlike what an artist has in
deciding what to paint or an architect in deciding what form a building
will take.

In fact, is there some underlying theory that drives the design of
antennas? For example, the computation of radiation patterns.

You're confusing design and analysis. Design is driven primarily by a
need for a particular set of performance parameters. Analysis is done by
computation. Analysis is an important part of the design process, in
that a design, once created, is analyzed to see if and how well it meets
design goals. The design is then often modified and re-analyzed many
times until the goal is reached, the design abandoned, or the goal
redefined. And yes, indeed, there's solid theory underlying antenna
operation.

I'm sure these days the computer would be an aid, but what theory and
application drove the development of varied designs before 1960?

Hopefully what I've said above has answered this. A lot more physical
modeling and experimentation were required before computers were
available, but design was still driven by need, and theory hasn't
changed significantly for many decades.

There's no magic computer program that you can put some numbers into and
out comes the optimum aircraft design, or bridge, or car, or house.
Antennas are no different. Computers can be used to optimize a
particular class of antenna (e.g., Yagi or patch) for a particular set
of performance criteria, just as they can be used to fine-tune an
aircraft or bridge once the basic structure is designed. But not to
design an antenna from nothing.

When did Maxwell's equations seriously get used for this?

They were used long ago as the basis of equations more directly
applicable to antenna analysis, and those equations are still used by
modern computer programs. In that sense, Maxwell's equations are still
being used, although not directly.

What suggested a tin can could become an antenna?

It's obvious to anyone who understands the most basic of principles. Any
conductor can act as an antenna. A tin can is a conductor.

How did anyone think up the idea of a microwave antenna?

Actually, some of the first experiments with radio waves by Heinrich
Hertz were done in the microwave region, so some of the very first
antennas were microwave antennas. As for modern microwave antennas, once
you have microwave energy (which first became available at high power
levels with the invention of the cavity magnetron in 1940), the
need for an antenna becomes obvious.

I would think that in the case of antennas that are used for
different parts of the EM spectrum a driving force would be the
consideration of the wave itself. For example, it would seem unlikely
an x-ray antenna (I believe there is such a thing on one of the
space satellites used in astronomy) would be anything like one used
to receive TV. Certainly the 'antenna' to collect visible light is
different than that for AM radio.

All antennas obey the same fundamental physical laws. But you're correct
that the wavelength of the signal to be transmitted or
received plays a big role in determining which antenna designs are
practical and optimum. That's just one of the many factors that have to
be considered when designing an antenna. All bridges obey the same
physical laws, but the optimum design for a bridge crossing a creek is
quite different from one crossing the Golden Gate.

In fact, if you change "antenna" to "bridge" in your questions, you'll
probably find most of the answers to be pretty obvious.

Roy Lewallen, W7EL