[TenTec] Re: Rudimentary SWR question...

[Dr. Gerald N. Johnson, electrical engineer]

The organ pipe is definitely a resonator. Excited by an air "blast" the
turbulence of the air makes broad band noise which the organ pipe
filters to a pleasing tone. 

Conversely we are not driving the antenna with wide band noise (except
in the case of UWB now coming into vogue to destroy narrow band
microwave communications) but with a narrow spectrum, nearly pure tone
and we use the antenna to match the impedance of the radio to that of
space. The horn at microwave or at audio is an example of that. (Same
boundary conditions, same differential equations, same solutions). In
the case of the horn speaker the driver has a small area but quite a bit
of motion and lots of thrust for that motion so it moves a small amount
of air at the horn throat which the horn then impedance matches to the
acoustical space. A horn antenna or an exponential flare of a balanced
transmission can do the same matching a low impedance power source to
377 ohms of free space. Horns do introduce more directivity than their
exciters.

Then yagis work with an array of phase shifting elements to make a slow
wave structure to combine that impedance matching with desirable
directivity.

Reflector antennas such as parabolas of revolution work the same for
audio, RF, and optical wavelengths, so long as the reflector is more
than a few wavelengths across. And still the feed element works to
provide an impedance match through the reflecting surface from the drive
source (or the receiver input) to free space while illuminating (or
seeing in the receive case) as much of the reflecting surface as
possible with as little spill over as possible.

The resonant organ pipe can be quarter wave and odd multiples or half
wave and even multiples. The first if the end is closed, the second if
the end is open. And its the ratio of those harmonics to the fundamental
that give the organ pipe a more pleasing sound than a steam whistle or a
pure sinewave synthesizer. Modified by the bandwidth of the resonator.

73, Jerry, K0CQ

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.