David Alkire Smith W8YZ
530 Hollywood Drive
Monroe, Mi 48161
(313) 242-5589
SUBJECT: Synopsis of transmission line and antenna
demonstration.
While the demonstration has been done before audiences ranging
from Cub Scout packs to college electrical engineering classes in wave
propagation, it was originally developed for demonstration before amateur radio
groups.
Physically, the transmission line consists of three four-foot
long sections of 200 ohm balanced line mounted on wooden bases. These sections
are connected together to form 12 feet of transmission line. A table or tables
at least 12 feet long is needed to support the transmission line. The line is
terminated in various antennas and loads during the demonstration.
Twenty-five watts of RF power in the amateur two-meter band is
used. Incandescent lamps are used as indicators, which include voltage,
current, forward and reflected power.
A three element yagi
is used to show antenna gain, directivity, polarization as well as to serve as
a field strength indicator using an incandescent lamp. Ten feet or more of open
space is needed at the end of the transmission line.
Forty five minutes is
required for a typical demonstration before amateur radio groups. This includes
a brief question and answer period. This permits time for a rather superficial
demonstration of antenna gain, polarization, directivity as well as the
properties of open and shorted quarter and half wave stubs. The effect of
proper as well as improper transmission line termination is shown together with
standing waves and the resultant reflected power. Stub matching is shown
together with matching using adjustable capacitance and inductance. Matching by
transformer action and the loss of phase velocity in a dielectric are also
shown. The antenna is presented as an impedance matching device. The voltage
relationships on a very short dipole, a half wave dipole and full wave dipole
are shown.
Both the title and
the content of the demonstration can be changed to best suit the interests of
the group before whom the demonstration is being presented. Several sample
titles together with brief descriptions are given.
1. STANDING UP TO STANDING WAVES:<):> How
important is it to prevent standing waves on a transmission line? Are standing
waves present on an antenna. See actual standing waves and how to deal with
them.
2. HOW TO TRANSFER MAXIMUM POWER TO YOUR
ANTENNA:<):> Matching techniques are shown using open wire line.
Incandescent lamps are used as SWR and field strength indicators. A discussion
of antenna problems will follow.
3. ANTENNA FORUM:<):> Dave W8YZ will use
his antenna and transmission line demonstration that has been shown at Dayton
and several ARRL conventions as a springboard for a discussion of antennas and
antenna problems
4. HOW TO LOAD UP A WET NOODLE:<):> Does
a wet noodle make an effective antenna? See Dave W8YZ actually use a wet noodle
for an antenna. This demonstration of antenna matching techniques will be
followed by a discussion of actual antenna problems.
5. A GRAPHIC DEMONSTRATION OF ANTENNA AND
TRANSMISSION LINE PHENOMENA:<):> The charts and diagrams in textbooks
come alive in this demonstration of transmission line phenomena using 12 feet
of 200 ohm balanced line. The indicating instruments show relative transmission
line voltage, current, forward power and reflected power with the use of
incandescent lamps. The use of transmission line sections as circuit elements
which correspond to lumped constants is demonstrated. Stub and slug matching
techniques will be demonstrated and discussed as will the loss of phase
velocity in a dielectric. The antenna as
a impedance transformer is treated.
As you can see, the
demonstration is quite versatile. It can be tailored to suit the needs of a
group. If you have any questions, feel free to contact me.
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