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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