| Anatomy of an
Antennas commonly thought of by most people as "just a piece of wire" are anything but! The appearingly simple device is in actuality the by-product of one of the most sophisticated and complex sciences known as "Electromagnetics". To be classified as a "real antenna" the device must be properly designed:
1. To have a resonance at a specific frequency, band of frequencies or group of frequency bands.
2. To assure that the electromagnetic field created by the antenna is correctly positioned for maximum performance.
3. To present a correct feed impedance to the scanner or receiver.
While sounding terribly complex, this is really quite simple to understand. As a matter of fact it is considerably easier to understand than in many instances to accomplish. Taking things point by point:
Radio signals are electromagnetic fields formed by the energy radiating from the antenna of the source transmitter. For the scanner listener, these sources most usually are the police and fire base stations, trucks,
cruisers, etc. These sources are operating on frequencies assigned to them by the Federal Communications Commission. Frequency is a way of describing the dimensions of an electromagnetic wave. Let's give it a try!
The Podunk Police Department is assigned the frequency of 155 MHz. [155 Megahertz, le, 155 million hertz.] By the way, megahertz used to be called megacycles, but a few years ago just when everybody had gotten real familiar with cycles, this was changed to hertz to honor the German scientist
Heinrich Hertz for his accomplishments in electromagnetics and related fields. This information should provide some comfort to those who feel overlooked as Heinrich died about 100 years ago.
OK- The equation: [A real brainburner.]
984 X 12 /Frequency in MHz=length of the wave in inches
For Podunk 984 x 12 / 155 = 76.180 inches.
The most common antenna configuration utilized for transmission and reception is a length of 1/2 wave.
492 X 12 / Frequency in MHz=1/2 wavelength
So for Podunk this would be 76.180 / 2=38 inches.
When an antenna is designed according to this definition it is said to be resonant. No! It is not exactly as simple as this, but this definition is fundamentally true, and it is correct that an antenna that is resonant is significantly more efficient than anything that is not. It is also useful in understanding the elementary antenna criteria, and that the
"whip" and the "duck" are not resonant at more than one frequency. So If your duck is 6" long and your whip is 15" long, they AIN'T working too good on the Podunk frequency.
Got it ? ?
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