What is an antenna? A length of wire is a basic antenna; but it is often associated in being a metal rod placed in an open area to receive and sometimes transmit as well. That antenna is making the receiver sensitive to micro-voltages at various frequencies that have been transmitted from various locations. Sometimes it is receiving commercial broadcast, or internet data, or even television broadcasts. But what they all share in common is the ability to receive and/or transmit information by weak electrical power over large distances at a given frequency.
An antenna is nothing more than a given length of conductive material that takes these weak electrical impulses and conducts them down into the radio. It is the radio's purpose to sample a given frequency by discriminating against all other electrical impulses/frequencies that are also present. After it picks out any one frequency, it then decodes it electrically back into sound waves thru a series of special circuits that can be understood by us. Of course it doesn't have to be a radio - a television or computer modem is doing much the same thing. But for the purpose of this article, we will consider radios as the basic medium.
It is one thing to have an antenna that receives, and another to have one both receive and transmit from the same antenna. The requirements are different. Just about any length of wire can pick up many different and random frequencies. But to transmit on that same wire or rod; it needs to be a resonant length
of that frequency. Any one 'cycle' mentioned is the full distance that it takes for any given frequency to go electrically from "+" to "-" just once!
"Frequency' and 'cycle' are the same thing. Resonant Peaks...
The most commonly used lengths are 1/4, 1/2, 5/8, 3/4, 7/8, or even one full wavelength of any given frequency. Any frequency has a wavelength in which there is a sharp resonant peaks associated with it. A given frequency - such as 28 megahertz - has a known wavelength of 10 meters in length. Which is roughly 33 ft
for a full cycle. So, a 10 meter antenna will need to be 1/4 to 7/8 in length of 33 feet. A 1/2 wave antenna will be 1/2 of that 33 feet - which is approximately 16 1/2 feet in length - as an example. A 1/4 wave antenna will be 1/4 of that 33 ft
- which is 8 1/4 feet in length. And so on. You can figure out the most commonly used resonant
points of any antenna - by knowing the wave length being used and the desired wave interval (1/4, 1/2, 5/8, and so on). For the purposes of transmitting properly, the antenna must
be resonant on one of those previously above mentioned fractions - to transmit the energy away from the radio and into the antenna to be heard miles away effectively - for any given frequency. 'Resonant Antenna' definition:
"An antenna for which there is a sharp peak in the power radiated or intercepted by the antenna at a certain frequency, at which electric currents in the antenna form a standing-wave pattern." (Thanks 'Doc'!)
As frequency changes; then so does the length of that wave.
As the frequency increases; the wave length becomes shorter and shorter.
As the frequency decreases; the wavelength becomes longer and longer.
At 144 mhz - the length is 2 meters for a single cycle.
At 3.2 mhz - the length is 80 meters for a single cycle.
Let's say that we are using a CB
radio. Let's say that it is on channel 20 - which is 27.205 megahertz
. The radio can both receive and transmit on that frequency. "Megahertz' in this case - means that the frequency is changing polarity ("+ to -") 27,205,000 times a second as it travels at the speed of light. When we transmit; it will need to do this efficiently so that all of the electrical power that has been converted into electrical frequency out of the radio and on to the antenna. It is the lack of efficiency that makes any antenna a poor transmitting candidate.
The antenna - more than any other component - is most responsible for making the radio effective. Not the other way around. Even a poor quality radio transmitter can be maximized by having an antenna if it is very efficient and resonant. If the radio does not have this efficiency when transmitting, it can also damage the radio. The electrical energy will be reflected back into the radio's components and cause damage because it cannot find the antenna resonant enough. So, we must maximize the antenna's efficiency and be sure that it is tuned correctly.
Both an antennas resistance to flow frequency dependant energy on to it, and its resonance at a given frequency are the two most important factors in determining any antennas efficiency.
Part 2 tomorrow... http://www.worldwidedx.com/cb-antenn...-part-2-a.html
This is meant for the beginner; if you found this page while browsing on the internet - save this page!
For more information, please read this: The Ultimate Guide to 11 Meter CB Antennas