Wednesday, June 18, 2008

Antennas 1, The Dipole (Bob, Week 5)

This week we begin an open-ended series on Antennas. Antennas are the one part of our hobby left where it is common and worthwhile to build your own gear. It's true that you can still build your own radio, amplifier, power supply, etc. and many do, but with the advent of modern electronics this is now much less common than it used to be. But antennas haven't changed too much over the years. There have certainly been some new and effective designs but the basic function and design is still as valid today as it was in the very early days of radio.

Today I start with the Dipole, also known as the Half-Wave Dipole since the length of the antenna is roughly one-half wavelength of your operating frequency. More on why it is only "roughly" one-half wavelength later. The dipole is good for illustrating some of the antenna basics and is a good and effective do-it-yourself antenna if you have the space and can get it high enough off the ground.

So, what is it? and why half-wave? A dipole at it's simplest is a long straight conductor that is broken at the middle into two separate halves. Each half is connected at the center by one of the two conductors of your feedline. Because of this way of connecting your feedline to the antenna we say that a dipole is a center fed antenna. As the RF current passes from the feedline to the antenna it is stopped by the endpoints of the antenna. So the current at the endpoints is zero and it is at a maximum at the feedline connection. The reason a half wave is chosen is so that the antenna will be resonant.

Resonance could be an entire topic on it's own and maybe someday it will be but I'm not prepared to go into it in depth today. Instead I will give you a musical analogy. When you blow across the top of a pop bottle you hear a tone of a given frequency. Let's not worry about harmonics just now. The reason you hear the tone you do is because the bottle is resonant at that frequency. When you blow across it, the sound you make isn't of any given frequency. That sound travels down to the bottom of the bottle and back up again in a certain time and reinforces the sound you made. The time it takes to go down and back determines the frequency of resonance. In a similar way, the resonant frequency will depend on how long it takes a radio wave to travel the length of the wire antenna and be reflected back. It turns out that the distance to do that is one-quarter wave. Since you have two of these quarter-wave pieces, end to end, you get a half-wave antenna.

When we talk about a wave-length we are talking about how far the wave would travel in the time it takes to go through one cycle. In general when we talk about the speed of radio waves we say that it is the same as the speed of light. But inside a wire it is not. It is actually a bit slower so we have to apply a factor to the calculation. So in effect the antenna is a half-wave long but it is a half-wave at the speed it travels in the wire.

I will fill this in more later...

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