The dipole is the most common antenna configuration for amateur radio, specifically the half-wave dipole. The dipole antenna is inexpensive and effective. The dipole consists of two identical conductive elements and can be placed vertically or horizontally.
Most dipole antennas look like a giant “T” sticking up out of the ground. Basically there are two metal rods or lengths of wire that are center fed. The length of the two wires corresponds to the electrical half wave of the desired frequency for a half-wave dipole.
So the formula to calculate how long both sides of the dipole need to be when added together is Length (in feet) = 468 / frequency (in MHz). This will tell you how long to make the two wires when adding their lengths together and feeding them from the center point.
This calculation is only approximate due to the resonant frequency of an antenna being affected by several different factors including how high off the ground it is placed and it’s proximity to metal objects.
The radiation pattern for the dipole antenna is at its maximum when perpendicular to the conductor and is 0 when it is parallel to the conductor. To help visualize this, imagine a long rod with a large donut at the center. The donut represents the radiation pattern.
The top part of the “T” shape is the conductor. When the conductor is placed parallel to the ground, this creates a directed antenna where the signal is really only going in two directions. When the conductor is placed perpendicular to the ground, this creates an omnidirectional antenna.
How it Works
Current is continuously flowing back and forth between the two conductors. This along with the applied voltage to the system is what creates an electromagnetic force that can be radiated out as a radio wave.
It works pretty much the same for picking up incoming signals. The two conductors making up the dipole both pick up and radiate one side of the waveform. When an incoming wave is picked up, the antenna converts the electromagnet energy into voltage and the voltage is what pushes the current.
You may be wondering about how the half-wave dipole only has to be half a wavelength long to pick up a certain frequency. Why not an entire wavelength? This works because antennas that are meant to resonate with one frequency will also resonate with that frequency’s harmonic frequencies. There are many different configurations of dipoles due to this including ¼ wave and ⅝ wave for some antennas.
Physical Length vs Electrical Length
You might be wondering what I meant when I said the length of the dipole must correspond to the electrical length of ½ wavelengths of the desired frequency. Electrical length refers to the length of the physical conductor, but it is possible to electrically lengthen or shorten the conductor without having to physically lengthen or shorten the conductor.
Most antenna systems are resonant which means that the circuitry is locked up until the desired frequency comes along and unlocks it. When a radio signal induces an AC or oscillating voltage, if the particular frequency is the resonant frequency for the circuit then the signal will be able to flow through with no resistance.
Because determining the length of a dipole is an inexact calculation, you may need to lengthen or shorten your dipole depending on what frequency you are really trying to hone in on. To lengthen or shorten your antenna electrically you can add either an inductor or capacitor in series with your antenna.
XL = 2fL where X is the reactance in terms of inductance, f is the desired frequency and L is the inductance.
XC = 12fC where X is the reactance in terms of capacitance, f is the desired frequency and C is the capacitance.
From the equations above, you can see that adding inductance will electrically lengthen the antenna and adding capacitance will electrically shorten the antenna.
A dipole consists of two conductors that are center fed. The current flows back and forth between the two conductors creating an oscillating energy that when associated with an applied voltage gets transformed into electromagnetic energy.
The most common type of dipole is the half-wave dipole and this works because an antenna system designed to resonate with a specific frequency will also resonate with the harmonics of that frequency.
You can electrically lengthen or shorten an antenna to pick up a desired frequency without having to physically lengthen or shorten the antenna. All you have to do is add a capacitor or inductor in series.
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