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Directional Antennas

An antenna is known as "directional" if its pattern strongly favors a certain direction. A directional works by concentrating the signal in one direction at the expense of other directions. It is also commonly referred to as the "Beam" antenna. I am going to start with the earliest type of beam discovered, the "Yagi" Beam. This type of beam was discover by Professor Uda but the english translation was done by Hidetsuga Yagi. This design goes back to the 1920s! One would think today there would be better designs. I believe there is, and that's why I am so interested in antennas!

The Yagi Beam

The yagi is very simple. The basic yagi consists of three elements, as shown in figure 1. The middle element is an antenna you are already familiar with, the simple 1/2 wave dipole antenna. This element is generically called the "driven element". This is because this is the only element that is connected directly to the radio, it actually drives the whole antenna. The other two outer elements are generically called parasitic elements. One is called the Reflector (some CBers call it the "back door") and the other one is called the director element. These elements get their name from the job they do. The reflector reflects RF energy, the director directs RF energy. There is no magic circuit located inside the elements, they are simply straight rods! The reflector element is typically 5 % longer than the driven element and the director is typically 5 % shorted than the driven element. How it works. See figure 1. As signal A comes in it strikes all three elements hence generates a current on each element. Remember we said that current on a wire causes it to radiate? Even though the current is very low, this current induced on the antenna actually re-radiates off the antenna again! Ok, back to the action, the signals are re-radiated by the director and reflector and arrive at the driven element in-phase with one another (the two re-radiated signals and the original signal). This basically means, the signals reinforce each other...and make the incoming signal much stronger coming from direction A.

When the signal comes from direction B and C, the same thing happens, except the signals arrive at the driven element out-of-phase with one another which simply means they cancel each other out, significantly reducing signals from direction B and C.

This very useful effect (signals arriving in-phase/out-of-phase) is caused by the special spacing and length of the director and reflector element in relation to the driven element.

yagiexp.jpeg

Figure 1 - See text for simple explanation of how a beam works! "Element" is the generic term given to each of three rods that actually make up the antenna. The boom is not actually part of radiating antenna, merely a supporting structure.

We can even add more directors elements to increase the gain. Adding more reflector elements has NO more effect on the gain of the antenna, however.

Here is a table with a guide for gain figures for some yagi beams:

 

Number of Elements

Gain (Over Dipole)

Front-to-Back Ratio (F/B Ratio)

Comment
2
5 dB
14 dB
Reflector element only
2
7 dB
Zero
Director element only
3
10 dB
15 dB
4
12 dB
25 dB
5
12.1 dB
26 dB
6
12.2 dB
30 dB
7
12.3 dB
22 dB
8
12.4 dB
32 dB

Note: This table is typical performance of Yagi's with the stated number of elements. Typically, the gain will be within 2 dB of the indicated gain. However, Front-to-back ratio can vary greatly (as much as 25 dB) from the indicated F/B. F/B is much more sensitive to adjustments to the element length and spacing.

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