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.

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 for gain figures for some yagi beams:

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.

Typically most 2 element Yagi’s use just the reflector element. If you would use just the director on your two element, you would have more forward gain, but you would also not have any rejection of signals coming from direction B, that is why its F/B or Front to Back ratio is zero. The Front to back is the ratio of gain of the forward direction as compared to the reverse direction. So, if we were receiving signal A, and we turned our beam around 180 degrees, how much would the signal be reduced? This ratio is known as Front to Back ratio, and is as important as gain to some. If you have a lot of CB neighbors, getting a beam that has a good F/B will reduce interference from them if you point your beam in opposite directions from them. There is another term, Front-to-Side ratio that works the same way as as the F/B…except it means when you turn you beam to the side (90 degrees away) from the signal how much is it reduce. Typically, Front-to-Side ratios are even higher than the F/B ratio. You can see the deep notches in the radiation pattern in figure 2 that indicate this is where the greatest rejection of signals occurs. It may not be directly at the side of the beam, it is mainly dependent on antenna design (spacing, length).

Figure 2 – How the Yagi distributes its transmit/receive power.

Ok then, we can have variations of this Yagi beam. We can actually still have a beam even if you take off the reflector element or director element and just have a 2 element beam. This beam would have less gain than the three element, but would still be quite directional. It would certainly have more gain than a 5/8 Vertical antenna.

As you can see from the table, it gets difficult to get more gain after 4 elements. Not only that the antenna gets huge, the antenna bandwidth goes down, and it is hard to tune! As a quick note, its better to “stack” or “co-phase” beams rather than go with a large number of elements. For instance, its better to go with co-phasing two 4 elements Yagi’s rather than using an 8 element beam. Read section the section “Performance Tips”, “Co-Phasing”.

I have seen some monsterous gain figures for the Maco line of beam antennas, especially their 6 and 8 element beams. In my opinion, these gain figures are really exaggerated! Be cautious, and read on.

We can see the pattern changing when we compare the radiation pattern of the 2, 3 and 5 element Yagi antenna, see figure 3.

Figure 3 – Comparison of radiation patterns. You can see how the higher number of element beams concentrate their power.

Lets check out some pictures of some yagi beams so you get a better idea what they look like. Figure 4 shows a 4 element Yagi in the horizontal position. It radiates a horizontally polarized signal. You can see a special matching device where the coax connects that looks a small “jumper rod” that connects a few inches out on the driven element. This matching device is called a “Gamma Rod” or “Gamma Match”. It is a device that simplifies adjusting the antenna. The gamma match is a type of matching transformer used to match the feedpoint impedance of the antenna (which rarely is 50 Ohm) to the 50 Ohm coax. This is especially necessary on beams with more elements (more than 4) because the impedance at the feedpoint is naturally low (around 20 Ohms).

Figure 4 – 4 Element Yagi in the horizontal position.

Figure 5 shows the real electrical antenna makeup. You can see the boom is not part of the radiating structure. Figure 7 show the electrical makeup of a 4 element yagi antenna with the Gamma Match.

Figure 6 – Electrical makeup of Yagi beam. Parasitic elements can be bolted directly to the boom OR can be insulated from the boom. The driven element has to insulated from the boom for proper operation. Coax is not shown to scale to clearly show the connection of the coax to the driven element.

Figure 7 – Electrical makeup of Yagi beam with a Gamma Match. Parasitic elements can be bolted directly to the boom OR can be insulated from the boom. The driven element is mounted directly to the boom in this case, it does not have to be insulated with this configuration. The shorting strap is slid up and down the rod to match the feedpoint impedance of the beam to the 50 Ohm coax, this way the operator does not have to adjust element length to tune the antenna. Coax is not shown to scale to clearly show the connection of the coax to the driven element. HERE is a detailed diagram for the proper dimensions of a gamma match for a typical CB beam (drawing courtesy of a friend of mine).

Figure 8 shows a 4 element Yagi in the vertical position. It is the same antenna as pictured in figure 4, just rotated 90 degrees to send out a vertical signal. This is good for talking to omnidirectional vertical antennas (such as the A99 vertical antenna).

Figure 9 shows how you could combine two antennas on the same boom so that you could use either horizontal or vertical polarization. Typically you still need to run two separate coax cable up to the antenna (It has two separate connections, one to the horizontal driven element and one to the vertical driven element). This antenna uses two separate gamma rods for each polarization. When this antenna is operating in either polarization mode (hor. or vert.). It has the same gain as the single antenna (figure 4 and 7). There is no magic to mounting the antenna’s this way.

Figure 8 – Same antenna as in figure 7, but its rotated 90 degrees so that it radiates a vertical signal.

Figure 9 – You do not have to settle for horizontal or vertical, put both on the same boom! This antenna is fed with two separate coax that can be switched at any time, to switch between horizontal and vertical polarization. Usually, just one antenna is used at a time. This is still considered a 4 element beam – some companies try to impress with numbers any way they can (Maco calls this “8” elements)

One last thing, the JoGunn antenna. Let me just say, it is a Yagi antenna. JoGunn came up cheaper way to make a crossed yagi (like in figure 9). Figure 10 shows the JoGunn driven element as if we are looking straight down the boom at it. You can see the horizontal and vertical elements share a ground element. I think this results in slightly lower gain than using a full crossed yagi. However this difference may not even be noticeable. Also, it offers the advantage of lower wind resistance. But do not be fooled when they say it has “the highest gain”. Lets face it this just a simple yagi beam with dipole antenna driving it!

Figure 10 – JoGunn driven element.

Article by scott 2rp789 originally available at http://signalengineering.com/ultimate/yagi.html

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