|World Prefix List||G3YRC Website Award||Club Information||Members Cards||Club Log||Members||Club QSL|
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 various advertised lines of beam antennas, especially for 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).
Home | Part 1 | Part 3 | Theory Index