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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.
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.
Our Thanks to for his permission to reproduce this article on our website.
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