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.
