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.
Also known as the "Beam" antenna.
Read the section on Yagi antennas if you are not familiar with
directional ("beam") antennas and how they work. Since the
Cubical Quad works off the same principles, you must first
understand the Yagi antenna.
The Cubical Quad
The next type of beam is the Cubical Quad, my favorite. This is the type of beam
that I use. There really is not any new principles involved here, the quad
works on the same principles as the Yagi. However, instead of using
the dipole antenna for the driver, director and reflector elements, we are going to use the
quad loop antenna. The quad loop was invented by an amateur radio operator by the name of Clarence
Moore. He was working for a broadcast station in Quito, Ecuador.
It is called a quad loop because most people configure
it as a square (quad = 4, 4 sides to a square). The quad loop measures exactly 1/4 of a wavelength on
each side. As you can see, this antenna actually is a Full (1) wavelength
antenna as compared to the 1/2 Wavelength driven element of the Yagi. The
loop is usually made from a 36 foot piece of copper wire. The Quad loop alone has
2 db of gain over the dipole antenna. So, using this as the driver element
our antenna already has at least 2 more db gain over a yagi antenna with
the same number of elements.
Take a look at the quad loop in figure 1. As you can see, each side of the loop
is 1/4 wavelength. The total distance around the loop then equals 1 full
wavelength or 36 feet. Figure 2 show the quad loop with a common supporting
structure. The arms that hold the wire (usually #12 or #14 copper wire)
are made from fiberglass, wood or bamboo. Really, anything that is an insulator
can be used. Keep in mind, it must be strong. Wire is light, but when the
wind starts hitting it or ice builds up on the wire, weak supports will break.
Figure 1 - The 1 Wavelength quad loop. Shown without supporting
Another point I would like to point out about the quad loop is what it is really
made up of. In figure 2 side A and side B can be thought of together as a dipole antenna. You can see
if you remove side C and side D you would just have a dipole who's legs (each 1/4
arm) slant up at an angle. Then you can see if we add sides C and D, and
join them all together, we really have just hooked two dipoles together! This is
equivalent to co-phasing two dipoles. Read the "Co-Phasing" section for more
information about what we mean. This arrangement doesn't quite give us the
3db we would expect, but it does up our gain over 1 dipole by 1.8db.
Figure 2 - The quad loop shown with a common way to physically support the
Figure 3 shows a drawing to scale that compares the sizes of the quad loop
element and the dipole. This is just to give you an idea of size requirements.
Figure 4 compares the radiation pattern of the dipole antenna to the quad. When
you are using the dipole, you can just feed the antenna in the same place (the
middle) and rotate the antenna to achieve a different polarization. But since
the quad loop is a loop, rotating it would not do you much good. To obtain a
certain polarization with a quad it matters where you attach the coax. Figure 5
shows the feedpoints to obtain a certain polarization. Each of these feedpoints
have the same impedance.
Figure 3 - Size comparison of the quad loop and dipole. Bother antennas
are on the same plane (the dipole is actually lying on the quad).
Figure 4 - You can see the increased gain of the quad loop compared to the
dipole in this polar plot.
Figure 5 - Feed points to obtain different polarization's. Unlike the dipole
that you can just rotate to obtain horizontal or vertical polarization, you have
to move the feedpoint of the quad. The shield of the coax goes to one side of
the wire and the center conductor goes to the other. See figure 1 for a close up
of how the coax is suppose to attach to the loop.
One of the biggest advantages to using the quad loop that it is impervious
to rain/sleet/snow/sand static noise. Have you ever had rain static pound your
receiver when a rain storm was rolling in? Things such as sand and rain carry an
electrical charge that cause a lot of noise on verticals and yagi beams.
Surprisingly, the closed loop of the quad does not respond (pick up) to this type
As a matter of fact, in Operation Desert Storm, Yagi beams where unusable because of
the sand storms that cause huge static noise problems! The U.S.troops had to use quad antennas
to communicate during sand storms.
Lets take a look at some 4 element cubical quads. Figure 6 shows a 4 element
cubical quad fed for horizontal polarization. The elements
create a beam pattern the same way a yagi does (see the "yagi" section). The
parasitic elements are closed loops, meaning they are not electrically broke at
any point around. All the wire loops must be insulated from the boom. Fiberglass
is usually used on durable quads. The reflector element is typically 10 % longer
(the distance around the wire loop) than the driven element and the director is
typically 10 % shorted than the driven element.
Figure 6 - 4 element cubical quad shown fed for horizontal polarization.
Another huge advantage of the the quad parasitic element is the fact that it is not
polarization sensitive. By this we mean, quad parasitic elements (reflector and
director(s)) respond to all types of signal polarization equally well.
Compared to the yagi elements, where the element is either in
the vertical plane (straight up and down) or the horizontal plane (side to side)
. These yagi elements only respond to signals that have the same polarization
as their self. All other signals (that do not match their polarization) are
reduced by 20db (I mentioned this under the "Antenna Basics" section)!
The quads parasitic elements are a continuous (closed) wire loop, that respond
equally well to all polarization's. This means they direct or reflect
horizontally polarized waves, vertically polarized waves and everything
in between (they direct/reflect all signals equally well regardless of
its polarization). Keep in mind the driven element is broken where the feedline
connects, which means the driven elements IS polarization aware.
What this mean is, during DX contacts when signals arrive at your antenna
with its polarization changing constantly (this is one reason signals fade and
pop back up suddenly), on the quad this effect is reduced because quad
parasitic elements still pick up these changing (flip-flopping polarization) signals. Notice I said
parasitic elements, the driven element still reduces signal strength
by 20db (if the signal doesn't match its polarization)..but the signal is stronger than it would be on the yagi because
the yagi parasitic elements would have also reduced the signal by 20db
before re-radiating it (for the driven element to then pick up).
If you are confused about polarization "flip-flopping" or changing during DX
contacts, you need to do some further
reading, check out my section of recommended books. You must keep in mind
signals bounce off of objects (water towers, radio tower, water, the
ionosphere) and their polarization gets rotated somewhere in between horizontal
and vertical most of the time. I can not cover every aspect of how signals
travel (called "propagation")...I would be writing for ever! Under the "Angle of
Radiation" section, you get get an idea of how "skip" (DX) signals travel.
Since our antennas are generally set up to receive only one polarization
at a time (usually horizontal or vertical), polarization changes due to reflections can cause signal
fade (signal strength waving up and down). This fading is reduced on
quad elements! Enough said.
Lets look at the 4 element cubical quad fed for vertical polarization. See
figure 7. Nothing new to really say, but you can see where the coax attaches for
Figure 7 - 4 element cubical quad fed for vertical polarization. You can see
it is not as easy to drop the coax straight off the antenna after it connects
like you can if you feed it for horizontal polarization (like in figure 6).
With these great features comes some disadvantages! First off, remember
in antenna basics when I talked about bandwidth and said that it
is mainly dependent on the antennas elements outside diameter? Well,
as you can see from the pictures, we usually make quads from #12 gauge wire.
This is small compared to the tubing you would normally use for a yagi,
and as a result the bandwidth of the quad is narrower than the yagi. Generally you can
just cover the CB band with a 4 element quad with a 2:1 SWR. If you are the type of operator
that is all over the place (even outside the CB band)...this antenna
will put a limit on your frequency range! More elements narrows bandwidth on
any antenna type (quad or yagi), so a 2 element quad has better bandwidth than
the 4 element quad.
Secondly, We are familiar that Yagi's can be cross mounted (see figure 8 under
the "Yagi" section), one for vertical
and one for horizontal on the same boom. Now think, two quad wires, its
not so easy, check out figure 8 to see what I am talking about. It is possible
to mount to wires side by side, but this is slightly more complicated.
A company by the name
of Signal Engineering is making quad antennas that deal with this problem
Figure 8 - Dual polarity quad. The parasitic element wires are left off for
clarity. You can see the driven element wires must be side-by-side (a few
inches). When one
loop is being used, the other must be an open circuit for the antenna to
function properly. Most coax switch boxes do not work like this, so if you are
going to try this arrangement, make sure you keep that in mind.
Maco has a few antennas that they call "quads". The V-Quad is a true
actually uses a full wavelength loop for the driven element.
If you shape
the quads elements like a triangle (three sides instead of four) it is
"Delta Loop". It uses the exact same principles as the quad and has
similar performance. The other antennas they call a quad is the
actually a hybrid - see the "Hybrid" section. To sum up the Y-Quad, it
a Yagi antenna (because its driven element is a dipole, not a full
quad loop...I am guessing the "Y" stands for Yagi). JoGunn's V-Series
the delta loop variety (they falsely state that they are "circular
polarized"). New tests
show that loop in a quad (4 sides) configuration yields slightly more
than the delta configuration. The best configuration would be
a loop arranged in a a perfect circle (instead of a 4 sided quad, or
sided delta), but that arrangement would be difficult and expensive, so
four sided quad is the closest practical way to form a loop at 27 MHz.
Here are some gain figures for some Cubical quads:
Number of Elements
Gain (Over Dipole)
Front-to-Back Ratio (F/B Ratio)
Reflector element only
Director element only
Note: This table is typical performance of Quad'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.
As you can see, the Cubical Quad antenna has the same gain as a yagi with
one more element..because the driven element of the quad has more gain than
the Yagi's driven element (2db more). Figure 8 shows a commercial 4 element
quad available in the Netherlands.
Figure 8 - A commercial 4 element quad. As you can see, you can
mount this antenna like an X also. You can still see the feedline hooks
to the same area on the loop regardless of how you mount the spreader arms.
A nice DX antenna! GB Antennas & Towers, Brielle, Netherlands.
I give construction details of my 4 element cubical quad under the "Antenna
Building" section. Check it out if you want to build a killer DX antenna!