The Quad antenna,
sometimes called the Cubical Quad, is an antenna which is used by many amateur radio operators. We owe it's
development to Clarence C. Moore
(patent no. 2,537,191), W9LZX,
(SK), an engineer at HCJB, (a shortwave missionary radio station in the Andean Mountains). He developed it to resolve
issues with large coronal discharges in the thin air with a beam antenna.
The quad was born!
Moore describes his
antenna as "a pulled-open folded
dipole". Ref:
Wikipedia
The 3 element 2 meter
(designed for 146mhz) quad in this project has a very small footprint
(boom length) of about 32 1/4 inches and according to the computer guru's
out there, it has about 9.45dBi gain with only three elements. 9.45
dBi equates to 9.45 - 2.15 = a whopping 7.3 dbd gain with over 20db front
to back ratio and a 2:1 swr bandwidth of 3.9mhz!!!!!!! Let's see a
standard dipole do that!
So with 3 elements on a 32
inch boom....you're more than multiplying your effective
radiated power by 5 times! If your 2 meter rig is getting 50 watts to the
3 element quad (assuming no line loss)...the station on the other ends
"thinks" your putting out over 250 watts! That's hard to beat on such a
short boom and only 3 elements. If your rigs puts 75 watts to the antenna,
that's a whopping 375 watts + erp! More than enough to do some
serious repeater "Kerchunking". Put a 5 watt handheld on this antenna and
it acts like over 25 watts erp out!
The standard 3 element yagi
gain is about 4 to 5 dbd and is usually longer in boom length compared to
the quad and sometimes difficult to tune for best
performance.
Other than the gain
for it's size and unlike most quad designs.... This
3 element 2 meter quad design has a major plus going for
it!
The feed point is about 53 ohms....a great match for direct
feeding with 50 ohm coax...no baluns, no fuss, no
muss.
So how do we build it? There
are many ways to build a quad antenna and the difficult part of
constructing the boom, attaching the support arms, etc will be left up to
your ham ingenuity. Most builders of VHF quads use PVC or wood as
the boom, and wooden dowels, PVC, fiberglass, or some other non-conductive
material for the spreader (support) arms. Wire is used for the element
loops.
You be your own judge and
build the antenna out of material that suits your construction skills and
talents....experiment.
Below are the exact measurements for building the 2 Meter 3
Element Quad.
Refer to the drawings supplied below for the
layout.
ELEMENT LENGTHS FOR #14 WIRE AT 146MHZ: This design only presents a good match to 50 ohm coax using
#14 wire and using the spacings and lengths below....do not change
unless you want to experiment!
Reflector total length in
inches = 86.76 inches (86 3/4 inches)
Driven element length total
inches = 83.05 inches (83 inches)
Director element length total inches = 79.33 inches. 79 3/8
inches)
SPACING: Reflector to driven element = 14.25 inches Driven
element to director = 18 inches
Spreader
arm lengths: If you can put any faith in computers, the lengths are
calculated:
Reflector spreader bar = 30.67 inches Driven element
spreader bar = 28.04 inches Director element spreader bar = 29.36
inches
BUILDING NOTES: Allow yourself a minimum of 31 inches from
tip to tip for the spreader supports. If that should happen to be too
long, you can always cut off to fit when done. Remember this is from one
end of the "+" thru the boom to the other end. It is best to layout
the individual loops for the quad in an exact square pattern so they won't
move. Just take the element length from above, divide by 4 for each side
of the "square". Then measure from one corner to the other and add an inch
or two. Don't "cut" yourself short! Do not change
the total length of the loops unless you have some really strange swr
readings when done. Do not change the element
spacing on the boom! According to the calculations, you should
have an extremely good match to 50 ohm coax by direct feeding. You
must fit the supports (spreader bars and boom to the length of the
loops...not the other way around. The size wire, lengths and spacing of
the elements are fairly critical for the near 50 ohm impedance of the feed
point. Your measured impedance may be slightly different due to your
construction techniques, etc, but you should be close according to the
program used. See below for the program
information and a source to download it. It is loaded with other
interesting designs.
Do not allow
the wire elements to sag. They must be secure and in as near a square
pattern as possible on the spreader bars.
Antenna
polarity:
The quad can be fed as in
the drawings below for vertical polarization suitable for FM repeaters,
mobiles, etc....or.....turn the feed point to the bottom or the top for
horizontal polarization for ssb work etc. It makes no difference which
side the feed point is on for vertical polarization.
Quad shown above in Vertical
polarization configuration
Note: The quad can be
mounted with the top and bottom sides horizontal to the earth, but then it
still must be fed from one vertical side in the center of one
side for vertical polarization or from either the top side or bottom
side for horizontal. This makes for difficulties with supporting the
feedline and extra measures must be taken to prevent strain on the coax
connection to the loop. Either way you mount it. it should perform the
same.
Feed point end detail on spreader bar
support
Attaching the feed line for the driven
element: There are many ways to attach the feed line to the
driven element while allowing good support for the feed line and also the
wire loops on the end. One example above on the driven element is
to drill 4 small holes big enough for the loop wire to pass thru and
twisted back on itself giving some strain relief. Then the coax center and
shield is extend out just enough to twist onto
the loop ends, soldered and then the coax ends and the solder connections
sealed from the weather. The feed line can be
nylon tied to the spreader bar, taped, etc. In doing this, remember to
keep the exact lengths of the wire loops unchanged. The length of the
radiator loop , (the driven) starts at the coax connection center
conductor, continues around the loop and attaches to the shield. Don't
leave three or four inches of center conductor or shield extended before
it attaches to the wire loop. This will detune the antenna because this
added length becomes part of the radiator!
The other element loops can
be simply run thru a hole drilled all the way thru the end of the spreader
bars at the appropriate distance from the boom while maintaining the exact
loop length for each element and the square shape of the antenna.
Attach them securely to the spreaders, twist together the ends and solder.
You should have 4 equal length sides on each element of this 3 element
quad. Looking at the finished quad from the side should look something
like this.... |-|-| You may also want to add a small 1:1 coax "air"
choke consisting of about 3 to 5 turns of the end of your coax feed line
wound into a coil in series with the feed line near the feed point to
help eliminate feed line radiation similar to the "Ugly
Balun"....your choice. It might be advisable to us a
short length of PVC heavy enough to support the finished antenna mounted
to the boom as a "mast" extended below the antenna, then attach it to your
final support mast. This keeps any metal mast out of the field of the
antenna which could possibly distort it's pattern. Use a rotor if you need
to "aim" it.
You should not have to "tune" this
design....unless the computer program has errors in the calculation
results. The lengths above were taken directly from a GWBASIC program
called:
QUAD.BAS
in HAMCALC
version 93 by
George Murphy, VE3ERP 77 McKenzie Street,Orillia, ON L3V 6A6,
Canada. e-mail <ve3erp@encode.com>
