EI7BA
Multiband Cubical Quad
" ...A
thing of beauty and a joy forever..."
I wanted an antenna that ....
1. Would not be too
expensive
2. Would cover as many
bands as possible.
3. Would not compromise
on performance
4. Would be at least
equal to a 3 El trapped commercial yagi on all bands, eg
Th6
5. Could be rotated by
an inexpensive rotator
6. Would withstand
severe weather conditions.
7. Could be built with
readily available parts.
8. Would lend it self to
future modification, experimentation etc
9. Could be fed simply
with a single feedline.
10. Reasonable size.
So, taking these requirements what was
available.....
Well first of all, virtually all commercial antennae were
ruled out on cost and/or performance considerations. I looked
seriously at the possibility of building a log periodic , but
ruled it out on the grounds of size and complexity. So that
left the Quad!!
The Quad meets all the 10 listed
requirements with a few disadvantages, these being -
1. Three dimensional, therefore
visually more noticeable (more beautiful??) than a Yagi.
2. Limits the use of your tower for
wire antennae (Dipoles etc.) because the quad arms hang
below the rotator.
3. Prone to damage from ice (we get
no ice in EI).
4. Very difficult to install on a
fixed tower. A tilt over tower is very desirable, as the
quad can be built onto the tower when tilted over.
( Pic of tower tilted
over, so that the quad is at the other side of the hedge on the
lawn.)
OK.. We'll start with a specification of my present Quad
(2 element).
It covers six bands, 20m-10m on HF, and also 6m. It is a
Boomless (spider quad, gem quad)design. It uses Glassfibre
arms (a must). It uses a single coax line to a homebrew
antenna switch. From there, a seperate feedline goes to each
of the Driven Element feedpoints. There is a homebrew Choke
Balun at each feedpoint.
Performance-
I have no accurate method to measure forward gain, but I
reckon it is the text book 6-7 db.. F/B ratio is consistently
5-6 S points on my TS850 S meter, on all bands
20-10m. I don't know how many dbs per S Point for my TS 850,
but it is surely at least 3db per S Point. So this translates
to a minimum of 15 db, and arguably as high as 30db. As with
all 2 element Quads, It has a wide beamwidth, about 60
degrees.
(Measured plot of
polar diagram..15m. Using software by I0JX and TS850 to
produce the plot)
Construction -
The spider was built for me by my friend Jim E18GS,(EI7M)
who is an Engineer (He also built my crank up, tilt over
tower). It is made of stainless steel, which of course, does
not rust. The boom is about 18 inches long ( not critical),
with the angle iron arms welded to the boom at an angle of
110degrees to the horizontal. The pipe, a foot long, is
welded at right angles to the boom in the centre. Depending
whether you weld the pipe to the flat side of the boom, or on
the corner, gives you either a diamond or a square shape
quad.
( Drawing of stainless
steel spider)
( Pic of spider )
(Pic of spider
mounted..First arm attached)
I use the square shape. This pipe slips into the 2 inch OD
pipe which comes from my rotator, in my case the pipe to the
rotator is about 5 feet long. The glassfibre arms that I use
are about 16 1/2ft long, but 14 1/2ft should be long enough
to accomodate the 20m loops. The arms are clamped to the
angle iron arms (which are each about 1ft long) using jubilee
clips (Hose clamps).
(Pic of element to
spreader attachment)
It is a good idea to put circles of insulating tape around
each arm at roughly the points where the elements will be
attached (see photo above). Later, when you are tensioning
the loops, you have a visible check , so you can have the
attachment points at equal distances from the spider.
( Attaching
the elements....Ger EI8HT strutting his stuff...Known in our
QTH as SUPERMAN..!!)
There are many different ways of joining the element wires
to the arms. I use two strong cable ties at each attachment
point. One cable tie is closed to form a circle of about one
inch diameter, and the other is threaded through this and
around the arm. This allows the tie point to be moved up and
down the arm to tension the wires. Do not drill the
arms as this weakens them. Bob K2US, uses the best
method that I have seen, of attaching the elements to the
arms. He puts loops of nylon webbing around the element
corners, and trap the ends of the webbing against the arm
using stainless steel hose clamps. This is a stronger and
more durable option, but is more expensive. When the Quad is
built, and everything is tuned and working OK, cut lengths of
strong wire about a foot long. Bind these around the elements
at the corners where they attach to the arms. This helps to
stop the wire from flexing and breaking at the corners.
( Pic looking up at one of the arms.
Note the circles of insulating tape, and the spiral wound
reinforcing wires at the corners. Also, note how the elements
are being pulled away from the arms, which improves
insulation.)
The elements-
Now for the controversial bit.....
The inherited wisdom for element size is 1005/f for the
driven elements, and 4% to 5% larger for the reflectors.
The actual size of element that you end up with (usually
after hours of adjusting , measuring and testing) ,will
depend on a few different things, such as Type of wire, wire
diameter, insulated or non insulated, etc.(In other
words....The Velocity Factor)
If you use the following guidelines, and formulae, you
can build this 6 band quad without tuning stubs, gamma
matches or any other complexities, and it will work properly
with little or no adjustment required......
The wire should be bare hard drawn copper, not less
than 1mm diameter, and not more than 1.5mm diameter.
(18-16swg). The formula for this type of wire is 996/f for
the driven element, and 2.5% larger for the reflectors.
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EI7BA
Two Element Quad Dimensions |
(feet) |
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Driven
Element |
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Reflector |
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996.00/f |
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1021.00/f |
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Freq |
14.17mhz |
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20m |
element |
length |
70.29 |
70ft 3 1/2ins |
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element |
length |
72.05 |
72ft 1/2ins |
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Freq |
18.13mhz |
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17m |
element |
length |
54.94 |
54ft 11 1/2ins |
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element |
length |
56.32 |
56ft 4ins |
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Freq |
21.20mhz |
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15m |
element |
length |
46.98 |
46ft 11 3/4ins |
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element |
length |
48.16 |
48ft 2ins |
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Freq |
24.94mhz |
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12m |
element |
length |
39.94 |
39ft 11 1/2ins |
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element |
length |
40.94 |
40ft 11 1/2ins |
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Freq |
28.50mhz |
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10m |
element |
length |
34.95 |
35ft |
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element |
length |
35.82 |
35ft 10ins |
A number of these quads have been
built, using these formulae, with slight variations in wire
diameter, but always using bare hard drawn copper
wire. All have delivered the goods with no adjustment of
element size, giving good gain, f/b, and bandwidth. I
emphasize once more, that if you use insulated wire, or bare
copper, different to what I have used, The above measurements
will not be correct. For instance, If you use insulated
wire, the loop size will be approximately 2% shorter,
depending on the type of insulation. This 2% represents a
300kHz frequency shift in resonance on the 20m band. Other
wire types and sizes will work just as well of course, but
you will end up with different loop sizes.
( Almost There...Put your
back into it Ger..! )
The elements should be mounted on the outside of the
spreaders, because when they are tensioned, they pull away
from the spreaders, and improve the insulation at the
attachment points. When the elements have been attached very
loosely to the spreaders, attach equal lengths of
nylon twine about 12ft long (4 off) between the 20m driven
element attachment points and the reflector 20m attachment
points. Adjust the 20m element spacing with these twines. I
use 11.5 ft spacing between 20m elements, which corresponds
to about .18w/l spacing. Do the same at the 10m or 15m
element attachment points with some more nylon twine, and
then tension the elements on the arms. You now have a nice
box like construction which is very strong.
So far, so good......
( pic showing feedlines and a choke
baluns)
Feeding...
In my experience, when feeding a 5 band quad, it is not
advisable to take too many short cuts. I found that if I
joined all the feedpoints together to a single feedline,
results were mediocre, with reasonable performance on 20m,17m
and 15m, but very poor on 12m and 10m. There is a lot of
interaction between elements, giving a bad pattern, poor f/b
ratio and very little gain on 10 and 12m..
There are two major detrimental interactions..
10m is an harmonic of 20m, so the 20m loop will accept power when
transmitting on 10m
10 and 12m are so close in frequency, that they tend to see each other's
elements as parasitics within their own band..
If you absolutely refuse to use seperate feeds, then you should at least
use two feedlines, and group 20, 15 and 12m on one line, and 17 and 10m on
the other.. This should go some way to improving
matters.
A Quad is a
balanced antenna, and should be fed in a balanced manner.
Although Gamma matching is an obvious solution, there is a
simpler one... Choke baluns...
I use a single feed of 50ohm coax to the centre of the
Quad spider. There I have mounted a stainless steel box. In
this box I have a homebrew remote antenna switch. This
consists of six medium sized 24v relays, with good
quality contacts. These act as single pole, double throw
switches. I run a multicore cable from the switch box in the
shack to the relay box. This 8core cable allows me to switch
bands by putting 24v on the appropriate relay. The unused
relays are wired so as to ground the feedpoints of the unused
driven elements.
( Pic and schematic of
Relay box . Only three bands shown, 17m selected. Note that all
unused loops are grounded at the relays )
I used to feed my Quad at the bottom corner,
i.e. half way between horizontal and vertical, or "cross
polarized". I did this for three reasons. For curiosity,
to limit the wind area, and to give me nice secure tie points
for the feeders against the quad spreaders. It performed well, and was
mechanically very good, as it held less wind with the feeds taped to the
quad arms. However, I have now reverted to the traditional feedpoint half
way along the bottom horizontal wire.
( Pic
showing the stainless box mounted on the spider,
with the homebrew switchbox inside. The stainless
steel box has a weatherproof lid, which was
removed for the pic)
Up to Christmas 1997, I fed the loops using 75 ohm 1/4 w/l
matching transformers which were connected back to the switch
box using 50ohm coax. This was to match the theoretical 100
ohm to 120ohm feed impedance of the driven loops, at the
wider spacing that I use. (.18 w/l, as against the more
traditional .12 w/l). However, having had damage to the quad
on Christmas Eve 1997 during 110mph gales, I decided to feed
the loops directly from the switch box with 50ohm coax as I
was in a hurry to get back on the air. I was amazed to find a
nice swr curve on all bands (1:1 in most cases at
resonance). I have'nt as yet fully figured out why this
should be, but for now I have no plans to change back to the
1/4 w/l transformers. (If it ain't broke...don't fix
it...)
The Choke Baluns....
Since I have moved back to the traditional feedpoint, I now use RG58
coax from the Remote Switch to each driven element. I wind as many turns
of the RG58 as I can, on an Amidon T200-2 iron dust core, wound as in the
following picture..
I cabletie all the feed lines together, and tie the chokes to the
feedpoint insulators. This arrangment is fairly light and strong, and survives
well.
The original chokes consisted of 5-10 ferrite cores slipped over
the coax at the point where the coax is joined to the driven
elements. I used six at each feedpoint. They create a high
impedance path to any RF that may feel inclined to run on the
outside screen of the coax, thereby spoiling all your hard
work, giving misleading SWR readings, screwing up the
pattern, and generally behaving in an anti social manner...
ADDENDUM.. An extra two bands..!
As I stated earlier, my quad arms are 16.5
ft long. This meant that they were longer than needed for 20m by about 1.5 ft.
As it is very easy to chop off the surplus length, and not so easy to add it
back on, I decided not to " look a gift horse in the mouth", and
use the extra length to my advantage. I have added two more bands..i.e.
40m and 30m. So now, it has 7 HF bands.
I have added three elements for 6m and 2 elements for the
European 4m (70mHz) band to the existing spider, and on a seperate 6ft boom
which is clamped to the spider, I have 5 elements for 2m, and 9 elements for
70cms.. A grand total of 11 bands..
On 40m, I built a 40m dipole, bent in a
square, and attached to the tips of the arms on the Reflector
side of the Quad. Because it is bent, the feed impedance was only about 22
Ohms, so I added a simple rolled up "hairpin match" at the
feedpoint to compensate. The matching coil consisted of an airwound coil of
about 8 turns, with a 1.5 inch diameter connected across the feedpoint.
Pull out, or compress the turns of the coil for best SWR.
I have now
updated this in anticipation of the extra 100kHz on 40m that we are to get in
2005. I needed to increase the SWR bandwidth to cover 7 to 7.2 mHz. I have done
this by converting the bent dipole to a folded bent
dipole. In one fell swoop, this has increased the feed
impedance to 50 Ohm, and has given me a 1.5:1 SWR bandwidth of about 200kHz
I found Andre
GM3VLB's pages had all the info that I
needed on his Mini-Delta page. Andre does a lot of "island hopping",
and has been giving out rare IOTA contacts for years. He is a past master at
creating effective portable antennas quickly, and for little or no
cost. His site is a must, if you are, or plan to be, a
mobile, or portable operator..
The diagram below should be
self explanatory..
On 30m, it is a full wave loop, attached to
the arm tips on the driven side of the quad. The two linear loading wires
(as per G3FPQ) are spaced about 6 inches in from the loop sides, and are
connected to the loop at their mid points. These wires are adjusted for
resonance.
Again, the drawing
should be self-explanatory
The performance on 40m is as you would
expect....That of a dipole at a reasonably good height, i.e. simple but
effective..
On 30m, it is excellent. The loop is fed with RG58 via a
choke balun.The SWR bandwidth is very good
There is NO f/b on these two bands of
course, but there are side nulls.
Glen WB4KTF's homebrew 5
band Quad
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