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SOURCES:
Article by Markus Hansen, VE7CA, in QST
magazine, November 2001.“Portable
Two Element Triband Yagi”.
See the
following links:
http://www.arrl.org/tis/info/pdf/0111035.pdf
http://www2.arrl.org/files/qst-binaries/
And also:
Presented in “Amateur Radio” magazine, Australia, February 2002, by
Gil Sones, VK3AUI.
DIAGRAMS:
DIAGRAM 1 shows the antenna with Markus’ dimensions,
but with my method of connecting the three driven elements at the common
feed-point.
DIAGRAM 2 shows Markus’ original feed-point design.
CONSTRUCTION:
I laid the antenna out on my back lawn, and
Markus suggests hanging it between two trees. Take your pick.
The end pieces of wood are about 2.45 metres (8ft 2”) long by 25mm
(1”) thick dowel. This seems to be strong enough for the job.
The dowel is painted to protect the wood and to render it less
conspicuous.
Black nylon marine grade cord was used throughout.
ACCURACY
OF CONSTRUCTION:
+/- 1 cm. (1/2”).
ELEMENT
MATERIAL:
Markus stipulates #14 hard drawn copper wire.
I used multi-strand PVC coated wire, as I had plenty of that on hand.
I needed to shorten my wire lengths by about 1.5% from the original
specifications to attain a good SWR.
This wire is flexible, and winds up easily for transport.
It will stretch with time, so better a little shorter than longer.
(NB: Markus recommends
shortening the wire by no more than 0.5% to allow for the effects of the
PVC insulation.
Before deviating from his recommended lengths, please read this entire
article.)
SPREADERS:
6.5 mm (1/4”) black PVC garden sprinkler risers.
STRESS
RELIEF AT CENTRE INSULATOR:
I looped my wires 1.5 times around the loop-holes in the
insulator.
This gives great strength, but probably compromises the tuning up of the
hairpin stub.
The bared wires are all wound and soldered together, and
waterproofed.
HAIRPIN
STUB:
I found the best length was 38 cms. Specification is for
43 cms.
I suggest that my difference is due to the way I applied stress relief
at the common feedpoints.
In retrospect, I would recommend Markus’ method.
I used 1.7mm copper wire (16 SWG) for the parallel lengths, and double
wound-wire of the same type for the shorting bar.
The parallel wires are supported by two 6.5mm PVC spreaders, and the
feedline is held to one of those spreaders by a nylon tie.
COMMON
FEED-POINT:
I used the 15 metre driven elements section as the
straight line.
Markus uses the 20 metre section (see diagrams).
The height above ground was about 5 metres (16 ft). This is a little
low. Markus’ feed point was about 11 metres (35 ft).
WATERPROOFING:
The common feed-point was waterproofed with two coats of
contact cement.
This works well – it sticks well, and is durable and flexible.
TUNING
THE ANTENNA:
Follow the original articles’ suggestions, viz.,
- Set
the hairpin match position for best overall SWR. Start at 43 cms.
- Adjust
the 15 metre driven and reflector elements.
- Adjust
the 20 metre elements for best SWR.
- Adjust
the 10 metre elements.
I
hung my antenna from our elevated timber deck at one end, and over a
tree branch at the other. It was easy to do length adjustments from the
deck. For the centre and other end adjustments, I had to raise and lower
the antenna by rope.
Height above ground is about 5 metres.
Initial
tuning was done from in the shack, with 13 metres of RG213 between the
rig and the RG58CU.
Final testing was done with a direct connection from the rig to the
RG58, and these results are published below. Very little difference was
noted between the two sets of results.
CHOKE/BALUN
FORMER:
I cut a length off a clear plastic drink bottle.
It’s very flexible, but light, strong and inconspicuous.
FEEDLINE:
10 metres (33 ft) of RG58CU.
(Markus used 80 ft (24 metres.))
SUMMARY
OF TESTING AND ADJUSTMENT:
|
|
INITIAL
VALUES (1)
|
FINAL
VALUES (2)
|
|
Hairpin
match length
|
43
cms
|
38
cms
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FREQ
|
Driven
Element Length
|
Reflector
Length
|
Driven
Element Length
|
Reflector
Length
|
SWR
|
|
14.0
|
4.88
m (x 2)
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10.65 m
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4.81 m (x 2)
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10.49 m
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1.1
|
|
14.15
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1.3
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14.3
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1.3
|
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21.0
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3.35 m (x 2)
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7.08 m
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3.30
m (x2)
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6.97 m
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1.1
|
|
21.2
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1.15
|
|
21.4
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1.3
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|
28.0
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2.54 m (x 2)
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5.31 m
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2.50 m (x2)
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5.23 m
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1.2
|
|
28.5
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1.6
|
|
29.0
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1.7
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29.3
|
1.8
|
|
29.7
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1.5
|
-
The lengths in these columns are as recommended by Markus.
-
My
final lengths correspond to a reduction of 1.5% from Markus’
original recommendations.
Markus points out that the front-to-back ratios will be affected by
changes in the lengths of the reflectors, and that 1.5% is probably
too much for optimum results.
Start at his values, and, if necessary, only reduce at 0.5% at a
time.
RETURN
to TOP
SWR:
Values were calculated from the formula, SWR = (fwd_pwr +
reflected_pwr)/(fwd_pwr - reflected_pwr).
I chose to use this method, as
it gave more consistent results than using my SWR meter.
TRANSPORT:
I found that I could easily roll up the antenna onto a
2.4 metre (8 ft) length of 80mm (3”) PVC storm-water pipe for
transport.
Placing the feed-line and choke/balun into a strong plastic bag provides
protection during transport.
Roll up and unroll carefully, to prevent wires and guys
entangling.
PERFORMANCE:
Markus states a gain of about 5dBd.
I have obtained good contacts with it, with quite good signal
reports.
DIRECTION:
The direction that the antenna faces can be easily
reversed by flipping the array over with the feed-line.
ACKNOWLEDGEMENTS
Thanks to Markus for his original design, and the extra
information that he provided to me.
RETURN
to TOP
CONVERSION
TABLE
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ORIGINAL
LENGTHS
|
MY
LENGTHS
|
|
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10 cms
43 cms
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4”
1.41 ft
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38 cms
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1.25 ft
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|
A
B
C
D1, D2
E1, E2
F1, F2
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10.65 m
7.08 m
5.31 m
1.88
m
3.35 m
2.54 m
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34.94 ft
23.23 ft
17.42 ft
16.0 ft
11.0 ft
8.33 ft
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10.49 m
6.97 m
5.23 m
4.81 m
3.30 m
2.50 m
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34.42 ft
22.87 ft
17.16 ft
15.78 ft
10.83 ft
8.20 ft
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|
J, K
L
M
N
|
9 cm
1.25 m
50 cms
38 cms
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3.5”
4.10 ft
1.64 ft
1.25 ft
|
|
|
|
|
1.43 m
1.93 m
2.31 m
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4.70 ft
6.33 ft
7.58 ft
|
|
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RETURN
to TOP
DIAGRAM
1.
If your browser
can't display this diagram, click here.
(Wait a bit - it's a big file.)
DIAGRAM
2
FEED
POINT DETAIL
NOTE:
-
This is before the centre
insulating cords were added.
-
I used two 50mm (2") long
insulators. These are pulled tight when antenna is erected.
-
Note the hair-pin match, with
adjustable bar during testing.
-
Choke/balun on section of former
plastic bottle. (No pun intended.)
RETURN to TOP
RETURN
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By
John Elliott
VK5EMI
Adelaide,
South Australia
May 2002. |
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EXTRA
NOTES FROM Markus, VE7CA
Two Element Tri Band Portable
Yagi
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The following links may need to be referred to when
reading this text.
http://www.arrl.org/tis/info/pdf/0111035.pdf
http://www2.arrl.org/files/qst-binaries/
"I have had a lot of response from the article
and many tri-band wire Yagi's have been built to date.
The gain over a dipole is approx. 5.5 db. It is very noticeable if
you hang a single dipole along side the triband Yagi and do some
listening.
The triband Yagi is always better, by quite a margin. It is very
similar on transmit to adding a linear. On receive the improvement
in the signal to noise ratio really helps as well.
Here are some further tips to help when you build it:
Assembly:
When you are ready to assemble the array, attach the 20 meter
reflector and the driven elements first and then hang the array between
two
supports, (trees etc.) at about 5 feet above the ground. Pull it
tight so
that the array is fairly flat. It won't want to stay horizontal
because
the combined driven elements are heavier than the 20 meter reflector so
it is
best to support one end of the 2" x 2's so that it is
horizontal. Add the
10 and 15 meter reflectors next using a little less tension than the 20
reflector.
Next attach the feed line.
The last step is to adjust the V slings as
mentioned below so that the antenna is balanced in the
horizontal plane.
All adjustment for lowest SWR if needed should be done with the antenna
raised to its typical operating position.
V Slings:
There are two rope V slings, one on each end. The secret is that
they are not equilateral in shape. The combined weight of attached
dipoles and feed line is heavier than the reflectors. If the
length of the sides of the V are equal, the array will want to turn with
the 2"x2" end supports positioned vertical, with the
dipoles facing the ground and the reflectors facing up. Increasing
the tension on the side of the V that is
attached to the end where the dipoles are also attached causes the whole
array
to turn toward the horizontal plane. Do this by shortening the
length of the side
of the V that is attached to the driven elements. It is quite easy
to adjust in the field. Once it is adjusted, it stays balanced.
Once it is in the horizontal plane, you can change direction 180 degrees
by giving the feed-line a pull. If you pull it hard enough, the
whole
array will begin to turn-flip over. Stop it from turning too far
by
holding on to the feed line once the array has swung over to face the
opposite direction.
Balun:
It is a little unclear regarding the coil of coax at the feed point in
Fig. 1. This is meant to be a choke balun however it is best to
let the coax feed line drop straight down and attach it to the centre of
the hair-pin shorting bar, then make a coil of coax 6 to 8 turns with a
diameter of 4 inches or so just below this point. This balun will
choke off any
RF from flowing along the outside of the coax shield. The centre
of the
shorting bar is neutral potential so no problem attaching the feed line
to it.
You don't have to use 2x2 wood spreaders, several hams have written and
told me they have used bamboo poles, fiberglass poles etc. What
ever
works, it only has to be strong enough that it won't buckle under the
pull
of the ropes at either end.
73
Markus Hansen"
ve7ca@rac.ca
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