Cheap Yagis for 2450 MHz.pdf

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By
K e n t
Britain," WA5VJB
Connecting t h e Radio t o t h e
sky
Cheap
Yagis
for
2450
MHz
his time, at the suggestion of one of
our readers we take the Cheap Yagi
construction projects to 2450 MHz
(photo A). While this is not the highest
frequency for which I have built a Yagi
(that is still 10,300 MHz), it is the high-
est frequency for a "Cheap Yagi" con-
struction project.
The design was more challenging than
I had thought it would be. I try to keep all
dimensions to .1 inch accuracy. Yes, I've
seen published 20-meter beam designs
with dimensions to .0001 inch, but that's
ridiculous. Also, the 2.4-GHz ISM (In-
dustrial, Scientific, and Medical) band is
83
MHz wide. That is pretty wide for a
long Yagi, but after several tries I have
two Cheap Yagis that fit the bill.
When things get small, little things
become big things. Just how the ends of
the elements are cut makes a difference
(figure
1).
My prototype wouldn't tune
up until
I
cleaned up the jagged ends of
the elements. A few quick strokes with a
flat file-or in my case, a second on the
belt sander-fixed the problem. The
antennas covered here have similar
dimensions (figure 2 and Table
I),
and
"1626
Vineyard, Grand Prairie,
7X
75052
Photo
A.
2400-MHz
Cheap Yagis.
I
C--C
2.2"
I
Side View
e-mail: <wa5vjb@cq-vh$com>
SWR
Tweak
1
Cleaned
Up
Driven Element
Figure
2.
Driven element of both the 6-element and
11-element
versions.
Ref
DE
Dl
2.1
1.3
2.1
1.3
'
D2
2.1
2.1
2.1
2.1
D3
2.0
3.0
2.0
3.0
D4
1.9
4.2
2.0
4.2
D5
-
-
D6
-
-
D7
-
DS
-
D9
-
-
1
As
Cut
Figure
I .
Element ends.
www.cq-vhf.com
6
elements
Length
Spacing
2.4
0
2.4
0
:'
.6
-
-
11
elements
Length
Spacing
.6
2.0
5.2
1.9
6.2
1.9
7.3
1.8
8.5
1.8
9.8
*Driven element is perfigure
2
for both Yagis.
Table I. Dimensions (in inches) of the 6-element and
11-element
Cheap Yagis.
Spring
2005
CQ
VHF
37
Photo B. The driven element.
Photo
C.
Dowel drilling.
Photo
D.
Rotated driven element.
that was not by accident. For the sake of simplicity, I forced the
designs to converge. There is no real loss in performance, but
it did take a bite out of my sanity to get to that point.
Photo
E.
Network-analyzer plot of the
I 1
-element antenna.
just so it will go back into the boom. The radius of the bend, and
the distance between the tip and body of the J are not a critical
dimension. Just make it fit and you'll be okay. If you have the
equipment to measure return loss or SWR at 2.4 GHz, the free
tip of the J can be trimmed for best SWR (photo
E).
All elements are l116-inch material.
I
used silicon bronze
welding rod, but 14-gauge copper wire or any .062-inch diam-
eter rod can be used (photo
B).
A drop of Super Glue@ was
used to hold the elements in place. For my prototypes
I
used
store,
'14-inch-square wood from the local home-im~rovement
as it's muih easier to drill.
If you do insist on using l14-inch dowel, taping it to a block
of wood makes it much easier to drill the elements in a straight
line (photo C). You also will need to use a lot of glue to hold
the driven element in place, or flip it 90 degrees when using
dowel (photo
D).
There are several technical reasons for not
wanting to flip the driven element. It creates quite a bit of uncer-
tainty as to exactly where the phase center of the element is.
However, it can be done. Note the flipped driven element on
the PC-board version later on.
The shield of your 50-ohm coax goes to the center of the dri-
ven element. The center of the coax goes to the inter tip of the J
element. Yes, they are offset slightly and not exactly in the cen-
ter of the
J,
but that has been allowed for in the final dimensions.
The bend in the end of the J is going to end up .2 to .25 inch wide
38
While the Yagi does cover the entire
83
MHz of the ISM
band, it does work a bit better in the ham portion of the band
Photo
F.
"But
I
built it exactly to your dimensions!"
CQ
VHF
Spring
2005
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top and bottom of the board, but the back lighting shows the
elements well.
Thus far
I
have developed nine different PC-board Cheap
Yagis from 434 MHz to 5800 MHz with over 10,000 of the
915-MHz versions in the field. In general, start with a design
for a free-space Yagi (figure 3). Shorten the elements about
30% and reduce the element-to-element spacing about 20%
when you generate the PC-board artwork. These are general
factors, and that .062-inch thick PC board looks a lot thicker to
a 11-GHz radio wave than it does to a 400-MHz radio wave
(photo H). Also, the
E,
of the fiberglass tends to drop as you
go up in frequency. While the fiberglass board may have a
E,
rating of 4.0, that is usually measured at 1 MHz and drops to
3.7 or 3.8 in the GHz range. All these factors really cause prob-
lems when designing PC-board log-periodic antennas from 400
Photo
G.
A
5-element Cheap Yagi etched on PC board.
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Figure
3.
Free-space Yagi and PC-board Yagi.
and has good performance in the AMSAT portion. How about
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Since wood doesn't conduct electricity, many seem to assume
it has no effect on the antenna. Well, that's not entirely true
(photo
F).Wood
contains cellulose and moisture, and these give
the wood an
E,,
or dielectric constant.
When light travels through water or glass, it travels more
slowly. The same is true when a radio wave travels through
plastic, air, wood, etc. The
E,
of wood varies quite a bit between
dry balsa wood and damp teak, but through typical construc-
tion woods, the radio wave travels at about half the speed it
does in air. Wood around a wire is a bit more complex, and the
effect varies greatly with wavelength. However, for 1 inch of
wood you need to make the element about one-tenth inch short-
er to allow for the effects of the wood. That's no big deal on 2
meters, but above 400 MHz or so a very thick boom can kill a
Cheap Yagi. Now just put the Yagi i d o n a dielectric material,
and you quickly learn about
E,
effects.
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Spring 2005
CQ
VHF
-
39
"
;
L
,
Photo
G
shows a Cheap Yagi etched on PC board. Taking
the photo was not a lot of fun, since there are elements on the
"GL
1
d
\
I
r
-
,
-
-
3
3
I
L
d L d L < -
-
yr
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11
A guide For Radio Amateurs
By
Ken Neubeck,
WB2AMU
&
Gordon West,
WBGNOA
/
Photo
H.
PC-board log-periodic antennas from 400
MHz
to
1I
G H i
1
In my case, the, is a
big box of prototypes
&
the shack with
trimmed elements that just weren't quite
right. However, once you get the dimen-
sions right, thousands and thousands of
high-quality antennas can be etched with
high repeatability.
I've been asked many times what com-
puter program I use to design PC-board
E, for Quad Builders
Bare Wire vs. Insulated Wire
Making that quad antenna designed for
bare
#12
copper wire out of insulated #12
copper wire moved it down in frequency
about
5%,
didn't it? Your 147-MHz quad
resonates in the 130s, and the element spac-
ing now is all wrong. You can compensate
for this. Make the elements about
95%
as
long, but the smaller elements won't cou-
ple quite as well, so the element-to-element
spacing has to be changed as well. In theo-
ry, all of this can be incorporated into the
new design-that is, when you know the
Er
of the plastic (see main text below)! Just try
going down to the local hardware empori-
um and asking the clerk for the
Er
of the
#12
insulation when measured at
147MHz.
Be prepared for that "deer in the headlights"
stare. The electrical wire insulation mate-
rials are optimized for voltage puncture,
UV
exposure, moisture resistance, flexibil-
ity, etc. The dielectric constant of the insu-
lation is not controlled from batch to batch
or manufacturer to manufacturer. Thus, a
quad design using insulated wire might not
be able to be replicated by others.
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antennas. To the best of mv knowledge,
there is no program for designing them.
For IE3D, HFSS, SONNET, etc., users,
you can use all that computing horse-
power to analyze an existing design, but
those programs do not design the anten-
na for you. These programs also require
you to know things that you rarely know,
such as the
E,
of the PC-board material
over an octave of frequency.
Last, I don't have the space here to
explain, but you
cannot
accurately fre-
quency sweep log periodics with these
programs. I'm waiting for the e-mails to
come pouring in, but you cannot accu-
rately frequency sweep a PC-board log
periodic antenna with these programs.
This is aroad
I
and others have been down!
Are you planning to be at the Dayton
Hamvention@? If the timing belt on my
car doesn't break, I should be in slot 915
selling 915-MHz PC-board antennas. I'll
be happy to show you some of the
200-
plus 13 MHz to
1
l
GHz antennas that I
have designed.
1
8
\,
,-
r r
??
" J d
,
L
2
www.cq-amateur-radio.com;
FAX
your order
t o
u s
a t
516 601-2926
Call
Toll-Free
800-853-9797
40
Next time I'll cover why you don't
e
want to use a l / ~ w a v antenna with those
new
RF
transceiver chips from Chipcon,
RFM, Micrel, etc. This applies to many
ham rigs as well. I'm also trying again to
come up with a 6-meter Cheap Yagi. It's
a mechanical, not an electrical, problem.
As always, some of my best ideas for
this column come and from your e-mails
and questions.
73, Kent, WASVJB
Visit
Our
Web
Site
CQ
VHF
Spring
2005

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