BUILD A 2 METER YAGI AND
6 METER
YAGI ANTENNA
USING THE VELOCITY FACTOR OF COAX
TO
DECREASE SIZE OF ANTENNA.
Reduce the
size of your antennas
by using these simple and effective
ideas!
An article and project by Steve, K4MMG
based on ideas by
John, N0KHQ
Dialup users
Please allow time for pictures to
load!
2
and 6 Meter Coax
Antennas
After reading John's (N0KHQ) article and his description for constructing a reduced size Moxon
antenna using coax, I decided
to expand on his concept but in a Yagi configuration. My purpose was to
ultimately end up with antennas that I could just use for monitoring to
catch band openings etc., and be smaller. Additionally I would not have to
use up my limited supply of aluminum tubing.
Each antenna that I will
describe uses John's formula for calculating the coax
lengths ( 984 x .66/your frequency/4= 1/2 of the driven
element), but I did not leave the 20% longer sections as I was
not going to use insulators. All coax ends are shorted together. (See
pictures below) Both antennas have 3-4db of forward gain, good side rejection
and the F/B is reasonable. I do not have any test equipment for lab type
of measurements, but what I have just noted are the results of on the air
tests, and some during band
openings.
2 ELEMENT SIX
METER ANTENNA
The 6 meter antenna has
two elements and is housed in 1/2 inch pvc tubing, using
T connectors to form the yagi configuration, and another T is located
mid-way between the two elements for support. End caps are used to enclose
the pvc ends. The coax feed is inserted into the DE T via a drilled hole
on the T's under side. I used clear silicone to seal the hole.
Using a
MFJ 259, I followed the guidelines as John has described them
in his
articleand cut both the
DE, and the REF coax to length for 50.125mhz. At the DE
remove the shield 1/4inch at
the feed points, attach your coax to these two center conductors
independent from each other . (See drawing below), I found that when the
DE was installed in the pvc tubing the center frequency shifted down
slightly, almost 50 kHz. I used RG58/U coax for my elements. To readjust
this, I cut off approx 1/16 inch off each end piece of
both the DE and REF. I used RG8X for the feed line. The band width for
this antenna is about 1.5mhz for a swr of <
1.3/1.
The final dimensions for the 6 meter
antenna are as follows.
The spacing between the DE and the REF is
31.250 inches.
The total coax length for the DE is
56 inches.
The length for REF is 59
inches.
(See pictures
below)
I
painted the pvc pipe with a combination of Wal-Mart cheap flat black and
walnut to allow it to blend in with the trees. I stuffed some rubber
padding into the pvc tubing to hold the coax elements tight against the
pvc walls.
2 METER THREE
ELEMENT YAGI (Prototype)
The 2 meter antenna
in picture above is a three element yagi and again it uses John's formula
for calculating the length for the DE and REF
@146.0Mhz.
The 1st DIR length was calculated by
reducing the dimensional length of the DE by 3%.
Remove the shield from the DE
about1/4 inch, attach your coax to
these two center conductors independent of each other.
I used
RG58/U for the elements, RG8X for the feed line. The boom for this antenna
is 1/2inch pvc tubing, and the
elements are wooden dowels used to support and hold the coax in place.
Since I will install these elements in pvc tubing later, I just used tape
to hold the elements in place.
The band width is 144-148 for a swr <
1.5/1.
Spacing
for the elements is as follows:
REF to DE 17.250 inches
DE to
DIR 13.250 inches
The REF length is 24.750 inches
DE is
23.50 inches
DIR 22.750 inches.
A picture of the coax configeration
is shown below:
Seal up all ends of the coax, and where
the DE is fed, all with silicone,
if you do not plan to enclose it in
pvc..
This antenna has been used in both a vertical and horizontal
configuration with the same performance results. You can make a two
element version if you want, a little less gain but very portable and
light weight.
An explanation of the abbreviations
DE= driven
element (dipole)
REF= reflector
DIR=
director
More thoughts by
Steve
After building
these antennas I believe they both could be built using a good swr meter
in lieu of the MFJ259. It may take a little longer, but it is
possible.
If you use only an swr meter, just use the
formula,
234/your frequency x .66 = 1/2 of dipole
This means
however you will have to make the complete dipole for testing. The reason
I say this is because you will probably be using a signal source that is
around a watt or more to get the swr meter to indicate. In this instance
unless the antenna under test is away from the signal source RF fields are
likely to cause false readings. This is the advantage of using the MFJ
259.
The performance parameters noted in the description for both
these antennas was referenced to each antennas DE alone, and then the
other elements were added for the final performance measurements.
As
with any homemade antenna the information that I have provided is what I
found out to be the case. If you build one of these you most probably will
end up with something slightly different for the coax element
lengths.
Construction of this type will provide you with a lightweight,
smaller, portable and operational antenna that you can truly enjoy because
you built it yourself! Fabrication does not require any special tools or
equipment.
73 and enjoy, K4MMG.....STEVE
Email Steve for
questions at the address below:
Please note that this is a spam fighting email
address.
You can't click on it!
Just copy it down
on paper and go from there!
marfa at ktc dot
com
Photos by
K4MMG
Editors notes:
Well Guys
and Gals, there you have it thanks to Steve, K4MMG.
This just goes to
show you that with a bit of time, determination and a "can do" attitude
along with a bit of brain storming and a litle money, you can have fun
experimenting with antennas!
So based on comparing the sizes of "normal" antennas using the
standard formulas for a half wave dipole versus using the velocity factor
ideas by John, N0KHQ....The end result is an antenna about 40 %
smaller.
Now who is going to submit plans, diagrams, pics, etc
for the following antennas using these ideas?
What about it Steve or
John?
160 Meter
Inverted Vee?
80/75 Inverted Vee?
40 Meter
Inverted Vee?
2 Element 40 Meter Beam?
Dipoles and on and
on?
Contact this
webmaster via email... n4ujw at hamuniverse.com...
when you have them
ready to share with others on this site!
Disclaimer From the
Editor: All information
on this page represents the author's opinion, experiences or results,
based on his background, or experience with the particular subject in good
faith. This does not necessary imply that the provided information has
been substantiated by actual theory if needed, depending on the article
content, and in no way intended to discredit actual theory.
It should only be regarded as an outlet for him
to publish his desire to
share his thoughts, experience, methods, results, performance, or his
ideas with you for your information if you choose to use it for reference.
Experimenting is the "fun" part of Amateur
Radio!
N4UJW
You can check out our latest gre preparation certkiller
and examsheets network+ certification exams
written by our certified teams to help you pass braindumps practice ged test.You can also purchase testking ccna books. Our
testinside security+ certification practice
test is simply excellent in quality.
