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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 picture 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 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 about 1/4 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 1/2 inch 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)
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
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!
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