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Author Topic: 2 meter yagi  (Read 4335 times)
WM8E
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Posts: 15




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« on: January 31, 2011, 08:46:03 AM »

I am building a 5 or 6 element 2 meter yagi on a square piece of boom 79 inches long (thats what I have to work with). I have some 3/8 inch alum tubing. I am going to feed it with a gamma match 11 inches long constucted of alum tubing with a piece of rg-8 insulated center conductor. Years ago when we built beams we simply used 16" spacing on all elements.My question is what would be the best number of elements I could use on a boom 79 inches long and if I am cutting them for 146.52mhz what would be the spacing and length of each element ( please in inches and feet not wavelengths).I am interested in what other folks have homebrewed. thanks and 73 WM8E
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WB6BYU
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Posts: 13353




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« Reply #1 on: January 31, 2011, 01:35:05 PM »

There are a lot of options out there.  I'm partial to the OWA antennas (from W4RNL and others.)
Here is one family of designs:

http://www.cebik.com/content/a10/vhf/2mowa1.html

The 6- and 7-element versions should work, even if you have to shorten it a bit.  One advantage
of these designs is that they use a direct 50 ohm feed so you don't need the gamma match, and
performance is pretty consistent across the whole 2m band.


Another source of designs is DK7ZB:  some have direct 50 ohm feed, but the 28 ohm versions have
better performance (and are fed using a pair of 75 ohm lines.)

http://www.mydarc.de/dk7zb/


My favorite yagi designs for simplicity are WA5JVB's "Cheap Yagi" designs.  I've built these for
several frequencies from 121.5 MHz to 732 MHz.  They give good gain with direct coax feed.

http://wa5vjb.com/references.html


Here is a set of high performance yagi designs from G0KSC:

http://www.g0ksc.co.uk/


Look through and see what designs attract your attention as being easy to build and meeting your
performance needs.  We can then adapt them for 3/8" elements if needed, and move the center
frequency around, shorten them to fit your boom, etc.  While constant element spacing can give
you pretty close to maximum gain, the newer designs with variable spacing allow much better
control of F/B ratio, sidelobe levels, and feedpoint impedance.

One other thing we need to know, however, is the size and material you are using for a boom.
There is a correction factor that needs to be applied due to proximity to a conductive boom,
and it is different if the element is attached to the boom, above it, or passes though it without
making contact.  Here is some further information on how element mounting affects length, as
well as some recommended construction notes:

http://www.ifwtech.co.uk/g3sek/diy-yagi/index.htm


So IF you are using a non-conductive boom, most sets of dimensions can be simply converted
to 3/8" elements (and shifted in frequency if needed.)  With a conductive boom we also need to
know the boom width and mounting method.  You are looking at a short enough yagi that it
could be built using PVC pipe or fiberglass rod for the boom.

Let us know what designs you like and we can make tweaks on it for you.
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WB6BYU
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Posts: 13353




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« Reply #2 on: January 31, 2011, 08:30:04 PM »

Here is a possible set of dimensions, a modified version of W4RNL's 7-element OWA yagi.
This has direct 50 ohm feed (under 1.3 : 1 across the whole band) and about 11dBi gain,
adjusted for 79" boom (but not allowing mounting space on each end) and 3/8" elements
and rounded off (mostly) to convenient measurements:

element . . .length" . . . spacing from reflector
ref . . . . . . . 41"
driver . . . . . 39.6" . . . . 9
director 1 . . 36.5" . . . . 13.5
director 2 . . 36" . . . . . 25
director 3 . . 36" . . . . . 40
director 4 . . 36" . . . . . 59
director 5 . . 34" . . . . . 79

This assumes an insulated boom - the elements will have to be lengthened for a metal boom,
depending on how they are attached to it.
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AA2JZ
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Posts: 18


WWW

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« Reply #3 on: February 10, 2011, 02:53:59 PM »

VE3SQB has a neat yagi-uda design software available for free...
hamuniverse.com/ve3sqbprog.html
Just follow the element diameter, etc and the software will give you the
required data....Ive built several vhf antennas and they all work just fine.
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KD5GR
Member

Posts: 102




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« Reply #4 on: November 17, 2012, 07:56:07 PM »

Here is a possible set of dimensions, a modified version of W4RNL's 7-element OWA yagi.
This has direct 50 ohm feed (under 1.3 : 1 across the whole band) and about 11dBi gain,
adjusted for 79" boom (but not allowing mounting space on each end) and 3/8" elements
and rounded off (mostly) to convenient measurements:

element . . .length" . . . spacing from reflector
ref . . . . . . . 41"
driver . . . . . 39.6" . . . . 9
director 1 . . 36.5" . . . . 13.5
director 2 . . 36" . . . . . 25
director 3 . . 36" . . . . . 40
director 4 . . 36" . . . . . 59
director 5 . . 34" . . . . . 79

This assumes an insulated boom - the elements will have to be lengthened for a metal boom,
depending on how they are attached to it.

How high off the boom would the elements have to be placed to use these dimensions on a 1"X1" aluminum boom?

Thanks,
Chas.
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WB6BYU
Member

Posts: 13353




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« Reply #5 on: November 17, 2012, 09:12:53 PM »

Good question!

I'd start by reading GM3SEK's Long Yagi Workshop page, especially the section
on Element and Boom Corrections (scroll down the page and look for the yellow block.)

You can also see how DK7ZB builds his antennas with a square boom here:
http://www.mydarc.de/dk7zb/2m-short/detail.htm

The elements are mounted on insulators just above the boom, held in place with a
single screw.  I suspect that approach would work in your case.  (I should note that
other designers would still insist on applying a length correction factor to these
antennas, but it may be small enough to ignore.)




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N8CMQ
Member

Posts: 379




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« Reply #6 on: November 24, 2012, 06:38:24 PM »

Two meters is a nice band to experiment with.
 
 If you have a field strength meter, you can play with
element spacing and element length to get maximum
front to back ratio or maximum gain.
 
 What you might find tho, is the antenna handbook
already has the theory worked out, all you need to do
is plug in the center of band frequency in the formulas
to get the measurements.
 Just be aware of the conductive vs. non-conductive
boom effect.
 
 I did that with a three element ten meter beam years
ago, and I was pleasantly surprised by the results!
 
 As with most antennas, reciprocal measurements can
be just as useful. You just have to have a two meter
signal close by to get an idea of how good the pattern is.
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