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Author Topic: 2m beam ideas/suggestions?  (Read 2824 times)
KF7GTU
Member

Posts: 36




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« on: July 16, 2012, 11:23:24 AM »

Hello folks...

I am looking to homebrew a 2 meter beam for long range (less than 150 mile) repeater and shorter distance simplex contacts. I need some advice on suggestions.

The reason I need advice is because I have 50 feet of 1/4" copper refrigeration tubing. I can't find a single plan out there that would use this material and I am beginning to wonder if it is even worth trying to use it for this project. I was thinking that some 1/4 copper tubing a a large wooden dowel or 2x2, etc. might make a pretty decent beam. If I was to dedicate 25 feet of the tubing, I'd image a pretty strong yagi setup.

Any suggestions, advice, alternatives would be appreciated. Thank to all who respond, have a splendid week.

Jason
KF7GTU
Bandon, Oregon
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KA4POL
Member

Posts: 2028




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« Reply #1 on: July 16, 2012, 11:42:42 AM »

Polarization is a problem. Repeaters are vertical and station to station is usually horizontal. You could consider a circular polarization antenna though.
Copper does not make a good material for an outside antenna. I'd use aluminum. Keep in mind that at these frequencies the currents do go via the skin, i.e. outside. You'll find lots of designs via Google or any other search engine.
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WB6BYU
Member

Posts: 13334




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« Reply #2 on: July 16, 2012, 12:40:17 PM »

In general, the element material doesn't make much difference in the electrical
performance of a yagi (unless it is steel or other magnetic material).  The element
diameter, however, is important.  In most cases you can take a yagi that
is designed for, say, 1/8" elements, and adjust it to use 1/4" elements.  So if
you find a yagi design that you like using someother material (such as 8mm aluminum
tubing) it can be adjusted to get similar results using 1/4" copper (by lengthening
the elements slightly in this case.)

Where the difference in element materials is important is the mechanical construction:
copper is softer and heavier than aluminum.  That means that the same length of yagi
will need a stronger boom and/or additional bracing when built using copper tubing.
The elements will sag more (if horizontal), and will tend to bend more over time due
to wind, birds sitting on them, etc.

So you don't see designs specifically calling for 1/4" copper tubing because it isn't
necessarily a good choice mechanically (and because aluminum is cheaper), but that
doesn't mean you can't use it to build yagis.  I used to have a spreadsheet around
that I wrote to scale yagis for changes in element diameter and frequency:  if you
need it I'll look around and see if I can dig it up.


My recommendation would be to start with something around 6 elements, perhaps
a boom length between 4 and 7 feet.  That is practical to build without worrying too
much about the mechanics, and it should give a noticeable improvement in
performance.  That will give you some idea of how much more gain you need, and
you can choose further designs based in that.


The WA5VJD "Cheap Yagi" designs are great for homebrew:

http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf


Here are some other links that might give you some ideas:

http://www.g0ksc.co.uk/
http://commfaculty.fullerton.edu/woverbeck/quagi.htm
http://www.k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf.html
http://www.mydarc.de/dk7zb/
http://www.yu7ef.com/           (he includes dimensions for 1/4" elements)
http://www.cebik.com/content/a10/vhf/2mowa1.html


GM0SEK has some good general information in his Long Yagi Workshop:

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


And you can model your own yagi designs online:

http://fermi.la.asu.edu/w9cf/yagipub/index.html
« Last Edit: July 16, 2012, 12:47:07 PM by WB6BYU » Logged
KA4POL
Member

Posts: 2028




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« Reply #3 on: July 16, 2012, 11:20:10 PM »

Let me add that increasing the diameter of aluminum against copper by a factor of 1.6 results in the same conductivity value as copper. The weight then is only 1/3 that of copper. Today aluminum alloys have very good data concerning strength. So be careful with those instructions asking for copper.
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N2RRA
Member

Posts: 645


WWW

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« Reply #4 on: July 23, 2012, 07:50:10 PM »

In general, the element material doesn't make much difference in the electrical
performance of a yagi (unless it is steel or other magnetic material).  The element
diameter, however, is important.  In most cases you can take a yagi that
is designed for, say, 1/8" elements, and adjust it to use 1/4" elements.  So if
you find a yagi design that you like using someother material (such as 8mm aluminum
tubing) it can be adjusted to get similar results using 1/4" copper (by lengthening
the elements slightly in this case.)

Where the difference in element materials is important is the mechanical construction:
copper is softer and heavier than aluminum.  That means that the same length of yagi
will need a stronger boom and/or additional bracing when built using copper tubing.
The elements will sag more (if horizontal), and will tend to bend more over time due
to wind, birds sitting on them, etc.

So you don't see designs specifically calling for 1/4" copper tubing because it isn't
necessarily a good choice mechanically (and because aluminum is cheaper), but that
doesn't mean you can't use it to build yagis.  I used to have a spreadsheet around
that I wrote to scale yagis for changes in element diameter and frequency:  if you
need it I'll look around and see if I can dig it up.


My recommendation would be to start with something around 6 elements, perhaps
a boom length between 4 and 7 feet.  That is practical to build without worrying too
much about the mechanics, and it should give a noticeable improvement in
performance.  That will give you some idea of how much more gain you need, and
you can choose further designs based in that.


The WA5VJD "Cheap Yagi" designs are great for homebrew:

http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf


Here are some other links that might give you some ideas:

http://www.g0ksc.co.uk/
http://commfaculty.fullerton.edu/woverbeck/quagi.htm
http://www.k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf.html
http://www.mydarc.de/dk7zb/
http://www.yu7ef.com/           (he includes dimensions for 1/4" elements)
http://www.cebik.com/content/a10/vhf/2mowa1.html


GM0SEK has some good general information in his Long Yagi Workshop:

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


And you can model your own yagi designs online:

http://fermi.la.asu.edu/w9cf/yagipub/index.html

I believe another difference using different diameter elements is band width. It's known that the wider elements are the broader in band width in reference to resonance you will obtain.

Try looking into building a Moxon. These generally are 2 element beams that are very broad banded antennas that give you great Front-to-back and forward gain results. You can do these easily with the copper tubing and the way their built are sturdy because of its semetrical shape. Ive built these of wire and have worked 100 miles away and even Florida on 2m SSB band openings.

Now if FM simplex and repeaters are your thing then another beam is a Bobtail beam. Look up 2m Bobtail beam. This might be just what you want with 13dbd of forward gain with should kick butt.

73!
« Last Edit: July 23, 2012, 07:52:31 PM by N2RRA » Logged
WB6BYU
Member

Posts: 13334




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« Reply #5 on: July 23, 2012, 10:37:44 PM »

Quote from: N2RRA

Now if FM simplex and repeaters are your thing then another beam is a Bobtail beam. Look up 2m Bobtail beam. This might be just what you want with 13dbd of forward gain with should kick butt.



But you won't get anywhere near that much gain out of a bobtail beam.

The late W4RNL modeled a 6-element Bobtail Beam (two Bobtail elements, one driven, one
operating as a reflector) and found the gain to be a little over 6dB better than a dipole.
A good 3-element yagi can achieve that with much less mechanical complexity.  The
6-element yagis will outperform the Bobtail beam by roughly 2dB, depending on the
specific design.



For geeks who want to follow the details, the article is here:

http://www.cebik.com/content/a10/vhf/hs3.html

The final plot shows a gain of 13.41 dBi (NOT dBd), which includes ground reflection.
His comment on the single Bobtail is that the gain is about 3dB better than a dipole,
and it was 10.15 dBi, so we can infer that the dipole would be around 7.15dBi at that
height over that particular ground.  That means that the final version of the beam
has 13.41 - 7.15 = 6.26dB gain over a dipole.  In free space, which is how most
beam patterns are displayed, that would be about 8.4dBi.  By comparison, I have
several 3-element yagi designs that achieve 8+ dBi.  One of the WA5VJB 6-element
"cheap yagi" designs optimized for the FM portion of the band shows about 10.8 dBi
and is less than 6' long.
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