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Author Topic: Homebrew beam (( What would you build?))  (Read 1487 times)
2ST123
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Posts: 56




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« on: April 07, 2007, 07:46:40 AM »

If you lived on a hill (( IN INDIANA WHICH IS REALLY HILLY TO BEGIN WITH ))with no trees say within 400 feet and was going to use 2 meter , what antenna do you think would be best for me to build that would be good at say a hight of 30 feet? I want a beam and ground plane . I don't know if it is possible but i have some neighbors within a few miles on citizen band frequencies that run quite a bit of power , so i need a beam that would have i think what is called (( GOOD REJECTION )) to keep them from getting into the back of the beam so bad.. I am just getting into ham radio  and we plan to take our test on the 11th..

                          Thanks
                        Roy & Mary
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W5JI
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Posts: 146




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« Reply #1 on: April 07, 2007, 08:56:36 AM »

It depends upon whether you intend to work FM only or both SSB and FM. On FM, you want a yagi designed to be mounted with the elements vertical and for SSB you need a yagi designed for horizontally mounted elements.

If you buy or build a yagi for FM, the top section of mast should be fiberglass or other non-metallic material to avoid interference with the yagi elements (or mount the yagi offset from the mast).

30-feet height is no problem since it is quite a few wavelengths on 2m. Higher is always better, but for 2m you'll be fine, especially since you have a clear view out some distance.

The CB'ers should not cause you any problem on 2M even if they are running high power. What you are looking for is high front-to-back ratio. Any yagi of 4 or more elements will have good enough f-to-b.

The forward gain will depend upon the number of yagi elements and boom length. If you want to work out to 20-miles or so, a 4 element should be fine. If you want to work the 200 to 500 mile range during band openings, look for 10 elements or more.

Cushcraft, M2 and many others make good yagis for 2m. Make an investment in good, low loss coax and it will be your best investment. The best RG-8 and RG-213 coaxes have a lot of loss at 2m. Try to find some 1/2-inch or 7/8-inch Heliax or cable TV hard line for your feedline. They have low loss at 2m and are a good investment. Times LMR-600 is also reasonable if you can't find some hard line.

As for your ground plane, I would highly suggest a commercial or homebrew J-pole instead. It will give you some gain and does not need a ground plane. There are some very good J-poles which cover both 2m and 440 which are made of aircraft aluminum and are in the $50 to $80 range. Mount the antenna at least 10-feet above your roof level and you should be able to hit the local repeaters.

You will enjoy ham radio. Good luck with your new antenna system.....73.....Jim  W5JI
 
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WB6BYU
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Posts: 13038




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« Reply #2 on: April 07, 2007, 12:04:55 PM »

> what antenna do you think would be best for me to build that would be good at say a hight of 30 feet?

There are lots of antennas that you can build - the choice will often
come down to what skills, material and equipment you have available
and how much you are willing to spend.  Also on what sort of coverage
you want - it doesn't require a really good antenna to cover 20 to 30
miles from a hilltop.  (I easily hit repeaters 100 miles away with my HT
on a 3dBd antenna, but it is almost line of sight.  Your mileage may vary.)

>  I want a beam and ground plane.

My suggestion is to put up the ground plane (or similar omnidirectional
antenna) first to see how it works for you.  With that information you
can decide what sort of a beam you need for those situations where
the ground plane isn't good enough.

A ground plane is easy to build - there are lots of plans for one built
on a chassis-mount coax connector.  I use 18" of brazing rod (not the
welding rod with the thin copper coating that rusts off overnight) for
the center whip and #14 bare solid copper wire for the radials (24"
each.)  The standard dimensions are a bit different, but this works for
me and lets me cut two center elements out of a single 36" length of
brazing rod.

There are also many plans on the internet for J-poles made from
copper pipe.  These work about the same as the ground plane but
are sturdier, though they also cost more in materials.

For a bit more gain I'd consider an interesting design in one of the
ARRL Antenna Compendium volumes that I can't find at the moment
that is basically a home-brew version of the IsoPole.  But I'd suggest
starting with something simple - just make sure that you design your
support mast to make it easy to try out different antennas as you
build them!


Beams - boy, there are a lot of options there.  I probably have 30 to 50
different designs in my database - just for 2m.  Aparently minor changes
in the element lengths and spacings can make 3-element yagis with
very different patterns.  For an understanding of the issues I'd suggest
visiting W4RNL's excellent antenna page, especially this article on choosing
a 3-element yagi:

http://www.cebik.com/yagi/3lyg.html

Although he uses some 10m yagis for examples, I've scaled them to 2m
and used them very successfully.  His 6-element "OWA" yagi is very good
also:

http://www.cebik.com/vhf/owa.html

and his comprehensive study of quads makes it easy to build one for
any desired frequency without any need to tune it:

http://www.cebik.com/quad/2mq.html

I've built these for 2, 3 and 4 elements - they work very well, and using
PVC pipe and copper wire makes them very inexpensive.

Another approach to building beams comes from WA5VJB, called the
"cheap yagi" because they are inexpensive and easy to build.  From a
mechanical perspective I recommend this approach because the driven
element is continuous rather than being split at the feedpoint:

http://www.clarc.org/ArticleRepo/uhf2.pdf



> I have some neighbors within a few miles on citizen band frequencies that run quite a bit of power...

This shouldn't be a problem at all.  The frequencies for 2m (146 MHz)
and CB (27 MHz) are so far apart that there shouldn't be any problems
at all from a couple miles away.  If you DO hear them on 2m, it means
they have so much distortion that they may also be interefering with
the local Police channels and Aircraft emergency frequencies, and folks
can get really upset about that...
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ONAIR
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Posts: 1735




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« Reply #3 on: April 07, 2007, 07:27:16 PM »

    You can build simple 2 meter beams with wires mounted on cardboard or wood.  Fun to build and experiment with!
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KC8VWM
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Posts: 3124




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« Reply #4 on: April 07, 2007, 09:12:07 PM »

...Build mine. Smiley

http://users.belgacom.net/hamradio/schemas/yagi_vhf_antenna.htm

73 de Charles - KC8VWM

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KC8VWM
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Posts: 3124




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« Reply #5 on: April 07, 2007, 09:23:47 PM »

I should mention that my antenna (In Ohio)  made a few VHF contacts into N.J. and another one 590 miles away into a town near Hartford, CT. I have regularly made contacts into Toronto, Canada - Buffalo, N.Y, Chicago, IL and otherwise in the 200-300 mile range on VHF SSB using 50 watts and the yagi antenna located just 35 feet off the ground as shown in the photos below.

http://users.belgacom.net/hamradio/homebrew.htm

73 de Charles - KC8VWM


 
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K5CQB
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Posts: 223




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« Reply #6 on: April 09, 2007, 09:49:56 PM »

http://www.moxonantennaproject.com/
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2ST123
Member

Posts: 56




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« Reply #7 on: April 10, 2007, 05:56:20 AM »

I found the moxon antenna place just before i came here.. Looks like a promissing place and i will study it.. I even posted a question there..

                             thanks alot
                               Roy
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WB6BYU
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Posts: 13038




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« Reply #8 on: April 10, 2007, 10:34:27 AM »

I've built several 2m Moxons.   They are small and lightweight, and are
my wife's favorite antenna for transmitter hunting on foot (at least for
horizontal polarization.)

This is a good example of the tradeoffs to make in choosing an antenna.
A Moxon doesn't have a lot of gain compared to other beam antennas.
For vertical polarization the pattern is very wide:  +/- 120 degrees or
so, with a sharp null in the rear.  So it gives a little gain in a lot of
directions.  Other designs will give more gain in a narrower range of
directions - it all depends on what you need in your specific situation.
If most of the stations of interest are within a 180 degree azimuth then
a Moxon can be fixed and will cover the whole range somewhat better
than a dipole or groundplane.  The Moxon is short enough that it can
be side-mounted on a mast, keeping metal supports out of the field
of the antenna.   For horizontal polarization the pattern is more narrow,
and again it gives a reasonable gain and good F/B ratio in a small space.

But in many cases, especially for VHF, a 2-element quad or 3-element
yagi is still a practical size, and they will have more gain than a Moxon
(and correspondingly a sharper pattern.)  Although somewhat larger,
they can still be built small and light enough to side mount on a mast
if necessary.  In my experience the Moxons were more difficult to build
than quads or yagis, but part of that is the choice of materials that I
happened to use.

As is so often the case on antenna topics, W4RNL's web site has designs
for quads, yagis, Moxons, half square beams, etc., along with pattern
plots and gain figures for each.  That allows you to compare them to
see what designs give the compromise between gain and pattern for
your specific needs.  Be sure to check his  Antenna Options series here:

http://www.cebik.com/ao/ao.html

especially the first three articles, which will help you understand the
tradeoffs in beam design and different construction methods.

Remember that there are at least four SEPARATE things to consider
about an antenna:    (1) the design, which determines the gain and
pattern;  (2) the mechanical construction - the materials used and how
they are connected together; (3) the feed method used to match it
to the feedline; and (4) the frequency it is built for.  Although there are
some interactions (for example, using a larger diameter material
requires a change in the element lengths to implement the same
design, and construction techniques that work for 2m may not be
practical for a 40m yagi) these tend to be independent.  For example, I
can choose a Moxon, 3-element yagi or quad with various options for
the element dimensions and spacings - that is the design. For each of
those I can build it using aluminum tubing, copper wire, or tape
measure blades for the elements, and a metal or insulated boom.
Then I might feed it with a gamma or delta match if I wanted a continuous
driven element, or split the element and connect the coax directly
across it, possibly with further matching from a shunt coil ('beta match")
or a quarter wave transformer.  That same design can be scaled to
2m or 440 MHz or 20m, with appropriate mechanical considerations.

So if you find a particular construction method that seems within your
building capabilities, don't feel constrained that you can only use it to
build the specific antenna you saw that used it.  Similarly you don't have
to build an antenna for the same band that you found in an article
(though you do have to be careful about the relative diameters of the
elements and mounting methods, especially for high-performance
designs.)  And, just because you find a design posted on the Internet
doesn't mean it really works as claimed!


Here are some other interesting yagi designs that are not hard to build:

http://www.mydarc.de/dk7zb/start1.htm
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