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Author Topic: 2M YAGI HIGH GAIN WIDEBAND FM  (Read 5618 times)
KA5KLA
Member

Posts: 5




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« on: April 14, 2001, 06:13:03 PM »

   ANYONE HAVE SCHEMATIC FOR A 2M HIGHGAIN 144-148
YAGI FM BEAM ANT.WITH LOW SWR CENTERED ON 147.00
MGZ FOR NEWBIE TECH.HAVE TO BUILD MY OWN DUE TO COSTS.
        THANKS,
             73
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WB6BYU
Member

Posts: 13331




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« Reply #1 on: April 16, 2001, 04:17:57 PM »

There are a lot of tradeoffs in yagi antenna design, so first, some
more questions:

1)  how much gain is needed?  5dB?  10dB? 20dB?

2)  what are the mechanical limitations for the desired application,
such as wind loading, boom length, portability?  Does it need to be
rotatable, or can it be fixed in one direction?

3)  what are the construction parameters - do you have a machine
shop available?  a lathe?  hacksaw? just a pocket knife?  What
materials do you feel comfortable working with - copper pipe?
welding steel?  threading aluminum?  PVC pipe?  wood?

4)  if the low SWR is centered at 147.0 MHz, what is the allowable
SWR at the band edges?

Most yagis up to 7 elements or so probably have enough bandwidth
to cover 144 - 148 MHz, so whether the minimum SWR is at 146
or 147 is really irrevelant.  Longer antennas will have more gain,
but typically narrower bandwidth (though there are some tradeoffs
to be made here.)  Another thing to remember is that the gain drops
off more quickly on the high frequency side than the low frequency
side (because all those directors start looking like reflectors at the
higher frequencies.)  The longer antennas become more complex
mechanically, needing a larger diameter boom and/or stays to keep
the boom from sagging.  Also, higher gain means a sharper pattern,
so aiming the antenna must be more precise to take advantage of
the extra gain.

The SWR is not necessarily the best indicator of the bandwidth of
a yagi, however.  The gain, side-lobe level, and front-to-back
ratio will all change with frequency.  Actually, the feed point
impedance for many yagis is NOT 50 ohms:  the yagi is designed
for best performance (as a trade-off among several parameters)
then some sort of impedance matching is applied to the driven
element to make it match the feedline.

A good resource on many of these design issues (including plots
of the gain vs. frequency, etc.) is W4RNL's site www.cebik.com.

All that said, here is some advice from my own experience.

1)  Antenna height is often more important that gain, particularly  on
VHF.  That is, a smaller antenna on a higher mast may work better
than a higher gain antenna on a shorter mast.

2)  If you are trying to reach a distant repeater, they you want
vertical polarization, of course.   But if you want to maintain simplex
communication with a specific site over an obstructed path, then
horizontal polarization (at both ends) may be better.

3) On an obstructed path, the strongest signal may not always be
in the direction of the other station.  Check around for reflections
from mountians or other sources of indirect paths.

4)  Instead of using a single long yagi, consider using a pair of
shorter ones mounted side-by-side.  The mechanics and
construction are often simpler, and if they are mounted on the ends
of a common cross-bar then the vertical mast has less effect on
the pattern.  The SWR bandwidth is generally wider when you use
several smaller yagis than one larger one.

5)  unless you have already run some tests, you probably don't
really know how much gain is required in your situation.  Perhaps
the best solution is to try out some inexpensive antennas to see
what works for you, then build one in a more robust version.
Yagis are actually quite simple to build.  I've built a number of them,
the latest took me 20 minutes or so.  The key to simple construction
is to use PVC pipe for the boom and #8 aluminum ground wire (from
Radio Shack, or large diameter aluminum electric fence wire)
for the elements.  Sure, this won't hold up in a
tornado, but you can try several designs for a very small investment
in parts.  Construction is simple:  cut a piece of thin-wall PVC pipe
a bit longer than the specified boom length.  Mark the element
positions on the boom and drill holes in the pipe with a 1/8" drill.
Straighten the wire enough to measure one element, cut it to
length, and stick it through the hole in the boom.  Secure with tape,
glue, or a rubber band so it doesn't slide through.

Now, the only thing remaining is to attach the feedline.  After trying
several approaches, this is what I have had the best luck with.
First, make a 4 : 1 coaxial balun.  (This consists of an electrical
quarter wavelength of coax with one end connected to the feedline
and both ends of the shield connected together.)  On the two ends
of the balun center conductor, attach about 12" (30cm) of #18
solid, insulated wire.  Mark the middle of the driven element and
measure out about 2" (5cm).  Lay the midpoint of the insulated wires
over this point on each side of the boom, and wrap the insulated
wire around the driven element in a snug, compact coil until you
run out of wire.  Attach the balun to the boom temporarily, point the
antenna straight up (preferably outdoors) and check the SWR.
Adjust the match by sliding the coils along the driven element,
keeping them about equally spaced either side of center.  You can
also do some matching by changing the shape of the wires from the
driven element to the balun somewhere between a triangle and a
"T" shape.

I built a pair of 5-element yagis using this technique and they
worked quite well.  (I don't have the dimensions handy, but can
probably find them if needed.)  In that case, after tuning each yagi
individually to 50 ohms, I connected each through a 3/4 wave
section of 75 ohm cable to a T connector, which gave a good
match for the pair of them.  The crossbar was also PVC, using
standard PVC "T" fittings at each end and for the boom.  I did
find that I needed a short length of dowell (or broomstick) in the
crossbar to keep it from sagging under the weight of the two yagis.

Another approach is a Quagi.  The ARRL Handbook and Antenna
Book have carried a number of designs by N6NB (formerly K6YNB).
These are simple to build in various lengths.

Also remember that the dimensions for a yagi are dependent on the
diameter of the elements and how they are connected to the boom.
So if you find a design which uses 1/2" tubing, or copper pipe
soldered to the boom, you can't just build it with the same
dimensions out of #8 wire in an insulated boom and expect to get
the same performance.

That should give you something to chew on.  If you post back with
more specific information I'll try to help you find a design which will
meet your needs.

Good luck! - Dale WB6BYU
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KA5KLA
Member

Posts: 5




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« Reply #2 on: April 16, 2001, 06:09:56 PM »

    THANKS FOR THE REPLY.I WOULD PREFER USING HEAVY DUTY PVC PIPE BECAUSE OF THE AREA THAT I AM IN IS TORNADO ALLEY AND WE GET A LOT OF FAIRLY HIGH WINDS,OF COURSE I KNOW I CAN'T BUILD TO BATTLE TORNADOS BUT AT LEAST I WOULD LIKE TO WEATHER THE REGULAR STORM WINDS
AS BEST I CAN.I HAVE A VERY LARGE RADIO SHACK VHF/UHF/FM TV ANTTENA THAT I COULD STRIP DOWN AND I HAVE A LOT OF 12 GAUGE BARE WIRE WHICH IS I GUESS NOT
LARGE ENOUGH.I WOULD MAINLY USE FM,I AM SHOOTING FOR SOMETHING THAT CAN REACH DISTANT REPEATERS APPROX.100
MILES AS WELL AS MY LOCAL REPEATER THAT I HAVE A LOT
OF TROUBLE KEYING UP DUE TO MY LOCATION ON MY LITTLE
RINGO VERTICAL,I WOULD ALSO LIKE TO USE SIMPLEX FOR
THE LOCALS.I HAVE AN ALL-MODE RADIO INCLUDING SATELLITE.A PAIR OF STACKED YAGIS SOUNDS GOOD FOR FM
MAYBE A PAIR STACKED VERTICAL FOR FM AND A PAIR HORIZONTAL?? FOR SSB/CW WITH A POLORIZATION SWITCH USING TWO COAXES?I HAVE ALL THE TOOLS I NEED WITH THE
EXCEPTION OF THE LATHE.
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« Reply #3 on: April 16, 2001, 10:33:47 PM »

hi,
I don't want to talk you out of the yagi,
however, how about a quad beam for 2m
five elements with 11 db gain for under
$ 10 to build and feed with coax ?

QST has plans for this antenna that I build
out of #12 bare copper wire (strip insulation
off of some romex house wire I had saved)
use a pressure treated 2x2 (better quality
and less warping then std. pine 2x2)

here is a link for you:
www.qsl.net/soundcardpacket/index.html

good luck, post back and tell us what you did.

plus, you can always do the yagi later !

73 clay
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« Reply #4 on: April 16, 2001, 11:25:02 PM »

link correction follows:

http://www.softcom.net/users/kd6dks/quad.html

sorry, this cut/paste is not too good for
those like me who can not type properly !!

by the way, you can also stack the quad beams
and they can be fed vertical or horizontal
polarity.

73
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WB6BYU
Member

Posts: 13331




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« Reply #5 on: April 17, 2001, 03:45:20 PM »

If you have a supply of #12 wire, then a quad is probably a better
way to go.  I've built several of them using PVC pipe and solid,
bare copper wire.  I use them for transmitter hunting - which means
sticking out the window of the car at freeway speeds, so the
construction is probably adequate for your prevailing winds (even
if it wouldn't survive a full tornado.)

One reason I use the thinwall pipe instead of the thicker stuff is
that the added weight of the thick-wall pipe makes the antenna sag
more.  Of course, you can use whatever materials you have handy,
including pipe, wood, fiberglass, etc.  But it still may be worthwhile
to build a cheap prototype which will allow you to assess whether
the antenna design has enough gain for your needs - if so, you can
plan to build a more robust version.

One advantage of a quad is that only the driven element needs to
be rotated to change the polarization.  Some hams have arranged
a way to rotate either the whole antenna or the driven element from
the ground to change polarization (perhaps using a rope tied to a
lever on the boom?)

Stacking one antenna above the other (instead of side-by-side)
gives about the same gain, but a wider pattern in the horizontal
plane (which makes aiming less critical) and a narrower pattern in
the vertical plane.

A 5-element quad seems like a good starter - if it seems to work,
you can add a second.  I'll dig up my dimensions for a 5-element
version that we tested the other day, which had a nice pattern.

The first volume of the "ARRL Antenna Compendium" had a good
article on quad antenna design, which included the dimension
corrections for different wire sizes.  It also showed the important
difference between designs with an odd number of directors and
those with an even number:  the bandwidth is better with an odd
number of directors.  This means that the constructional tolerances
are not as critical with 5 elements as they would be with 4 or 6
elements.

A month or so back in this Forum there was a discussion on 2m
yagis which included several good web sites for antenna
construction plans.

Good luck! - Dale WB6BYU
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« Reply #6 on: April 17, 2001, 07:24:01 PM »

while you visit www.cebik.com

go to vhf section (technicals)
go to half square for 2 meters.

You can toss this together for $ 2.00
in pvc pipe and #12 bare copper wire.

It has 3 units more gain then the jpole
and is very easy to raise up a pole.

It has good front/back results with
deep nulls on the sides.

how about using a hor. and vert. feed quads
together, ready for the satellites ?

73 clay
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KA5KLA
Member

Posts: 5




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« Reply #7 on: April 17, 2001, 09:49:45 PM »

   I have been getting a lot of good ideas and I like
both the stacked yagi and quad ideas.I surely appreciate the input.This is a great site and a great
bunch of guys.
                THANKS AND 73
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WB6BYU
Member

Posts: 13331




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« Reply #8 on: April 19, 2001, 12:09:15 PM »

Here are specific dimensions for two 2m antennas:  my 5 element
quad and a scaled version of N6NB's 8 element Quagi.

5 element quad using #12 bare wire:
reflector circumference 85 5/8" or 216.5cm
driven element circumference 82" or 206.7cm
director circumference 79 1/8" or 200.7cm
spacing: ref to de and de to dir1 is 11" or 28cm
remaining directors are spaced 13" or 33cm

The elements are strong enough that, in most cases, they
can be supported on two sides by a single crossbar connected to
the boom.  I use PVC pipe for the boom and spreaders, with 4-way
couplers to connect them.  A very rugged version can be built with
lengths of round fiberglass (used for fence posts for electric fences)
on the diagonals.

N6NB's original Quagi was designed for weak signal work centered
on 144.5 MHz.  I've scaled the dimensions to center it on 147 MHz.
The Quagi uses a quad loop for the reflector and driven element,
and yagi directors.  This has 8 elements on a 14' boom.  It features
direct connection of the coax to the driven element.

Reflector:  84 3/16 " or 213.8cm
driven element 80 5/8" or 204.7cm
(both loops made from #12 wire, though it appears that he used
insulated house wire and left the insulation on.)

directors: 35 5/16" to 34 3/8" in 3/16" increments
......or 89.7cm to 87.2cm in 5mm increments
made from 1/8" or 3mm diameter wire or rod

Spacing:  reflector to driven element: 21" or 53.3cm
driven element to first director 15 3/4" or 40cm
first director to second director 33" or 83.8cm
second director to third director 17 1/2"  or 44.5cm
and the remainder of the elements 26.1" or 66.3cm

This was built on a 14' (4.3m) boom made of 1" x 3" (7cm x 2cm)
wood, tapered to 1" (2cm) square at the ends.  Optimum stacking
distance for two such antennas is about 11' (3.35m).  Other
materials can be used, but it must be non-conducting.

I hope this helps.  Good luck! - Dale WB6BYU
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