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Author Topic: 1/4 wave yagi?  (Read 2669 times)
N6XXX
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Posts: 6




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« on: April 14, 2004, 06:24:31 AM »

Hi, looking to make a tiny hand-held yagi for the 2-meter band.  At over a yard long the 1/2 wave driven elements are too unwieldy to be carrying around.  Can 1/4 wave elements be used?

Thanks!
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W3JJH
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Posts: 1422


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« Reply #1 on: April 14, 2004, 12:22:28 PM »

You can build a directional antenna array using 1/4-wave elements, but it won't be a yagi.

In fact, most directional antenna for AM broadcast stations are phased arrays of 1/4-wave verticals.

One solution to the portabilty problem is to build your antenna so that it fold or rolls up for transport.  An easy way to do that with a 2-m yagi is make the antenna elements out of sections cut from the metal tape in a tape measure.  The elements can be rolled up and secured to the antenna's boom with  shoe lace or piece or string for transport and will quickly snap out for use when you need them.  Google "tape measure antenna" and check out the resulting links.
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WB2WIK
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« Reply #2 on: April 14, 2004, 02:49:22 PM »

A Yagi requires 1/2-wavelength elements (usually the reflector is longer than this!); however, you have alternatives that perform equally well and are less "wide" for transport:

-A cubical quad uses full-wavelength loop elements, so they're only a quarter-wavelength "wide".  For any given boom length, works every bit as well as a Yagi, and some believe even better.

-You can also take a standard Yagi and bend the elements to form horizontal "vees."  This reduces the "width" of the antenna, but increases its "length" slightly.  The elements can be bent so that the included angle of each "vee" is about 90 degrees, and this hardly impacts anything about the way the antenna works.  The antenna then "points" towards the openings of the vees.  It doesn't matter if you use the antenna horizontally or vertically polarized, the vee-beam Yagi will behave about the same as if the elements were not bent.

WB2WIK/6

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N6XXX
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Posts: 6




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« Reply #3 on: April 15, 2004, 03:27:20 PM »

Very good.  Thanks for the replies and the help!

N6XXX
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WB6BYU
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Posts: 13166




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« Reply #4 on: April 16, 2004, 05:51:26 PM »

I know the problem - I use 2m yagis for Radio Direction
Finding competition - literally running cross-country
through the woods with them.  (Well, OK, I don't run
THAT fast, but you get the idea.)  I'm always looking
for something that will be smaller and ligher, while
still having a good directional pattern.

But first let's look at your particular application.

Is it more important to have something that is smaller
to USE, or smaller to CARRY?  Most of my antennas are
full-sized yagis, but they use tape-measure blades for
the elements - this allows them to bend when they hit
a branch, and even be rolled up for transport.  It
still is full sized for taking bearings, but easily
fits in my carry-on luggage when travelling to an event.

Actually it is possible to make rather high gain 2m
antennas that will pack in a small space - for example,
a wire-and-string rhombic or 8-element yagi, though you
can't easily swing around in your hand!

The quad has a smaller width or height, but is a 3-
dimensional antenna instead of two dimensions.  There
are some that fold up nicely for transport, but I have
problems with the closed loops catching on tree branches
when I try to use one in the woods.  However, it is my
favorite antenna for transmitter hunting from my car
because it doesn't stick up as high above the roof.

K0OV has a design for a "Shrunken Quad" that he uses
for close-in hunting - the loops are about half as
wide as a standard quad.  You can find some information
about it on his web site at www.HomingIn.com

The next smaller antenna I want to try is the Moxon,
which is a closed rectangle perhaps 3/4 as wide as a
standard yagi.  This is a 2-element beam and doesn't
have as much gain as some other designs, but has a very
clean pattern.

Let's take another break and look at what the important
electrical properties are of the yagi for you.  In my
application I want a clean sharp pattern for taking
bearings, but unless the signals are very weak the
absolute gain isn't important.  There are other
applications, such as backpacking, where the gain to
reach a distant repeater is more important than the
sidelobe level or front/back ratio.  This will affect
your choice of antenna also.

Now that we have taken some detours, let's return to
your original question about a 1/4 wave yagi.  Yes, it
can be done, and there two different approaches.  First
is to use quarter wave monopoles fed against ground,
though this isn't very convenient for hand-held use.
(I've seen this done with several mag-mount whips on
a car roof.)  The other approach is to use loaded
elements:  for example, by making all the elements of
a yagi about 18" long and using a loading coil in the
center of each element.  With the right amount of
inductance in each coil you can get a reasonably good
pattern out of such an antenna.  And I may try it out
at some point.  However, there are a number of issues
that you will have to address.

Any method of shortening a dipole will reduce the
bandwidth, and usually results in increased current and
higher losses.  Putting any dipole element in a yagi
has a similar effect due to the lower impedances and
higher currents.  As you shorten an antenna element the
Q goes up and the tuning becomes more critical (again,
the bandwidth decreases.)  All these effects combine
when you are trying to make a shortened yagi!

First you need to use relatively rigid elements, since
the tuning is easily shifted as the elements move.  The
loading coil needs to be high-Q to keep losses down.
The settings of the coils will be critical for a good
pattern, so adjustable coils would be handy, except that
most of them have relatively low Q, which means the
losses will increase.  You may end up with a nice
pattern but only 2dB of gain, or you may find that it
works well only across 500 kHz of the 2m band.  Whether
these are suitable tradeoffs will depend on your specific
application.

Some good reading on the subject is on the W4RNL web
site (www.cebik.com/radio), particularly his articles
on shrunken 40m quads, short beams and operating
bandwidth, and the overview of small loaded yagis.  That
should give you a good idea of the tradeoffs.

Good luck! - Dale WB6BYU
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