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Author Topic: Help With Tape measure antenna design  (Read 7669 times)
N9VKC
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Posts: 33




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« on: June 10, 2004, 11:09:17 PM »

Hello all

  I'm putting together a Foxhunting antenna and installing my offset attenuator into a boom section.  my question is for those of you who have built these little antennas do you prefer the 2 element or the 3 element versions and why do you prefer what you have.  Right now I'm leaning towards the 3 element and my integral offset attenuator.

Chris  n9vkc
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WB6BYU
Member

Posts: 13337




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« Reply #1 on: June 11, 2004, 04:07:25 PM »

Chris -

I've made several versions of tape measure yagis, all
with 3 elements.  The primary advantages I find are
(1) bearing accuracy (narrower beamwidth) and (2) you
still have some directivity if you happen to break an
element while bashing through the bushes.  Generally
2-element yagis with good front/back ratio have a low
input impedance, which often means higher Q (reduced
bandwidth.)  Whether this makes a difference to you
will depend on the range of frequencies you use for
hunting.

As a side note: the orignal WB2HOL tape measure yagi
works reasonably well with just the driven element and
reflector if you need to use a shorter antenna for some
reason.

Most of the 2-element antennas I see in competitions are
the HB9CV-style phased arrays, which are capable of
better performance than straight parasitic elements.  
Currently I'm experimenting with a Moxon 2-element
beam, though doesn't seem suitable for tape measure
construction since the elements are bent.  But it is
nice for a smaller antenna - about 11" wide and 30" long.

I strongly suggest a visit to W4RNL's antenna page for
turorials on the characteristics of 2- and 3- element
yagis, phased arrays, Moxons, etc.:

www.cebik.com/radio.htm

I particularly liked his series on making a 3-element
yagi, and all the tradeoffs that involved.  (I think
this appears in the "upper HF" section of the index,
since it was done for 10m.)  I've adopted his "middle
length" design for my 2m tape measure beams, and the
"short length" one for yagis on 121.5 MHz because the
boom lengths are practical, but you may find that a
longer or shorter boom is better suited to your common
hunt conditions.

Personally I look for beamwidth (more than total gain)
and no spurious responses within 20dB of the peak as
desirable traits.  I always hunt the peak instead of
the null, so the exact lobe structure in the rear of the
pattern doesn't matter that much.  Some of my yagi
dimensions appear in the Equipment section of the
IARU Region 2 ARDF web site:  www.ardf-r2.org
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WB6BYU
Member

Posts: 13337




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« Reply #2 on: June 11, 2004, 04:12:38 PM »

One construction note:

I've seen a number of tape measure yagis break at the
screw holes in the elements.  The best method I've
found so far is to use fiberglass strapping tap to
secure the element to the PVC pipe fittings - this
avoids the localized stress of attaching it with screws
through the tape.  And it is really quite simple!

Typically I make the elements from two sections of
tape:  one full length, plus a half-length (or so) piece
centered in the middle for extra stiffening.  Bind the
ends of the shorter one to the longer elements with
tape or heatshrink.  A single screw through both pieces
in the center helps to keep them aligned properly, but
the strapping tape takes the stress and keeps the
metal from tearing at this point.
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KA8VIT
Member

Posts: 40


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« Reply #3 on: June 24, 2004, 11:52:23 AM »

I have been a couple of three elements.  The design has a nice 50db null off of the back end.

Works well.  I have a schematic for a nice attentuator
as well.

You can see one of mine here:
http://www.qsl.net/ka8vit/TMBeam/TMBeam.htm

73,

Bill  KA8VIT
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N9VKC
Member

Posts: 33




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« Reply #4 on: July 05, 2004, 12:21:43 PM »

Thanks for all the info.  I Plan on making the antenna tonight.  Our next fox hunt is this friday evening and I'm the fox   so I guess I won't need the antenna til next month however I can get some early practice in hi hi.   I think I'm going to stick with a fairly easy 3 element beam to start with and see how that does.

Thanks again EV1

73

Chris N9VKC
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WB6BYU
Member

Posts: 13337




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« Reply #5 on: July 06, 2004, 03:11:08 PM »

There are two common widths of tape measure:  5/8" and
1" (or thereabouts.)  The optimum dimensions will change
somewhat depending on the size you use.  (I think the
WB2HOL design used 1" material.)  If you are modeling
an antenna in software, the effective diameter of a
flat surface is about half the width, so a 1" tape is
reasonably modeled as a 1/2" diameter cylinder.  Except
for the capacitive coupling between the elements, which
changes depending on the orientation of the elements
to each other.  Some day I'll run a test to see how
much difference this REALLY makes!

The 1" tape is best for general use, though I've used
the thinner stuff for light weight versions.
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WB6BYU
Member

Posts: 13337




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« Reply #6 on: July 06, 2004, 03:14:43 PM »

By the way, broom handle holder clamps are a convenient
way to mount equipment to the boom of a yagi.

There are a couple different designs, but they are
designed to screw onto the wall and have a broom handle
pushed into the bracket.  Usually just a bent piece of
spring steel.  Mount it on your receiver/attenuator or
sniffer and it will just snap onto the boom.
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AE7GL
Member

Posts: 62




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« Reply #7 on: July 06, 2004, 08:23:16 PM »

Question:  Are these just for recieve or can you transmit as well?

It looks like a neat project.
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WB6BYU
Member

Posts: 13337




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« Reply #8 on: July 07, 2004, 03:58:05 PM »

You certainly can transmit through them - makes it a
good choice for backpacking, etc. because if you don't
glue the PVC pipe together you can take it apart and
literally roll up the elements to fit it in a small
space.

The antenna efficiency is not quite as good using the
steel elements as it would be using aluminum or copper
due to hysteresis losses.  I haven't quantified the
difference - may have a chance to run comparisons with
a standard yagi later this month.  But I don't think
the difference is more than 1dB or so.

I had an occasion to transmit on my DF antenna several
years ago when I flew into Dayton for Hamvention and my
ride was waiting in Cincinatti.  Couldn't hit the
repeater with the HT, but after assembling my DF beam
(in front of the airport) I was at least able to break
someone on reverse pair and make the necessary
arrangements.
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N7AOB
Member

Posts: 10




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« Reply #9 on: July 07, 2004, 04:26:58 PM »

Go with the three element version. Also, using double sided tape to hold the elements along with screws will prevent breakage.
bill n7aob
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K2GW
Member

Posts: 535


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« Reply #10 on: July 12, 2004, 11:46:59 AM »

I use the three element design with the "Aussie Box" receiver mounted on the boom between the driven element and the reflector.  I used hose clamps instead of screws to affix the elements to the tee's.  A compass with a reverse bezel overlay for direct reading of azimuth is mounted between the director and the driven element.

A six inch piece of PVC pipe runs out of the tee along the bottom reflector and both are covered with foam pipe insulation for a pistol grip.

A piece of stiff plastic stiffens the middle half of the reflector.  Don't stiffen the elements too far out. You want to be able to fold the elements back and stick the ends into tees for moving in heavy brush. The antenna still works well in this folded manner and it gives about a  quick 30 dB of attentuation.

At the Dayton Foxhunting forum, a version with the spacing of the director and reflector reversed was shown.  Not sure of the theory, but it supposedly works with a sharper null on the short distance foot ARDF events.

73

Gary, K2GW
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K9DCI
Member

Posts: 9




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« Reply #11 on: March 22, 2005, 03:33:02 PM »

Better late than never...

>> I've seen a number of tape measure
>> yagis break at the screw holes in
>> the elements.

YIKES!  screws??
  Put the tape elements INSIDE the T's and Crosses.  Then just shove a 1" piece of pipe in the end to hold the elements in place.  I used a Dremmel tool and the sanding drum to remove any shelf inside the fittings for a straight through shot for the tape elements.  With careful planning, the hairpin matching loop can be put INSIDE the boom and cross for a really snag-free design.  The coax then exits at the rear handle.  A few screws in the fittings to hold to the boom and you're done.

I have a 6" dia, heavy duty cardboard tube that it slips into in the trunk.
73, Steve
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WB6BYU
Member

Posts: 13337




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« Reply #12 on: March 23, 2005, 01:11:16 PM »

YIKES!  Screws in the fittings!

Smiley

Having carried with my antennas about a fair bit, I rely on
the slip fit to hold the antenna together just fine in the
field, while still allowing me to pull it apart when I
need to stuff it in my carry-on luggage.

Using standard PVC pipe fittings, a 1" tape measure
element will fit inside of a 3/4" fitting, and a 5/8" tape
will fit inside a 1/2" fitting.  This allows me to fold
the elements in thirds, rather than in half, so it stores
in an even smaller space.  See the 6th photo here:

http://members.aol.com/homingin/ncohpix2.html


Regarding different dimensions:  the WB2HOL design is
specifically for a good null off the back, while other
dimensions may be optimized for gain, 3dB beamwidth,
or lowest side lobes (which will reduce the chance of
taking a bearing on a spurious lobe.)  Personally, I
never use the rear null for ARDF, so my criteria are
"clean pattern" (all spurious lobes down 20dB or more)
and reasonable gain consistant with boom length.  One
problem I sometimes have with the WB2HOL dimensions is
finding a place to mount the receiver and compass while
still having room to hold the boom near the center of
gravity.  Using a wider spacing between the driven
element and the reflector helps this.  (In the photo
you can see that I put the compass near the driven
element - but not too close, since the element is steel  -
and the receiver on a short piece of pipe behind the
reflector.  This specific antenna uses equal spacing,
and is developed from a W4RNL design.)

I've seen several European designs (which probably
pre-date the availability of yagi modelling software)
where the driven element is much closer to the director
than the reflector.  In fact there are many different
yagi designs used in competition, from 2 elements to
5 or 6.  Part of the choice will depend on how well your
equipment can indicate a 1dB signal strength difference:
if this is difficult, then the half-power beamwidth is
critical.  The VK3YNG and VK4BRG receivers with a VCO
make this relatively easy, so the sharpness of the main
lobe isn't as critical for bearing accuracy.

It is all a bunch of trade-offs.  There is no "right
answer" or "best design"!
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