[ARDF] Tape Measure yagis

[Jay Hennigan]

On Sat, 20 Sep 2003, Kuon & Dale Hunt wrote:

>       I think most ARDF competitors are using tape measure yagis
>    or something similar, though there are some exceptions.  If you
>    look at the photos from the recent events you will the original
>    WB2HOL design with relatively close spacing between the reflector
>    and the driven element, though I have scaled some of the W4RNL
>    3-element yagi designs and they work well for me.  The short
>    2-element antennas with black elements are the VK4BRG version of
>    the HB9CV phased array, with the outer element sections made
>    from tape measure covered with heat shrink tubing.  (The phased
>    array is capable of better front/back ratio than a similar
>    2-element yagi in a short beam.)
>
>       Number of elements:  my wife likes 2 elements because it is
>    easier to use and less cumbersome. I like 3 elements as a
>    reasonable tradeoff between antenna size and directivity.  Besides,
>    if you break one element you can still take bearings - somewhat.
>    4 elements can give you better directivity on a longer boom, or
>    (in some cases) a cleaner pattern on the same boom length as 3
>    elements.  But most of use are still not skilled enough at plotting
>    bearings that the sharper pattern will make a practical difference
>    in competition.

I use a tape-measure log periodic design based on the "Pounder" 2m
antenna described in the ARRL antenna book.  It has a 12-inch boom
with four elements.  If you think about it, the HB9CV is very close
in principle to a 2-elemant LPDA.

The front-to-back ratio of the LPDA isn't as good as the WB2HOL tape
measure yagi (26 dB vs. 50) but it has a more distinct front lobe
and the low-Q design is not as readily affected by near field items
such as trees, people, etc.

The LPDA is trickier and more time-consuming to construct than the
WB2HOL beam.  I use half-inch tape measure material which requires
some stiffening in the form of "tripling up" the elements near the
boom and staggering the stiffening sections, similar to the method
used on the Ukranian integrated yagi/receiver units.  I use a PVC
fitting to extend the back of the boom behind the handle.  A platform
behind the handle supports the receiver with Velcro which puts the
center of gravity closer to the carrying handle.  This arrangement
lets me get the entire antenna "out front" without too much wrist
effort to keep it pointed at the horizon.

>       Peak or null?   I always hunt the peak, even though the null
>    is capable of much sharper bearings (as is evident on 80m.)  The
>    reason is that any relfections (or change in frequency) can shift
>    and/or fill in the null in the pattern, while the main lobe is
>    not affected.  This will also depend on how sensitive your receiver
>    is for indicating the peak of the main lobe:  an audio S meter
>    generally has much better resolution than the bar-graph meter in
>    many HTs.

It's also somewhat counter-intuitive to hunt the null, especially with
the way most antennas are constructed.  To hunt the null you either have
to point the front lobe at yourself (reflector) or run away from the fox.

>       Size of tape:  Currently I use the 1" tape for my 121 MHz yagis
>    and 5/8" tape for 2m.  I have 16' x 5/8" tapes for $1 at Dollar Tree
>    stores, or 25' x 1" for $5 on special at WalMart and some specialty
>    tool stores.  In both cases I reinforce the center section of each
>    element with an additional half-length piece of tape to help it
>    snap back into position quickly.  Even at that, the 5/8" tape is
>    a bit on the flimsy side on 2m, but works OK.

Heat-shrink over the doubled-up part can help the flimsiness.

>    The Aussies use fibreglass or Kevlar rod covered in coax braid.  Quite
>    rugged, but the elements don't fold conveniently for transport.  (Ask
>    them how their antennas worked in Slovokia in 2002!)

I would think that the fiberglass rod might break if you take a spill
with it.  It has appeal for me in the LPDA realm as it's an insulator
and might make the split-elements easier to construct.  How flexible is
the rod?

Aussies on the list, I'm asking!  How did your antennas work in Slovakia?

>    I used copper-covered welding rod on one antenna with removable
>    elements - worked fine for a month or two until the copper corroded
>    off.  Brazing rods will also work, but vibrate more, which may make
>    it harder to get a bearing if it affects the pattern significantly.
>    (You would have to try this with your particular antenna design.)

The first LPDA units I made had brazing rod, but it doesn't spring back
into place like the tape-measure material.  I'm using brazing rod for
some 2m turnstile ARDF transmitting antennas.

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