[ARDF] Tape Measure yagis

[Kuon & Dale Hunt]

WB9SBD wrote:
>I have yet to try one of those tape measure yagis.
>
>They seem to be an excellent cure to that "Bushwhacking" problem because you
>need not fear the breaking (or even bending ) of elements.
>Who here uses them and what size?  I've seen two, three, and four element
>versions.
>
>And most importantly,, what dod you think of them?
>


   Joe -

      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.

      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.

      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.

      (If you use 1/2" PVC fittings for your antenna, you can fold the
   5/8" elements through the cross holes in the T's to store it.  The
   1" wide tape requires 3/4" PVC fittings to do this.)

      What do I think of them?  So far they are the best solution I've
   found, especially if you have to fly to ARDF events.  Other options
   and/or comments:

   The steel tape measure sections will affect your compass, so you
   probably want to mount it mid-way between the widest spaced elements.

   Aluminum window blind sections:  these are also curved and should pop
   back into shape, but the aluminum is not strong enough to hold up
   to repeated flexing.

   Tape measure elements DO break:  ask the GAOC folks, who always carry
   an extra tape measure in the truck for emergence replacements.  This
   is a particular problem if the elements are attached with sheet metal
   screws.  Hose clamps are more rugged, but heavier (and ore expensive.)
   I've had good luck with a good wrap of fiberglass strapping tape.

   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 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.)
   I've seen spring steel recommended, but haven't tried it.  Again,
   all my antennas have break down for transport, and I haven't found
   a convenient way to do this yet.  Since the tape measure works well,
   I haven't had much incentive to do much else.


   Happy hunting!
        - Dale WB6BYU


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