[ARDF] Tape Measure yagis

[Matthew Robbins]

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?
>

Two things I haven't seen anyone say here, but which have been noted (by 
others) many times:

1.  Build it with the tape measure material's concave side up when you hold 
the antenna level.  This is so when you run through fight:

2.  The elements just bend back when you lift the front as you crash 
through.  If you build it with the convex side up, you have to put the tip 
down when you go through stuff.  It is very hard on the tape measure 
material to bend the wrong way.  If you fall on the antenna, the concave up 
design is a whole lot more robust than convex side up.

It's also okay to just hold it level as you go through stuff.  The elements 
easily bend back.  With the concave up design, it's very nice to hold it 
tip-up as you go through spider webs and to use the PVC boom to push small 
branches.  I couldn't believe how many spider webs were on mine after Ft. 
Ancient.

They do break.  They do wear out.  I'm not going to abuse it in fight unless 
I have to.  But they work fine, and they're cheap.

A few of my comments about the TMB:

1.  One website recommends not putting anything on the sharp element tips to 
avoid mistuning the antenna.  I think that's completely unsafe.  I finished 
my second antenna in a hurry, and I didn't put on "tip treatments" right 
away.  I cut myself twice on those stupid things.  I almost cut someone else 
when I popped an element out one day.  I don't want to sound like Mom here, 
but you *could* put your eye out.  "You could hurt somebody.  You could hurt 
yourself!"

2.  Most (not all) of the TMBs I see have a handle built aft of the 
reflector.  I find that really tiring.  I want to hold the antenna at the 
Center of Gravity. (Jay Hennigan mentioned this too.)  Being slightly more 
than a little geeky, I weighed all the components with a gram scale, 
measured the size and depth of insertion for all the PVC, including the 
weight per inch for the linear components, and wrote a spreadsheet to 
calculate cut lengths and the center of gravity.  (For the readers who use 
SI units and who cringe at goofy American methods, I calculated moments in 
gram-inches.  If I had to do it over, I'd use cm or m for length.  I hate 
converting 6.659 inches to fractional inches so I can mark a cut.)  I wanted 
the CG to be centered between the driver and the reflector.   The 
spreadsheet let me play with different components and radio positions to get 
the CG close to where I wanted it.  In the end, the spreadsheet was just an 
estimate because I decided I didn't need to include every bit of glue and #8 
screw or be perfect in modelling the position of the coax, but it's close 
enough and very comfortable.  It's amazing how light it feels when you don't 
have to apply force continuously to hold it.

Just like the guy who recommends not putting stuff on the tips of the 
elements, I recognize that having my hand in there and the antenna closer to 
my body is detuning it and changing the pattern, but unlike that guy, I 
think this is a reasonable compromise, especially if makes it easier to use 
for two or three hour events.  (Also, a lot of the high-end competition gear 
is designed to have your hand between the elements, so you can't be giving 
up much performance.)

Ken: I saw your article in Orienteering North America. Great Job!  I thought 
I had it bad---I ran out of water 30 minutes from the end.  You must have 
really been suffering.  I got water before I turned in my punchcard, and 
then I almost passed out.  It was the climb out of one that did me in.

Matthew
AA9YH
Cincinnati, Ohio

_________________________________________________________________
Get MSN 8 Dial-up Internet Service FREE for one month.  Limited time offer-- 
sign up now!   http://join.msn.com/?page=dept/dialup