YAGICAD question
 

Am trying to design a 3 element tape measure antenna for 2 meters
with a 10x10 mm aluminium boom. After calculating the dimensions
with Yagicad 6.1 the tapes get cut and mounted.

http://img837.imageshack.us/img837/8209/snc14096a.jpg
(Before cable and hairpin match)

The driven element sits on a slider which allows to adjust
for best F/B ratio over the working frequency span of the antenna.
Normally the F/B degrades quickly when tuning off center.

http://img90.imageshack.us/img90/7112/fbratio.jpg

With the driven element on a slider you can move the peak
to the desired frequency, within a certain range.
About 10 MHz span is possible with such a 2 meter antenna


Measuring with a scalar analyzer and tests with actual radio signals
reveal that YAGICAD does calculate too short elements.
Everything is a few MHz too high in frequency, the gain maximum,
the F/B optimum, the resonant frequency, on the antenna,
when built with the YAGICAD dimensions.

The only parameter I can vary in YAGICAD is the element diameter.
The tape measure is iron, 19 mm. Iron can be selected
in YAGICAD, but that only affects little.

I optimize the design for best F/B ratio, because that is what you
want for a direction finding antenna.

I have to add 4 cm to the passive elements and 8 cm to the driven
dipole to get a working antenna, having selected 0,02 m element
diameter in the YAGICAD program. Entering less diameter
worsens the situation.
The boom adds only 1 or 2 mm to the elements, but the final
dimension results are several centimeters off!
The SWR is excellent with the hairpin calculated by YAGICAD.
At least something that works.

Luckily I have a good test equipment and see live on the screen
what the antenna does when I change elements.
But that YAGICAD is a mysterium to me.
After so many revisions I would expect some more realistic results.

w.