Thanks for the help with the tapered elements in NEC. I didn't realize
the taper correction included shortening the elements.
After plugging in the new numbers from Roger, the models look just as expected.
I tried two things with the 105CA model.
First I tried moving the 1st director on the 105CA about 3" closer to the
driven element.
I may need to do this when I side mount the antenna (side mounting on a self
supporting
tower with a wide face - 28") to have enough clearance. Moving the element
didn't have
too much of an effect. If I shorten the 1st director by 0.125" on either side,
then
the gain curve is almost identical. The impedance shifts a little, but I'm not
sure
what the SWR will look like after the beta match. The real impedance shifts up
by about
2 to 2.5 ohms from 28.0 - 28.6. Hopefully this won't make too much of a
difference.
Next was to try NW3Z's OWA techniques to see if I could get a good 50ohm match
and better bandwidth (gain & swr) by adding one more element. Natan already has
a design for a 10m 6-el on a 24 ft boom. The problem I had is that I'm trying
to
side mount this antenna, and one of the elements in his design is just 11" off
of
the mounting point. So I tried leaving the 105CA elements in place, adding
one element about 8-10" from the driven element, and then re-tweaking the
element
lengths to see what I could get.
Here's about the best I could do: (gain in dBi freespace SWR is 50ohm)
28.0 10.02 dBi 1.65:1
28.2 10.20 1.45:1
28.4 10.32 1.37:1
28.6 10.34 1.37:1
28.8 10.23 1.40:1
29.0 9.95 1.78:1
The gain isn't quite as high as the original 105ca (but close). The SWR
bandwidth is
a little better, and is direct 50 ohm feed. But is it worth an extra element
just
to get a 50-ohm match??? Probably not. I think I'll just leave well enough
alone.
I couldn't get anywhere near the curves that Natan got with his 6-el beam. He
gets better
that 10dBi from 28.0 - 29.2 (but the peak gain isn't as high as the stock
105ca), and
50 ohm SWR is better than 2:1 from 28.0 - 29.2. Having the 2nd director out a
little ways seems to make the difference.
--Lee - KT4ZX
--
FAQ on WWW: http://www.contesting.com/antennaware-faq.html
Submissions: antennaware@contesting.com
Administrative requests: antennaware-REQUEST@contesting.com
Problems: owner-antennaware@contesting.com
|