Ed,
Hate to say it, but what you have modeled is a fairly standard Extended
Double Zepp with feedline. With the construction of the feedline you have
used (+/- 0.285'), the feedpoint impedance at the base is very close to 50
ohms. Change the length of the feedline, and the impedance changes at the
feedpoint, but not at the antenna.
The wire you used at the bottom has too many segments, violating the
recommended segment length of adjacent segments for NEC-2, but it creates
no special harm. You can reduce it to 3 segments and increase the number
of segments on the other wires for a closer match of segment lengths. If
you go to 3 segments, use a non-split feed at the 50% point.
EZNEC-M (NEC-2) shows a bi-directional gain of 10.81 dBi over
Sommerfeld-Norton Ground as you specified--Mininec type ground usually
gives overly optimistic gain numbers with NEC-2. The gain is just about
right for an EDZ over a dipole at that height (about 8 dBi at 5/8 wl up).
One can achieve the same gain with just the wire and 450-ohm cable to an
ATU, but the nice part of your design is that the home-made feedline gives
a good match to 50-ohm cable for 40 meters--and can be fed with an ATU on
all other bands.
EDZs are very good antennas, considering that they are "just wire." If a
bidirectional pattern is fine for 40 meters, then you can get nearly the
gain of a 2-element Yagi in 2 directions at a lot lower cost (although
rotating the thing is open to ingenuity).
-73-
LB, W4RNL
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|