Topband: Shunt feed vs sloper

[R. Dean Straw]

I have received some good feedback about my posting concerning EZNEC files.
These model a quarter-wave sloper on a tower, with a Yagi on the top as
"counterpoise." I wanted to clarify several points:

1. The Yagi I used in the models is a 5-element 20-meter monobander
utilizing "plumber's delight" construction, where the elements are
electrically bonded to the conductive aluminum boom.

2. The original model was for a 3.8-MHz sloper off a 70-foot high tower with
the Yagi on the top. The length of the sloper wire was adjusted to be close
to resonant.

3. I had a typo in the first model I sent to several respondents. The
sloping wire diameter should have been "#12" (12-gauge) not "12" inches in
diameter! I have corrected that and will provide new EZNEC input files for
whomever wants them. The wires lengths again have been adjusted to be close
to resonant for all the models in the zipped file after the wire-diameter
was corrected.

4. I have added another model for a 140-foot high tower with the Yagi on top
and the sloper wire. This tower is roughly halfwave high on 75 meters. This
model shows more current flowing in the tower, although the feed-point
impedance still is mainly determined by the sloping wire (and top Yagi as
counterpoise) than it is determined by the current flowing in the tower. The
gain and pattern is also not materially affected by the tower current (you
can check this by removing the tower from the model and comparing the
pattern/gain).

5. The calculating engine I use with EZNEC Professional is NEC-4. Most
people using EZNEC 3.0 or EZNEC 2.0 are using the NEC-2 engine. NEC-2 will
complain about the bottom of the tower being grounded, but the computations
are still accurate.

Again, e-mail me if you wish copies of the EZNEC data-input files.

73, Dean Straw, N6BV