I went through a rigorous attempt at modeling the four element Steppir
on all its bands several years ago. My results did not differ
substantially from those who used a less rigorous approach. I have all
of our results tabulated, if you're interested in the data. Here's how
I described my approach to the issue of wire diameter and resistivity.
(BTW, W9CF teaches physics or EE at ASU. and you can get to the
equivalent diameter calculator here:
http://fermi.la.asu.edu/w9cf/equiv/index.html).
Wire diameter
The Steppir elements are made from a rectangular perforated Cu-Be alloy
tape that is 0.56" wide and 0.018" thick. The perforations are 0.125"
in diameter and there are five perforations/inch. If one calculates the
surface area of the tape, accounting for the perforations, and then
calculates the equivalent cylindrical wire diameter, a value of 0.33" is
obtained. (Based on postings on the Steppir reflector, this is the
diameter that Steppir recommends for modeling purposes.) However, if
one obtains an equivalent diameter based on self-impedance using W9CF's
on-line calculator, a value of 0.30" is obtained. Although the
difference between the two calculation methods is not major, I used the
0.30" diameter in my models. (Since the calculator didn't account for
the perforations in the Steppir tape, I asked W9CF about their effect.
He said that the impact on equivalent wire diameter would be small and
would tend to reduce it.)
Wire resistivity
It appears that the standard Cu-Be alloy used in electronic applications
is hardened 2% beryllium, and I am assuming this is the material used in
the Steppir tape. The conductivity for this alloy is reported as 22%
ICAS or a resistivity of 4.55 times that of copper (1.74E-8).
The W9CF calculator also provided an equivalent cylindrical diameter for
a rectangular conductor based on resistivity and the result was 0.20".
I asked W9CF why this value differed so much from the equivalent surface
area calculation (0.33") and he explained that for a rectangular
conductor most of the current is confined to the edges. Since
resistance per unit length is inversely proportional to cross-sectional
area, I accounted for the effect of the resistivity based wire diameter
by multiplying the Cu-Be resistivity by the ratio of the square of the
diameters. Thus, I used a resistivity of 4.55 X 1.74E-8 X (0.32/0.22) =
1.78E-7. As you will see, the difference between this calculated value
and the resistivity of copper only reduced gain by about 0.2 dB.
73, Joe
K2XX
Jim Lux <mailto:jimlux@earthlink.net>
Wednesday, April 08, 2015 11:54 PM
On 4/8/15 1:42 PM, Richard (Rick) Karlquist wrote:
In general, you can use a round element with circumference equal to
the width of the tape (or half the width of the tape.. opinions vary).
I just did a bunch of models of a "tape" dipole at work where we were
comparing the results from HFSS (modeling a 1cm wide tape) and NEC2
(modeling it as a tube with radius 1mm.. 0.628 cm circumference) and
they were "pretty close".
The variations from the dielectric properties of the tube will have a
bigger effect.
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Richard (Rick) Karlquist <mailto:richard@karlquist.com>
Wednesday, April 08, 2015 4:42 PM
The fiberglass tubes lower the resonant frequency by an
uncontrolled amount, since they are just fishing poles
that do not have any exact permittivity value. It is
analogous to FR-4 PC boards where the impedance is
to some extent an accident of the characteristics of
the epoxy glass. SteppIR has sourced the tubes from
various vendors over the years which further complicates
the issue. Any correction factor you would use would
be specific to a particular run of tubes. SteppIR
seems to gloss over this issue. There is a parameter
you can set on the controller that scales all elements
by a fudge factor that can be determined experimentally
for the particular copy of the antenna that is being used.
Another issue is: what size round element is equivalent
to the copper tape?
Rick N6Rk
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William Osborne <mailto:wosborne44@gmail.com>
Wednesday, April 08, 2015 4:05 PM
Does anyone have experience modelling an antenna in EZNEC and then loading
those elements into a SteppIR antenna (I am using 4E so that is of most
interest). If you have, how does your antenna perform with your new
element
dimensions versus the factory default ones? Mine does not seem to resonate
at the same frequency as the model, so I think a correction factor is
required to the EZNEC model elements?
Bill--K5ZQ
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