> To: L. B. Cebik <email@example.com>; firstname.lastname@example.org
> Subject: Re: [TowerTalk] modeling the 402CD
> Date: Saturday, July 12, 1997 12:55 PM
> Is it not conceivable that some of these problems comes from the fact
> US is not using metric terms ? I guess the antennas are calculated in
> lengths ( meters ) and converted to the medival British empire terms ?
> 73 Rag OZ8RO
> > From: L. B. Cebik <email@example.com>
> > To: firstname.lastname@example.org
> > Subject: [TowerTalk] modeling the 402CD
> > Date: Saturday, July 12, 1997 12:14 AM
> > The discussion of results from modeling the 402CD deserve some comment,
> > because the antenna is not an easy one to model on NEC2. First, using
> > EZNEC, with its stepped diameter correction feature, can be misleading
> > because the feature is not invoked by the program. The structure of
> > antenna, with the spikes just past the loading coils, does not meet the
> > criteria for automatic employment. Consequently, stepped diameter
> > are possible. Second, the values of the loading coils have been handed
> > down--some reasonable assurance that they represent factory spec values
> > correctly measured values is needed. Something similar is needed to
> > verify the Q (or series resistance) values also handed down.
> > The model received here was not satisfactory in terms of segmentation
> > hence would not converge within a reasonably low number of segments.
> > Failure of convergence generally indicates that the results are not
> > trustworthy, although it does not say that they are necessarily
> > inaccurate. Equalizing segment lengths, with a consequent large
> > in the total number of segments in the model, did bring about
> > in NEC-4. However, enough questions remain about input values and
> > accuracy of the programs with respect to structural complexities to
> > my confidence in the results of the model.
> > Note that this lack of confidence is in a model; NOT in the antenna.
> > The proper range comparison for the determination of antenna gain at a
> > site with a height is not the ideal free space dipole gain of 2.15 dBi.
> > Rather, it is the gain of a comparison dipole at the same height. That
> > antenna may run 7.5 dBi or more, depending on height and constitution
> > its elevation angle of maximum radiation, and the elevation angle of
> > maximum radiation for the 2-element Yagi may be 1-2 degrees less (where
> > the dipole may show slightly less gain). Whatever the precise test
> > figures in this overall ballpark, the roughly half S-unit of gain is
> > significant in many operating conditions, as is a couple of S-units of
> > F-B, especially on a band where having any such gain and F-B is the
> > exception rather than the rule. Moreover, your terrain features may be
> > helping or hindering you more than you know.
> > Incidentally, heights of 5/8 wl and 1 1/8 wl will generally show gain
> > maxima while 7/8 wl up will show a gain minimum if you raise most
> > dipole and Yagi antenna models systemmatically. This phenomenon is
> > confirmed by standard grund reflection equations. For 40 meters, gain
> > maxima occur between 85-90' and 155-160' with a minimum around
> > F-B maxima and minima do not correspond to those for gain. And
> > impedance wanders around in its own third curve. Significant
> > maxima/minima wash out around 1 3/4 to 2 wl up. The lower the height,
> > steeper the curves. So if you want to advertise your home brew antenna
> > a true winner by virtue of modeling, be sure to model it at a gain
> > maximizing height, while modeling your neighbor's home brew antenna at
> > gain minimizing height.
> > Modeling is as much art as it is calculational science. Complex
> > structures require uncommon care in modeling to ensure reliable
> > The 402CD models give suggestive results--even some guidance in
> > modifications and adjustments--but even with NEC-4 they require further
> > refinement to achieve reliability, especially in comparing such models
> > models of equally complex but different structures.
> > -73-
> > LB, W4RNL
> > --
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