[TowerTalk] 160 vertical question (Top Hat)

[Grant Saviers]

NEC4 can analyze stepped diameters without the approximations that are 
used in NEC2.  That might help with tube verticals with wire top hats.  
A recent passive 4sq I designed came close with aluminum tube verticals 
and a 4 wire 45deg hat.  Acute conductor angles create another modeling 
problem.

NEC4 still has difficulty with big steps in diameter such as grounded 
element to boom mounting plates.   Maybe there is a workaround but for 
now I agree with others that insulated elements on yagis are a way to 
get more accurate results.

NEC2 is ok with radials very close to the ground.   The EZNEC 6 manual 
discusses this in detail.  NEC4 does buried radials but for many 
situations exactly where "ground" is in the Z plane relative to the wire 
may be a within the NEC2 criteria.  Laid on the ground, worked into the 
turf, laid on the forest leaves, buried 1 inch, etc. - there is no hard 
surface "copper plate" transition.   I did a little NEC4 playing with 
small steps of radial heights +Z to -Z and observed that the results are 
fairly continuous.  That seems logical but perhaps I'm missing something.

Grant KZ1W

On 2/5/2018 5:00 AM, john at kk9a.com wrote:
> Interesting data.  I also avoid short segments, I just add the length to a
> nearby tube. Perhaps NEC4 would be more accurate for an inverted L but
> having the NEC2 model still gave you a good starting point which saves a
> lot of time.
>
> John KK9A
>
>
>
> To:	towertalk at contesting.com
> From:	Wes Stewart <wes_n7ws at triconet.org>
>
> Although what follows is in regard to an inverted-L antenna, not a Tee,
> it might be of interest.
>
> My L is composed of telescoping aluminum tubing starting with 2 1/2" OD
> and tapering to 1 3/8".  The total height is 55' 10". From the top of that
> runs a 14 AWG copper wire that is 90' long, with the far end at ~45' above
> ground.  The vertical is insulated at the base and driven via a 6" 10 AWG
> pigtail against (currently) twenty-four 55' long insulated radials laid on
> the ground and four 11', 6 AWG bare wires running to four 8' ground rods
> in a 16' square.  All of these connect to a DX Engineering ground plate
> about an inch and a half above the dirt.  The 1/2" Heliax connects to a
> type N connector mounted to the ground plate.  All in all, rather
> unremarkable, except for the surprising amount of money invested, a good
> part of that the cost of the DX Engineering foldover mount.  (What
> happened to the beer-can-vertical with the Coke bottle insulator?)
>
> Measuring the feedpoint impedance at the N connector on the ground plate
> using a DG8SAQ VNWA shows Z = 29.2 j0 at 1850 kHz.
>
> I created a model of this in AutoEZ invoking EZNEC+ V.  There are some
> issues with this however.  Normally one would use the built-in stepped
> diameter correction, but this only works when all segments are collinear.
> The horizontal wire isn't.  Placing a source on the 6" long wire is
> problematic since there is a huge difference between the wire and the
> tubing diameters. Segment tapering is a fix, but a 6" length is already
> too short to satisfy guidelines.  The compromise solution is to eliminate
> the 6" wire and connect the tubing to "ground" and place the source at 0%
> from the end.  Likewise, to simulate ground loss with the Mininec type
> ground, a resistance is also placed on the bottom wire at 0% from the end.
>
> Using the AutoEZ optimizer with the simulated ground resistance and the
> length of the "L" wire as variables, I let it adjust the variables to get
> the same Z in the model as the measured data. By plotting this on the
> Smith chart and then saving the result as an S1p file I was able to import
> that file into the DG8SAQ program and overlay it on the measured data. For
> the limits on the 2:1 VSWR circle (~1.8 to 1.9 MHz), the traces overlaid
> each other very nicely.  The simulated ground resistance to bring this
> about was 15 ohm..
>
> The length of the horizontal wire in the model was 83.1 feet for a total
> length of ~139 feet.  However, the physical wire is 90 feet long for a
> total length of ~146 feet, a considerable difference. Both of these
> dimensions are longer than the ~133 free space quarter wavelength perhaps
> indicating that the radial field is still resonant and at a higher
> frequency.  This is something that I can't model with the NEC-2 engine.
> Severns mentions this in "Experimental Determination of Ground System
> Performance for HF Verticals Part 4 How Many Radials Does My Vertical
> Really Need?", QEX May-June 2009.   But he observed a change in resonant
> frequency depending on the number of radials.  I have not seen this, only
> a reduction in the real part of the feedpoint Z with more radials.
>
> The point of all of this is that modeling is a great tool and I'm a firm
> believer in it, but it has its limitations.
>
>
> FWIW,
>
> Wes  N7WS
>
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