[TowerTalk] 160 vertical question (Top Hat)

[john at kk9a.com]

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