The recent "trimming elevated radials" thread prompted me to re-read the N6LF
QEX two-part article "A Closer Look at Vertical Antennas With Elevated Ground
Systems", available here:
http://rudys.typepad.com/files/qex-mar-apr-2012.pdf
and here:
http://rudys.typepad.com/files/qex-may-jun-2012.pdf
For those of you who have the AutoEZ program (or its predecessor, which is what
N6LF used) I have created a general purpose model which may be used to
duplicate many of the charts shown in the article as well as do other studies
concerning elevated radials. Almost all aspects of the model are controlled by
variables. I kept the N6LF usage for variables H, J, L, and N plus added a few
others. The complete set is:
H: Vertical element length (May be 0 to model radials only)
J: Base height (May be 0 for use with Perfect or MININEC ground)
L: Radial lengths
N: Number of radials (May be set to 0, 1, 2, 4, 8, 16, or 32)
T: Top wire A (above +X axis) length (Set to 0 for no top wires)
U: Top wire B (above -X axis) length (see below)
V: Angle of top wire(s), down from horizontal
All dimensions are in feet. Variables T,U,V allow you to create a "T" or
"Inverted L" radiator with either horizontal or sloping top wire(s).
As mentioned, one use of this model is to recreate the N6LF charts under
different conditions. For example, here is Fig 12 from Part 1 of the article.
In this case Average Gain is being used as a proxy for antenna efficiency. The
length of the radials is swept from 0.05 WL to 0.6 WL and the chart shows a
large drop in Average Gain as the radial length approaches 0.45 WL, less so
when more radials are used.
http://ac6la.com/adhoc/MCVertFig12a.gif
Here's a similar chart, tailored for Top-Banders, produced by AutoEZ.
http://ac6la.com/adhoc/MCVertFig12b.gif
Another example relates to the section "An Explanation for the Dips in Ga"
(Part 1 pg 40) in which N6LF discusses the large current peaks that develop on
the radials as the length approaches 0.45 WL. These large currents increase
the E and H-field intensities in the ground. He then states: "Since the power
dissipation in the soil will vary with the square of the field intensity, it’s
pretty clear why the efficiency takes such a large dip when the radials are too
long." This is illustrated with his Figs 24-26 (not shown here).
A alternate way of showing what is happening, not possible with a print
publication, is to animate the E-field pattern as the length of the radials is
increased. Here is the case for N=4 with the radial lengths ranging from
L=0.05 WL to L=0.6 WL, the same range as the previous chart. Temporary
variable "A" is being used to set the radial length in WL units which is then
converted to the actual "L" in feet. Values for both A and L may be seen to
the right of the chart. E-field was calculated at 1 foot below ground. With
most browsers, press Esc to stop the animation in order to take a closer look
at any frame, press F5 to restart.
http://ac6la.com/adhoc/MCVertNF3D.gif
Another way to use this model is for comparison between alternate scenarios.
For example, W8JI suggested starting with two opposite radials trimmed to be a
resonant dipole, then adding additional radials of the same length, then adding
the vertical element and adjusting its length for feedpoint resonance.
Modeling this scenario at 1.85 MHz, H=0 (to start), J=10, and N=2, the AutoEZ
"Resonate" button yields L=127.2 ft (0.24 WL) for "dipole resonance". Then
with N=8 the "Resonate" button yields H=132.0 ft (0.25 WL) for a feedpoint Z of
39+j0 ohms.
Although the SWR is already low, to minimize feedline loss you might put a
matching network at the antenna base. The AutoEZ "Create Impedance Matching
Network" button allows you to easily add this to your model. With a Lo-Pass L
network (coil in series, capacitor in shunt), the computed values are
coil=1.804 uH and cap=931 pF to get 50+j0 ohms at the antenna. This is with
real-world (lossy) components, assuming a coil Q of 200 and a capacitor Q of
1000, both at 1 MHz and adjusted as necessary for other frequencies.
Call this Scenario 1. For Scenario 2 suppose you have room for radials that
are only 100 ft long, you can only get the top of the vertical up to 80 ft, and
you'll add two top wires to make a "T" for the remaining radiator length.
With N=8, H=70 (puts the top at 80 with J=10), and U set to "=T" (that is, U
will change as T changes so that symmetry at the top is maintained), the
"Resonate" button sets T=42.8 ft to give a feedpoint Z of 25+j0 ohms. Then
again adding a matching network, the computed values are coil=2.154 uH and
cap=1684 pF to get 50+j0 ohms at the antenna.
Here's what the two antennas look like. Remember, both were created from the
same model file, the difference is just how the variables were set.
http://ac6la.com/adhoc/MCVertView.gif
Finally, how do the two scenarios compare? Here are the azimuth patterns at 22
deg elevation:
http://ac6la.com/adhoc/MCVertPolar.gif
It's easier to discern the small difference in gain as well as the slight
asymmetry of Scenario 2 when the pattern is shown in rectangular rather than
polar format:
http://ac6la.com/adhoc/MCVertRect.gif
And here's the SWR comparison with the matching network in place in both cases:
http://ac6la.com/adhoc/MCVertSWR.gif
The AutoEZ model can be downloaded (right click and select Save As) here:
http://ac6la.com/adhoc/Multi-Config_Vertical.weq
And for more information on AutoEZ see:
http://ac6la.com/autoez.html
Dan, AC6LA
http://ac6la.com/
All good topband ops know fine whiskey is a daylight beverage.
_________________
Topband Reflector
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