For those of you who have AutoEZ here's a step-by-step guide to modeling the
Jim Brown "Pacificon pitch" ( http://k9yc.com/PacificonSmithChart.pdf ) 80m
dipole. Paraphrasing, “…tune the antenna for 3675 kHz. Feed it with a half
wave of 50 ohm line beginning at the antenna, then a quarter wave of 75 ohm
cable…”
Start by defining a simple dipole using a variable for the length. Variable
"L" was used in this example, any variable name will do. (The links below go
to various screen grabs.)
http://ac6la.com/adhoc/K9YCdipole1.png
Make sure the source is set to Wire 1 / 50%. Tab to the Calculate sheet, set
the frequency to 3.675 and give "L" an initial value of something in the
neighborhood of a half-wavelength on 80m. With cell C11 still selected click
the "Resonate on Selected Cell" button. The value for "L" will be
automatically reset to produce resonance, ~129.3 ft in this example. This was
for #12 copper wire at 70 ft over Real/Average ground. Your value will vary
depending on the modeling parameters.
http://ac6la.com/adhoc/K9YCdipole2.png
Now add the two transmission line segments that Jim described. In this screen
shot T1 is the 180° (half-wave) segment of 50 ohm line, T2 is the 90°
(quarter-wave) segment of 75 ohm line. Change the source placement to virtual
wire "V1". The connection sequence is V1 (Source) to 75-ohm RG-11 to V2 to
50-ohm RG-213 to antenna feedpoint (Wire 1 / 50%).
http://ac6la.com/adhoc/K9YCdipole3.png
The line characteristics (Zo, VF, Loss) were set via this dialog. You can
choose from roughly 100 different built-in line types.
http://ac6la.com/adhoc/K9YCdipole4.png
For the precise physical lengths corresponding to 180° and 90° you need not dig
out your pocket calculator. You can use this dialog.
http://ac6la.com/adhoc/K9YCdipole5.png
Now tab back to the Calculate sheet and use the "Generate Test Cases" button to
set up a frequency sweep from 3.5 to 4.0 MHz.
http://ac6la.com/adhoc/K9YCdipole6.png
Click "Calculate All Rows" then tab to the Smith sheet. You'll see this red
trace which shows the impedance values at the input (toward the shack) end of
the 75 ohm section. I captured the other traces by doing intermediate
calculations as the model was being built. The green dot marker at 3.950 MHz
shows that almost the entire 80m band is within the 2:1 SWR circle.
http://ac6la.com/adhoc/K9YCdipole7.png
And here's a conventional SWR chart as produced via the Custom sheet tab.
http://ac6la.com/adhoc/K9YCdipole8.png
You can compare the above Smith chart to pg 46 of Jim's Pacificon pitch. The
small difference is because we used slightly different transmission line
characteristics.
In his pitch Jim also describes how to use the SimSmith tool to design various
types of matching networks. AutoEZ has similar capabilities, the difference
being that AutoEZ will calculate the correct component values and then add the
network to the model. For example, this dialog is used to implement series
section or stub transmission line matching networks.
http://ac6la.com/adhoc/K9YCdipole9.png
And this one is used to automatically calculate and add T, Pi, and L discrete
component networks.
http://ac6la.com/adhoc/K9YCdipole10.png
If you'd like to play with this model without bothering to build it from
scratch here's the AutoEZ format (.weq) model file.
http://ac6la.com/adhoc/K9YC80mDipole.weq
For those unfamiliar with AutoEZ you can find complete details here, including
a link to download a free demo version. Be sure you have the pre-req software
as specified.
http://ac6la.com/autoez.html
Dan, AC6LA
http://ac6la.com
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|
