[TowerTalk] 80M Inverted V - CW and SSB

[Dan Maguire]

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