[TowerTalk] Modeling LPDAs with AutoEZ + EZNEC

[Dan Maguire]

Recently I assisted an AutoEZ customer with a project involving LPDAs.  Along the way I created a couple of model files that might be of interest to others.

Model "LPDA by Tau+Sigma.weq" lets you create an LPDA by specifying a set of parameters.  Here's a screenshot of the Variables sheet tab of the AutoEZ workbook with that model opened.

http://ac6la.com/adhoc/lpda1.png

A second model file, "LPDA by Elems+Boom.weq" is similar but lets you set the desired number of elements and the boom length; tau and sigma are deduced.  In both cases, by design, the models created will be very close to those produced by the popular LPCAD34 program by Roger Cox, WB0DGF.

Since the parameters can be modified at will and the model is then changed immediately (including a different number of elements if necessary), you can easily run frequency sweeps to compare different parameter sets.  Here's a sweep comparing two tau/sigma combinations.

http://ac6la.com/adhoc/lpda2.png

And here's one comparing different phase-line Zo values, both for the same tau/sigma (0.89/0.06).  Note the aberration between 24 and 25 MHz when using a 75 ohm phase-line.  That spike can be moved to a different position, perhaps to a frequency not to be used, by changing the rear stub.   

http://ac6la.com/adhoc/lpda3.png

Once you have the design finalized you may be tempted to use the AutoEZ "Create Set of Stepped Diameter Wires" dialog to replace each cylindrical element with a "taper schedule" set of wires representing your proposed physical construction.

http://ac6la.com/adhoc/lpda4.png

Unless you are using EZNEC Pro/4 with a NEC-4 engine you probably do *not* want to do that for LPDAs.

When a model contains "stepped diameter" sets of wires, EZNEC (except Pro/4) will attempt to use the Leeson correction to convert each set back to a cylindrical (mono-taper) equivalent.  However, the correction can only be applied when the element is within about 15% of resonance.  With LPDAs, at any given frequency only some of the elements will be within that resonance range and hence can be "Leeson corrected".  Other elements will be left as stepped diameter, producing inaccurate results under NEC-2.

Here's a comparison.

http://ac6la.com/adhoc/lpda5.png

Note that this caution does not apply to single-band Yagis.  In that case all the elements can normally be "Leeson corrected" without problems.  There are some tricks involving the AutoEZ Resonate button that make it easy to create an "equivalent" set of stepped diameter wires for each Yagi element.  I'll be happy to describe how to do that in another posting if there is any interest.

Current AutoEZ users, please get the latest program revision by using the download link in your purchase confirmation email.  If you need assistance contact me directly.  The two new LPDA models are included in the "Sample Models" folder and the AutoEZ workbook itself has been modified slightly to accommodate them.

For those unfamiliar with AutoEZ you can find complete details here, including a link to download a free demo version.

http://ac6la.com/autoez.html

AutoEZ requires both EZNEC and Microsoft Excel.  It cannot run under Open Office, Office Libre, or Kingsoft Office.  Although those programs can handle basic Excel workbooks they do not fully support the 27000 lines of Visual Basic code contained in AutoEZ.

Dan, AC6LA
http://ac6la.com