The WB0DGF Antenna Site

LPCAD software

(1 Nov 2011)

LPCAD - Log Periodic CAD

LPCAD version 3.40 is now available.  LPCAD 3.40 runs fine under Windows 2000, XP, Vista and Win 7.  Version 3.4 allows a minimum frequency of 1 MHz, which is useable now for 160m LPDA designs.  Sorry, LPCAD does not contain a digital signature, so you may get an error message the first time you run this program. Please email me at rgcox2 (at) gmail.com if you find bugs or to suggest improvements.

Contact Roger at rgcox2 (at) gmail.com to obtain the latest software version (LPCAD34.zip) .  LPCAD34.exe was created on 7-29-2011 at 9:01pm.  The file size is 223K.  Or if you can, download this file here.

Or if you still use DOS, download this older file: LPCAD33A.zip.

I am working on version 3.41 currently. Some of the things that I hope to add are:

1.  Rounded xyz values and Tau-tapered diameters in SAVE: AO

2.  Support for MMANA file format in SAVE:

3.  Option to add director and reflector element(s) for a Log-Yag design in SAVE:NEC and SAVE:AO

If you have other suggestions, please email me at rgcox2 (at) gmail.com. Thanks!

updated 20 Oct 2011

An example of a 2300-6000 MHz LPDA on a PCB - designed with LPCAD

Here are a few more LPDA antennas that I have designed:

24-59-05V-MMCX,

ALP-450, ALP-600, LP1009, LP1010, TH11DX

and a few more that I have worked on:

LP-1001, LP-1002, LP1005

Log-Periodic Dipole Array Links:

Hy-Gain LP-1010 video

LPDA Antennas used in Cosmic Ray Detection

Log Periodic Dipole Array Design

In 1957, R.H. DuHamel and D.E. Isbell of the University of Illinois published the first work on what was to become known as the log periodic dipole array.  These remarkable antennas exhibit relatively uniform input impedances, VSWR, and radiation characteristics over an extremely wide range of frequencies.  In essence, log periodic arrays are a group of parallel dipole elements of increasing length strung together and fed alternately through a common balanced transmission line. 

The log periodic antenna works the way one intuitively would expect.  Its “active region,” -- that portion of the antenna which is  radiating or receiving radiation most efficiently -- shifts with frequency.  The longest element is active at the antenna’s lowest usable frequency where it acts as a half wave dipole.  As the frequency shifts upward, the active region shifts forward.  The upper frequency limit of the antenna is a function of the shortest elements.

Figure 1

The basic LPDA geometry is that shown in the above Figure 1.  Each element is shorter than the element to its left.  Ratio of each dipole element to each adjacent element is constant, and is referred to as Tau (t). The other critical dimension is the ratio of the spacing between adjacent elements to the longer element length, designated Sigma (s).  Sigma is sometimes called the relative spacing constant.  These 2 constants, along with the minimum and maximum frequencies are enough to describe most log-periodic dipole arrays.  Another relatively important constant is the L/d ratio (length to diameter of each element).

                         F/B:  Yellow = 8-11 dB  Green = 12-14 dB  Blue = 15-19 dB  Pink = 20-24 dB Red = 25 dB+

Figure 2

The curves shown in Figure 2 have been used for many years to estimate the gain of LPDA designs, based upon the constants Tau and Sigma.  These should be used with caution, as they are only an approximation of performance, based on one particular L/d and an octave bandwidth.  Generally, higher values of Tau also give higher F/B (front to back ratios) and higher values of Sigma generally give higher gain.  Rarely are LPDA's designed using values above the optimum Sigma line as these would be extremely large.  For compact LPDA's, I like to use Tau in the range of 0.84 to 0.90 and Sigma between 0.04 and 0.06. This produces gain between 6.0 and 6.5 dBi with F/B between 15 and 20 dB.  For F/B greater than 25 dB, Tau needs to be greater than 0.92.

I have always wanted to see how close Figure 2 is to what the NEC models give.  Here is my attempt.  Stay tuned - it may take a while.

GAIN

LPDA median Gain (dBi) based on 250-500 MHz (octave) design with L/D = 50 (0.46")

LPDA median F/B (dB) based on 250-500 MHz (octave) design with L/D = 50 (0.46")

Example 1:

Suppose that I want to design an LPDA to cover the frequencies of 200 - 500 MHz with approximately 7.5 dBi gain and F/B of 23 dB or more.  The largest diameter element should be made from 0.5" aluminum tubing or rod.

From the curves in Figure 2, I will initially select Tau = 0.92 and Sigma = 0.08.  This should give me approximately 7.5 dBi gain and 22-24 dB F/B.  In LPCAD, I use these numbers in "Enter Design Parameters".  The LPCAD34 screen should look like that below.

When I select "Edit / Review Parameters" after entering the above data, I see this screen below.

This might be a little low for the 22-24 dB F/B that I wanted, but lets continue to see what this design gives. The boomlength of about 40 inches (1 meter) is OK and the smallest element diameter of 0.156 inch is also OK.  For actual construction, I will likely round the element diameters to something commonly available such as 0.125", 0.1875", 0.25", etc.

Next, press "N" to return to the Main Menu, then press "C" for "Calculate Design". LPCAD calculates the element lengths and spacings and gives the L/d ratio as 57.9.  The screen should look like the one below.

Press "Enter" to continue.  LPCAD will ask you what input impedance is wanted, then after you enter the value, it will get as close as possible.  The feeder "Z" lower limit is set at 73 ohms, which along with the elements, determines the input "Z".  See the screen shot below. Unless you use K6STI's AO (Antenna Optimizer) program, you can normally ignore the question about calculating the geometry of the feeder.  NEC (4NEC2, GNEC, etc.) gives you the TL (transmission line) parameter, which is very easy to use. If you Save/Export the design to the NEC format, this will be used automatically.

Press "N" to return to the Main Menu. At this point, if you are satified with the design, you can save it in 3 ways.  Press "S"  to go to the "Save/Export" menu.  Press "D" to save the design in the LPCAD format.  You can later recall this file to edit or review the design. You can also save in the K6STI AO format if you happen to use this software.  The most popular file format to save to is NEC - Numerical Electromagnetics Code. Press "N" to save to this format.  LPCAD will then ask you some questions during the SAVE-NEC process.

In the screenshot above, I typed in "LP200500" as the file name.  The extension "NEC" will be added automatically.  I like to use the beginning and ending frequencies as part of the filename, however you can use any naming convention that you want.  The description, name and date are also just suggestions for the first 2 comment lines of the Nec file. I would recommend adding a rear termination to all LPDA designs.  This is a short circuit at the end of a short transmission line at the rear end of the LP. This termination helps, but does not completely eliminate the narrow-band VSWR spikes that are normally found in the frequency swept response. At each of these "spikes" the F/B and gain also degrade. The best way to minimize these "spikes" is to use a TAU of 0.93 or higher along with a rear termination.

Other options in the SAVE-NEC option include changing the polarity  from horizontal to vertical, placing the antenna above ground, and entering the start frequency and frequency step size.  In 4NEC2, you can override these frequency settings by choosing the Frequency Sweep option.  Once you see the "Press Enter to Continue" line, the NEC input file has been saved in the current folder (where LPCAD is located).  I like to place the LPCAD.exe file in the model folder of 4NEC2, so I can easily find the NEC files when I use 4NEC2.

To see how this design looks in 4NEC2, continue to page 2.

Comments are welcome!

contact Roger: email to WB0DGF @ ARRL.NET

or

rgcox2 (at) gmail.com

Roger Cox WB0DGF - Spring Lake, MI