Tuesday, March 31, 2009

160 m Windom antenna modeling

Recently W4BXI provided some details (left) of his 160 m Windom antenna, since I had offered to model it with the 4nec2 software.

The screen shot below is a representation of the antenna structure data as entered into the program. Click on any image for an expanded view.The white lines are the wire elements and the blue lines are the feedline. A level ground is used for convenience, but the actual antenna site has some sloping ground as noted in the details sent to me.

Below are some of the results. The main program window shows impedance, etc. at the end of the 450-ohm feedline. Impedance at the transmitter end of the feedline is calculated to be quite high: 3797 + j861 ohms. I intend to check this using alternate tools (e.g. Smith chart). I expect it to be very dependent on the length of the feedline.

A sweep of frequency from 1.6 to 2.0 MHz yields the following SWR and reflection coefficient curves. This is based on a 50-ohm source with no matching. Naturally, the SWR looks better when using a 72-ohm feeder.

The program provides a tool for calculating several types of matching networks for matching to the designated 50-ohm source. Here are the results.

Also available are several forms of far field pattern plots in horizontal and vertical planes. Here is the horizontal gain plot.

3D plots of gain in various forms are also available. Here is one example that uses color coding to represent intensity. The antenna structure is also shown in this plot.

Text output files are also available if you want to examine or separately plot all the results.

Monday, March 30, 2009

Electronics Recycling

Several web sites enable you to search for companies near you that recycle electronic equipment that you may be unable or unwilling to sell, trade or reuse. Two that I can suggest are:

An example of a company I found in Birmingham is Technical Knock Out.

Saturday, March 21, 2009

Antenna Modeling & Analysis

Some powerful software tools are available for modeling and analyzing many forms of antennas that are popular among hams. These can be used to guide the designs of home-brew antennas and to gain insight into measurements made on actual antennas that you are using. Models can include antenna elements, detailed in 3 dimensions, transmission lines, matching / loading components, sources, ground surfaces (with various characteristics) and surrounding objects. Program outputs include: impedance, SWR, radiation patterns, current distributions, front/back ratios, impedance matching. Currently available versions of the program can sweep frequency inputs or component values (e.g. antenna height, wire size, loading coil inductance) to produce plots depicting the variation of results with the swept parameters.

The granddaddy of antenna analysis programs is the Numerical Electomagnetics Code (NEC) family of programs originally developed at Lawrence Livermore Labs in 1981, on a contract to the U.S. Navy.

NEC2 was released to the public. Browsing the NEC2 User's Manual reveals the punch-card FORTRAN genesis of this program, and the terminology continues in current versions of the program that hams can run today on their personal computers. Historical documentation and information links are collected at the NEC2 web site.

Among the links at the NEC2 web site is one to the EZNEC versions of the program developed by W7EL. A free demo version of this program is available and it is a very good place to start to learn what this program can do. The free version is full-featured, only limiting the complexity of antennas (20 segments max) that can be modeled. Paid versions of the program permit much greater complexity of antenna systems.

Another implementation is the 4NEC2 program developed by Arie Voors. You can get an appreciation of this program's capabilities from the large collection of screenshots. It is a free program and, like EZNEC, includes a good number of pre-built antenna models, many developed by W4RNL. These are good for getting familiar with the program and as starting points for developing models specific to your needs.


Wednesday, March 11, 2009

Antenna Tuner Discussions

The topic of antenna tuner operation and design was popular today. Here are some links that Phil and I have dug up that may be helpful:
How to User Your Antenna Tuner is a fairly long article with several additional web links and many posted comments. Among the included links are:
  • Antenna Tuner Operation is a short article by ARRL about how to use a common tuner type.
  • A T-Network Simulator is a Java application that lets you practice using a software simulated tuner. You can turn the knobs on the tuner to try your solution and then click the autotune button for comparison. The simulator can be run or the web page or downloaded to run locally on your Java-enabled PC.
  • Antenna Transmatch Tips provides some details of popular tuner designs that are commercially available along with tips for similar homebrew units. It includes some discussion of common component limitations.
Other links provided in Phil's recent email:
A subtopic of particular interest is the ability to find more than one group of settings in T-network tuners to provide a match. This property is illustrated in the 1995 QST article cited above. See the article's section labeled Practical T-Network Tips for some recommendations on how to avoid overheating inductors and arcing capacitors.

Many additional articles are available at this DX Zone web page.

Your comments are welcome.


Dalton Hamfest 2009 Photos

Photos from this event are available for viewing here. Thanks to N4OLN and W4UOA for their camera work!