I’ve had two weeks now to flex my new-found antenna modeling skills, courtesy of Antenna Modeling For Beginners by Ward Silver N0AX.
Long story, short – I like the book and consider it money well spent (although I spent significantly less than the cover price by buying from Amazon before they raised it).
For others who may be considering the purchase of Antenna Modeling For Beginners, there are a few things you ought to know.
While this book is a how-to on antenna modeling, it is more specifically a book on the free version of W7EL’s EZNEC antenna modeling program. I mention this because the book and the software go hand in hand – and the free version of EZNEC has serious limitations.
That’s not as bad as it may seem – the techniques described in the book translate to more advanced versions of EZNEC – just know that to accurately model antennas more complex than dipoles, loops and verticals that you’ll have to pay for software that will allow it.
The primary difference between various versions of EZNEC are in the number of “segments” you’re allowed to use to define antenna element portions. The more segments, the better the acuracy. But it is a book for beginners, as the title clearly states.
Also, there are corrections to various typos as well as other pertinent info regarding the book online here – be sure to check that out if you buy the book.
So on to the book itself:
The limitations and capabilities of of antenna models vs. real-world antennas are discussed, as are the factors contributing to those differences. Tables, charts, plots and traces are all described and made use of via exercises.
In Chapter 4, you build your first antenna – a dipole. Simple, right? Once built, you then learn to rotate the elements into an inverted V configuration.
The method of the book is this: describe the how and why of a given topic, then perform a related exercise in the program, then use the program to analyze the results. The way the book is laid out is a perfect blend of description and hands-on with the software, with each activity enforcing the other.
A lot of learning takes place in these early exercises – how to connect wires (elements), the effect on input impedance as the dipole becomes an inverted V, the effect on bandwidth as aluminum elements are used rather than copper wire, etc.
Later, an antenna model’s height is increased and the ground type is changed.
Any ham worth their ticket already knows the basic results of these changes without having to model them but the point of these particular exercises is twofold:
- to learn how to manipulate the software to correspond to your intended design, and
- to demonstrate that a given change in the design will result in a valid change in antenna characteristics
Now that the software has earned your trust, the book contains other exercises that will be needed to design more realistic models. You learn to add taper to aluminum elements in one exercise and add an inductive load in another.
Step by step.
It didn’t take long for me to discover that the real value of antenna modeling (for me anyway) is not in designing new antennas from scratch but rather to manipulate existing designs in order to predict new results. To do this, I had to spring for the full version of EZNEC ($89) and was happy to do so knowing that I could use it to its almost-full potential.
Some examples of what I mean in my particular case:
- A model of my Yagi, a Force 12 C3, already exists and is included on the CD in the ARRL’s Antenna Handbook. With EZNEC and the knowledge to manipulate it, I could now add the optional 10m reflector and analyze results on 10m and other bands. Is the additional gain and F/B worth the $99 price of this option?
- Force 12 also offers a 40m aluminum dipole that can be added to the C3’s boom. I added this to the model to see what it might offer me over my fixed wire dipole for that band. And how it might interact with the C3’s performance on other bands (negligible).
- If I add the 40m element, how will it perform at the antenna’s current height vs. 20 feet higher in terms of take-off angle, etc?
One thing missing from the book but contained (even in the free version) in EZNEC is the topic of adding and defining transmission lines used to feed the modeled antennas. As it is, the feed source for all models is placed directly at the feedpoint. This is probably a minor point for most antennas that I would design/manipulate – at least for now. Perhaps Ward’s next book, Advanced Antenna Modeling, will address that issue!
As I mentioned at the beginning, I consider this a very useful and well-written book. It’s one thing to know how to do something; knowing how to teach someone else to do that something is an entirely different skill and Ward Silver has it.