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ANTENNA NOTES FOR A DUMMY

Restricted Space Antennas

by Walt Fair, Jr., W5ALT

Before anyone gets upset over the title, let me explain that these notes were written for me. I am an engineer by profession - a petroleum engineer, not an electronics engineer. When it comes to electronics and antennas, Iím still a dummy, but Iím trying to correct that situation!

It hasnít always been easy - electronics experts have written a lot of the literature. Many times I thought I understood something, only to find out I was wrong. Other times Iíve been totally confused and only after a lot of studying, finally managed to get some concepts into my thick skull.

So, these notes are written for this specific dummy. If they serve to clarify things for others, Iím certainly happy to share them.

Introduction

Unfortunately much of my ham radio ďcareerĒ has been in places where antenna space has been limited. This has been partly due to formal restrictions, such as operating from a college dormitory where I wasnít even supposed to have an antenna. Sometimes it's been due to other restrictions, like working overseas where my employer specified where I lived and it happened to be an apartment in the middle of a high-rise condominium or from a hotel room while traveling on business. Other times it was due to a desire to stay on good terms with my neighbors. Needless to say, this has been a constraint, but it has not stopped me from operating. As a result of the constraints of limited space, I have invested a lot of effort into understanding more about the performance of small antennas and through experimentation and theory devised ways to continue my ham radio operating even though limited by antenna size.

This document is meant to summarize some of my notes and experiences in design, installation and use of small antennas. By small, that means in terms of wavelengths, not necessarily physical size. For example, a full size dipole for the 2 meter band is less than 40 inches long, so there is rarely a need to use a compromise antenna for that band. On the other hand, even a 40 ft antenna for 160 meters is small in terms of wavelength, since a wavelength is around 500 ft long. So even though my main motivation is for space limited antennas, the notes and concepts described here are also applicable to physically large antennas for the lower bands where the natural antenna size is large compared to normal space limitations.

Please understand that these notes are not meant to show that limited space antennas are better than full sized antennas. That simply is not true. The adage that the more wire you can get up, and the higher you get it, the better it will perform, is usually true. However, as will be seen, antennas are governed by some fairly well defined physical phenomena. As we understand these phenomena, it is possible to construct limited space antennas that perform relatively efficiently. They wonít compete with full sized beams, but they will allow operation, and that is what matters.

Acknowledgments

First, I gladly acknowledge the input of lots of people, from personal and on-line discussions, to magazine articles to textbooks. Especially useful in compiling these notes were The ARRL Handbook and The ARRL Antenna Book. I've also drawn extensively from things I've read in both QST and QEX. I've also found the late Joe Carr's books useful, especially Practical Antenna Handbook. And, of course, nothing major can be done in today's world of antenna design without some good software. I have made extensive use of MultiNEC by Dan Maguire, AC6LA. You can be assured that there is nothing here that I have invented or have any claim of originality.

However, if you find some errors, mistakes or blunders, I must accept full responsibility. Any comments on these notes should be addressed to me at w5alt@comportco.com.

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