Radio Propagation and Antennas: A Non-Mathematical Treatment of Radio and Antennas

By Steve Cerwin

It is from the hands-on perspective of a lifelong ham radio operator turned professional “RF and antenna guy” that this book is written. The intense mathematical antenna descriptions given in most antenna handbooks is more befuddling than enlightening for many. So in this book the intuitive is emphasized and mathematics is minimized, though many formulas are given to calculate selected parameters if desired. The purpose of this book is to provide a basic understanding of antennas and radio propagation for both professionals and amateurs alike. Many of the technical explanations were developed for a 5-day antenna course in which the requirement was to take students from zero to antennas in one week. The characteristics of many antenna types are discussed and construction recipes are given for building selected antenna types. The intent is to provide enough basic understanding so that the interested readers can select an appropriate antenna for their application and then design and build one for themselves. More than anything this book is intended to give the reader a basic understanding of what radio waves are, how they behave, and insight to the creative thought processes used to build the antennas that launch and receive them.

• Language: English
• Publisher: AuthorHouse
• Release date: Jul 24, 2019
• ISBN: 9781728320328

Steve Cerwin

Radio has held a fascination for me since 1963 when at age 12 I became licensed as WA5FRF and entered the wonderful world of Amateur Radio. Just the word “radio” holds wonder for me even now though I have been an active practitioner of radio art for over a half-century. Antennas and the way radio works have been at the forefront of this fascination. I will never forget the wondrous sounds emanating from an army surplus receiver when I hooked it up to the first antenna I ever made: a 40-meter dipole in the backyard of my parent’s house. Nor will I ever forget the 1-inch long arc that jumped from the open end of the coax to my finger one day. Could radio signals really be that strong or was it static electricity from the thunderstorm that was brewing nearby? I would not know until sometime later, but it certainly sparked an increased interest in radio and antennas. It was that early interest in ham radio that kindled my career in science. I spent most of my career at Southwest Research Institute, achieving the level of Institute Scientist and obtaining eleven patents. My formal education is as a physicist, though I have been designing and building antennas and RF systems throughout my professional career. Additional interests include flying full scale and radio controlled aircraft, and scuba diving.

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2019 Steve Cerwin. All rights reserved.

No part of this book may be reproduced, stored in a retrieval system, or transmitted by any means without the written permission of the author.

Published by AuthorHouse 12/05/2019

ISBN: 978-1-7283-2034-2 (sc)

ISBN: 978-1-7283-2033-5 (hc)

ISBN: 978-1-7283-2032-8 (e)

Library of Congress Control Number: 2019910311

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Because of the dynamic nature of the Internet, any web addresses or links contained in this book may have changed since publication and may no longer be valid. The views expressed in this work are solely those of the author and do not necessarily reflect the views of the publisher, and the publisher hereby disclaims any responsibility for them.

CONTENTS

Preface

Chapter 1 Fundamentals of Radio

Chapter 2 Terrestrial Propagation Modifiers

Chapter 3 SWR, Reflection Coefficient, and Transmission Lines

Chapter 4 Dipole Antennas

Chapter 5 Verticals and Monopoles

Chapter 6 Loop Antennas

Chapter 7 V-Beam, Rhombic, and Vivaldi Antennas

Chapter 8 Yagi and LPDA Antennas

Chapter 9 Slot Antennas

Chapter 10 Circularly Polarized Antennas

Chapter 11 Patch Antennas

Chapter 12 Antenna Arrays

RADIO ANTENNAS and PROPAGATION

Steve Cerwin

WA5FRF

PREFACE

Radio has held a fascination for me since 1963 when at age 12 I entered the wonderful world of Amateur Radio. Just the word radio holds wonder for me even now though I have been an active practitioner of radio art for over a half-century. Antennas and the way radio works have been at the forefront of this fascination. I will never forget the wondrous sounds emanating from an army surplus receiver when I hooked it up to the first antenna I ever made: a 40-meter dipole in the backyard of my parent’s house. Nor will I ever forget the 1-inch long arc that jumped from the open end of the coax to my finger one day. Could radio signals really be that strong or was it static electricity from the thunderstorm that was brewing nearby? I would not know until sometime later, but it certainly sparked an increased interest in radio and antennas. It was that early interest in ham radio that sparked my career in science. I spent most of my career at Southwest Research Institute, achieving the level of Institute Scientist.

It is from the hands-on perspective of a lifelong ham radio operator turned professional RF and antenna guy that this book is written. My formal education is as a physicist, though I have been designing and building antennas and RF systems throughout my professional career. The intense mathematical treatment given in most antenna handbooks was more befuddling than enlightening to me. So in this book the intuitive is emphasized and mathematics is minimized. However many formulas are given to calculate various parameters. Much of what I understand of antennas came from the excellent publications of the American Radio Relay League and the rest from OJT. It is in the spirit of intuitive understanding used in many of their publications that this book is written. Certain fundamental level illustrations in this book are reprinted from League publications with the gracious permission of the ARRL. This is because they are done so well that little improvement is possible.

The purpose of this book is to provide a basic understanding of antennas and radio propagation. Cookbook type recipes are given for building selected antenna types. The characteristics of many antenna types are discussed to help the reader determine which might be appropriate for a given situation. Although there are many antenna types, the ones described here address most antenna requirements. Many design techniques are given that adapt to field expediency where it may be necessary to construct a specific antenna type using only whatever materials might be at hand. More than anything this book is intended to give the reader a basic understanding of what radio waves are, how they behave, and insight to the creative thought processes used to build the antennas to launch and receive them.

I would like to thank Randy Roush, Bill Liles, Gordon Mitchell, and John Swartz for their invaluable help, insights, and most of all, their encouragement.

CHAPTER 1

FUNDAMENTALS OF RADIO

1.1    ELECTROMAGNETIC RADIATION

1.jpg

Figure 1.1 Electromagnetic

Radiation

Radio, light, X-rays, and gamma rays are all the same thing: a propagating electromagnetic (EM) wave. The wave is comprised of oscillating electric and magnetic fields oriented mutually perpendicular to one another, and perpendicular to the direction of propagation, Figure 1.1, reprinted with permission from the ARRL Antenna Book, shows the orientation of the electric field (E), the magnetic field (H), and the direction of propagation (X). The fields oscillate sinusoidally and are in-phase in that each peaks and wanes at the same time.

There are three interrelated quantities that completely quantify a propagating EM wave: the characteristic frequency of the signal, the velocity of propagation, and the wavelength. The wave propagates at the speed of light, 300x10⁶ m/sec or 186,000 miles/sec in vacuum and very nearly that through the earth’s atmosphere. The wave travels more slowly through dielectric materials like plastic, glass, or wood. In radio, the ratio of wave speed in a material to that in free space is referred to as velocity factor. In optics, the ratio of the speed of light in a vacuum to the speed of light through a material like glass is defined as the index of refraction of that material. For all antennas constructed in the open air on planet earth, the velocity is taken as that in vacuum, referred to as the free-space velocity.

65345.png

The definition for frequency of a radio wave is the number of complete positive to negative oscillations per second. One-and-a-half complete cycles are depicted in Figure 1.1. One cycle per second is called one Hertz (Hz), in honor of Heinrich Hertz, an early pioneer of radio. The convention used to quantify progressively larger numbers of cycles per second follow the standard kilo-, Mega-, Giga-, and Tera- designations that describe quantities that increase by factors of 1,000. These are defined in Table 1-1.

The frequency spectrum used for radio has acquired letter descriptors over the years. The evolution of the nomenclature has followed partly the evolution of radio technology, partly the need for secrecy during the accelerated pace of development during the Second World War, and partly from input from the peacetime engineering community. Table 1-2 lists this nomenclature and associated uses for each frequency band. The F at the end of all the abbreviations stands for Frequency. The first letters stand for Extremely Low, Very Low, Low, Medium, High, Very High, Ultra High, and Extremely High. After a few of these it becomes difficult to come up with unambiguous adjectives to describe ever higher frequencies. The letter designators beginning with L were born in the secrecy of the war years and also address this issue.

Table 1-2 Frequency Designations and Uses

The Possible Uses column is not intended to be either all- inclusive or exclusive. There is great overlap in uses, especially at VHF and above. The propagation characteristics of a radio wave vary greatly with frequency. Almost every advance in electromagnetic technology has been accompanied by the ability to generate and receive ever increasing frequencies. Radio dawned at low frequencies, where a propagating wave hugs the surface of the earth and travels via what came to be called ground wave. This wave must have the e-field perpendicular to the earth to propagate, lest ground conductivity short it out. It was ground wave that first spanned the Atlantic. Today, omnidirectional navigation beacons still use the ground following characteristics of LF to guide aircraft towards airports (though VHF Omni Range (VOR) and GPS are largely replacing them). The 60 kHz WWVB time synchronization signals propagate via ground wave and atmospheric duct. At least two advanced radio navigation systems used VLF and LF signals. The now-defunct Omega radiolocation system operated near 10 kHz and could actually be heard at the RF frequency in a pair of headphones connected to an amplifier and a long wire antenna. LORAN-C (acronym for Long Range Navigation) used the time-difference-of-arrival of the signals transmitted from chains of transmitters operating at 100 kHz to unambiguously determine position on the surface of the earth. Though completely outmoded by modern GPS, in 2010 this system was still held in reserve as a backup to GPS and an enhanced in a version known as Enhanced LORAN, or eLORAN evolved.

The next leap in radio technology came with advancement into the HF band and the discovery of worldwide propagation afforded by the return of HF signals to earth by the upper regions of the ionosphere. This mode of propagation, known as sky wave, leaves the earth towards the horizon from a transmitter where it enters the layers of the ionosphere. It is expedient to think of the ionosphere as a mirror, albeit one more resembling a ball made of rumpled and rumpling aluminum foil than a shiny spherical ball. This prevents radio waves from leaving the earth and instead reflects them back to earth to land great distances away. Ultraviolet rays from the sun ionize the molecules in the upper atmosphere by separating electrons from nuclei. It is actually the free electron density that causes an HF ray to be bent or refracted downwards back to earth to be intercepted by a receiving antenna hundreds to thousands of miles away. This marvelously complex phenomenon undergoes cyclic variations in effectiveness that varies with frequency, time of day, time of year, and sunspots that come and go in a nominally eleven-year cycle. While it is true that modern communications technologies like cell phones, repeaters, and satellites have provided alternate means of communicating, the HF spectrum affords the only mechanism capable of worldwide communications without the use of specialized electronic appliances (like communications satellites). It should be considered a natural resource and protected as such.

And then came the war years. Advancement in the radio art exploded, driven by the need for reliable and secure communications and by the development of radar. The term RADAR was coined in 1940 by the U.S. Navy as an acronym for radio detection and ranging. It afforded long range detection of inbound enemy aircraft by detecting the reflection of short transmitted pulses from metal aircraft skins. It drove development of transmitters and receivers well into the VHF frequencies and above.

Besides radar and communications, VHF and UHF frequencies are used for radionavigation, FM and TV broadcasting, and satellite communications. Frequencies at VHF and above are not returned to earth by the ionosphere, but instead penetrate it. It is this ability that enables space communications.

At still higher frequencies many interesting and useful effects appear. Frequencies in the 10’s to 100’s of GHz can penetrate clothing but not knives or guns, enabling sophisticated security screening systems. Radiometers use these frequencies to image through obscuring media like smoke and fog that would block light. Frequencies above 1 THz (1000 GHz) are selectively absorbed by certain chemicals. Analyzing these absorption characteristics (called spectroscopy) can provide positive identification of only trace quantities of the chemicals. Keep going up in frequency and you leave radio and enter the optical realm. The lowest optical frequencies are infrared, followed by visible red, yellow, green and blue. EM frequencies again become invisible as deep violet turns into ultraviolet – invisible yet capable of causing sunburn and skin cancer. Continue up in frequency and you leave optical and enter the ionizing radiation realm of X-rays and finally gamma-rays. These frequencies penetrate solid objects and provide imaging capability of the interiors of people, machines, and structures. They can also break apart chains of biological DNA, which can lead to the cellular mutation known as cancer.

The point is that from low radio frequencies through gamma-rays, the stuff is all the same: a propagating electromagnetic wave comprised of an electric and a magnetic field, mutually perpendicular to each other and to the direction of propagation, as depicted in Figure 1.1. The fact that different names like radio, light, and X-rays came into being was only a result of our monocular view of how they were discovered and came into use.

1.2    FREQUENCY AND WAVELENGTH

The wavelength of a radio wave is defined as the distance between similar points on adjacent cycles, e.g., from crest-to-crest or from zero-crossing to zero-crossing. A wavelength is one complete cycle of a sine wave and contains 360 electrical degrees. Exactly 1.5 wavelengths are depicted in Figure 1.1. The wavelength (l) is related to frequency (f) and the speed of light (c) by the equation l = c / f. The dimensions of a wavelength for frequency specified in MHz are

                            l = 300 / f meters (Eq. 1-1)

                            l= 984 / f feet (Eq. 1-2)

                            l = 11,811 / f inches. (Eq. 1-3)

Almost all antennas are constructed as specific numbers or fractions of a wavelength. Therefore the above formulas are crucial to antenna building and will be used throughout the remainder of this book. Of the very few imperative mathematical equations used in this book, this is the first.

1.3    DIPOLE RADIATION

The half-wave dipole is the fundamental antenna element. A conductor exactly one half-wavelength long is the simplest structure that can radiate an electromagnetic wave as a real antenna. Theoreticians might argue that the isotropic radiator is the fundamental antenna type. An isotropic radiator is the notional concept of an infinitesimal point source that radiates equally in all directions, like the spherical optical wave emanating from a point source of light. In this case, equal means all frequencies with equal amplitude, equal polarization, equal everything. But in the world of radio waves, this is a notional concept only, as no real antenna with these properties can be constructed. The concept of an isotropic radiator is nonetheless useful in that it can be used as the yardstick by which the performance of all other antennas can be compared.

5.jpg

Figure 1.2 Formation of a Half Wave

Dipole by End Reflections

When a radio wave is launched onto a long wire, the voltage and current propagate along the wire as a traveling wave. Figure 1.2 shows how a dipole is formed from a linear conductor by first terminating one end of the wire to reflect the wave back towards the source, then terminating it again exactly one half wave away. To simplify the figure, only the waveform for current is shown. Since the wire is just long enough to hold one half-wavelength the reflections that occur at both ends set up coherent back-and-forth oscillations in which the current distributions are congruent, i.e., they line up on top of one another. Two simultaneous phenomena happen at the end reflections: 1) the direction of propagation reverses, and 2) the phase flips polarity. Because of the simultaneous reversal of both phase and direction at each end, the characteristics of the current waveform repeat, with current always going through zero on the ends and always reaching a maximum value in the center. Therefore a wire that is exactly one-half wavelength long is the shortest possible straight wire that can contain a self-replicating current distribution that now appears stationary. This distribution is now called a standing wave. The voltage waveform is present as well, 90-degrees out of phase so that voltage is always maximum on the ends and zero in the middle.

6.jpg

Figure 1.3 Counter-

Propagating Charges

Reverberating

on a Dipole

Dipole antennas that contain exactly one-half-wavelength are said to be resonant. The current distribution remains fixed: zero on the ends, and maximum in the middle. This standing wave of current gives rise to the acceleration of electric charge that gives rise to electromagnetic radiation. The continuous back-and-forth oscillations of charge on the half-wave dipole constitute simple harmonic motion in which the charge is continuously accelerated. Current flow (movement of charge) generates a magnetic field and separation of charge sets up an electric field as illustrated in Figure 1.3. When charges moving in opposite directions come together the field closes in on itself and departs as a wave at the speed of light. The result is a radiated electromagnetic wave that flows outward, broadside in all directions from the dipole.

An interesting analogy is to compare oscillating charges along a dipole to the effect of a permanent magnet oscillating back and forth inside a coil of wire. A magnet with N-S aligned with the coil axis produces the same polarity current in the coil when passing left-to-right as when passing right-to-left. In other words the South Pole entering from the left produces the same induced current sense as a North Pole entering from the right. Similarly, a positive charge accelerating to the right along a dipole produces the same far field effect as a negative charge accelerating to the left. Both effects use a double-negative to make a reinforcing outcome.

Up close to the antenna, the electric and magnetic fields are 90-degrees out of phase. But in the propagating wave shown in Figure 1-1, they are in-phase. The region near the antenna is called the near-field, and regions far from it are called the far-field. One of the things that happen as near-field transitions into far-field is that the initial 90-degree phase shift between the electric and magnetic fields changes into the in-phase relationship. In the near-field, there are both storage fields and radiating fields while in the far-field there is only the radiating wave. This is one of the reasons the region near an antenna must be kept free from lossy or absorbing dielectric materials: their presence strikes at the very heart of how an antenna works by absorbing the crucial storage fields. The distance from the antenna required for the near-field to transition to the far-field scales with the size of the antenna in terms of wavelength. For a simple antenna like a half-wave dipole, the transition happens quickly: within the first wavelength. For antennas that are very large in terms of wavelength, such as a large dish antenna, it can take hundreds of wavelengths.

1.4    SIZES OF DIPOLE ANTENNAS

Once understanding is obtained on how various antenna types work, a very accurate estimate can be made of the operating frequency by noting the antenna dimensions. As an illustration, consider the required sizes of full-size half-wave dipoles for various frequencies. First consider 1 MHz, a frequency in the middle of the AM broadcast band with a wavelength of 300 meters. A half-wave antenna would have to be about 150 meters, or 500-feet long! Dipoles for HF sky wave antennas range in size from 5m (16-ft.) at 30 MHz to 50m (160-ft.) at 3 MHz. These are typically made from wire suspended between poles or trees. At VHF and UHF, dipoles are only inches to a few feet long, and become self-supporting when made from stiff