Thursday, November 23, 2017

Glass Jar to Ground Plane Antenna

Did the ground plane antenna evolve from a glass jar?  Of course it did!  Can't you see the logical progression?  Just as all earthly creatures evolved from marine life: from fish to man; in our case, glass jar to ground-plane antenna.

From a radio communications standpoint, the ground-plane antenna has been the fundamental antenna used in radio communications from almost the beginning.  Simple and basic; yet, an efficient means for launching electromagnetic waves.  But, you ask, how is it related to a glass jar?

The jar in question is the Leyden Jar as shown in Figure-1. This device was not named after its founder; but rather, a University in Holland where it was discovered.  The jar was founded by accident by a scientist named Pieter Musschenbroek sometimes before 1745. To his surprise, Musschenbroek received an electrical shock when he picked up the jar to move it.

Musschenbroek's construction was simple. The device was simply a glass bottle half filled with water and sealed with a cork.  A nail penetrated the cork into the water, and the jar was charged through the nail using an electrical machine.  When first discovered by Musschenbroek, water served as the inner coating and Musschenbroek's hand served as the outer coating. Touching the jar completed the ground connection, creating an electrical shock. An early electrical component, the Leyden Jar is the ancestor to today's capacitor.  Back then it was called an accumulator or a secondary battery.

To charge the jar, an electrical machine is used.  One of the oldest types of machine is called the Cylinder Electric Machine shown in Figure-2. The device is constructed using of a glass cylinder mounted on a horizontal bar with a handle on one end.  The upper half of the cylinder is covered with a flap of silk.  The silk is attached to a cushion comprised of rubber, leather, horsehair and powdered amalgam zinc or tin.  To the right of the device is a brass cylinder called the prime conductor, which is mounted on a glass stand. On one end, a comb-like device made up of fine metallic spikes is mounted near the glass cylinder. The other end is a rod with a brass knob on the end.  When the handle is turned, friction is applied between the amalgam covered leather and the glass cylinder.  The prime conductor collects the charge through the spikes and forms a charge at the brass ball.  To charge a Leyden Jar, the jar's brass ball is placed near the prime conductor’s brass ball. The outer coating is connected to ground by a wire or chain.  The jar collects an electrical charge.
Later, the jars were constructed using tinfoil.  A glass jar was coated inside to a certain height with tinfoil.  Tinfoil also covered the outside jar.  A brass knob connected to the end of a brass rod is extended through the cork lid and connected to a brass chain, which makes contact with the inner foil. Just like the electrolytic capacitor we're all familiar with, an electric charge is collected on the glass dielectric between the two tinfoil plates [as discovered by Benjamin Franklin - prior to this time, scientists thought that the charges accumulated on the tinfoil]. Figure-1 shows a Leyden Jar being discharged. The inner and outer tinfoil of a charged Leyden jar is brought together by the discharger, creating a spark across the air gap.

The Leyden Jar was well known and commonly used in electrical experiments.  Scientists experimented with the jar's discharges and its electrical characteristics. Experimentation showed that jars could be connected in series/parallel arrangements with other jars to produce bigger discharges [ranges greater than 10,000 volts] as shown in Figure-3.  This jar arrangement was also known as a secondary battery. The jar could hold a charge for several hours or even several days.  Up to this time in electrical history, the Leyden jar was the only device that could store electrical energy until Volta created the battery in 1800.  This was nearly a half a century later.  The Leyden jar was an early electrical discovery and was experimented vigorously before its association with electromagnetic wave theory.

In 1867, James Clerk Maxwell, through his celebrated mathematical equations, proved that electromagnetic waves existed, and that they traveled at the speed of light.  Maxwell was a genius who could couple theories developed by Gauss, Ampere, and especially Faraday.  His theory married electrical and magnetic motion in the form of waves in a medium called the ether [aether]. Light was a high frequency electrical phenomenon while heat was its low frequency counterpart.  Electromagnetic waves are found in between.

Maxwell's work inspired both Victorian and modern-day scientists to research and experiment with electromagnetic theory.  His discovery was exceptionally revolutionary, since Newtonian physics at that time asserted that electric waves could not exist.

Maxwell's theory set the stage for major change.  It showed that changes in electric and magnetic forces sent waves into space.  A phenomenon that had a difficult time of being accepted by the current day scientific community. No other scientist would show this brilliance until the emergence of Einstein at the turn of the 20th century.

Maxwell's theory of electromagnetic radiation, with the relationship between electric and magnetic fields, can be illustrated as follows:  The output of a transmitter's final amplifier feeds an alternating current to the antenna via a transmission line. The resulting current flowing in the antenna produces an alternating magnetic field in the space surrounding it.  This alternating magnetic field, in turn, produces an alternating electric field in the space surrounding it, but is offset by 90 degrees.  The magnetically induced electric field in turn produces a second magnetic field in the space surrounding it.  This electrically induced alternating magnetic field will produce yet another alternating electric field; and so on, and so forth - each field alternately being reproduced by its reciprocal field force in space.

Still, during Maxwell's lifetime and several years thereafter, there was no physical electrical apparatus that could transmit or receive the electromagnetic waves that his equations predicted.  In 1883, one scientist by the name of G. F. Fitzgerald, who initially refuted the existence of Maxwellian waves, suggested that Leyden Jar discharges could be a source of electromagnetic waves.  This essential fact eluded scientists for decades because there was no way of detecting electromagnetic waves that the Leyden Jar was thought of generating.

Eventually in 1887, the German scientist Heinrich Hertz achieved verification by experiment, proving that Maxwell’s theory was correct.  Hertz's simple circuit [Figure 4] consisted of a battery, sparking coil, interrupter, and spark gap. 

The device generates an electromagnetic wave when the battery provides a high voltage DC current into the induction coil primary.  This event causes a dc voltage pulse in the secondary, and the interrupter provides a constant alternating current in the coil’s secondary.  At this point, the spark gap acts like a very fast acting switch that positively charges one of the plates.  The other plate is charged negatively. The voltage at the gap breaks down and causes a spark to arc across the open air-gap connecting the two plates.  The energy in the gap is sustained by interrupting the spark coil current using a device called an interrupter. A wide spark, bright white in color, launches electromagnetic energy into space - radio waves!

Hertz was able to use these as components to generate radio waves.  As it turned out, the detection of the electromagnetic waves turned out to be quite simple device.  Hertz's receiver, which he called a resonator, was a simple circular piece of wire with an air gap. See figure-5.

In the time span between Maxwell's discoveries and Hertz's experiments, roughly twenty years, an important technological evolution unfolded. 

Examining Hertz's circuit closely uncovers an important transformation.

Referring to Figure-4, Hertz's apparatus consisted of two metal plates connected on the ends of a thin brass rod.  Hertz felt that this arrangement could be charged like a Leyden Jar by cutting the rods at the center and making an air gap using two spherical brass balls.  This arrangement is known as a Hertzian Dipole. 

Now, examine Hertz's transmitter diagram very closely, especially those rectangular plates at the far ends of the air gap.

If you were to pull out the inner tinfoil of the Leyden Jar, it would closely resemble a rectangular plate. Peel off the outer tinfoil from the jar and it too resembles another rectangular plate.  Now, when both plates are extended out and away from each other horizontally, it begins to resemble Hertz's dipole transmitter apparatus.  The other two ends form an air gap.  There it is!  The first evolutionary development.  From Maxwellian waves to Hertzian waves.

As everyone knows, a 22-year-old Guglielmo Marconi developed the first commercial wireless system in 1896.  His system consisted of various components incorporating a transmitter and a receiver.  These components included a Ruhmkorff coil, a spark-gap, a modified Branly coherer (metal filings in a glass tube), Volta’s batteries, and Hertz’ dipole.  Other scientists and experimenters developed all these devices, but Marconi was the first to use them in concert to develop a wireless information transmission system - wireless telegraph.

Marconi was not only the first to successfully launch electromagnetic waves to great distances, but also contributed to the development of the technology.  In parallel with his research, Marconi was also the first to bring wireless telegraphy technology to the for-profit marketplace. Before his appearance on the wireless scene, scientists involved in electromagnetic study were only for academic purposes and the pursuit of science.  No one thought of developing the technology for commercial purposes.  Scientists were only interested in proving Maxwell's theory and to expand on Hertz's experiments.

However, Marconi's involvement was significant as he gave birth to the wireless industry.  He continued to cultivate technical improvements.  For example, he theorized that elevating the antenna to greater heights would launch electromagnetic waves to even greater distances.  No one else had thought or done this before.

In those early days, the antennas used by Marconi and other experimenters were horizontally polarized Hertzian dipoles, consisting of a wire with capacitively loaded plates at each end of an air gap, resembling Hertz's device.  Each arm of the dipole was a quarter-wavelength.  One major difference of Marconi's dipole from Hertz' was that Marconi's system was physically larger due to the lower frequency used. Hertz's experiments were in the UHF range.

Again, we are on the brink of another transformation.  Here is an interesting development: Marconi rotated the horizontal dipole arrangement 90-degrees.  One of the plates was elevated to a vertical position by hanging it on a wooden pole [Fig 6].  The other metal plate was left lying on the ground.  The result was a notable improvement in the distance that a signal could be transmitted.  Soon, with more experimentation, Marconi buried the plate lying on the ground and found that the antenna became even more effective.  The earth now replaced one of the dipole arms.  This antenna configuration is called a grounded vertical.

Marconi's experimental antennas were elevated by raising antenna wires on poles, and at times with kites or even balloons.  His experiments found that the higher the wire, the greater the distance.  Before long, along with the improvement of his wireless apparatus, Marconi could achieve greater distances, when in 1901 he transmitted the first wireless message across the Atlantic Ocean.  The Morse code letter for “S” was successfully transmitted and received between England and Newfoundland.  The antenna was attached to a kite because the antenna supporting structure was destroyed a few days earlier due to stormy weather.

Now, if you eliminate the plate at the end of the dipole, you have a vertical end-fed or ground plane antenna that we are familiar with today.  What happened to the plates on Hertz's dipole and Marconi's grounded vertical?  First, they were never necessary.  The only purpose they had was to add capacitance, increasing the effective length of the antenna.  The plates had little or no influence to the total electromagnetic field strength generated or on the resonant frequency of the antenna.  Hertz possibly used them as a holdover from the inner and outer tinfoil of the Leyden jar. Marconi initially used them as a holdover from Hertz's experiments.  In both cases, it would have been much easier for both men to abolish the use of the plates entirely. 

 Today's ground plane antennas [Figure 7] are found everywhere on rooftops and towers. The ground plane portion of the antenna consists of extended rods usually made up of 3 or 4 elements perpendicular or angled to the vertical radiator.  These are commonly referred to as ground radials, providing the “missing” radiating element of the ½ -wave dipole. They also provide an “elevated” ground, simulating the earth's ground at a location above the earth.  These radials are physically independent from the earth ground, but supply the needed antenna electrical ground.

History provides interesting hindsight - 250 year's worth. Glass jar to ground plane antenna.