While this article is directed
at amateur radio operators who want to explore working in the microwave bands,
it is good fodder for anyone who wants a fundamental introduction to waveguides,
resonant cavities, distributed elements, and atmospheric propagation. If that
describes you, and particularly if you have formulaphobia, then start reading.
Even though the article appeared in a 1952 issue of Radio & Television
News, the list of frequency band allocations are not much different than
today so the information is useful. Unknown to many is that in the early part
of the last century Amateurs pioneered the use of microwave bands when the Federal
Communications Commission (FCC) allocated the spectrum to them since many "experts"
considered it unusable.
Microwaves for the "Ham"
By Samuel Freedman, W6YUG
Fig. 1 - A 6V6 tube mounted in a wave guide and cavity
system for generating microwaves with a conventional receiving tube.
Fig. 2 - Graph of attenuation vs frequency for a 1
x 1/2 inch wave guide suitable for 10,000 to 10,500 mc. ham band operation.
Table 1 - Currently authorized ham bands.
Fig. 3 - How coils and condensers, alone or as series
or parallel circuits, can be eliminated by moving along a quarter wavelength
within any over-all half wavelength.
Fig. 4 - Commercial vs homemade wave guide for the
3300-3500 mc. band. This 3" x 1 1/2" wave guide has a power handling capacity
of nearly 3,200,000 walls.
Fig. 5 - Nature's wave guide on low frequencies vs
fabricated wave guides on microwaves. Wave guide is shown at cut-off, above
cut-off, and at even higher values.
Fig. 6 - Cutaway view of reflex klystron. The negatively
,biased repeller atop the tube turns the electrons back to the positively biased
grids. A cavity circuit is set up between pairs of grids. A tube of similar
type can be used at 3300-3500 mc.
Fig. 7 - A reflex klystron tube with its output coupled
into a wave guide as required for operation in the 10,000 to 10.,00 megacycle
Fig. 8 - Circuit of one-tube microwave receiver using
Fig. 9 - One-tube microwave receiver using single 6F8G
tube. The horn unit comprises the antenna and transmission line when placed
over the tube.
The growing problem of TVI will eventually force hams to "move upstairs"
into the microwave bands.
Since May 1945 the Federal Communications Commission has reserved seven bands
of microwave frequencies for the exclusive use of hams. Ideally located between
420 and 22,000 mc., this allocation provides over 2,000,000 kilocycles of spectrum
as compared to the 16,000 kilocycles now used for approximately 99 percent of
all ham transmissions.
It is now seven years since these bands were assigned to the hams yet, with
the exception of the 420-450 mc. band, activity in these bands is negligible.
This state of affairs is inconsistent with the ham's traditional role as a "radio
The fact that 15,000,000 television receivers are now in the hands of the
public with more to come as new stations bring larger areas of the country within
the fold will provide a powerful impetus in "kicking the amateurs upstairs."
If there is to be any lasting peace and harmony between the millions of televiewers
and the thousands of amateurs, the only real solution lies in a voluntary, or
perhaps involuntary, exodus of radio hams to the high frequency, or microwave,
side of the television frequency bands.
Ham transmissions on the present popular amateur bands will continue to cause
interference in nearby receivers with the attendant protests from irate TV fans.
Although it is technically and theoretically possible to eliminate such interference,
it is sometimes financially unfeasible to do so. The mere fact that one amateur
station can operate in a certain area without causing interference in nearby
television receivers is no guarantee that the equipment will continue free from
TVI. All the ham has to do is change frequencies or equipment or for a neighbor
to buy a different type of TV receiver and the problem of interference becomes
The public's investment in television receivers now aggregates three billion
dollars as compared to the approximately thirty million dollars which hams have
tied up in their equipment. Thus the hams' stake in radio gear has been exceeded
a hundredfold by the public's investment in home receivers. In terms of the
number of households affected (or in votes at election time), the relationship
is even more top-heavy. At the present time there are over 200 times more television
households than there are ham radio homes. Even the ham often finds himself
in the unenviable position of having his ham equipment interfere with his own
television reception! A showdown is in the making at the present time and eventually
the ham will wind up in the microwave region and, in the opinion of the author,
when this happens it will be the greatest "blessing in disguise" ever vouchsafed
the radio art.
The situation is even more critical than it was in 1922 when radio broadcasting
developed virtually overnight into a hydra-headed monster of gigantic proportions.
At that time, amateur radio had to move up in the radio spectrum and operate
on the high frequency side of radio broadcasting. This enforced move made the
ham's equipment and techniques obsolete and unsuitable. Specifically he had
to discontinue operations on frequencies of 1500 kc. and lower where he had
employed a spark coil or rotary spark transmitter, crystal receivers, monstrous
an-tennas, and wireless telegraphy. He was forced to operate in the band from
1500 kc. upward with equipment requiring the use of vacuum tubes and careful
attention to impedance matching.
In a matter of months this enforced move resulted in several important developments.
The move led to the discovery of the Kennelly-Heaviside layer, reliable determination
of sky wave skip phenomena, and the establishment of the advantages of short-wave
The ham was delighted to discover that with a fraction of the power needed
on the lower frequencies he could communicate world-wide - even to the antipodes.
Even a small receiving tube, such as the now-obsolete UV201A, was sufficient
to serve as a transmitter to reach all the way to Australia. The range of communication
jumped from the usual hundred miles or less to distances circling the globe.
Furthermore the ham could make his contacts by voice or radio-telephony instead
of code or radio-telegraphy.
Today a similar situation exists except that now the amateur will move to
a much higher and more spacious spectrum. He can achieve efficiencies in circuitry
never before possible since he can dispense with lumped or specially-provided
inductances, condensers, and even resistors with their losses. He can develop
and control the electric and magnetic fields through distributed inductance,
capacitance, and impedance by means of the physical arrangement of simple metallic
On microwaves, the amateur will again be recognized as one of the nation's
most valuable sources of original research and experimentation instead of a
mere nuisance as he has now become in the opinion of millions of televiewers.
On microwaves, where amateur radio now more properly belongs, hams by their
very number and geographical distribution will open a new era in amateur radio.
They will quickly overtake the billions of dollars' worth of professional microwave
development that has thus far taken place without his participation. He can
ultimately save the taxpayers untold sums which are now going into microwave
research and development. In the past decade it has been demonstrated that professional
microwave activities have been unable to keep microwaves simple and inexpensive
enough to encourage their widespread usage. Only the radio amateur is in a position
to substitute empirical (cut-and-try) methods for the calculated complex and
planned procedures of government and industry. There are relevant discoveries
yet to be made which can best be made by a free exchange of information and
experiences by amateurs operating largely "without rhyme or reason" techniques.
The radio amateur today has a large number of frequencies in which he is
free to operate. These frequency bands are listed in Table 1.
It is estimated that most of the licensed amateur radio activity in the United
States is concentrated in the high frequency band, representing a total of only
3570 kc. out of the total 2,256,670 kc. assigned to hams. They, plus the bulk
of the rest of the hams operating on the 50-54 mc. and 144-148 mc. bands, are
the source of the TVI. The hams have congregated into 11,670 kc. of the amateur
spectrum subject to TVI while doing little to equip themselves and engage in
operations on the balance of the 2,245,000 kc. assigned to them. They are jammed
into less than one-half of one percent of the spectrum and are ignoring the
more than 99% percent of the spectrum in which TVI would be virtually nonexistent.
Microwaves have been generally recognized to be the frequencies between 300
and 3000 mc. (known as ultra-high frequencies) and between 3000 and 30,000 mc.
(known as super-high frequencies). The FCC has allocated 9.8 percent of all
ultra-high frequencies and over 7.3 percent of all super-high frequencies for
the exclusive use of hams. On a non-exclusive basis, the amateurs may also use
all frequencies above 30,000,000 kc. on to infinity or cosmic rays, including
the bands known as "infrared," "light," "ultra-violet," "x-rays," "gamma rays,"
Basically, the difference between microwave operation and transmissions at
the lower frequencies is a matter of equipment. In the case of microwave operation
there is no need for specially-provided transformers, coils, condensers, or
resistors. As shown in Fig. 3, all of these components are replaced by
positions taken along a closed or shorted pipe (called a wave guide). In practice.
this pipe is usually rectangular with its wide dimension exceeding a half wavelength.
Fig. 4 is a photograph of a commercially-available model and an improvised
unit made of screen wire. The home-built unit can be made out of foil or any
other conductive or non-conductive material as long as the inner surface is
a good conductor. If a simple can is used instead of a pipe, the unit is called
a "cavity." Only the frequencies which have electric and magnetic field distributions
that fit inside of such a can or cavity will exist in same. Thus, it is a frequency
controlling element hat replaces quartz crystals on the lower frequencies. For
the amateur frequencies, the cavities can have a "Q" on the order of 10,000
On the frequencies that the radio amateur best understands (frequencies below
450 mc.) , he has been conveying energy by means of conductors such as circuit
wiring. On the microwave frequencies, he conveys the current by means of electric
and magnetic field displacements within the wave guide. In other words, microwaves
are characterized by the displacement technique while conventional frequencies
use conduction or the cumbersome electric power line technique.
On microwaves, the phenomenon of space radio propagation is extended to the
passage of energy within the equipment itself and the transmission line system.
The method by which this takes place is shown in Fig. 5. One side of the
wave guide pipe (Fig. 4) simulates the ionosphere while the opposite side
simulates the earth. Fig. 5 shows a several-hundred-foot medium frequency
broadcasting tower used for sky wave transmissions by reflections between the
ionosphere and the earth. Fig. 5A shows a rectangular pipe (artificial
or fabricated wave guide) which replaces "Nature's wave guide" and performs
the same function. In Fig. 5A the pipe is less than a half wavelength or
at cut-off. Energy will not proceed down the pipe and attenuation is maximum.
Fig. 5B shows what happens if the wave guide is wider than a half wavelength.
Energy will propagate down the wave guide. Fig. 5C shows what happens if
the wave guide is made even wider. Energy will be propagated even better with
less attenuation or losses. In order to keep this explanation simple it is desirable
that the dimension of the guide not approach or exceed a full wavelength. If
the guide is wider than a full wavelength, the energy divides itself and becomes
similar to two wave guide pipes. Two energy patterns or modes would then exist
side by side. In addition, the narrow side of the rectangular wave guide would
accommodate a pattern. The narrow side walls function to keep the other two
walls properly spaced. They also determine how much power can be handled by
the wave guide. The wave guide of Fig. 4 can handle up to 3,200,000 watts
of power without breakdown or flashover. Fig. 2 is a graph of the performance
of a wave guide suitable for the 10,000 to 10,500 mc. amateur microwave band.
The rectangular pipe has an inside dimension of .9" x .4". Part A in Fig. 5
corresponds to 6562 mc. on the graph of Fig. 2. Part B in Fig. 15
might correspond to 7500 mc. on the graph of Fig. 2. Part C might correspond
to 10,500 mc. on the graph of Fig. 2. At 13,123 mc., two modes of energy
will form, changing this particular energy designation from "cut-off frequency
of transverse electric mode 1,0" to "cut-off frequency of transverse electric
It is preferable to operate in the dominant or first mode for reasons of
simplicity. It is feasible, and research has been conducted along these lines,
to use several modes, each a separate channel of communication. If the wave
guide is two wavelengths in width, there would be four modes of energy. If it
is three and one-half wavelengths in width, there would be seven modes of energy,
etc. Fig. 2 also shows the attenuation in decibels-per-foot for a particular
size wave guide. In this case, in the 10,000 mc. amateur microwave band, it
is less than .035 db-per-foot. The wave guide could be over 28 feet long before
the energy would be attenuated 50 percent. By selecting an appropriate size
wave guide, minimum attenuation can be obtained for any frequency. This same
concept holds true even on low frequencies except that at 4000 kc., for example,
the wave guide pipe would have to be substantially greater than a half wavelength,
or 123 feet in width. It is only because of the shorter wavelengths which make
possible convenient physical dimensions that it is possible to take advantage
of microwave techniques that would be impossible or unfeasible to employ on
lower frequencies. The technique would otherwise function on any wavelength
as long as physical dimensions and associated costs are not prohibitive. The
amount of power which such a wave guide can handle depends upon the height of
the guide. For the wave guide of Fig. 2, the power handling capacity is
235,000 watts. The smallest size wave guide, such as the one required for 21,000
to 22,000 mc., will still exceed 60,000 watts power handling capacity. Since
communication at microwave frequencies can be carried on with a fraction of
a watt power (even microwatts) there need be no concern that the user might,
in any way, exceed the power handling capacity of a wave guide.
To further appreciate wave guide phenomena, one need but recall what happens
to an auto radio receiver when the car is driven through an underpass. The underpass
is, in reality, a wave guide. Since the broadcast might be 1000 kc. (300 meter
wavelength), such an aperture or wave guide would have to exceed 500 feet in
diameter in order for the signals to go through. Police radios and two-way vehicular
systems have no difficulty in communicating in such a wave guide since their
operating wavelength is substantially shorter and will fit inside such boundaries.
The wavelengths involved for the amateur microwave bands range from as little
as a half inch on 22,000 mc. to as much as 14 inches on 1200 mc.
There are many other methods of handling microwave energy of which the "G
string" and the "helical coil" are particularly interesting examples. In the
case of the helical coil, the coaxial inner conductor connection is extended
into a coil which serves as a wave guide. With the "G string," the coaxial connector
inner connection extends as a straight wire while the coaxial connector outer
connection flares out into a horn which focuses the energy onto this straight
Tubes for Producing Microwaves
There are several tube types or tube techniques for generating microwaves.
Where one can be purchased at surplus, a reflex klystron is a useful means of
generating microwave frequencies. Fig. 6 is a cross-section view of a type
which is approximately correct for the 3300-3500 mc. amateur band. It has a
cathode, a pair of grids, and a repeller. A repeller is equivalent to a plate
but is biased negatively instead of positively. The grids operate at a high
positive potential. A cavity connects to the grid extremities to form a tuned
circuit. If modulation or audio is impressed on the repeller voltage, the tube
will serve as an FM transmitter.
Fig. 7 shows how a reflex klystron tube is coupled to the wave guide.
The output electrode extends into the wave guide as if it were a quarter-wave
grounded antenna, in low frequency applications. Energy then propagates down
the wave guide. An adjusting screw tunes the cavity contained within the tube
itself. Such a tube and wave guide is nearly correct for the 10,000 to 10,500
mc. amateur microwave band.
Fig. 1 shows a conventional tube enclosed within a wave guide cavity.
In this application only the tube frequencies which can exist for that size
microwave plumbing are available and utilizable. The grid and plate leads can
be adjusted external to the guide. The photograph also shows an elaborate wave
guide attenuator consisting of a carbon-coated resistor that can be inserted
into or withdrawn from the wave guide. A gauge is used to indicate how much
attenuation is being inserted.
Other means of providing microwave energy include:
1. Tubes having very close inter-electrode spacings while maintaining low
orders of interelectrode capacitance by their geometrical design.
2. By using conventional tubes with the transit time between cathode and
plate equal to more than a period of oscillation in order to maintain proper
phase relations even though the transit time is too long with respect to the
same period or cycle of oscillation. It can be corrected for a subsequent period.
The electron transit time may take two or more periods of time to reach the
plate from the cathode but it must arrive at the plate during the correct part
of the period. This is accomplished by means of suitable voltages.
3. By use of a spark gap within a shielded wave guide. A spark gap generates
the frequency spectrum while the wave guide plumbing enclosing or connected
to it permits only the microwaves to propagate.
In its simplest form, a microwave transmitter is merely a signal source which
may be a tube or a spark gap and a wave guide pipe. The outer end of the pipe
will squirt energy into space from the end of such a wave guide. If a horn extends
from that end, the energy may be concentrated or directed as desired. The beam
may be sharpened or broadened by changing the length and angle of the flared
The simplest microwave receiver is a silicon or other type of receiving crystal
connected to a pair of headphones. A more elaborate receiver may consist of
a crystal detector followed by several stages of audio or video amplification.
Still more elaborate is a crystal mixer stage in which the crystal output is
mixed with a local oscillator (which may be the transmitting tube) to yield
an i.f. frequency which is then handled by a superheterodyne circuit similar
to the one used on the lower frequencies.
Fig. 8 is the schematic of a one-tube microwave receiver used by the
author in his laboratory experiments. The same tube serves as a combination
r.f. amplifier, detector, first audio amplifier, as well as providing for possible
a.v.c. connections. Fig. 9 shows how this receiver appears, complete with
its antenna system made of brass foil. The horn and wave guide section slips
over the tube with a coaxial tuning plunger connecting to the grid of the first
half of the Type 6F8G tube. The antenna system comprises an electromagnetic
horn, tapering to a round wave guide. The coaxial plunger, consisting of a movable
short, permits adjustment of the wave guide system.
The statement or belief that microwaves can only be used within the unobstructed
horizon is completely erroneous. Such ideas were also prevalent before the amateurs
opened up the short-wave band in 1922 and when "five-meter" radio opened up
in 1932. Skepticism was rampant when police two-way radio began expanding on
v.h.f. around 1935 and when radar on microwaves moved up into the 200 mc. region
and above in 1940-41. In every case, equipment has operated beyond the horizon,
with many instances having been recorded showing transmissions of several thousands
of miles. Once this fact was established, our research experts were able to
layout study programs for yielding an explanation as to how this could occur.
New and relevant factors became known. On short-waves, it was the Kennelly-Heaviside
layer, first believed to be a single layer and later found to consist of several
layers - each of which was responsible for a new set of radio communications
ranges. On very-high frequencies, it was the dispersion effect at the horizon
plus natural wave guide paths resulting from walls of buildings, sides of hills
or mountains, walls of a canyon, or boundaries set by wayside wires and fences,
On microwaves, the possible range of operations is unlimited if the following
conditions affecting propagation through space are recognized.
1. Direct path communication within the unobstructed
horizon. This is approximately equal (in miles) to 1.41 times the square root
of the antenna height (in feet) above the intervening terrain plus the same
conditions for the second station. For example:
Station 1 has a radio horizon of 1.41 * √2500 = 1.41 * 50 or 70.5 miles
while Station 2 has a radio horizon of 1.41 * √9 = 1.41 * 3 or 4.23 miles.
The two stations can thus intercommunicate over a distance of 74.73 miles by
2. Indirect path or reflected communication. This type of transmission may
exist either within the horizon, beyond the horizon, or by a reflection within
the horizon passing the energy on to another reflecting or pickup point beyond
the horizon of the originating station. To understand how "this happens, one
should consider the source signal as a beam of light and every solid object
encountered enroute as a reflecting mirror. Whatever a mirror of such shape
would do to light, a similar thing will happen with respect to microwaves. Reflections
will be more effective when obstructions enroute are substantially larger than
the wavelength. This is quite likely to happen since microwaves are normally
less than one foot long. Even dense cloud formations have reflective possibilities.
At their greatest height, they can develop great ranges, even for stations operating
at sea level with very small unobstructed horizon.
3. Wave guide paths. Microwave energy may recognize the space between two
wires as the equivalent of the two walls of a wave guide pipe. It will treat
one wire as if it were the ionosphere, and the other the earth, and try to propagate
skywave fashion between such boundaries. The limit of such a communications
range is the limit of the availability and existence of suitable wire arrangements
around the country. Even the space between railroad tracks can serve as a wave
guide, as can tunnels, underpasses, canyons, gorges, etc.
4. Atmospheric ducts. These are a function of weather and can make microwaves
a tremendously valuable tool in weather forecasting. Microwave propagation at
great distances can be tied in with weather conditions. Microwave energy recognizes
the boundaries of a stratification of temperature or pressure aloft as a wave
guide. It also considers the adjacent boundaries of two strata a wave guide.
If signal energy from a transmitter can enter one of these atmospheric ducts
or wave guides, the range of communication can become very great - often up
to thousands of miles. This fact has been verified on many occasions and is
undergoing continuing research by governmental and subsidized institutions.
In investigations of this type the hams will be invaluable because of their
large groups, geographical distribution, and the number of hours they spend
on the air. The phenomenon is often missed by the professional groups working
the modern 40·hour week.
Although thousands of persons are currently employed in the microwave industry
involving the expenditure of billions of dollars, very few of these persons
and only a small portion of the total funds are actually used for propagation
studies. Instead, most of the time and money has gone into the design and construction
of complex and expensive radar and microwave relay systems.
The author feels confident that when the hams really get into microwaves
in sufficient number, the "CQ" call will yield just as interesting responses
as those enjoyed now. Microwaves also offer infinite possibilities for a ham
organization like the ARRL to live up to its name. Microwaves are an excellent
medium for radio relaying and for working out communications networks with a
wide selection of echelons to communicate during emergencies and civil defense
To utilize the microwave frequencies, the radio ham has to become more of
a mechanic than an electrician. He must get used to pipes called wave guides
and metallic structures having certain shapes, configurations, and dimensions.
He will use "cut-and-try" methods and simple arithmetic in his computations.
He will be required to perform simple sheet metal and machining operations in
building his apparatus but will probably purchase certain of his gear such as
the wave guide probes and coaxial connectors, if they are readily available,
otherwise he will build or improvise them from whatever is at hand. There is
not one single thing connected with microwave operation on the ham bands that
cannot be built or improvised very cheaply if the ham is willing to experiment.
Microwaves offer a real challenge to the alert ham-a challenge very few hams
will be able to resist!
Posted July 5, 2019
(updated from original post on 5/27/2013)