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
Thanks to Terry W. for providing this
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
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
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 ham band.
Fig. 8. Circuit of one-tube microwave receiver using conventional-type
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 pioneer."
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 critical.
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
antennas, 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 operation.
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
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," and beyond.
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 or more.
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 mode 2,0."
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
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 horn.
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
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 direct path.
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 May 27, 2013