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 1950 issue of Radio & Television News
list of frequency band allocations are not much different than today
so the information is useful.
See all available
vintage Radio News
.Thanks to Terry W. for providing
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 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
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."
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
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
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.
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," 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 wire. 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 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. Propagating Characteristics
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, etc.
On microwaves, the possible range of operations is unlimited
if the following conditions affecting propagation through space are
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
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!