November 1949 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
Here is the final installment
of C.W. Palmer's "Microwaves" series of article in Radio−Electronics magazine.
Topics for all seven parts are shown below. Unlike the previous parts, this one
discusses uses for waveguide below its cutoff frequency for switching and attenuation
purposes. Of course there is also the filter application as well which exploits
the high attenuation in the cutoff region. Since these pieces were written in the
pre-solid state semiconductor era, vacuum tubes appear as control and amplifier
devices rather than diodes and transistors, but don't let that deter you from benefitting
from the useful waveguide characteristics lessons presented.
Microwave Series -- Part 1: How Radio Waves
Can Be Transmitted Inside Pieces of Pipe (4/49), Part II: An Introduction to Standing
Waves, Cavity Resonators, and Representative Examples of u.h.f. Plumbing (5/49),
Part III: Tubes for the Microwave Frequencies, Giving Special Notice to the Lighthouse
Triode, Velocity-Modulated Tubes, and the Magnetron (6/49),
Part IV: How
Waveguides Are Joined and Tuned for Lowest Possible Loss (8/49),
Part V: Special
Sections of Waveguide Are Employed as Transformers (9/49),
Part VI: Some Equipment
Used for Measuring Frequency, and Crystals for Receiver Frequency Conversion (10/49),
Part VII: Action of Below-Cutoff Attenuators and of TR and Anti-TR Switches
VIII: Receiving and transmitting antennas for microwave communication.
Part VII - Action of below-cutoff attenuators, and of TR and anti-TR
A TR assembly for waveguide insertion. Courtesy Sylvania Electronic
By C. W. Palmer
We discussed waveguide attenuators in a general way in an earlier part of this
series and showed how a resistance strip could be inserted into a waveguide to introduce
an adjustable amount of attenuation.
Another type of attenuator used extensively in microwave work is called the "waveguide-below-cutoff"
attenuator or sometimes simply the cutoff attenuator.
A wave propagates through a waveguide with very little loss, provided the diameter
or width of the guide is greater than the cutoff point. If these dimensions are
below cutoff, then there is no longer any real wave propagation; instead the magnetic
and static fields of the r.f. waves are attenuated very rapidly down the length
of the guide.
If the guide diameter is made small compared to the free-space wavelength, the
attenuation is independent of frequency over a very wide range. The modes generally
used for such attenuators are the TE1,1 and the TM0,1. The
methods of exciting waveguides in these two modes are shown in Fig. 1. Here a co-axial
line is terminated in either a coupling loop or disc and the reduced power is picked
up in another co-axial line. The distance between the exciting and pickup loops
controls the amount of attenuation. Since the relationship is linear, a scale on
the movable loop or disc can be calibrated directly in decibels. Attenuators of
this type usually have an insertion loss of about 10 to 20 db at the position of
maximum coupling and more as the coupling is reduced.
Fig. 1 - Coupling elements are loops or discs.
When co-axial line is coupled into a cutoff attenuator with loops, a serious
mismatch to the line results, a co-axial line terminating in a loop being practically
short-circuited. Three methods of reducing the bad effects of such mismatch may
be used. The most common is to pad the input and output ends of the cutoff attenuator
with lengths of high-loss co-axial cable. These add about 10 db of attenuation,
and their resistance damps out the effects of reflection from the mismatched co-axial
Another way is to use resistor discs, little circles of graphite or carbon, made
to fit the co-axial cable, with a hole in the center to contact the inner wire.
The resistance of these discs should be equal to the characteristic impedance of
the line so that the line is terminated correctly.
A third method is to make the loops of a resistance material and adjust the resistance
of the loop to equal the characteristic impedance of the line.
Cutoff attenuators are also made to work in waveguide at the higher frequencies
where co-axial line is not desirable because of high losses. Fig. 2 shows how this
is done. A rectangular or circular guide is joined to the small "below-cutoff" section
with a pickup probe near the termination of the large guide. This probe ends in
a fixed loop for exciting the small guide. A second (movable) loop used as the pickup
point ends in a probe extending into another section of large guide to continue
the waveguide circuit.
The space between exciting and pick-up loops is adjusted by one of several mechanical
methods, the simplest of which consists of two telescoping metal tubes, each of
which contains a loop and is terminated in the co-axial lines or waveguides. A rack-and-gear
drive controls, the amount of telescoping and, consequently, the spacing between
loops, which varies the attenuation.
TR and ATR units
Fig. 2 - Attenuator between waveguides.
Fig. 3 - The principle of the TR switch.
Fig. 4 - Voltage is highest across gap.
Fig. 5 - TR switch in antenna circuit.
In radar and microwave communication systems in which the same antenna is used
for both transmitting and receiving, it is necessary to use a fast-operating transmit-receive
switch to prevent transmitter power from reaching the sensitive crystals and vacuum
tubes of the receiver and also to pre-vent the received signal from being absorbed
in the transmitter.
A transmit-receive (TR) box is an electronic switch which operates in a fraction
of a microsecond. It must provide an excellent short-circuit for the receiver, since
even a small part of the transmitter power would burn out a silicon or germanium
Some form of gas discharge device is generally used for this purpose. Note Fig.
3. Here the transmitter power builds up a voltage across the gap, which then arcs
over so that most of the transmitter power goes out to the antenna. This simple
scheme could be employed in either a co-axial line or a waveguide, but unfortunately
it doesn't offer enough protection to the receiver.
The simplest way to improve it is to insert a voltage step-up transformer before
the gap and a step-down transformer after it. And this is just what is done, in
the form of a resonant cavity in which the gap is placed.
This resonant cavity may take the form of a cylindrical box with perfectly conducting
walls and with two posts in the axis of the cylinder, separated by a gap, as shown
in Fig. 4. In the lowest mode that will function in such a cylinder, the electrical
field is parallel to the axis of the cylinder and increases toward the center. The
magnetic field lines are circles perpendicular to the axis of the cylinder and currents
tend to flow radially and up and down the center posts, as shown in the cross-section
Energy may be fed into and out of such a cavity either through windows in the
cylinder walls or by coupling loops inserted into the cavity. The step-up ratio
of the transformer is controlled by the size of the coupling windows or loops, the
ratio increasing as the window or loop size is decreased.
When weak microwave currents pass through the waveguide or co-axial cable, the
TR tube permits power to pass through. But if a strong wave - such as would be set
up by applying power to the magnetron transmitter of Fig. 5 - passes down the guide,
the tube breaks down and becomes a short circuit. The shorting of the TR gap applies
a "solid wall" at the junction of the T side arm of the waveguide, and sets up a
strong standing wave in the side arm which prevents the transmitted signal from
reaching the receiver.
In receiving, the magnetron is not fired, and since most magnetrons have a considerable
change in impedance be-tween hot and cold conditions, it is possible to tune the
waveguide to provide a matched impedance condition when the magnetron is fired,
thus introducing a gross mismatch when the magnetron is not fired. This sets up
a standing wave in the line between the TR tube and the magnetron so that most of
the received power goes through the cold TR box to the receiver.
Some magnetrons, particularly those on 3 cm and shorter wavelengths, do not change
impedance enough to prevent an excessive loss of received signal. In these instances,
an anti-TR box is used.
The anti-TR box is very similar to the TR box except that it has only one coupling
window instead of two. It is placed in a T side arm between the TR and the magnetron.
On transmit, .it fires just as the TR box does and reflects a solid wall at the
junction of the T side arm, thus allowing maximum power to reach the antenna.
On receive, however, being situated a quarter-wavelength from the TR box and
tuned in length so that when it is not fired it reflects signals coming from the
direction of the antenna, it thus prevents loss of signal in the magnetron. If the
distance from the anti-TR to the TR is correctly chosen, a maximum received signal
will pass through the unfired TR to the receiver.
A TR switch may not fire in the first few cycles of the transmit signal and the
high-voltage pulse may damage the receiver. To prevent this a "keep-alive" electrode
is often built into the TR tube. An auxiliary electrode or gap near the main gap
of the TR tube, it is connected to a source of voltage sufficient to keep a small
arc always fired in the TR to supply the necessary ions to cause the main gap to
fire on the first pulse from the transmitter. This causes a small loss of received
signal, but prevents damage to the delicate receiver parts and is thus a worth-while
The use of TR and anti-TR switches in a microwave communication system permits
duplex operation. Transmitter power can be applied momentarily to the antenna when
it is desired to talk; but reception is possible at all times that the transmitter
is not active. It also permits a single antenna and reflector system to be used
for transmit and receive. This reduces cost and allows focusing on point-to-point
transmissions to be simplified greatly. An even more important advantage is in radar,
where the rotating antenna makes it extremely convenient to use one antenna for
transmission and reception.
Posted November 30, 2021