Microwaves Part VII - Below-Cutoff Waveguide Attenuators & TR Switches
November 1949 Radio-Electronics

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 (11/49), Part VIII: Receiving and transmitting antennas for microwave communication.

Part VII - Action of below-cutoff attenuators, and of TR and anti-TR switches

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 TE_1,1 and the TM_0,1. 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.

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 line termination.

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

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 crystal.

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 drawing.

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 compromise.

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