Microwaves Part IV - How Waveguides Are Joined and Tuned
August 1949 Radio-Electronics

August 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.

A well-planned and installed waveguide system is always impressive. With enough switches, couplers, transitions, adapters, and various and sundry other accoutrements, the result can be downright artistic in appearance. Although usually the most costly form of electromagnetic wave transmission, it has the decided advantage of having by far the lowest path loss and highest isolation of any of the other forms of transmission - over the air, coaxial cable, twin lead, etc. This, Part IV of an VIII-part series on microwaves by author C.W. Palmer, appeared in the August 1949 issue of Radio-Electronics magazine. He discusses E- and H-planes, inductive and capacitive "windows," tuning screws, proper joining of waveguide sections, and more.

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.

How Waveguides Are Joined and Tuned for Lowest Possible Loss

Part IV - How waveguides are joined and tuned for lowest possible loss

By C. W. Palmer

DeMornay Budd Photo

So far we have learned how waveguides are used to transfer microwave radio power from an oscillator or transmitter to an antenna, and from the antenna to the receiver's amplifying and detecting circuits.

We have learned that the familiar radio quantities - inductance, capacitance, impedance, reactance, and resistance-are found in r.f. plumbing, but that their appearance is entirely new. And since we are dealing with wave propagation instead of conduction of r.f. currents as in low-frequency radio, we must learn a new set of rules.

In this part of the microwave series we will attempt to express those rules in a form that will help the radio man to understand better the do's and don'ts of r.f. plumbing.

One of the most important things to learn in using waveguides is to avoid discontinuities or changes in the internal mechanical shape of the guide from one section or piece of apparatus to an-other when joining them together in a "circuit."

Look at Fig. 1 as an example. It shows two types of L bends used extensively in r.f. plumbing. The first is known as an E bend, and the second is called an H bend. It is easy to remember which is the E and which the H bend if you think of the E as the "easy" and the H as the "hard" bend (if it were possible just to bend a piece of straight guide to make a right-angle turn, which it isn't).

In manufacturing these bends, deviation from the inside dimensions of the straight section by even a few thousandths of an inch in the bent portion will increase the loss from a nominal 0.02 decibel for such a section to several decibels, with a corresponding increase in the standing-wave ratio.

Fortunately such discontinuities can be taken care of by introducing into the guide obstacles that produce reflections which cancel the unwanted ones.

The matching devices most commonly used are diaphragms and tuning screws. The diaphragm, or window as it is sometimes called, is an aperture of thin metal placed across the waveguide. Such a window introduces either inductive or capacitive reactance depending on the direction of the slit.

Fig. 2 shows inductive and capacitive windows. For an inductive window the edges of the slit are parallel to, and for the capacitive window perpendicular to, the electric field. Usually these windows are soldered in place and are not variable. Where large amounts of power are to be carried in the waveguides, inductive windows are preferred because the capacitance type breaks down, causing arcing and loss of power.

Several examples of the use of fixed windows in waveguide circuits are shown in Fig. 3, which shows E and H tees used for branching or splitting the waves into two paths. The windows in these tees balance out reflections that would otherwise be introduced by the branch line and would introduce losses in the main waveguide path.

Sometimes it is desirable to have a variable reactance in a waveguide set-up to permit balancing out undesired reflections. In such cases tuning screws - small cylindrical posts projecting into the broad face of the guide as shown in Fig. 4 - are used. These screws provide capacitive reactance which varies with the penetration of the post into the guide. A single screw may be sufficient, but usually three screws are provided at quarter-wave-length intervals along the guide.

Sometimes it is desired to insert in a waveguide a device that will either pass a desired mode (modes of propagation were discussed in Part I) and no other, or reflect completely the power in a certain mode. Resonant diaphragms or windows are used for this purpose. A thin rectangular ring of the proper dimensions placed across the inside of the rectangular waveguide and separated from it by insulation will reflect, for example, all the T E_0,1 mode transmitted through the guide.

If a thin metal diaphragm across the guide is provided with an opening of the proper size, all the power in the T E_0,1 mode will be transmitted.

Resonant slits in the waveguide diaphragms are also useful for passing waves of low power and rejecting those of higher power. The slit is so narrow that breakdowns occur, and the resonant condition is temporarily removed. A device of this sort is useful for preventing the direct power of a transmitter from reaching and damaging a receiver connected to the same antenna system, during transmit periods, while allowing incoming radio waves to be received normally. In a radar system a special form of such a device is called a TR or ATR box, and will be discussed in detail later.

A microwave primer

It is frequently necessary to use long stretches of waveguides, and it is very unusual for them to proceed in a straight line. Bent and twisted sections with the bends in both E and H planes, tees for branch lines, etc., must be used to fit the needs of the individual installation.

In using these lengths with tee's, L's, etc., certain rules must be observed to obtain the desired results. These are summarized here in the following eight items.

1. A shorted end of waveguide (as the side arm of a tee or a stub) an odd number of quarter wavelengths long reflects an "opening" where it joins another waveguide. Waves in the main arm would travel into such a side arm as well as traveling through the main arm.

2. A shorted end of waveguide any number of half wavelengths long reflects a "solid wall" where it joins another waveguide. Waves in the main arm of a tee would travel through without entering such a side arm for this reason.

3. A quarter-wave section of wave-guide has opposite impedances at its ends (if impedance is high at one end, it is low at the other) just as in ordinary types of co-axial and parallel transmission line. (This summarizes rules 1 and 2.)

4. The Q of a waveguide is a function of frequency and also depends on the ratio of volume to inside area of the guide. Q's of 25,000 are not uncommon in waveguides and resonant cavities.

5. The characteristic impedance differs with different modes of operation. In a rectangular waveguide the impedance is proportional to the narrow or b dimension of the guide. It varies from about 475 ohms to zero as the b dimension is reduced.

6. The wavelength in a hollow wave-guide (as measured in a slotted waveguide section) is always greater than the wavelength of the same wave in air, due to the multiple reflections from the walls of the guide.

7. Sections of open and closed waveguides may be used as switching circuits by applying the principles of Rules 1 and 2 above.

8. Standing waves in waveguides are checked in a manner similar to that used for co-axial lines. A section of waveguide with a narrow slot parallel to the axis of the guide is used. A probe with a crystal detector or a small fuse (1/200 ampere) heated to almost the blowing point by direct current is used to detect the presence of standing waves as with a slotted co-axial line at lower frequencies.

Joints in waveguides

Sections of waveguide may be soldered together end to end. If the sections line up and touch, losses and reflections introduced by the joints are negligible.

However, for maintenance purposes and to make waveguide apparatus useable in more than one installation, it has become common usage to terminate waveguides with flanges on the ends which are soldered to the guide and machined flat on the ends. These flanges are bolted together. Experience has shown that this type of joint can be made better than even the average soldered guide-to-guide joint if care is taken to make uniform contact.

To reduce further the possibility of losses due to waveguide joints, it is considered good practice to use the "choke flange" joint, butting a choke flange always against a flat one.

The principle on which this choke flange works can be readily understood from waveguide Rule No.2 above. See Fig. 5. This shows a cross-section view of a choke-to-flat joint. The slot left by the junction of the two waveguide sections is a half wavelength long at the optimum frequency of operation of the waveguide, which means that the side cavity A reflects a "solid wall" to the main guide so that there can be neither leakage of r.f. nor dicontinuity to cause reflections in the main guide path.

In addition to the above, the point B where contact is actually made between the two waveguide sections is at a point of zero current, and perfect electrical contact need not be made between the two sections as is necessary in joining two flat flanges.

Such choke couplings are frequently used as "wobbly" or nonrigid connections between waveguide sections. They are used, for example, at the junction of an antenna where it is desired to shift or rotate the final section of guide to orient the antenna for peak response.

As a rigid connection the loss in a choke-flange joint is in the order of 0.02 db, compared to about 0.05 db for a well-made contact joint. The nonrigid connection mentioned, with a gap of about 1/16 wavelength between the choke and flat flange, has a leakage of about 0.3 db.

Plungers for shorting bars

In terminating side arms as described in the rules above, it is sometimes desirable to make movable shorting plates or plungers so that the lines can be tuned exactly to a desired quarter- or half-wavelength point.

Plungers can be either solid blocks, cup terminations, or choke terminations. The solid blocks must make good electrical contact at all points around the edge with the inside of the guide, or behavior will be erratic as the plunger is moved and power may leak past the termination.

Better results are obtained with a cup contact as shown in Fig. 6. Contact is made with the inside walls of the guide a quarter wavelength from the plunger, where the flow of current in the walls of the guide is zero. Losses in this type of termination can be held to as little 0.08 db.

An improved termination is the choke plunger which uses the same principle as the choke coupling. As shown in Fig. 7, no mechanical contact with the inside walls of the waveguide is made at the front surface of the plunger. Contact is made at B where the current is zero. Choke plungers have losses in the neighborhood of 0.02 db.

Dielectric in waveguides

The fact that the introduction of a dielectric inside waveguide will decrease the "cutoff" wavelength has been used practically in a so-called "line stretcher." This device introduces controlled amounts of dielectric into the guide to tune it.

The effect of an ideal dielectric in a waveguide is to increase its apparent size. It also lowers the impedance in all but one TM (transverse magnetic) mode. The losses of a guide filled with solid or liquid dielectric are higher than for an air-filled or gas-filled guide. However, the effect is slight.

An exception to the above and a point of caution to the experimenter in microwaves is the effect of water. Small amounts of water condensed on the inside of a waveguide may introduce losses up to 1/2 decibel per foot of waveguide. This is caused, not only by the large dielectric loss characteristic of water at high frequencies, but also by the high dielectric constant of water. This is why some waveguide installations are pressurized or, charged with an inert gas.

Posted October 4, 2021