Transmission lines

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Transmission lines

Jan 1, 2006 12:00 PM, By John Battison, P.E., technical editor, RF

For many engineers a transmission line is simply a piece of plumbing that is forgotten until something goes wrong, such as a burned bullet or some idiot with a rifle and a different kind of bullet punctures it. This is unfortunate because not only are transmission lines interesting pieces of equipment, but they are also critical pieces of transmitter installations.

Basically transmission lines are designed with only one purpose in mind. This designated purpose is to transmit power from a generator to a load with zero or minimum power loss. There are many types of transmission line such as for alternating current, direct current and RF. In this article we shall be concerned only with RF transmission lines. Waveguides, although doing the same job as �lines� operate with different rules and will not be covered here.

A transmission line cannot be described in a Gertrude Stein manner as �a transmission line is a transmission line, is a transmission line, is a transmission line�.� It is far more than that. It can be a simple two-wire RF line, a balanced line or an unbalanced coaxial line. The most familiar type in use today is the coaxial line. This of course is an unbalanced line because the entire outer conductor is grounded.

DC transmission lines are generally controlled by Ohm's Law and the power and voltage requirements, and will not be discussed here.

Traditional concerns

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Figure 1a. A transmission line can be electrically represented as a network of resistance, capacitance and inductance. The parameters RD, LD and CD represent the distributed electrical values for each unit of length. This is a balanced line. Click here to enlarge this image.

For the average broadcast engineer's professional life he has to deal with capacitors and inductances that are often called �lumped constants.� This means that the capacitive or inductive reactance is an entirely separate unit � either a capacitor or a coil and its effect is basically confined to one area of a circuit.

In the case of �lumped� individual units, stray capacity, inductance or leakage resistance occurs in the vicinity of the individual component and can be considered during the design of the circuit. In the case of a transmission line, which may be several hundred feet long, these effects are spread out over the whole transmission line and are known as distributed constants. Such values are more difficult to handle.

A transmission line is basically a network consisting of capacitance, inductance and resistance distributed throughout its length. The arithmetic values of these characteristics are described as x units of capacitance, inductance and resistance per unit length.

In most cases the broadcast engineer is not greatly or specifically concerned with these spread-out values. He selects his transmission line based on its characteristic impedance, propagation velocity, attenuation, frequency and power handling characteristics per unit length. The unit length can be any convenient measure; it is often a given in 100 feet units.

Transmission lines for ac and RF are governed by similar laws although the application is generally quite different. In low-frequency ac power transmission work the line length has little or no effect (because the wavelength is so large), and the voltage at the sending and will be about the same value as at the receiving end. Nevertheless long line effect must be considered in high power transmission. However, as the operating frequency increases the effects of various multiples of quarter wavelengths of line become extremely important. This will be discussed later.

The most familiar type of line in use today is the coaxial line. This of course is an unbalanced line because the outer conductor is grounded. Variations of coax line occur, such as dual coax line and triax. Triax is generally used for TV cameras to reduce the number of conductors in the studio lines and make them easier to handle. Dual coax is used mostly for special circuits.

The familiar twisted-pair is often used in audio work and is sometimes used in small transmitters to convey RF drive between stages. Old time hams will recall their use when hams built their own equipment. Today, small coaxial cable is used.

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Figure 1b. This is an unbalanced line. Click here to enlarge this image.

The basic requirement for efficient transmission line operation are absence of standing waves, line impedance properly matched to the transmitter and load impedance (antenna) and capable of carrying the required RF power without breaking down. Protection from mechanical damage by burial or adequate support is also required.

It is also important to ensure that all transmission lines feeding directional antennas have similar environmental conditions. This means that if part of one line is buried, a similar length is buried on all the other transmission lines to the array. This ensures the net all lines are affected in the same way by changing weather and temperature conditions.

The odd length

As the frequency increases to a point at which the transmission line length is equivalent to an appreciable portion of the wavelength, such as a quarter wave, strange things begin to happen. In most cases, the voltage at the receiving end is quite different from the voltage at the sending end.

A transmission line can transform impedances in the same fashion as a tee or a pi network. However, such results are not confined to RF operation alone and a 60 cycle line of sufficient length will exhibit the same kind of problems. Although the technique of handling such phenomena is the same as RF solutions, solutions to ac power line problems require physically larger equipment and larger electrical reactances.

In the world of transmission lines, quarter wave lines have many extremely useful properties. A transmission line that is not designed to be attenuating, and consists of an odd number of quarter wavelengths has the interesting faculty that an open circuit at the receiving end results in a short circuit at the transmitter and vice versa.

It was mentioned earlier in this article that a quarter-wave transmission line can be used as an impedance-changing device in RF work over a fairly narrow range of frequencies. Its characteristic impedance is designed to be the geometric mean between the transmitting and receiving ends. However, in most cases I have found a simpler to use a tee network.

Another use for a quarter-wave line is transforming voltage to current or current voltage by a known relationship. This characteristic makes possible a measuring instrument that does not place an appreciable load on the circuit under measurement. An RF ammeter is connected directly across the end of a quarter-wave line. This short across the line results in an infinite impedance at the far end. The current in amps multiplied by the characteristic impedance of the line gives the voltage at the other end.

Lines with an even number of quarter waves i.e. half wave or more, and that are non-attenuating are useful to measure an impedance in an inaccessible location, such as up a tower. The unknown impedance is connected to one end of the line and measured in the usual way at the accessible end on the ground.

It is important that the line is exactly a multiple of half waves long. This is easily determined by shorting the far end and measuring the impedance on the bridge. This should, of course, be zero. If it is not zero there will be a purely resistive reading and a low impedance.

Transmission lines are quite tough and standup to weather. But vibration and swinging movements are not beneficial. Include regular inspection of lines along with your other routine equipment checks.

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