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Impedance Matching CB Antennas
July 1961 Electronics World

July 1961 Electronics World

July 1961 Electronics World Cover - RF Cafe  Table of Contents 

People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Electronics World was published from May 1959 through December 1971. See all Electronics World articles.

You would be forgiven in this era of ubiquitous cellphone usage for thinking maybe Citizen Band (CB) radios are only used these days by techno-throwbacks like myself, but the fact is many truckers still use them for convenience as well as to avoid having all their communications intercepted, monitored, and recorded by government agencies. It can be a deceiving sense of privacy though, because police officers often monitor CB radio transmissions while in patrol cars, and even solicit the assistance of other CBers in identifying and apprehending suspected transgressors - an advantage of public, unencrypted conversation afforded law enforcement which is not available with cellphones. Also, CB transmission, even though usually regarded as 'hearsay' in legal venues, has many times been admitted as evidence in cases where "present sense impression," "excited utterance," or some other special clause has allowed for it.

Cobra 29 LX 40-Channel CB Radio - RF CafeA lot of cars you see with vertical whip antennas attached to a bumper or roof that you might assume have Ham radio operators aboard are actually for CB radios. Unlike Ham radio, CB radio is still unlicensed and unlike Ham radio, CB radio rules permit business transmissions as well as private. So, if you like radio and don't have a Ham license or want to be able to communicate with a non-licensed operator at a home base station, you might want to give CB radio a try. It's pretty cheap.

Impedance Matching CB Antennas

By Hartland B. Smith, 19W1375

Don't waste power by mismatching your CB transceiver. Here are practical suggestions for improving performance.

Impedance Matching CB Antennas, July 1961 Electronics World - RF Cafe

The FCC's five-watt input restriction places a premium on CB equipment performance. Unless your mobile and base station transceivers and antenna systems are all working at peak efficiency, the communication range will be seriously restricted. A knowledge of transmission-line theory and antenna-matching procedures can be very helpful to the CB operator who seeks top-notch results.

The most convenient location for a transceiver is seldom a good place for an antenna. Consequently, these two items are usually interconnected with an r.f. transmission line, an undesirable parasite which contributes nothing to the signal. As a matter of fact, no matter how well a line is constructed, a measurable amount of the power which is fed into one end never reaches the other. This power loss is caused mainly by the series resistance of the two conductors which make up the line and by the leakage resistance of the insulation between them. Obviously, it pays to use as short a feeder as possible, in order to minimize power waste.

Solid dielectric coaxial cable is most often chosen for CB installations. Less efficient than twin-lead or open-wire line, coax is preferred because it provides shielding, has little radiation loss, and is more convenient to work with. Coax may be buried in the ground, run through metallic conduit, or taped to a steel mast without affecting its electrical characteristics.

Although CB transceivers are usually designed to work into 52-ohm cable, 75-ohm coax may be used if the antenna bas a feed point resistance of this value.

Impedance Matching

The impedance rating of coaxial cable is determined by the ratio between the inside diameter of the shield and the outside diameter of the inner conductor. Thus, two cables, one thick and the other thin, may have exactly the same characteristic impedance. However, in order to maintain the correct diameter ratios, the center conductor of the thin cable must be much finer than the center conductor of the thick cable. Resistive losses, therefore, are greater in the thin cable than in the thick one. Whether you decide to purchase light or heavy coax depends on how much money you wish to spend and upon how much loss can be tolerated at your particular installation. The characteristics of four popular cables are given in Table 1 to help you make a suitable choice.

Maximum transmission efficiency occurs when a pure resistance is connected across the antenna end of the coax which matches the impedance of the cable. (This assumes that the output impedance of a CB rig is purely resistive.) 52-ohm cable, for example, requires a 52-ohm terminating resistance while 75-ohm cable works best with a 75-ohm resistance.

Power, Voltage, Current on Matched Transmission Line - RF Cafe

Power, Voltage, Current on Matched Transmission Line (50 Ω to 50 Ω)

ower, Voltage, Current on Unmatched Transmission Line (50 Ω to 267.5 Ω) - RF Cafe

Power, Voltage, Current on Unmatched Transmission Line (50 Ω to 267.5 Ω)

Power, Voltage, Current on Unmatched Transmission Line (50 Ω to 10.7 Ω) - RF Cafe

Power, Voltage, Current on Unmatched Transmission Line (50 Ω to 10.7 Ω)

Fig. 1 - Power, voltage, and current along RG-58/U coax under matched and mismatched conditions. A wavelength on this line is about two-thirds of its free-space length.

Fig. 1 graphically shows how line loss is increased by improper termination. In Fig. 1A a CB transceiver with 3 watts output is connected to 100 feet of RG-58/U cable which is terminated by a non-reactive composition resistor. A composition resistor is employed because, unlike a wirewound unit, it has practically no inductance or distributed capacity. For our purpose, it may be considered as a pure resistance. Under the matched condition depicted in Fig. 1A, 1.94 watts of r.f. power reach the resistor. The balance of the transceiver's output is swallowed up by the cable.

A drooping ground-plane antenna has a feedpoint resistance of 50 to 55 ohms. If a 27-mc. antenna of this type is connected to the cable in place of the 53.5-ohm resistor, the curves of Fig. 1A will remain substantially unchanged.

In Fig. 1B, a 267.5-ohm terminating resistor is used. Since 267.5 ohms is five times the characteristic impedance of the cable, the line is no longer correctly terminated. As a result of this 5 to 1 mismatch, some of the energy which reaches the resistor is reflected back toward the transceiver. When the reflected energy arrives at the transceiver end of the cable, it joins with new power being supplied by the transceiver and returns to the resistor. Each time reflected energy traverses the feedline it must overcome the loss resistance of the cable. In the process, a significant amount of power is wasted. As a matter of fact, in Fig. 1B only 1.31 watts are available at the resistor. More than half of the transceiver's output, 1.69 watts, is wasted in the feedline because of the extra trips made back and forth by the travelling wave of reflected energy.

Fig. 1B shows what happens when the right transmission line is hooked to the wrong antenna. The situation depicted is approximately equivalent to using 53.5-ohm coax to feed a 27-mc. folded dipole (Fig. 3D). Often employed by radio amateurs, the folded dipole is not recommended for CB use because it is incompatible with coax.

In Fig. 1C the terminating resistance is only 10.7 ohms. Line losses are the same as in the previous example, because the ratio between the line impedance and the terminating resistance is again 5 to 1. A mismatch of this magnitude will occur if coax is connected directly to the driven element of a beam antenna (Fig. 3F) instead of through some form of impedance-matching device.

The current and voltage curves of Fig. 1A are smooth. Although there is a slope to the right denoting a power loss, no undulations or peaks and valleys are visible. The standing-wave ratio (s.w.r.) , which is the ratio between a voltage or current maximum value to an adjacent minimum value, is, therefore, said to be 1 to 1.

In Figs. 1B and 1C the voltage and current curves show rather large fluctuations. The maximum values or peaks, more correctly referred to as loops, are 5 times as high as the adjacent troughs, or nodes. The s.w.r. is 5 to 1.

The power curves of Figs. 1B and 1C are drawn smooth, despite the 5 to 1 s.w.r., because the voltage and current excursions cancel each other. When the current is up the voltage is down and vice versa.

Power losses for the most commonly used coaxial cables - RF Cafe

Table 1 - Power losses for the most commonly used coaxial cables when properly matched and when mismatched by 5 to 1. See text.

Standing Waves

The standing-wave concept isn't an easy one to grasp. Radio waves travel along a coaxial cable at approximately two-thirds the speed of light. For this reason it is rather difficult to visualize how something can stand still while traveling so rapidly. Nevertheless, standing waves are very real. So real, in fact, that you can actually feel their effect. A badly mismatched cable, when carrying an appreciable amount of power, will become hot enough to melt the insulation at the high current points. Yet at the current nodes, the cable will hardly be warm to the touch.

Many have the mistaken notion that a standing wave is a radio wave that has come to a screeching halt. This, of course, is not true. Radio waves move just as rapidly in a cable plagued with standing waves as in a cable blessed with a 1 to 1 s.w.r. When someone speaks of standing waves he is merely referring to the variations in meter readings which can be detected as an r.f. voltmeter or ammeter is moved along an improperly terminated line, as graphically portrayed in the curves of Figs. 1B and 1C. These undulations are caused by the reflected energy which alternately bucks and reinforces the new energy emerging from the transceiver. A careful study of Fig. 2 will disclose how this process takes place.

In this highly imaginative sketch a transmitter is shown generating alternately positive and negative 3-volt charges which flow into a 53.5-ohm cable of infinite length. At the left of the transmitter is a clock. While the action depicted in Figs. 2A through 2E takes place, the clock hand rotates once, charges A, B, C, and D move steadily to the right and two new charges, E and F, are generated. The needle of a non-polarized, peak-reading voltmeter placed on the cable at any position between I and VII will be deflected to 3 by the passing charges. There are no standing waves on the coax, but all the waves are traveling steadily from left to right.

In Fig. 2F a short piece of cable is terminated with a pure resistance of 53.5 ohms. As far as the transmitter is concerned, the cable still appears as though it were infinitely long. There are no standing waves because all of the energy reaching the end of the coax is absorbed by the resistor.

At 2G, however, the resistor has been removed, leaving an open circuit. Water may flow from the end of a pipe, but electrical energy isn't likely to fall off the end of a wire. Consequently, when charge A reaches position VIII, it rebounds like a rubber ball from a brick wall and starts back toward the transmitter. On its return trip, whenever charge A encounters another positive 3-volt charge, the two add to create a 6-volt potential. Where A coincides with a negative 3-volt charge, cancellation takes place and the resulting voltage is zero.

Standing waves occur when outgoing energy is reinforced or cancelled by the energy that is reflected back along line - RF Cafe

Fig. 2. Standing waves occur when outgoing energy is reinforced or cancelled by the energy that is reflected back along line.

By the time A has arrived back at the transmitter (Fig. 2N) all outgoing charges are bumping into reflected charges. There are now standing waves on the line. The voltmeter will always read 6 when placed at position II, IV or, VI because, whenever two charges meet at these points, they are of the same polarity and so their sum is 6. This is not true at I, III, V, and VII, where the meter will read zero, because charges of unlike polarity always pass each other at these positions. The s.w.r. is 6 to 0, an infinitely high figure.

In Figs. 2O and 2P a resistor of the wrong value terminates the line. The reflected charges have a potential of only 2 volts, since some of their energy is absorbed and dissipated by the resistor. When the returning 2-volt charges encounter outgoing 3-volt charges, they add to 5 or drop to 1, producing a 5 to 1 s.w.r.

If either capacity or inductance is associated with the terminating resistor, the s.w.r. on a transmission line will rise sharply, even though the resistor itself may closely match the line. When an antenna is too long or too short for the operating frequency, it exhibits inductance or capacity, as well as resistance. Thus, it acts as an impure resistance and boosts the s.w.r. Low line losses cannot be achieved unless the antenna is accurately resonated at the operating frequency. The dimensions and feed-point resistance shown in Fig. 3 will be affected to some extent by height above ground, the proximity of other objects, and the size and type of material used in construction. Therefore, it is always a good idea to check a new antenna system with suitable test equipment to learn whether or not it is correctly tuned.

The best location for a mobile antenna, from the viewpoint of best performance, is usually in the center of the vehicle's roof. A fairly good spot is on one of the front fenders. The least desirable mounting place is the rear bumper.

Checking S.W.R.

Use a 52- or 53.5-ohm cable for your mobile installation. Since only a short length of coax is required between the whip and the transceiver, loss resulting from the terminating mismatch will be small if you employ a quarter-wave antenna. A quarter-wave whip should not be pruned in an attempt to improve performance. However, a coil-loaded vertical or a spiral-wound Fiberglas whip, one that's physically shorter than a quarter wave, may do a more efficient job if it is carefully adjusted for minimum s.w.r. as indicated on a reflected-power meter. Instruments suitable for making the necessary measurements include the Globe TM-1, Cesco CM-52, the Heath AM-2, and others. The procedure which follows applies specifically to the AM-2. When using a different brand of meter, follow the instruction manual supplied by the manufacturer.

A number of popular CB antennas - RF Cafe

Fig. 3 - A number of popular CB antennas. In all cases, 1/4λ = 8'8" and 1/2λ = 17'4". FC is feed point for coax center conductor and FS is feed point for coax shield. (A) Ground plane, about 35 ohms impedance (52 ohms with built-in matching arrangement). (B) Drooping ground plane, about 52 ohms impedance. (C) Coaxial vertical, about 72 ohms. (D) Folded dipole, about 280 ohms. (E) Dipole, about 72 ohms. (F) Three-element beam, under 30 ohms (52 ohms with built-in matching arrangement shown). (G) Mobile whip, about 35 ohms. (H) Coil-loaded short mobile whip antenna, less than 30 ohms impedance unless it utilizes a built-in matching circuit.

Disconnect the coax from the transceiver and place it in the output socket of the reflected-power meter. Run a short length of coax from the transceiver's antenna socket to the input terminal of the meter. If the cable fittings of the transceiver and meter do not mate, you can obtain adapters from your electronics parts dealer.

Set the meter's function switch to "Forward." Turn on the transceiver and adjust the sensitivity control for a full-scale meter reading. Some transceivers may not put out enough power to move the meter to full scale, even with the sensitivity control fully advanced. Although this will cause the s.w.r. reading to be over optimistic, it is of little consequence, since we are more interested in achieving the lowest possible s.w.r. than in knowing the precise value of s.w.r. which is present in a certain setup.

Throw the function switch to "Reflected." Trim the antenna a little bit at a time. The closer you come to the operating frequency, the lower will be the meter reading. Don't be surprised, though, if you are unable to obtain a 1 to 1 s.w.r. A loaded antenna may have an impedance below that of 52-ohm coax and so a perfect match is unlikely.

During the antenna tuning process it is helpful to have crystals for Channels 1, 11, and 22 on hand. Then you can switch from one frequency to another to find where the lowest s.w.r. is. If the meter is lowest on Channel 1, the antenna is a little long. If the lowest s.w.r. is on Channel 22, the antenna is too short. A low reading on 11, of course, means that the antenna is right on the nose for the middle of the band.

After the whip has been cut or telescoped to frequency, switch back to "Forward" and reduce the sensitivity control until the meter is at approximately half scale. On a mid-band channel, adjust the transceiver's final tuning control or coil slug for maximum output as indicated by the meter. Be careful, of course, not to exceed the 5-watt input limit.

Remove the reflected-power meter and reconnect the coax directly to the transceiver. Tune in a weak signal and adjust the slug of the receiver input coil to produce the greatest possible speaker volume. Since no two antenna systems are ever exactly alike, you will be wise to repeak the transceiver input and output circuits whenever you change from one vehicle to another.

A commercially built base-station antenna, when erected according to instructions supplied by the manufacturer, will normally require no pruning. However, if a check of the completed installation shows a s.w.r much greater than 2 to 1, it will pay you to experiment with different element lengths. When doing this, you may connect the reflected-power meter at either the transceiver or antenna end of the cable. The latter position will provide slightly more accurate s.w.r. measurements.

A reflected-power meter is a one-impedance device. If wired for 50-55-ohm line, it will not perform correctly with 75-ohm cable. Since 50-55-ohm coax is generally employed for CB work, a 50-55-ohm meter will prove most useful. Should you find it necessary to check the s.w.r. on a 75-ohm line, the Heath AM-2 can be converted to this impedance by substituting two 100-ohm fixed resistors for the two 150-ohm resistors which are located inside the instrument's case.

Antenna Suggestions

Don't try to save money by utilizing a quarter-wave mobile whip as a base-station antenna. If you do, the results are likely to be disappointing, because a quarter-wave antenna must be operated in conjunction with a good ground.

The ground serves as an electrical mirror which produces an image antenna as illustrated in Fig. 4. The whip, plus the image, is equivalent to a resonant half-wave antenna. During mobile operation, the car body forms the image. When a whip is perched atop a pole, however, there is no ground to furnish an image. The whip merely acts as a non-resonant antenna and offers a very poor match to the cable. Consequently, the s.w.r. is high.

A ground may be simulated by mounting radials directly beneath the vertical as is done with the ground plane antennas of Figs. 3A and 3B. Adding radials to a quarter-wave whip in an effort to make it work properly, is a rather foolish procedure, though. You'll save time and money if you put up a correctly designed base-station antenna right at the start.

Although a 1 to 1 s.w.r. is the ideal toward which all CB operators should strive, the reader who has just purchased a base-station antenna may hesitate to attack his shiny new acquisition with a hacksaw, drill, pliers, and screwdriver, unless he can be certain that the tuning process will provide a marked improvement in performance. In general, if a check reveals that the s.w.r. is below 2 to 1, you will gain very little by attempting antenna adjustments. When an extremely long line run is required, beyond 150 feet for example, a 1.5 to 1 s.w.r. is worth going after if you want to squeeze the last bit of distance from your equipment.

A factor which is sometimes overlooked by the CB operator is antenna polarization. Maximum range can only be achieved if both the receiving and transmitting antennas are in the same plane. Try to avoid cross polarization. In other words, don't use a vertical antenna at one end of the link and a horizontal at the other. Vertical to vertical is best for mobile work. Horizontal to horizontal is superior for point-to-point communication between fixed stations.

Here is one final suggestion. Don't reduce the effectiveness of a good antenna system by hooking it to a misadjusted transceiver. Make sure that the input and output circuits of the base-station rig are carefully peaked as previously described for mobile units.

Quarter-wave whip plus image reflected by car body - RF Cafe

Fig. 4 - Quarter-wave whip plus image reflected by car body produces the equivalent of a resonant half-wave antenna.

Suggestions for Improving Performance of Your CB System

1. Mount mobile antenna on roof or front fender of vehicle, if possible.

2.Prune or telescope coil-loaded short whip for lowest s.w.r.

3.Mount base-station antenna as high as law allows.

4.Use shortest possible transmission line.

5.Maintain base-station s.w.r. below 2:1.

6. Tune transmitter final stage for maximum output, without exceeding legal limit.

6. Peak receiver input stage to provide best weak-signal response.

8. Avoid cross-polarization of antennas.

 

 

Posted September 18, 2015