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July 1961 Electronics World
Table
of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
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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.
A
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.
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 (50 Ω to 50 Ω)
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)
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.
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.
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.
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.
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
