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Antenna and

Transmission-Line

Measurements

Chapter

The principal quantities measured on transmission lines

are line current or voltage, and standing-wave ratio (SWR).

You make measurements of current or voltage to determine

the power input to the line. SWR measurements are useful

in connection with the design of coupling circuits and the

adjustment of the match between the antenna and

transmission line, as well as in the adjustment of these

matching circuits.

For most practical purposes a relative measurement is

sufficient. An uncalibrated indicator that shows when the

largest possible amount of power is being put into the line is

just as useful, in most cases, as an instrument that measures

the power accurately. It is seldom necessary to know the

actual number of watts going into the line unless the overall

efficiency of the system is being investigated. An instrument

that shows when the SWR is close to 1:1 is all you need for

most impedance-matching adjustments. Accurate

measurement of SWR is necessary only in studies of antenna

characteristics such as bandwidth, or for the design of some

types of matching systems, such as a stub match.

Quantitative measurements of reasonable accuracy

demand good design and careful construction in the

measuring instruments. They also require intelligent use of

the equipment, including a knowledge not only of its

limitations but also of stray effects that often lead to false

results. Until you know the complete conditions of the

measurements, a certain amount of skepticism regarding

numerical data resulting from amateur measurements with

simple equipment is justified. On the other hand, purely

qualitative or relative measurements are easy to make and

are reliable for the purposes mentioned above.

indicator. In many cases, particularly with a screen-grid tube

in the final stage, minimum loaded plate current does not

occur simultaneously with maximum power output.

RF VOLTMETER

You can put together a germanium diode in conjunction

with a low-range milliammeter and a few resistors to form

an RF voltmeter suitable for connecting across the two

conductors of a coaxial line, as shown in Fig 1 . It consists of

a voltage divider, R1-R2, having a total resistance about 100

times the Z 0 of the line (so the power consumed will be

negligible) with a diode rectifier and milliammeter connected

across part of the divider to read relative RF voltage. The

purpose of R3 is to make the meter readings directly

proportional to the applied voltage, as nearly as possible, by

swamping the resistance of D1, since the diode resistance

will vary with the amplitude of the current through the diode.

LINE CURRENT AND VOLTAGE

A current or voltage indicator that can be used with

coaxial line is a useful piece of equipment. It need not be

elaborate or expensive. Its principal function is to show when

the maximum power is being taken from the transmitter; for

any given set of line conditions (length, SWR, etc). This

will occur when you adjust the transmitter coupling for

maximum current or voltage into the transmission line.

Although the final-amplifier plate or collector current meter

is frequently used for this purpose, it is not always a reliable

Fig 1

RF voltmeter for coaxial line.

C1, C2

0.005- or 0.01-

F ceramic.

Germanium diode, 1N34A.

J1, J2

Coaxial fittings, chassis-mounting type.

0-1 milliammeter (more sensitive meter may be

used if desired; see text).

6.8 k

Ω

, composition, 1 W for each 100 W of RF

power.

680

Ω

, 1 / 2 or 1 W composition.

10 k

, 1 / 2 W (see text).

Ω

Antenna and Transmission-Line Measurements

27-1

You may construct the voltmeter in a small metal box,

indicated by the dashed line in the drawing, and fitted with

coax receptacles. R1 and R2 should be carbon-composition

resistors. The power rating for R1 should be 1 W for each

100 W of carrier power in the matched line; separate 1- or

2-W resistors should be used to make up the total power rating

required, to the total resistance as given. Any type of resistor

can be used for R3; the total resistance should be such that

about 10 V dc will be developed across it at full scale. For

example, a 0-1 milliammeter would require 10 k

Ω

, a 0-500

microammeter would take 20 k

, and so on. For comparative

measurements only, R3 may be a variable resistor so the

sensitivity can be adjusted for various power levels.

In constructing such a voltmeter, you should exercise

care to prevent inductive coupling between R1 and the loop

formed by R2, D1 and C1, and between the same loop and

the line conductors in the assembly. With the lower end of

R1 disconnected from R2 and grounded to the enclosure,

but without changing its position with respect to the loop,

there should be no meter indication when full power is going

through the line.

If more than one resistor is used for R1, the units should

be arranged end-to-end with very short leads. R1 and R2

should be kept 1 / 2 inch or more from metal surfaces parallel

to the body of the resistor. If you observe these precautions

the voltmeter will give consistent readings at frequencies

up to 30 MHz. Stray capacitance and stray coupling limit

the accuracy at higher frequencies but do not affect the utility

of the instrument for comparative measurements.

Ω

Fig 2

A convenient method of mounting an RF

ammeter for use in a coaxial line. This is a metal-case

instrument mounted on a thin bakelite panel. The

cutout in the metal clears the edge of the meter by

about 1/8 inch.

precaution being that the capacitance to ground, chassis, and

nearby conductors should be low. A bakelite-case instrument

can be mounted on a metal panel without introducing enough

shunt capacitance to ground to cause serious error up to

30 MHz. When installing a metal-case instrument on a metal

panel, you should mount it on a separate sheet of insulating

material so that there is 1 / 8 inch or more separation between

the edge of the case and the metal.

A 2-inch instrument can be mounted in a 2

Calibration

You may calibrate the meter for RF voltage by

comparison with a standard such as an RF ammeter. This

requires that the line be well matched so the impedance at

the point of measurement is equal to the actual Z 0 of the

line. Since in that case P = I 2 Z 0 , the power can be calculated

from the current. Then E Z

inch metal box, as shown in Fig 2 . This is a convenient

arrangement for use with coaxial line. Installed this way, a

good quality RF ammeter will measure current with an

accuracy that is entirely adequate for calculating power in

the line. As discussed above in connection with calibrating

RF voltmeters, the line must be closely matched by its load

so the actual impedance is resistive and equal to Z 0 . The

scales of such instruments are cramped at the low end,

however, which limits the range of power that can be

measured by a single meter. The useful current range is about

3 to 1, corresponding to a power range of about 9 to 1.

= 0 . By making current and

voltage measurements at a number of different power levels,

you can obtain enough points to draw a calibration curve

for your particular setup.

RF AMMETERS

Although they are not as widely available as they used

to be, if you can find one on the surplus market or at a

hamfest, an RF ammeter is a good way to gauge output

power. You can mount an RF ammeter in any convenient

location at the input end of the transmission line, the principal

SWR Measurements

On parallel-conductor lines it is possible to measure

the standing-wave ratio by moving a current (or voltage)

indicator along the line, noting the maximum and

minimum values of current (or voltage) and then

computing the SWR from these measured values. This

cannot be done with coaxial line since it is not possible

to make measurements of this type inside the cable. The

technique is, in fact, seldom used with open lines because

it is not only inconvenient but sometimes impossible to

reach all parts of the line conductors. Also, the method is

27-2

Chapter 27

subject to considerable error from antenna currents flowing

on the line.

Present-day SWR measurements made by amateurs

practically always use some form of directional coupler or

RF-bridge circuit. The indicator circuits themselves are

fundamentally simple, but they require considerable care in

construction to ensure accurate measurements. The

requirements for indicators used only for the adjustment of

impedance-matching circuits, rather than actual SWR

measurement, are not so stringent, and you can easily make

an instrument for this purpose.

BRIDGE CIRCUITS

Two commonly used bridge circuits are shown in Fig 3 .

The bridges consist essentially of two voltage dividers in

parallel, with a voltmeter connected between the junctions

of each pair of arms , as the individual elements are called.

When the equations shown to the right of each circuit are

satisfied there is no potential difference between the two

junctions, and the voltmeter indicates zero voltage. The

bridge is then said to be in balance .

Taking Fig 3A as an illustration, if R1 = R2, half the

applied voltage, E, will appear across each resistor. Then if

R S = R X , 1 / 2 E will appear across each of these resistors and

the voltmeter reading will be zero. Remember that a matched

transmission line has essentially a purely resistive input

impedance. Suppose that the input terminals of such a line

are substituted for R X . Then if R S is a resistor equal to the

Z 0 of the line, the bridge will be balanced.

If the line is not perfectly matched, its input impedance

will not equal Z 0 and hence will not equal R S , since you

chose the latter to be equal to Z 0 . There will then be a

difference in potential between points X and Y, and the

voltmeter will show a reading. Such a bridge therefore can

be used to show the presence of standing waves on the line,

because the line input impedance will be equal to Z 0 only

when there are no standing waves.

Considering the nature of the incident and reflected

components of voltage that make up the actual voltage at

the input terminals of the line, as discussed in Chapter

24 , it should be clear that when R S = Z 0 , the bridge is

always in balance for the incident component. Thus the

voltmeter does not respond to the incident component at

any time but reads only the reflected component

(assuming that R2 is very small compared with the

voltmeter impedance). The incident component can be

measured across either R1 or R2, if they are equal

resistances. The standing-wave ratio is then

Fig 3

Bridge circuits suitable for SWR measurement.

At A, Wheatstone type using resistance arms. At B,

capacitance-resistance bridge (“Micromatch”).

Conditions for balance are independent of frequency

in both types.

1–

SWR

(Eq 2)

where k = E2/E1.

The operation of the circuit in Fig 3B is essentially the

same, although this circuit has arms containing reactance as

well as resistance.

It is not necessary that R1 = R2 in Fig 3A; the bridge

can be balanced, in theory, with any ratio of these two

resistances provided R S is changed accordingly. In practice,

however, the accuracy is highest when the two are equal;

this circuit is most commonly used.

A number of types of bridge circuits appear in Fig 4 ,

many of which have been used in amateur products or

amateur construction projects. All except that at G can have

the generator and load at a common potential. At G, the

generator and detector are at a common potential. You may

interchange the positions of the detector and transmitter (or

generator) in the bridge, and this may be advantageous in

some applications.

The bridges shown at D, E, F and H may have one

terminal of the generator, detector and load common. Bridges

at A, B, E, F, G and H have constant sensitivity over a wide

frequency range. Bridges at B, C, D and H may be designed

to show no discontinuity (impedance lump) with a matched

line, as shown in the drawing. Discontinuities with A, E and

F may be small.

Bridges are usually most sensitive when the detector

bridges the midpoint of the generator voltage, as in G or H, or

in B when each resistor equals the load impedance. Sensitivity

also increases when the currents in each leg are equal.

+12

SWR

(Eq 1)

–

where E1 is the incident voltage and E2 is the reflected

voltage. It is often simpler to normalize the voltages by

expressing E2 as a fraction of E1, in which case the formula

becomes

Antenna and Transmission-Line Measurements

27-3

Fig 4 Various types of SWR indicator circuits and commonly known names of bridge circuits or devices in that

they have been used. Detectors (D) are usually semiconductor diodes with meters, isolated with RF chokes and

capacitors. However, the detector may be a radio receiver. In each circuit, Z represents the load being measured.

(This information provided by David Geiser, WA2ANU)

27-4

Chapter 27

Resistance Bridge

The basic bridge configuration shown in Fig 3B may

be home constructed and is reasonably accurate for SWR

measurement. A practical circuit for such a bridge is given

in Fig 5 and a representative layout is shown in Fig 6 .

Properly built, a bridge of this design can be used for

measurement of SWRs up to about 15:1 with good accuracy.

You should observe these important construction

points:

1) Keep leads in the RF circuit short, to reduce stray

inductance.

2) Mount resistors two or three times their body diameter

away from metal parts, to reduce stray capacitance.

3) Place the RF components so there is as little inductive and

capacitive coupling as possible between the bridge arms.

proportional to the RF voltage) and no voltage calibration

curve is needed. D1 is the rectifier for the reflected voltage

and D2 is for the incident voltage. Because of manufacturing

variations in resistors and diodes, the readings may differ

slightly with two multipliers of the same nominal resistance

value, so a correction resistor, R3, is included in the circuit.

You should select its value so that the meter reading is the

same with S1 in either position, when RF is applied to the

bridge with the line connection open. In the instrument

shown, a value of 1000

was required in series with the

multiplier for reflected voltage; in other cases different values

probably would be needed and R3 might have to be put in

series with the multiplier for the incident voltage. You can

determine this by experiment.

The value used for R1 and R2 is not critical, but you

should match the two resistors within 1% or 2% if possible.

Keep the resistance of R S as close as possible to the actual

Z 0 of the line you use (generally 50 or 75

Ω

In the instrument shown in Fig 6 , the input and line

connectors, J1 and J2, are mounted fairly close together so

the standard resistor, R S , can be supported with short leads

directly between the center terminals of the connectors. R2

is mounted at right angles to R S , and a shield partition is

used between these two components and the others.

The two 47-k

). Select the

resistor by actual measurement with an accurate resistance

bridge, if you have one available.

R4 is for adjusting the incident-voltage reading to full

scale in the measurement procedure described below. Its use

is not essential, but it offers a convenient alternative to exact

adjustment of the RF input voltage.

Ω

resistors, R5 and R6 in Fig 5, are

voltmeter multipliers for the 0-100 microammeter used as

an indicator. This is sufficient resistance to make the

voltmeter linear (that is, the meter reading is directly

Ω

Testing

Measure R1, R2 and R S with a reliable digital ohmmeter

or resistance bridge after completing the wiring. This will

ensure that their values have not changed from the heat of

soldering. Disconnect one side of the microammeter and leave

the input and output terminals of the unit open during such

measurements to avoid stray shunt paths through the rectifiers.

Check the two voltmeter circuits as described above,

applying enough RF (about 10 V) to the input terminals to

give a full-scale reading with the line terminals open. If

necessary, try different values for R3 until the reading is the

same with S1 in either position.

With J2 open, adjust the RF input voltage and R4 for

full-scale reading with S1 in the incident-voltage position.

Then switch S1 to the reflected-voltage position. The reading

should remain at full scale. Next, short-circuit J2 by touching

a screwdriver between the center terminal and the frame of

the connector to make a low-inductance short. Switch S1 to

the incident-voltage position and readjust R4 for full scale,

if necessary. Then throw S1 to the reflected-voltage position,

keeping J2 shorted, and the reading should be full scale as

before. If the readings differ, R1 and R2 are not the same

value, or there is stray coupling between the arms of the

bridge. You must read the reflected voltage at full scale with

J2 either open or shorted, when the incident voltage is set to

full scale in each case, to make accurate SWR measurements.

The circuit should pass these tests at all frequencies at

which it is to be used. It is sufficient to test at the lowest and

highest frequencies, usually 1.8 or 3.5 and 28 or 50 MHz.

Fig 5 Resistance bridge for SWR measurement.

Capacitors are disc ceramic. Resistors are 1 / 2 -watt

composition except as noted below.

D1, D2

Germanium diode, high back resistance type

(1N34A, 1N270, etc).

J1, J2

Coaxial connectors, chassis-mounting type.

0-100 dc microammeter.

R1, R2

, 1 / 2 -W composition (see text).

R3 See text.

Ω

50-k

Ω

volume control.

R s

Resistance equal to line Z 0 ( 1 / 2 or 1 W

composition).

SPDT toggle.

Antenna and Transmission-Line Measurements

27-5

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