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October 1963 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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Here is a nice article on various types of power measurement
instruments and their uses. The calorie wattmeter, calorimeter
wattmeter, photometric wattmeter, thermo-ammeter, RF voltmeter,
reflectometer, in-line meters, and slotted line are covered.
Suggestions for selecting the proper instrument for measurement
and operation is touched upon. Although the article was written
in 1963, many of these instruments - or close descendants of
them - are still in use today.
R.F. Power Output Measurements
Such measurement on communications transmitters is now specified
by FCC for many radio services. Methods and types. of equipment
are described.
By R. L. Conhaim
Although power input is often used as a measure of transmitter
capabilities, the measurement of r.f. power output is becoming
increasingly important. In some cases, power output measurements
are demanded by the FCC or are required as part of the technical
specifications for various services. Domestic Public Radio Services,
Aviation Radio Services, and proposed Citizens Band class D
requirements all specify maximum power output. Broadcast stations
- AM, FM, and TV - are all required to measure and monitor power
output. In addition, power-output measurements can give an over-all
check of the efficiency of a transmitter especially when compared
with previous readings, and some methods even allow a rough
check of modulation capabilities. In some types of instruments,
feedline and antenna efficiencies can also be computed or directly
read in terms of v.s.w.r.
The reading of r.f. power is by no means as simple as reading
power at d.c. or low-frequency a.c. Electrodynamometer-type
wattmeters, such as are used to read power at 60-cycle a.c.
are completely unusable at radio frequencies. As a result, r.f.
can only be read by some form of conversion - either converting
the r.f. by rectification to d.c. or by converting to some other
form of energy and then calibrating the indicating instrument
in terms of r.f. watts. Let's consider a few of the more common
basic methods.
Calorimeter Wattmeters
One common and accurate method is called the calorimeter
technique in which the r.f. power is converted to heat. In these
instruments, some medium for absorbing heat is required. Water,
oil, ammonia, and solid dielectrics have all been used, depending
on the type of calorimeter and the amount of power which must
be dissipated. These systems employ either a static or circulating
medium, and a thermopile or other temperature-difference device
which indicates such differences on electrical current-reading
meters calibrated in watts. Most such instruments, while quite
accurate, are slow since time is required to heat the dissipative
medium.
One calorimetric power meter, the Hewlett-Packard1
434A, employs an unusual technique in which a self-balancing
bridge is combined with a highly efficient heat transfer resulting
in a response time of 5 seconds or less. As can be seen from
the simplified diagram of Fig. 1, the unknown r.f. power is
checked against a 1200-cps comparison power in the bridge circuit.
Two temperature-sensitive resistors serve as gauges. In operation,
the unknown r.f. heats an input load resistor. This resistor
and one gauge are in close thermal proximity so that heat generated
in the input load heats the gauge and unbalances the bridge.
The unbalanced signal is amplified and applied to the comparison
load resistor which is in close proximity to the second gauge,
and nearly rebalances the bridge. The meter measures the power
supplied to the comparison load to rebalance the bridge. Efficient
heat transfer from the loads to the temperature gauges is accomplished
by immersing the components in an oil stream. While quite accurate
and reasonably fast for a calorimetric system, this instrument
will read powers only to 10 watts, at frequencies from d.c.
to 12.4 kmc. Being a laboratory instrument, it is quite costly
for applications involving routine service work.
Fig. 1. Simplified diagram of a calorimeter
power meter.
Photometric Methods
Converting the r.f. power to light has been used and is known
as the photometric technique. In some applications, a special
lamp containing two identical filaments is used. One filament
is fed from the source of unknown r.f. power, while the other
is fed from d.c. or low-frequency a.c. When the two filaments
are of equal brilliance, the r.f. power is assumed to be the
same as the d.c. or low-frequency a.c. power. A single filament
lamp, read by a photocell, may also be used but some method
of calibration is required. These photometric systems have limited
usefulness because lamp filaments make poor dummy loads. They
have considerable reactance above 2 mc. and the resistance may
vary with the amount of current passing through the filament
of the lamp.
Full-scale values as low as 10 ma. and as high as 15 amps.
are available from some manufacturers. External thermocouples
are offered by some firms for use with their meters.
Thermo-Ammeters
The r.f. current measuring systems which employ thermo-couple
ammeters, or thermo-ammeters as they are usually called, are
quite accurate in common usage and make one of the simplest
instruments when only r.f. power transferred into a dummy load
or into an antenna system is to be measured.
Thermo-ammeters consist of a thermocouple and a d.c. moving-coil
meter movement. The thermocouple is made of two dissimilar metals,
joined at one end. If the junction of these metals is heated,
a d.c. voltage is produced at the free terminals. This voltage
is proportional to the heat difference between the hot and cold
ends. A practical thermocouple meter consists of a heater through
which the r.f. current flows, a thermocouple attached to the
heater element, and a d.c. moving-coil meter connected to the
free ends of the thermo-couple. The d.c. voltage at the free
ends causes direct current to flow through the meter which is
calibrated in r.f. current. Power is then computed from Ohm's
Law, P = I2R, where R is the value of the load resistor
used.
Because deflection of the meter is proportional to the amount
of heat in the heater wire, which is proportional to the square
of the current passing through it, the thermo-ammeter has a
square-law scale. In this type of scale, the lower end is quite
crowded. For best accuracy, the full power should be read at
about 70% of full scale, but the meter is readable over the
range of 3 or 4 to 1, that is, a thermo-ammeter with a full
scale of 1 ampere can be read down to about 0.3 ampere. Such
ammeters cannot be shunted for use on other ranges, since any
shunt will make the meter quite frequency sensitive. Thermo-ammeters
are good up to about 200 mc. depending on the construction of
the heater wire. Some thermo-ammeters are made with a thin-wall,
hollow heater wire which behaves much like a waveguide, making
the instrument usable at higher frequencies.
Regular thermo-ammeters are made by a number of meter manufacturers.
In appearance, they look like any other panel meter, except
for the square-law scale. They are comparable in price to other
good-quality panel meters. Fig. 22,3 shows two such
units. The larger meter provides a more easily read scale. Both
have proved quite accurate and easy to use with CB transmitters
when equipped with proper dummy loads. They may also be used
in series with the center conductor of a coaxial cable as an
indicator of power being transferred to the antenna, However,
they should not be used with the antenna feedline as a load
for tune-up purposes. When used in this way, they may have a
tendency to add forward and reverse currents, giving a false
indication of output power, resulting in mistuning of the transmitter
coupling circuits. They are also useful as a rough indication
of modulation capabilities since speaking or whistling into
the transmitter microphone will cause the meter to move up scale.
Fig. 2. Typical panel-type thermo-ammeters
mounted in cases with built-in dummy loads. Power is computed
from P = I2/R
Fig. 3A is a schematic of a typical thermo-ammeter with dummy
load. The load is made up of 2-watt carbon resistors in parallel.
The closer the resistance of the dummy load matches the antenna
feedline impedance, the more accurate will be the readings on
the thermo-ammeter. For up to 8 watts, three 5% 220-ohm and
one 5% 200-ohm carbon resistors (all rated at 2 watts) in parallel,
will come close to approximating a 52-ohm load. For higher power
transmitters, parallel resistor combinations can be made in
the same way.
Special non-inductive carbon resistors are available for
this purpose or standard commercial dummy loads can be used.
If the load is constructed, leads should be kept as short as
possible and adequate ventilation allowed for heat dissipation.
Fig. 3. (A) Thermo-ammeter schematic showing
use of dummy-load resistors. (B) The basic peak-reading r.f.
voltmeter.
For power up to 13 watts into 52 ohms, a 0 to 500-ma. meter
will serve quite adequately. Table 1 lists commonly available
full-scale values, together with interpreted wattage ratings
for both the 52- and 72-ohm loads.
Table 1. Common thermo-ammeter full-scale
currents, powers.
R.F. Voltmeters
Radio-frequency voltmeters are commonly used as r.f. power
measuring devices. Power is derived from the Ohm's Law formula,
P = E2/R. In some commercial models, these units
are combined with systems which also read v.s.w.r. directly
or indirectly, or both forward and reflected power. Crystal
diodes are commonly used, but since such rectifiers have a tendency
to vary in resistance with applied voltage, a swamping resistor
(R1 in Fig. 3B) is used. The time constant of C1-R1 is made
large with respect to the period of the lowest radio frequency
to be measured. This condition can be met if R1 is at least
10,000 ohms and C1 is 1000 pf. C2 merely provides additional
r.f. filtering for the meter. Such voltmeters have to be calibrated
and for this reason are not too popular as construction projects.
A typical commercial meter, the Electro Impulse Laboratory4
Model AM-6 is shown in Fig. 4. This meter is useful over the
range 100 kc. to 200 mc. It is a dual-range type, employing
a voltage divider for full-scale values of 1.5 to 6 watts, and
a full-scale accuracy of 5%. The voltage-divider characteristics
are such that the voltage divisions are the same for d.c., 60-cycle
a.c., or r.f. This makes for easy calibration and checking against
known standards. Other models are made for full-scale wattage
readings up to 1500. This meter is intended for transmitter
measurements only and cannot be employed for feedline or antenna
measurements.
Fig. 4. A typical commercial dual-range r.f.
wattmeter unit.
Other examples of this type of device, but able to measure
higher powers, are shown in Fig. 5.
Fig. 5. These Cesco absorption wattmeters
use circuits of the type shown in Fig. 3B. Units are available
for powers up to 1500 watts. The dummy load resistors that are
used are special non-inductive types that are submerged in oil.
Reflectometer and In-Line Meters
These instruments are bridge-type voltmeters, consisting
of one or two voltmeters, and employing resistor, resistor-capacitor,
or capacitor bridges. Rather than being directly connected to
the feedline or transmitter, they employ a short length of coaxial
line and a pickup loop so that energy is induced by mutual inductance
and capacitance from the traveling r.f. wave. When two bridges
or detectors are used, the instruments can be made to read either
forward or reflected waves on the feedline and can thus be calibrated
in both watts and v.s.w.r. In some commercial instruments, the
pickup device is a special element which can be placed in the
instrument in one of two ways so that the forward or reverse
waves can be read merely by reversing the element physically.
A typical reflectometer circuit is shown in Fig 7. For relative
readings and the computation of v.s.w.r., no special calibration
is required. But for accurate power measurements, the instrument
must be calibrated for each specific frequency range, or furnished
with plug-in elements for different frequencies and different
power ranges. Because instruments of this type draw relatively
little power, they may be left in the feedline as permanent
power monitors or to determine antenna conditions.
Fig. 7. Basic circuit diagram of a reflectometer
unit.
Fig. 6 shows a typical in-line instrument manufactured by
Cesco5 and calibrated in power for CB use. This is
a basic dual-bridge instrument with a single indicator. Forward
or reverse power is selected by a switch. The v.s.w.r. is read
by adjusting a potentiometer for full-scale reading in the forward
direction, then switching to the reflected direction and noting
the reading directly in v.s.w.r. Like all diode-type instruments,
it shows few effects from sidebands and so is not particularly
useful for indicating modulation conditions. This instrument
may also be used for measuring relative field strength.
Fig. 6. Internal view of in-line r.f. wattmeter
for CB.
One of the in-line-type instruments most popular with manufacturers
and professional service engineers is the Bird6 "Thruline"
wattmeter, dubbed the "Birdie" by many of its users. This instrument
has a number of unique features and, as a professional instrument,
enjoys widespread use in both military and civilian applications.
This wattmeter employs changeable plug-in elements, provided
in a variety of ranges, for different frequencies and different
power applications. Elements are made in six frequency bands
from 2 to 1000 mc. and in various power levels from 5 watts
full-scale to 1000 watts full-scale, with an accuracy of 5%
of full-scale. This instrument and a coupling element are shown
in Fig. 8 (left). A simplified schematic is given in Fig. 9.
A coaxial resistor, designed to be used as a dummy load for
transmitter bench checks is shown in Fig. 8 (right). These resistors
are made in a variety of power ratings. The one shown is an
air-cooled, deposited-carbon type designed for 5 watts dissipation.
Fig. 8. (left) "Thruline" wattmeter with
changeable power element. (Right) A coaxial dummy load resistor
rated at 5 watts.
Fig. 9. Simplified schematic of Bird wattmeter.
R.f. power can be measured in either direction depending on
the plug position.
The Bird wattmeter consists of a short, uniform section of
air line, the characteristic impedance of which is exactly 50
ohms. The coupling element is prominently printed with an arrow
indicating the direction of the traveling wave being read. Energy
is produced in the coupling element by both mutual inductance
and capacitance from the traveling waves of the line section.
Inductive currents flow according to the direction of the wave.
Capacitive currents are independent of traveling-wave direction.
Therefore, assuming the element remains stationary within the
air line, current produced from the waves of one direction will
add in phase, while those traveling in the opposite direction
will subtract in phase. As a result, and because of the design
of the element, only the wave desired (forward or reflected)
will be read, while current from waves of the opposite direction
will be cancelled out almost entirely. A directivity always
higher than 35 db will result. While v.s.w.r. can be computed
with this instrument, the manufacturer recommends the user think
in terms of power ratios, forward to back. A ratio of 10% gives
a v.s.w.r. of less than 2 to 1 which is, for all practical purposes,
a good antenna installation in communications work. Lower standing-wave
ratios yield little in the way of improved performance, although
in TV, v.h.f. omnirange transmitters, and FM multiplex systems,
the lowest possible standing-wave ratio is desired.
The r.f. line section of the Bird wattmeter may be removed
and permanently installed in the transmission line. The d.c.
meter is connected by cable to this line and may be installed
at considerable distances from the line section. Cable lengths
up to 25 feet are available from the manufacturer, although
any shielded cable such as RG-58/U may be used.
Another instrument employing the reflectometer principle,
but also including a built-in load resistor and a field-strength
meter, is the General Radiotelephone7 Model 615.
This instrument is calibrated for 27 mc. but may be used in
the range 27-54 mc. As a power meter, it will read either power
in the feed line, forward or reflected, or power absorbed by
a calibrated internal load resistor. Power may be read on either
a 5- or 25-watt scale. A peaking coil is provided for field-strength
measurements on frequencies higher than 27 mc. A schematic of
this instrument is shown in Fig. 10.
Fig. 10. Schematic diagram of the General
Radiotelephone multi-function unit. Device can also be used
as field-strength meter.
The Slotted Line
At frequencies in the u.h.f. range, the slotted line is often
used, especially for reading v.s.w.r. While this type of instrument
measures with a high degree of accuracy, it is quite expensive
since it is a precision-machined device slightly longer than
one-half wavelength. Consequently, it is considered impractical
at frequencies below 460 mc. It is certainly not something that
can be constructed in the shop, unless precision machine tools
are available. Basic information on slotted lines can be found
in standard texts.
Selecting & Using R.F. Power Meters
The type of wattmeter you select will depend on many factors
- your intended use, your pocketbook, accuracy desired, the
band on which you are operating, as well as other considerations.
You can pay as little as $19.95 for a factory-assembled and
calibrated CB instrument to as high as $1600 and more for a
laboratory calorimeter. In between are the professional standards
used by manufacturers and communications engineers, ranging
in price from $40 to $200 or more.
If you are a radio amateur or CB enthusiast, you will find
the less expensive reflectometers and multi-function bridges
quite adequate for your purposes. If you are a professional
communications service engineer, you will probably require the
higher accuracy of the more expensive instruments.
Use will also determine which type of meter you want. If
you are reading strictly transmitter power capability on the
bench and into a dummy load, the straightforward wattmeters
are all you will need.
For base-station communications equipment employing a single
frequency, feeding a high-quality resonant antenna with very
low standing-wave ratio, the simple wattmeter will be sufficient.
But, for multi-channel equipment, or wherever antenna matching
problems may be encountered, reflectometer or in-line types
of meters are much to be preferred because of their ability
to read both forward and reflected waves.
Instruments of the in-line type, which are to be left in
the line, should be considered as part of the transmission line
when tuning a line to an antenna, especially at high v.h.f.
and u.h.f. frequencies. If they are removed from the line, an
equal length of coaxial cable should be fitted with the same
type of connectors and installed in its place.
Where antennas require tuning, the best procedure is not
to whack away at the feedline, inch by inch. Instead, do this:
First, tune the transmitter to a wattmeter equipped with a dummy
load equal in impedance to the system you are using. Next, if
the antenna requires tuning, cut the antenna to the desired
length for the principal frequency being used. Do not attempt
to alter the design of the antenna. Install it according to
the manufacturer's recommendations. If this is done, you can
be fairly certain the v.s.w.r. of the antenna will be less than
2 to 1. If you measure the v.s.w.r. and find it less than 2
to 1, leave it alone. You won't improve communications noticeably
by any matching techniques. But, if you are finicky about v.s.w.r.
and have nothing better to do, whack away. Who knows, you may
achieve what the author once heard on CB, "My v.s.w.r. is less
than 1 to 1!"
References
1. Hewlett-Packard Company, 1501 Page Mill Road,
Palo Alto, Calif.
2. The Triplett Electrical Instrument Co., 286
Harmon Rd., Bluffton, Ohio
3. Simpson Electric Company, 5200 W. Kinzie St.,
Chicago, Illinois
4. Electro Impulse Laboratory, Inc. 208 River
St., Red Bank, N.J.
5. Continental Electronics & Sound Co., Inc.,
6151 Dayton-Liberty Road, Dayton 18, Ohio
6. Bird Electronic Corporation, 30303 Aurora
Road, Cleveland 39, Ohio
7. General Radiotelephone Co., 3501 W. Burbank
Blvd ., West Burbank, Calif.
Posted March 24, 2015
