BIRD Thruline APM-16 User manual

RF Directional
Thruline®Wattmeter
Model APM–16
Operation Manual
©Copyright 2016 by Bird Technologies, Inc.
Instruction Book Part Number 920-APM16 Rev. E
Thruline® and Termaline® are registered trademarks
of Bird Electronic Corporation

i
Safety Precautions
The following are general safety precautions that are not necessarily related to
any specific part or procedure, and do not necessarily appear elsewhere in this
publication. These precautions must be thoroughly understood and apply to all
phases of operation and maintenance.
WARNING
Keep Away From Live Circuits
Operating Personnel must at all times observe general safety precautions. Do
not replace components or make adjustments to the inside of the test
equipment with the high voltage supply turned on. To avoid casualties,
always remove power.
WARNING
Shock Hazard
Do not attempt to remove the RF transmission line while RF power is present.
WARNING
Do Not Service Or Adjust Alone
Under no circumstances should any person reach into an enclosure for the
purpose of service or adjustment of equipment except in the presence of
someone who is capable of rendering aid.
WARNING
Safety Earth Ground
An uninterruptible earth safety ground must be supplied from the main
power source to test instruments. Grounding one conductor of a two
conductor power cable is not sufficient protection. Serious injury or death
can occur if this grounding is not properly supplied.
WARNING
Resuscitation
Personnel working with or near high voltages should be familiar with modern
methods of resuscitation.
WARNING
Remove Power
Observe general safety precautions. Do not open the instrument with the
power on.

Safety Precautions
ii
Safety Symbols
The caution symbol appears on the equipment indicating there is
important information in the instruction manual regarding that
particular area
Note: Calls attention to supplemental information.
Warning Statements
The following warnings appear in the text where there is danger to operating
and maintenance personnel, and are repeated here for emphasis.
Refer to page 11.
Refer to pages 11 and 13.
WARNING
Warning notes call attention to a procedure, which if not correctly
performed, could result in personal injury.
CAUTION
Caution notes call attention to a procedure, which if not correctly performed,
could result in damage to the instrument.
WARNING
Never attempt to connect or disconnect RF equipment from the transmission
line while RF power is being applied. Leaking RF energy is a potential health
hazard.
WARNING
RF voltage may be present in element socket. Keep element in socket during
operation.

RF Directional Thruline® Wattmeter Model APM–16
iii
Caution Statements
The following equipment cautions appear in the text and are repeated here for
emphasis.
Refer to page 7.
Refer to pages 11 and 23.
Refer to page 19.
CAUTION
For low reflection measurements, do not rotate the reflected power element
to read forward power. Damage to the element or wattmeter could result.
CAUTION
To prevent damage from battery leakage, remove the battery if the unit will
not be used for more than two weeks.
CAUTION
Do not attempt to remove the RF center conductor. This will damage the line
section.

Safety Precautions
iv
Safety Statements
USAGE
ANY USE OF THIS INSTRUMENT IN A MANNER NOT SPECIFIED BY
THE MANUFACTURER MAY IMPAIR THE INSTRUMENT’S SAFETY
PROTECTION.
USO
EL USO DE ESTE INSTRUMENTO DE MANERA NO ESPECIFICADA
POR EL FABRICANTE, PUEDE ANULAR LA PROTECCIÓN DE
SEGURIDAD DEL INSTRUMENTO.
BENUTZUNG
WIRD DAS GERÄT AUF ANDERE WEISE VERWENDET ALS VOM
HERSTELLER BESCHRIEBEN, KANN DIE GERÄTESICHERHEIT
BEEINTRÄCHTIGT WERDEN.
UTILISATION
TOUTE UTILISATION DE CET INSTRUMENT QUI N’EST PAS
EXPLICITEMENT PRÉVUE PAR LE FABRICANT PEUT
ENDOMMAGER LE DISPOSITIF DE PROTECTION DE
L’INSTRUMENT.
IMPIEGO
QUALORA QUESTO STRUMENTO VENISSE UTILIZZATO IN MODO
DIVERSO DA COME SPECIFICATO DAL PRODUTTORE LA
PROZIONE DI SICUREZZA POTREBBE VENIRNE COMPROMESSA.

RF Directional Thruline® Wattmeter Model APM–16
v
SERVICE
SERVICING INSTRUCTIONS ARE FOR USE BY SERVICE - TRAINED
PERSONNEL ONLY. TO AVOID DANGEROUS ELECTRIC SHOCK, DO
NOT PERFORM ANY SERVICING UNLESS QUALIFIED TO DO SO.
SERVICIO
LAS INSTRUCCIONES DE SERVICIO SON PARA USO EXCLUSIVO
DEL PERSONAL DE SERVICIO CAPACITADO. PARA EVITAR EL
PELIGRO DE DESCARGAS ELÉCTRICAS, NO REALICE NINGÚN
SERVICIO A MENOS QUE ESTÉ CAPACITADO PARA HACERIO.
WARTUNG
ANWEISUNGEN FÜR DIE WARTUNG DES GERÄTES GELTEN NUR
FÜR GESCHULTES FACHPERSONAL.
ZUR VERMEIDUNG GEFÄHRLICHE, ELEKTRISCHE SCHOCKS, SIND
WARTUNGSARBEITEN AUSSCHLIEßLICH VON QUALIFIZIERTEM
SERVICEPERSONAL DURCHZUFÜHREN.
ENTRENTIEN
L’EMPLOI DES INSTRUCTIONS D’ENTRETIEN DOIT ÊTRE RÉSERVÉ
AU PERSONNEL FORMÉ AUX OPÉRATIONS D’ENTRETIEN. POUR
PRÉVENIR UN CHOC ÉLECTRIQUE DANGEREUX, NE PAS
EFFECTUER D’ENTRETIEN SI L’ON N’A PAS ÉTÉ QUALIFIÉ POUR CE
FAIRE.
ASSISTENZA TECNICA
LE ISTRUZIONI RELATIVE ALL’ASSISTENZA SONO PREVISTE
ESCLUSIVAMENTE PER IL PERSONALE OPPORTUNAMENTE
ADDESTRATO. PER EVITARE PERICOLOSE SCOSSE ELETTRICHE
NON EFFETTUARRE ALCUNA RIPARAZIONE A MENO CHE
QUALIFICATI A FARLA.

Safety Precautions
vi
RF VOLTAGE MAY BE PRESENT IN RF ELEMENT SOCKET - KEEP
ELEMENT IN SOCKET DURING OPERATION.
DE LA TENSION H.F. PEAT ÊTRE PRÉSENTE DANS LA PRISE DE
L'ÉLÉMENT H.F. - CONSERVER L'ÉLÉMENT DANS LA PRISE LORS
DE L'EMPLOI.
HF-SPANNUNG KANN IN DER HF-ELEMENT-BUCHSE ANSTEHEN -
ELEMENT WÄHREND DES BETRIEBS EINGESTÖPSELT LASSEN.
PUEDE HABER VOLTAJE RF EN EL ENCHUFE DEL ELEMENTO RF -
MANTENGA EL ELEMENTO EN EL ENCHUFE DURANTE LA
OPERACION.
IL PORTAELEMENTO RF PUÒ PRESENTARE VOLTAGGIO RF -
TENERE L'ELEMENTO NELLA PRESA DURANTE IL
FUNZIONAMENTO.

RF Directional Thruline® Wattmeter Model APM–16
vii
About This Manual
This manual covers the operating and maintenance instructions for the
following models:
Changes to this Manual
We have made every effort to ensure this manual is accurate. If you discover
any errors, or if you have suggestions for improving this manual, please send
your comments to our Solon, Ohio factory. This manual may be periodically
updated. When inquiring about updates to this manual refer to the part number
and revision on the title page.
Literature Contents
Chapter Layout
Introduction — Describes the features of the Bird APM-16.
Theory of Operation — Describes how the APM-16 operates and describes
measurement methodologies.
Installation — Describes battery care, connecting elements to the APM-16,
and system connections.
Operation — Describes the process for measuring forward and reflected
power, includes charts for calculating VSWR, and provides information on load
matching.
Maintenance — Lists routine maintenance, repair, calibration and
troubleshooting procedures for common problems. This section also includes
APM-16 specifications, parts information, and Bird Service Center contact
information.
APM-16

viii
Table of Contents
Safety Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
Safety Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii
Warning Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii
Caution Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Safety Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
About This Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Changes to this Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Literature Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Chapter Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Chapter 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Chapter 2 Theory of Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Traveling Wave Viewpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Coupling Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Load Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Standing vs. traveling Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
 vs. 
Low Reflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Transmitter Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Component Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Frequency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Impedance Mismatch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Chapter 3 Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Connecting RF Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Absorption Wattmeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Chapter 4 Operating Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Normal Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Load Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Chapter 5 Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Maintenance Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

RF Directional Thruline® Wattmeter Model APM–16
ix
Contact Adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Zero Adjust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Repair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
“QC” Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Replacing the Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Replacing the Instrumentation Module . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Calibration Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Equipment Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Calibrating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Customer Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Replacement Parts List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Available “QC” Type Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Limited Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

1
Chapter 1 Introduction
The Bird APM-16 is an insertion wattmeter designed to measure RF power and
load match in 50 ohm transmission lines. It has a maximum VSWR of 1.05 for
frequencies up to 1000 MHz. The meter provides direct readings in watts with
an expanded linear scale for easy reading. The scale is graduated for 25, 50, and
100 full scale. Elements are available in a variety of power and frequency ranges
(See the Bird Electronic Corporation Catalog for details).
Description
The Bird APM-16 is portable and rugged, with an aluminum housing. For
additional protection, the microammeter is specially shock mounted.
Bumpers on the base and back allow the meter to stand or lie flat. Refer to
Figure 1 to identify components.
At each end of the line section are Bird Quick-Change RF connectors that may be
interchanged with any other Bird “QC” connector. The wattmeter housing does
not interfere with connector changes.
Figure 1 Outline Drawing









!


"
#

$!
!



$




%
&
!
'()
*(
*+()
,
,
*-

2
Chapter 2 Theory of Operation
Traveling Wave Viewpoint
The easiest way to visualize Thruline operation is from a traveling wave
viewpoint. In transmission lines the voltages, currents, standing waves, etc., on
any uniform line section result from the interaction of two traveling waves:
The forward wave (and its power) travels from the source to the load.
It has RF voltage Ef and current If in phase, with Ef / If = Zo.
The reflected wave (and its power) originates by reflection at the load
and travels from the load back to the source. It has an RF voltage Er
and current Ir in phase, with Er / Ir = Zo.
Each wave is mathematically simple and has a constant power:
Wf = Watts Forward = Ef2 / Zo = If2 Zo = Ef If
Wr = Watts Reflected = Er2 / Zo = Ir2 Zo = Er Ir
Zo is the characteristic impedance of a uniform line section. For useful lines it is
usually a pure resistance of 50 ohms. The RF circuit of the Bird APM-16 is a
length of uniform air line with Zo = 50 ohms.
Coupling Circuit
The Bird Plug-In Element contains a coupling circuit that samples the traveling
waves. The element circuitry and its relationship to the rest of the Bird APM-16
are illustrated in Figure 5 .
Figure 2 Schematic

















Theory of Operation
3
Current is produced in the coupling circuit by the traveling waves in the line
section. Both inductive and capacitive coupling contribute to this. Inductive
current flows in the direction of the traveling wave, while the capacitive current
is independent of the direction of the traveling wave. Therefore, the inductive
current produced by one traveling wave will add in phase with the
corresponding capacitive current, while that produced by the wave traveling in
the opposite direction will subtract. The additive or “arrow” direction is
assigned to the forward wave.
The electrical characteristics of the element are carefully adjusted so that, for
the reverse traveling wave, the inductive current will completely cancel the
capacitive current. The result is directivity greater than 25 dB. Thus, the element
is sensitive at either of its settings, but to only one of the two traveling waves.
Thruline Wattmeter measurements are also independent of position along the
transmission line.
Like similar diode devices, the Bird APM-16 indicates the carrier component of
amplitude modulation, with very little response to side band components
added by modulation.
Load Power
For loads with a VSWR of 1.2 or less, the power dissipated in a load (Wl) is equal
(with less than one percent error) to the forward power (Wf). When appreciable
power is reflected, as with an antenna, it is necessary to use the exact load
power, given by:
Wl = Watts into Load = Wf – Wr
Good load resistors, such as Bird Termaline Loads, will give negligible reflected
power.
Standing vs. traveling Waves
As mentioned previously, the Thruline Wattmeter reacts to forward and reverse
traveling waves to measure power in a transmission line. The standing wave
viewpoint, also widely used, is highly developed both in theory and in practice.
This viewpoint can be traced to the early use of slotted transmission lines.
The slotted line measures the standing wave ratio by mechanically positioning a
voltage detector at peaks and nulls along a length of line section. Its drawbacks
are that it is usually too long, too expensive for good accuracy, not portable, and
too slow. These problems grow rapidly as the measurement frequency drops
below 1000 MHz. The Thruline Wattmeter by comparison is fast, convenient,
and accurate. It provides the same information as a slotted line except for the
phase angle of the reflection coefficient (distance, load to minimum).

RF Directional Thruline® Wattmeter Model APM–16
4
 vs. 
The simple relationships:
can be used to convert between the standing wave ratio ( and the reflected/
forward power ratio , which can be directly read from the Thruline
Wattmeter. The relationship between  and  is graphed in Figure 3 and
Figure 4 .
Note: Attenuation, measured in dB, can be derived from the
power ratio by the equation
Ndb = 10 log .
VSWR scales and their attendant controls for setting the reference point have
been intentionally omitted from the Bird APM-16. Experience using the Thruline
Wattmeter for transmitter tune-up, antenna matching, etc. will show that the
power ratio measurement is as useful as the standing wave ratio.
A trial is suggested – forget about VSWR for a few days and think in terms of  =
Wr / Wf. The meter readings, Wr and Wf, give a useful, approximate picture of
the results without bothering to calculate the power ratio exactly. Consider
that, for an antenna matching problem, the main objective usually is to
minimize Wr . Anything done experimentally to this end will be seen when the
element is in the reflected power position.
and Where  = VSWR
and  = Wr / Wf
1+ 
1–
----------------= 1–
1+
------------=
2

Theory of Operation
5
Figure 3 Percent Reflected Power vs. VSWR (1.0 – 8.0)

RF Directional Thruline® Wattmeter Model APM–16
6
Figure 4 Percent Reflected Power vs. VSWR (1.0 – 1.3

Theory of Operation
7
Low Reflection
 = 10% ( = 2) is the typical limit of antenna match. Further effort is frequently
not worthwhile because below this level reflected power is hard to measure,
and Wl can not be significantly increased. TV and VHF transmitters are examples
of systems requiring lower reflected power but for reasons other than
maximizing power transmission.
When the same element is used to measure both forward and reflected power,
meaningful readings are possible down to about  = 5% ( = 1.5). For accurate
measurement of very low levels of reflected power, i.e.  = 0.6% ( = 1.17), use
a second element rated at one tenth of the full scale power of the forward
element. This method should not be used with element ranges differing by
more than 10:1.
For example, consider an 80 watt transmitter and a Bird APM-16 with 100 and
10 watt elements. Measure Wf with the 100 W element. Measure Wr with the
10 W element (make sure the arrow points towards the transmitter). Wr can be
measured down to at least 0.5 W, so that  = 0.5 / 80 or about 0.6%,
corresponding to  = 1.17.
Transmitter Monitoring
The Thruline Wattmeter can be used for continuous monitoring of transmitter
output or reflected power, for instance while checking intermittent antenna or
line faults.
Component Testing
The Bird APM-16 is very helpful in component testing, and may be employed in
several ways:
1. Insertion VSWR or  can be measured by placing the component between
the wattmeter and a good load resistor.
2. Attenuation (power lost by heat in a line) as well as insertion VSWR or  can
be measured by inserting the unknown line between two Thruline
Wattmeters, or between a Thruline Wattmeter and a Termaline Absorption
Wattmeter.
CAUTION
For low reflection measurements, do not rotate the reflected power element
to read forward power. Damage to the element or wattmeter could result.

RF Directional Thruline® Wattmeter Model APM–16
8
Note: Very small attenuations require allowance for normal
instrument errors. To correct for this without any calculations, con-
nect the wattmeters directly, with no line between them, and
adjust their zero settings.
3. Line loss using open circuit calibration: The high directivity of elements can
be exploited in line loss measurements, because of the equality of forward
and reflected power with the load connector open or short circuited. In this
state the forward and reflected waves have equal power, so that f = 100%
and r = ¥. Open circuit testing is preferred to short circuit, because a high
quality open circuit is easier to create than a high quality short.
To measure insertion loss, use a high quality open circuit to check forward
and reverse power equality, then connect an open-circuited, unknown line
to the wattmeter. The measured f is the attenuation for two passes along
the line (down and back). The attenuation can then be compared with
published data for line type and length (remember to halve Ndb or double
the line length to account for the measurement technique).
This measurement should be supplemented by either time domain
reflectometry or DC continuity and leakage checks, since the attenuation
measurement alone cannot account for faults such as open or short circuits
partway down the line.
Note: Very small attenuations require allowance for normal
instrument errors. Make sure to note exact readings, or their differ-
ence, on the initial equality check, and correct for this.

Theory of Operation
9
Frequency Response
Bird Plug-In Elements have a flat frequency response over their specified
operating range. A sample set of curves is shown in Figure 5 . Notice that for the
low power element, the rolloff outside its frequency band is more pronounced
than for the high power elements. For example, at 40 MHz the 10C element will
have a loss of 4 dB, giving a reading of about 40% of the true value For the 100C,
the loss will only be about 1 dB, for a reading at 80% of the true value, and the
500C should be within the normal 5% of full scale tolerance.
Figure 5 Representative Frequency Response
These curves are typical for all element types (H, A, B, C, D, ...) at their
respective frequencies. Since C elements have a frequency range of 100 - 250
MHz, response curves for other element types can be approximated by
replacing the 100 and 250 MHz points on the chart with the extremes of the
element’s frequency range, and recalculating the other frequency points
accordingly. For example, for a B element (range 50 - 125 MHz) simply divide all
frequencies by two. For an E element (range 400 - 1000 MHz) multiply all
frequencies by four.
Harmonics or subharmonics that lie outside of the frequency range of the
element may exist in the circuit under test. A rough approximation of the
element’s response to harmonics can be made with these curves. Using an
element for measurements outside of its frequency range is not recommended.
The response curves presented are only typical, and not guaranteed.

RF Directional Thruline® Wattmeter Model APM–16
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Impedance Mismatch
There may be cases where it is necessary to use the Bird APM-16 with a non-50
ohm transmission line. If the reflected power is less than 10% and the frequency
is below 200 MHz, the resulting mismatch will not be too serious. At higher
frequencies or higher reflected power levels, the load impedance will change
when the wattmeter is removed from the circuit.
When the line and load impedances are known, the system’s VSWR can be
calculated by dividing the larger impedance by the smaller. Remember that the
VSWR ratio must always be greater than 1.
For example, consider using a Bird APM-16 to tune a 70 ohm line. If the load
impedance is also 70 ohms, the wattmeter will measure a VSWR of 70 / 50 = 1.4.
However, if you remove the wattmeter, the VSWR will actually be 1.0. If the
load impedance is 35.7 ohms instead, the VSWR will be 50 / 35.7 = 1.4 with the
wattmeter and 70 / 35.7 = 2.0 without it. Caution must therefore be used, since
both good and bad matches can have the same measured VSWR. In this case,
the correct impedance can be determined by slightly changing the load
impedance. When the load impedance is near 70 ohms, the Bird 43 will read
increasing VSWR as the load impedance is increased.
Note: When working with non-50 ohm lines, it is especially
important to calculate the load power by subtracting the reflected
power from the forward power.