Instruction manual

From March 2009 QST © ARRL
PRODUCT REVIEW
Mark J. Wilson, K1RO

Product Review Editor

[e-mail]
Bottom Line
Reviewed by Bob Allison, WB1GCM
ARRL Test Engineer
A standalone wattmeter is a “must have”
in any Amateur Radio station and is the first
piece of test equipment that many hams own.
Whether you’re operating in your shack,
car, boat or out in the field, your wattmeter
shows you at a glance how much power your
transmitter is putting out. Many models dis-
play power and standing wave ratio (SWR)
simultaneously, so you can keep an eye on
antenna system conditions too.
Wattmeters are common on many modern
radios, but standalone wattmeters can be
used at any point between the transmitter
and antenna to help with troubleshooting.
For example, you can calculate feed line
loss by comparing the power into and out of
a length of coax.
My first “wattmeter” as a young Novice
(WN1TDN) was a 60 W light bulb, used
as a dummy load and made bright by my
Heathkit DX-60B’s RF output. Many hams
simply tuned their transmitters for a dip in
plate current and hoped for the best. I burned
out my final amplifier tube twice before I
purchased my first SWR meter. Fortunately,
today’s hams have a lot of good choices,
many at reasonable prices.
Features and Testing
Four of the five units tested for this
review are “cross needle” type, employing
two movements on a single meter for simul-
taneous display of forward power, reflected
power and SWR. The other has a single
movement and a switch to choose forward
or reflected power or SWR.
Four of the units reviewed here display
both average and peak power (PEP). There
are two different types of peak reading watt-
meters: active and passive. Active models use
some type of circuit requiring an external dc
supply or internal battery. Passive models
require no power supply and use damping
circuits for peak readings — usually charging
a small capacitor quickly and discharging it
slowly. PEP mode is good for measuring the
peak power of an SSB transmission.
The tables show ARRL Laboratory test
results at several frequencies and power
levels. The “CW” measurements are with
a steady carrier (100% duty cycle), and the
“50%” (for 50% duty cycle) measurements
are made with the transmitter sending a string
of CW dits at 60 WPM. Two-tone SSB testing
(700 and 1900 Hz tones) was performed at
14 MHz. SWR accuracy testing used resistive
loads that created 1:1 and 2:1 SWR through
50 MHz. We did not have an amplifier avail-
able for 1 kW tests at 50 and 144 MHz.
Each meter was measured against a cali-
brated, laboratory grade power meter with
attenuators (see July 2006 QST, page 63, for
a description of the ARRL Lab test setup).
It is important to remember that even with a
calibrated lab-grade setup, accuracy of ±5%
is typical. For most Amateur Radio applica-
tions, a wattmeter with reasonable accuracy
is sufficient. In actual operation, a few watts
higher or lower makes no difference at all.
Ease of operation and SWR accuracy are
more important to most users. The wattme-
ters tested are all good tools for any ham and
are a far cry from my light bulb wattmeter. In
alphabetical order, here they are.
AMERITRON AWM-30
A cross needle meter, the Ameritron
AWM-30 is the most compact meter in this
review except for the Comet CMX-1 that
has a separate sensor. With just two forward
power ranges of 300 W and 3 kW, this meter
is more suitable for high power operation.
Tick marks on the 2.5 × 2 inch meter face
are at 5 W; in 10 W steps from 10 to 200 W;
and 25 W steps from 200 to 300 W. Multiply
by 10 for the high power range.
Front panel pushbuttons
include HIGH/LOW pow-
er, PEAK/AVG and power
ON/OFF. The AWM-30’s
manual does not state its
frequency range. I inquired
about that, and while wait-
ing for a call back from the
factory I tested at 50 MHz.
The results were reasonable
and consistent with HF read-
ings even though the factory
specified 1.8 to 30 MHz.
This meter will not func-
tion without a power source
— either 12 V dc via the
back panel or (unique in
this group) an internal 9 V
battery. The battery does not
light the meter lamps but
powers the meter’s active
circuit for portable opera-
tion. To prolong battery life,
QST Compares Analog HF/VHF Wattmeters
These reasonably priced HF/VHF
wattmeters offer power and SWR
measurements at several power
levels. Each model offers something
different, but one is sure to be a
match to your station requirements.
Ameritron AWM-30, serial number 01620
Frequency range 1.5-30 MHz
Power range 1-3000 W
Power requirement 12 V dc or 9 V battery
PEP measurement Active
Size (height, width, depth) 4.5 × 4.25 × 5.25"
Price $135
Actual Power (W) Indicated Power (W)
Frequency (MHz) 2 14 28 50
5 W CW 8.5 7.6 7.6 7.6
5 W 50% 8.5 7.6 7.6 7.6
100 W CW 104 105 100 96
100 W 50% 104 105 100 96
100 W two-tone – 96 – –
1 kW CW 1130 1150 1050 –
1 kW 50% 1180 1240 1140 –
1 kW two-tone – 1190 – –
SWR Accuracy
1:1 SWR 1.0:1 1.0:1 1.0:1 1.1:1
2:1 SWR 1.8:1 2.0:1 2.0:1 2.3:1
Insertion loss (dB) <0.1 <0.1 <0.1 <0.1
– = Not measured.