June 1997
71
Mike Tracy, KC1SX, and Mike Gruber,
W1DG, of the ARRL Lab, for their help in
conducting this product review.
Manufacturer: AOR Manufacturing Ltd,
4E East Mill, Bridgefoot, Belper, Derbyshire
DE56 2UA, UK; tel +44 1773 880788;
fax +44 1773 880780; e-mail [email protected].
uk; WWW http://www.demon.co.uk/aor/.
Reviewed by Steve Ford, WB8IMY
Managing Editor
Someone once said that SWR meters are
the inventions of the Devil because they
cause so much suffering. There is a lot of
truth in that statement. Many hams grieve
needlessly over antenna systems that regis-
ter “horrible” 1.8:1 SWRs. Their diabolical
meters compel them to spend hours tweak-
ing and re-tweaking in desperate attempts
to reach Nirvana—the “perfect” 1:1 SWR.
And for what?
But bedeviling aspects aside, SWR
meters do serve useful purposes. Most
modern transceivers won’t tolerate more
than a 2:1 SWR before they begin reducing
output, so it pays to have some knowledge
of your SWR conditions. In addition, your
SWR meter often provides the first indica-
tion of trouble in your antenna system. If
you leave it in the line at all times, you can
watch for sudden variations that might sig-
nal a broken wire, a corroded connector, an
arcing antenna tuner and so on. And if your
SWR meter includes a wattmeter function,
you can also use it to keep an eye on your
radio’s output level.
In recent years, the digital SWR meter
has been encroaching on a market domi-
nated by analog meters. Digital meters tend
to be expensive, however, which limits
their appeal. Radio Shack hopes to win
the hearts of hams with a new approach—
an economical model known as the model
21-527. Can they succeed?
Features
The 21-527 lets you measure SWR and
power from 1.8 to 30 MHz in ranges from
1 W to 2 kW. You can select power ranges
manually (a button on the display unit steps
through the 20, 200, and 2000-W scales), or
leave it to the meter’s autoranging function.
I preferred the convenience of autoranging.
The SWR readings appear on a compact,
easy-to-read, backlighted LCD display. If
the forward power input to the meter is less
than 0.5 W, it cannot calculate the SWR
and simply displays
Lo
. If the SWR is too
high, it displays
InF
. You can also select a
peak or average-power display (more on
that later).
The 21-527 includes a remote sensor
unit, which lets you sample the RF at one
location and view the results at another. The
sensor connects to the display unit via a
four-foot-long, three-wire cable and a mini-
stereo plug. (The manual states that you can
Radio Shack Model 21-527 Digital SWR/Power Meter
BOTTOM LINE
An economically priced digital SWR/
wattmeter with good accuracy and good
looks. It includes a remote sensor, and
it’s entirely adequate for typical, non-
critical ham use.
use a cable of greater length, but it doesn’t
mention a maximum length.)
A 12-V dc power cube is included with
the meter. For this review I simply used my
station power supply. There are only so
many ac outlets to go around!
On the Air
The 21-527 was installed in minutes. In
my station, the only issue was finding a
place to hide the remote sensor, which takes
up a bit more space than the digital control
box. The remote sensor has a mounting
bracket. The control box has threaded holes
in each side that appear to be designed to
accommodate a mounting bracket, but none
was supplied.
I subjected the meter to power levels
from about 0.5 W to 600 W. Readings in the
average power-setting—the power-up de-
fault—were confusing. Unless it was read-
ing a steady carrier, the display flashed a
bewildering cascade of digits that never
seem to settle down. As a result, I used it in
the peak mode at all times. Tests in the
ARRL Lab indicated, however, that the
meter was not really providing a true PEP
reading, but something that was more than
average power but less than PEP. To put it
in perspective, however, the meter was
about as accurate on PEP readings as many
of the wattmeters we looked at in a com-
parative review of SWR/wattmeters a few
years ago (see “Product Review,” QST, Feb
1991). The auxiliary bar graph at the bottom
of the display is a nice touch. It augments
the digital readout by providing a relative
indication of power or SWR—depending on
the mode you’ve selected.
SWR readings were a snap. My only
complaint—and this applies to many other
digital SWR meters—was that the display
was slow to adjust to abrupt changes. I use
a venerable Johnson Matchbox antenna
tuner with a dipole that’s center fed with
450-
Ω
ladder line. With my analog meter
it’s easy to see the sharp SWR dips as I ad-
just the Matchbox. If you rely solely on the
digital readout of the Radio Shack 21-527,
it’s possible to miss these tuning points be-
cause of the display’s slow response. On the
21-527, however, you can use the auxiliary
bar graph across the bottom of the display
instead of the digital readout to see the SWR
“dip.” Of course, if you are using a resonant
antenna system, or an automatic antenna
tuner, this isn’t an issue.
We spot-checked the accuracy of the
wattmeter in the ARRL Lab, using it to
measure power from 10 W to 1500 W on
80, 20 and 10 meters. For the most part,
measurements made using the Model
21-527 were within 10% (in most cases,
well within 10%). Things started to veer
out a bit above power levels of 1000 W or
so on 28 MHz, but the meter still was only
approdimately 10% off at 1500 W. That’s
quite respectable for a meter that costs
around $60.
Conclusion
The Radio Shack 21-527 is a fine value
if you want the convenience of a versatile
digital SWR/power meter at a reasonable
price. But you’ll have to hurry to get one—
Radio Shack is closing out this product.
The
±
10% full-scale accuracy is more than
adequate for most amateur applications. It
certainly would be suitable for mobile use.
The ability of the 21-527 to provide read-
ings from QRP to QRO power levels is a
big plus not found on many HF SWR
meters. The stylish 21-527 design and the
remote sensor are icing on the cake.
A special thanks to ARRL Educational
Programs Coordinator Glenn Swanson,
KB1GW, for his contribution to this review.
Manufacturer: Tandy Corporation, 1900
One Tandy Center, Ft Worth, TX 76102; tel
817-390-3700. Manufacturer’s suggested
retail price, Model 21-527, $60.
Manufacturer’s suggested retail price: $1400.
NB7030 audio notch and RF noise blanker
board, $315; SM7030 service kit, $90;
FPU-7030 enhanced features CPU, $110.