by Peter Parker VK3YE - first appeared in Amateur Radio, August 2000
It’s
boring, but necessary. That just about
sums up many peoples’ attitude towards test equipment. Though it might not get as much use as the
station transceiver, it can be worth its weight in gold when something goes
wrong and you need to fix it fast.
This
month we look at five items of test equipment most commonly found in the
amateur shack. We describe each instrument, list its uses around the shack and
point out features to look for when buying.
The
multimeter is the fundamental item of
test equipment that all amateurs should own.
The cheaper multimeters (around $30) allow voltage, current and
resistance measurement as well as transistor, diode and audible continuity
testing. More expensive instruments
may include features such as capacitance measurement, frequency counters,
bargraphs, temperature ranges, computer connections and mains voltage
ratings.
Practical
uses for multimeters around the shack include:
·
Testing antenna and
power connections with continuity tester function.
·
Verifying transceivers
are being fed with the correct voltage.
·
Checking polarity of
power connections.
·
Measuring the current
drawn by station equipment.
·
Making voltage and
current checks when developing or troubleshooting circuits.
There
are two main types of multimeters – analogue and digital. Both have their pros and cons.
Digital
meters are so cheap these days that no amateur need be without one. They are easy to use and fairly
accurate. There is no need to estimate
the indicated value when the meter needle is between two closely-spaced
markings. The cheapest digital meters
also have functions (eg transistor tester) that are missing from analogue
meters of equivalent price.
Analogue
meters have advantages over digital for some purposes. Analogue movements are particularly good at
displaying varying voltages, such as audio signals. Also, when aligning transmitters, the fact that you’ve reached a
peak (or dip) when making an adjustment is often more important than the actual
value of the voltage (or current). An
analogue movement is better at displaying such trends. Some of the better digital instruments have
a bar graph function that combines the best features of both meters in one, but
some users still prefer to keep the analogue meter handy.
Other
features that amateurs should consider when buying a meter are: 20 amp DC
current range (most HF transceivers draw up to 20 amps), audible continuity
indicator (though missing from budget meters, it’s very useful), capacitance, inductance and frequency
measurements. The last functions may
not work as well on the multimeter as on specialised instruments designed for a
single task, but are still useful for much amateur work, especially when
budgets are tight.
SWR
and power meters cover a wide span. The
cheaper meters provide relative indication of the standing wave ratio (SWR)
only and do not measure transmitted power.
Slightly more advanced meters include RF power output and field strength
indication as well. Most of these
meters were designed for the 27 MHz CB market, but give useful relative
indications up to 148 MHz. At lower HF
frequencies (around 3.5 MHz) the sensitivity of these meters falls off
dramatically so they can be useless at low transmit powers.
The
better meters, such as the Revex range sold in Australia, operate over a wider
frequency range than the CB-type meters mentioned above. Their sensitivity is more uniform across the
specified frequency range, which may be as much as 1.8 to 1300 MHz. Accuracy is also better, and the use of
N-type connectors reduce losses and impedance variations at UHF.
Practical
uses for SWR and power meters include:
·
SWR measurements –
These are almost mandatory for anyone who installs or constructs antenna
systems and wishes to obtain the best performance from them, especially with
modern equipment.
·
RF power measurements –
useful for testing transmitters or ensuring one is adhering to licensed power
limits.
·
Field strength
measurements – useful for crude checks of handheld transceivers or antenna or
feedline radiation. Measurements given
are relative only. Not all SWR/power
meters include this function, but a separate field strength meter is very easy
to build (See NN
April 97).
The
SWR/power meter runs a close second to the multimeter as the test equipment
item of most use around the amateur shack.
The SWR function is most important, as modern HF transceivers do not
deliver their full output power if the SWR is high. For such tests, even a relative-reading meter is sufficient. Those who repair, align or construct
transmitting equipment are advised to obtain one of the better quality meters
with output power indication.
A
dip oscillator is one of the main instruments used by the radio
experimenter. People who experiment
with antennas or build and align tuned circuits as used in HF transmitters and
receivers will get most use from them.
Applications for dip oscillators include:
·
Testing tuned circuits
in receivers and transmitters. A dip
oscillator can give a reasonable indication of resonant frequency.
·
Checking resonance of
antennas such as mobile whips.
·
Measuring unknown
capacitors and inductors (especially handy for un-marked variable capacitors
and inductors).
·
An RF signal generator
to provide test signals to align homebrew receivers or IF strips.
·
As a crude beat
frequency oscillator (BFO) to allow an AM receiver to tune SSB/CW signals.
·
To monitor the quality
of AM transmissions and listen for clicks on CW – some dip oscillators have an
earphone socket for this purpose.
·
RF field strength meter
for antenna, feedline and RF leakage tests (though the author prefers to use a
separate instrument with antenna for this).
The
dip oscillator does all this and more in one or two transistors. It consists of a wide range RF oscillator
and a meter. When the dip oscillator’s
coil is brought close to a tuned circuit that is resonant at the oscillator’s
frequency, the meter needle dips. What
is happening is that the tuned circuit being tested is sucking RF energy out of
the dip oscillator’s coil, thus causing the meter needle to dip towards
zero. The resonant frequency of unknown
tuned circuits can be determined by holding the dip oscillator coil close to it
and tuning the oscillator until the meter current drops. The dip oscillator’s tuning control is
normally calibrated in MHz to allow a direct reading of approximate resonant
frequency.
Most
dip oscillators come in a long narrow case with plug-in coils on the end. This is so that they can be stuck deep into
the innards of radio equipment.
Commercially-made dip oscillators can be hard to find and quite
expensive new. However they are very
easy to build and require just one specialised component (dual gang variable
capacitor – common at hamfests). This
makes them popular amateur construction projects.
Dip
oscillators are not known for their accuracy and long-term frequency
stability. The need to perform
mathematical calculations is another drawback compared to direct-reading
instruments. However for a cheap and
simple test instrument that can do lot, the dip oscillator is hard to beat.
RF
signal generators provide a signal at a frequency set by the user. The best RF signal generators have good
frequency coverage and stability, easy tuning (possibly via keypad as well as
knob), in-built digital frequency readout, synthesised frequency generation and
calibrated output levels. These come in
19-inch rack cabinets, and being intended for the professional, have price tags
to match. For most amateur
applications, however, cheaper hobbyist-type instruments (eg Dick Smith Q1312)
will do the job quite nicely. Alternatively,
older valve instruments occasionally pop up at hamfests. If you are willing to tolerate the frequency
drift and poor dial accuracy, they can still be useful instruments.
Like
the dip oscillator, RF signal generators are versatile instruments. However, due to their larger dial, better
frequency stability and calibrated output levels, signal generators are
superior for many purposes. Amateur
uses for RF signal generators include:
·
Test oscillators for
receiver construction and alignment.
The ability to directly inject signals (rather than rely on RF pickup)
and control output levels makes signal generators ideal.
·
Receiver
converters. A signal generator can be a
makeshift local oscillator when testing converters or mixer stages.
·
Certain antenna tests,
especially when it is not desired to cause interference to others by radiating
a high power signal.
·
A BFO for AM receivers
when receiving CW/SSB signals. The
ability to vary RF output level and easier tuning on the signal generator makes
this technique superior to using a dip oscillator.
·
A low power
transmitter. People have had CW
contacts merely by connecting a keyed signal generator to an antenna! However best results will be achieved if
attention is paid to matters such as impedance matching to the antenna, quality
of keying, frequency stability and suppression of harmonics.
Leaving
aside those lucky few with spectrum analysers, RF test sets and other exotic
equipments with five figure price tags, the cathode ray oscilliscope (or CRO)
is the most advanced piece of test equipment that most of us can reasonably
aspire to own.
If
you intend to experiment with receivers and build the odd transmitter, you will
not need a CRO. You can certainly get a
homebrew CW, AM, FM or DSB station on the air without a CRO. However, if you wish to get the best
performance and signal quality from homebrew or repaired equipment, a CRO is
the way to go. Amongst other things, a
CRO allows you to see waveforms from transmitters and oscillators. As you peak a tuned circuit, you can see the
signal getting stronger. If you adjust
a transmitter’s power output setting too high, you may see the waveform depart
from a smooth sine wave to one with odd troughs and bumps. If using an RF power meter, the needle might
suddenly jerk up, but the signal still sounds good in the receiver. With a CRO you see things you don’t always
hear on a receiver and, by moving the probe back from the output stage, you can
identify the stages that are introducing distortion.
CROs
are more expensive than any other test equipment item described here. They might not be used often. However they are extremely valuable when
used properly, and can provide a better insight into the actual operation of a
circuit than any other instrument. For
amateur purposes, maximum frequency that a CRO will go up to is important. The author’s unit will go up to over 50 MHz
– sufficient for most amateur work.
Dual trace CROs are preferred.
In
addition to the test equipment items mentioned above, ownership of an HF
communication receiver (preferably with a digital readout) would be an
advantage. The general coverage
receivers included in recent HF transceivers are fine, though a separate
receiver is preferred if your workshop is some distance from the main
station. For VHF/UHF experimenters, a
tunable VHF/UHF receiver will also be desirable. A Uniden Bearcat UBC9000XLT scanner, though it lacks SSB and
misses most UHF TV channels, should be adequate for most. A frequency counter is nice to have, but
not essential if you already have a good receiver with accurate digital
readout.
This month’s column has looked at the items of test equipment that the amateur should own. If your interests are mainly operating, the first two items are only really necessary. However, if you’d like to keep your equipment in top operating order, wish to make repairs, modifications or build new projects, all of the items described above will be useful. Plans for simple test equipment to build appeared in the April 1997 Novice Notes.
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This page was produced by Peter Parker VK3YE parkerp@alphalink.com.au. Material may be copied for personal or non-profit use only.