by Peter Parker VK3YE - first appeared in April 1996 Amateur Radio magazine
A longing to build a small transmitter,
receiver or piece of test equipment is commonly expressed by many amateurs. All
too often, however, the longing remains merely that, due to perceived
difficulties in obtaining components, a lack of test equipment, or not having a
suitable circuit diagram. Yet, these difficulties can be overcome, and the
satisfaction of successfully completing a project is immense. This article aims
to answer a few of the questions aspiring homebrewers ask.
SELECTING A PROJECT
The first step is to determine what you want
out of a project that you are considering starting. Is the device being built
for the experience and pleasure that its construction provides, or is it to
test a particular circuit technique or component? Maybe the project is because
commercially made equivalents are unavailable. Alternatively, it could purely
be the satisfaction of working the world with a transmitter you built yourself,
or of making measurements with test equipment that would be unaffordable if
purchased commercially.
Whatever the reason for building, it is
important that the features you want are defined, so that a design can be
selected to suit your needs. It may happen that you find a description of a
project with all the wanted features, and, furthermore, all parts for it are
obtainable. A kit for it could even be available. Otherwise, the constructor
may prefer to borrow stages from a range of circuits, ending up with a unique
device that meets all requirements. This particularly becomes the case for the
more experienced experimenter who seldom builds exactly to published designs.
For the beginner though, it is preferable to
work from the one design, and not from parts of several. The decision as to
which one depends on available components, features provided, the completeness
of the project article, along with complexity and cost considerations.
SOURCES OF INFORMATION
Homebrewers normally have a wealth of
material on which to base projects. This is obtainable from:-
As well as being stocked by the larger
newsagents, various local and overseas magazines are carried by public, TAFE
and university libraries. These normally provide photocopying facilities,
available on a cents per page basis.
In addition, QRP (low power)
enthusiasts have their own publications. Probably the best known is 'Sprat',
published by the G-QRP Club, renowned for its technical articles and circuit
ideas. The US-based QRP Amateur Radio Club International issues 'QRP
Quarterly', while the Australian-based CW Operators' QRP Club produces
'Lo-Key'. These magazines widely read by those interested in constructing low-powered
transmitters and receivers.
TOOLS AND TEST EQUIPMENT REQUIRED
To complete most projects, only basic hand
tools will be required.
A 15 to 25 watt soldering iron with a fine
tip for electronic work will serve the experimenter well; an expensive
soldering station is not needed. Ownership of a reliable multimeter (digital or
analogue) is a must, while a general coverage receiver is valuable for checking
the operation and stability of VFOs.
A device to indicate the presence of RF is
handy for those working with transmitters or receivers. This can be as simple
as an RF probe for your multimeter, or could be a stand alone instrument, such
as an absorption wavemeter or field strength meter. An RF power meter/dummy
load combination is also desirable if building a power amplifier or
transmitter.
A dip oscillator is a simple but useful
piece of test equipment. It can provide a local signal if aligning a receiver,
aid in adjusting antennas, and perform a range of other measurements. Building
one is highly recommended, and suitable circuits appear in many of the
references cited above.
While a dip oscillator can be used to
measure inductance and capacitance, stand-alone instruments provide more
accurate results. Some inductance and/or capacitance measurers simply attach to
your digital multimeter to provide a reading, while some more expensive
multimeters have this circuitry built in as standard. Those continually making
tuned circuits, whether for transmitters, receivers or ATUs will find the
ability to measure inductance and capacitance indespensible.
A frequently overlooked item of test
equipment is the humble portable AM shortwave receiver. Provided the set tunes
a large portion of the HF spectrum (say from about 2 to 18 megahertz), it can
give a rough indication of a VFO's frequency, verify the operation of crystal
oscillator stages, or even act as a crude signal generator. This is made
possible by the use of the local oscillator signal generated from within the
set; its frequency will normally be 455 kHz below that indicated on the dial.
A frequency counter is a valuable
instrument, particularly if you lack a calibrated general coverage receiver. A
basic unit covering up to 1 GHz will suffice for most work. Such counters are
available off the shelf in ready-built or kit form.
Some form of benchtop power supply is
important when testing equipment. Using the standard 13.8 volt, 20 amp supply
for experimenting is asking for trouble; most experimental projects require
just a few hundred milliamperes, and any accidental short circuits or faulty
components could result in spectacular fireworks and further component damage.
For this reason, a smaller supply, preferably with short-circuit protection is
recommended.
Those experimenting purely with low powered
transmitters and receivers will find that 12 volts at one or two amps is all
that is required, while other projects incorporating high power linear
amplifiers, op-amps or TTL ICs have other requirements. Kits for suitable
supplies are readily available, while the use of pre-wired plug packs mean that
some can be assembled without performing any 240 volt wiring at all.
CIRCUIT CONSTRUCTION METHODS
While the through-hole printed curcuit board
was almost universally used for manufactured electronic equipment (though it is
now being replaced by surface mount technology) and kits, there is no reason
for the home constructor to use this form of construction for their projects.
While conventional circuit boards look neater than some other techniques, they
suffer from the disadvantage of requiring a new board to be etched if
substantial modifications to the project are desired. Further time is wasted if
these do not perform as envisaged. Thus, unless you know the circuit is
reliable, it is worthwhile to consider alternatives to the conventional PC
boards, particularly if your project uses only discrete components.
One such alternative is to use an etched
printed circuit board, but solder components onto the copper side of the board.
This obviates the need to drill holes, and makes it easier to make changes. If
no ICs are used, there is no need to use chemical resist pens or photographic
methods to produce a board; the use of small pieces of adhesive tape placed on
the parts of the board where you want the copper to remain will suffice for
smaller projects.
The 'paddyboard' form of construction,
popularised by Drew Diamond, VK3XU is also suited to smaller projects for which
the ultimate in small size is not required. While, like the above methods, it
uses PC board material, paddyboard requires no etching; component leads not
connected to the copper surface are soldered to small 5x5 mm square insulated
pads, made from spare PC board material. These pads may be glued or soldered to
the main board. It is very easy to add extra components and even modify circuit
layout. Again, paddyboard is most suited to circuits not containing ICs, though
this limitation can be overcome if ICs are mounted on small pieces of vero or
matrix boards beforehand. The use of high-value resistors (several megohm) as
standoff insulators, soldered to the main board is another approach that has
worked well. All of the construction methods mentioned so far are suitable for
audio, HF, VHF, and perhaps UHF projects.
If compact construction is required, but the
constructor is unwilling to use a conventional PC board, matrix board is a good
alternative. Having holes punched every 2.5mm, IC projects can be quickly
assemled. Matrix board has worked well for RF projects well into the VHF
region, and is stocked by the normal parts suppliers.
A refinement of matrix board is veroboard.
This is matrix board with a series of parallel copper strips, which can be cut
as required by using a drill bit held in the hand. While suitable for power
supply and audio projects, the capacitances between the long parallel strips may
impair the performance of RF projects. Veroboard can be made into matrix board
simply by immersing it in a bath of PCB etchant solution.
APPROACH TO CONSTRUCTION AND
TROUBLESHOOTING
Once all components to build a particular
project have been gathered, and a construction method has been decided upon,
the project can now be assembled. If a simple project, or a well-known design,
the entire board can be assembled in the one sitting. Otherwise, if the project
is an unfamiliar circuit, or has various stages derived from several sources,
it is preferable to build, test, and experiment one stage at a time, before
moving on. For this type of construction, where the developmental prototype
becomes the final model (possibly after several changes), one can easily see
that an adaptable construction method, such as 'Paddyboard' or the use of
matrix board is preferable to a PC board, where significant changes require a
new board to be etched.
For a large project, such as a large
receiver or transceiver, it is desirable that, rather than mounting the entire
circuitry on one large board, several smaller boards be used instead. This
modular approach to construction permits the project to be an evolving piece of
equipment, with additions able to be made as time, inclination and funds
permit. This method is also compatible with the 'build and test' approach
recommended previously and the desirability of having RF-sensitive stages
shielded from one another.
If the project involves RF (especially if it
is a transmitter or power amplifier), the box housing it should be shielded.
This does not neccesarily mean a conventional metal case is required; boxes
made from printed circuit board material is also effective.
The most important aid to trouble-shooting
is an ability to think logically. The posession of most of the test equipment
mentioned above, plus a schematic diagram of the circuit under test are also
desirable. Generally, with trouble-shooting, one checks the overall equipment,
by identifying which functions do and do not work, and then attempts to isolate
the area of the fault.
In the case of home built equipment under
development, it is often not so much a fault, but a performance deficiency that
needs to be remedied. This may simply entail the use of a slightly different
circuit value for a particular component, or may require the redesign of a
whole stage to perform to the specifications required; hence the earlier
emphasis on flexible construction methods.
SAFETY
It is not out of place here to discuss
electrical safety. The construction of equipment containing high voltages
require a change to one's working habits, to minimise the risk of
electrocution. The following list, while not exhaustive, shows examples of
precautions that should be always taken:-
CONCLUSION
While the impression may be conveyed that
constructing equipment is an activity calling for a high degree of specialised
knowledge, and that it is all too hard for the average amateur, nothing can be
further from the truth. By starting with simple one and two transistor projects
and commercially available kits, one's knowledge will steadily increase to a
point where more complex projects can be confidently tackled. By this time, you
will be able to construct an item merely from a schematic diagram, and start to
develop one's own designs from sections of circuits gleaned from various
publications.
USEFUL REFERENCES
PUBLICATIONS FOR THE EXPERIMENTER
QST, AR, SC, R&C, PW, E&WW, Elector
Electronics, Lo-Key, QRP Quarterly, Sprat
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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.