Projects & Technical
by Peter Parker VK3YE - first appeared in Amateur Radio, May 2001
Response
to December's column was so favourable that we return to crystal sets and
allied topics this month. Keep reading to find out how to improve
your AM broadcast reception, make your crystal set deliver hi-fi audio and set
a fashion trend with the all-new 'wearable wireless' pedestrian mobile crystal
set.
A frame antenna for AM
radio reception
Portable
AM broadcast receivers normally have no external antenna connection and are not
designed for long-distance reception. However the addition of a
directional antenna can greatly improve reception, even on cheap
receivers.
Figure
One shows a rotatable frame antenna that can be used with any portable
receiver. It consists of a large pick-up coil wound around a cardboard
box or plastic storage box. The receiver is placed inside the box.
A variable capacitor brings the antenna to resonance on the frequency of
reception.
To
construct, wind about 27 metres of insulated wire around the box, which should
be around 400 millimetres square. No spacing between the coil turns is
required. Make a pair of holes or use adhesive tape to anchor each end of
the winding. Connect the coil to an old 10 - 415 pF air-spaced variable
capacitor. One side of the coil should go the capacitor's case (which is
electrically connected to the moving plates) and the other to one of the
gangs. As discussed in December, a plastic dielectric variable capacitor
can also be used, but at the expense of tuning range.
To
use, tune the radio to a weak AM signal near the middle of the band. Then
place it inside the box. The windings on the set's ferrite rod should be
parallel to the windings on the box. Carefully adjust the variable
capacitor until an increase in signal is noted. Then rotate the box until
the signal level peaks. The received strength should be very much greater
than when the receiver was outside the box. Note that the tuning on the
box antenna is very sharp – re-tune with every receiver frequency adjustment.
When
turning the box, you will find nulls, where the signal level falls
dramatically. This directivity can be used to null out interference from
power lines or electrical equipment. Also when tuning to a weak
signal adjacent to the frequency of a local station, rotating the box to null
out the local station can be helpful in reducing adjacent-channel
interference. In some cases, the null can be sharp enough to allow two
stations using the one frequency to be separated, just by rotating the box.
If
you find the frame antenna does not cover the whole broadcast band (do tests
about every 100 kHz between 530 and 1600 kHz to verify this), change its
coverage by adding or removing coil turns. Adding turns reduces
frequency and removing turns increases frequency. Use an alligator
clip and install a tap near the middle of the coil if extended high-end
coverage is desired. This could be useful for reception of the narrowcast
stations above 1602 kHz and 160 metre AM operators. Another modification
is adding an extra winding of a few turns near the main winding. This
allows the frame antenna to be used with receivers that have external antenna
sockets, such as communications receivers.
Despite
its small size, the rotatable frame antenna dramatically improves AM
reception. It does this by increasing sensitivity and selectivity while
reducing interference. Signals marginal without the antenna become
pleasant listening with it. Even during the day you will be amazed at
what can be heard on a cheap receiver assisted by a frame antenna. Build
it and hear for yourself!
Figure One (use back button to return to this page)
Nearly all published
crystal sets designs require at least an outside antenna to
operate. This one is different. Its larger than average
coil picks up signals with no antenna and earth connection required,
particularly in urban areas.
The portable crystal set
shown is identical to the frame antenna described above except for the addition
of a diode detector, capacitor and headphones. Use is similar to
the frame antenna – tune in a station and rotate the loop for maximum signal
strength. Again the prototype was constructed on a spare plastic packing
crate about 40 centimetres square, though a cardboard box would perform equally
well.
The receiver was tested
in suburban Melbourne. Approximately ten stations were received, but at
low volume. This was gratifying considering its relatively small
size. However, unless you live near the local AM broadcast stations, this
receiver is only really suitable for use in a quiet room.
After
the encouraging results with the frame crystal set, an improved version was
constructed. Its main difference is the much larger area enclosed by the
coil. This resulted in greatly increased volume with no loss of
selectivity.
The
design concept was of a wearable 'pedestrian mobile' receiver, wholly powered
by the signal of the incoming station. It was originally built for
a local hamfest crystal set competition and would have almost certainly won the
'most unusual entry' prize, had it been completed in time!
The
Wearable Wireless looks like a single-element quad loop antenna for about 70
MHz. The circuit is identical to the set described above but the physical
construction is different due to the larger coil. As with the receiver
described in the December issue, a polyethylene chopping board forms the heart
of the set. Extending from the corners are wooden spreaders 600 mm
long. In the centre of the board is a plastic box that houses the tuning
capacitor and diode detector. Four banana sockets are used for the coil
and headphone connections. An old hot water bottle provides padding for
the wearer. If necessary this can be stuffed with old newspaper or other
filling to further improve comfort. A discarded belt is attached to
the chopping board to allow the set to be tethered to the listener. For
most convenient operation, the set should be worn so that the hot water bottle
fits in the small of the user's back.
Clothes
pegs are glued in the far end of each spreader to hold the windings. With
the 60/160pF tuning capacitor used ('O' and 'A' terminals wired together) 6
turns were required to provide broadcast band coverage. Again this
requires 27 metres of thin insulated wire. Heatshrink tubing or
insulating tape can be used to keep the wires bunched together.
Though
not quite as loud as a crystal set with a good antenna, the set works
remarkably well. Selectivity is good, and is all that is required for
reception of local signals. The set has two main
shortcomings. The first is that changing the station requires one to have
the skills of a contortionist in order to reach the tuning knob behind your
back. Secondly the antenna's sharp null means that you may lose reception
when going around a corner. Though the Wearable Wireless will not necessarily
be your most used receiver, its construction will challenge your ingenuity,
have great novelty value and make you the centre of attention at the next radio
club meeting or hamfest!
Felix VK4FUQ has been
experimenting recently with crystal sets. Along with a very informative
letter and article from the local 'Backscatter' club newsletter, he sent Novice
Notes a cassette recording of reception from his latest crystal set.
Overall fidelity was excellent and, to my ears at least, comparable to FM
radio.
Felix made the following
observations:
·
A double tuned circuit
always dramatically improves selectivity, regardless of coil tappings.
·
Gold bonded germanium
diodes (eg 0A47) provide outstanding clarity and efficiency.
·
Voltage doubler
detectors using two diodes do work, but produce somewhat muddy and ill-defined
audio, as confirmed by listening tests and spectrum analyser measurements.
·
No improvement has been
noted when adding bias to germanium diode detectors. However bias applied
to detectors using silicon diodes (1N914, etc) is highly beneficial.
·
It is important to have
a DC load on the diode detector. Felix used a 10k potentiometer.
·
When using the crystal
set as a tuner for a stereo amplifier, it is a good idea to add a one
transistor audio preamplifier. Use a BC549C and metal film resistors to
minimise noise. Feed output to both left and right stereo inputs tied
together.
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& Technical Page
This page was produced by Peter Parker VK3YE parkerp@NOSPAMalphalink.com.au. Material may be copied for personal or non-profit use only.
