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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
Antennas & earthing systems
for crystal sets
Crystal sets and other low-sensitivity receivers
require good aerial and earth systems to give
their best performance. Here’s what you can do
to improve broadcast band AM radio reception
on these simple receivers.
Due to the improved sensitivity of
receivers after the mid-1930s and the
increasing power of AM broadcast
transmitters, large antennas quickly
become redundant as far as the general public was concerned. In fact,
most people now don’t like having
to extend the whip antenna on a
portable FM receiver to get the best
reception.
Unfortunately, the use of metallic/
bituminous insulation paper in the
walls of many modern homes now
acts as an RF shield, reducing the level
of the radio signal penetration. For
this reason, good reception on older,
less-sensitive receivers may require an
outside antenna and this is particularly so for crystal sets.
In the November 1998, December
1998 and January 1999 is
sues, we
looked at various methods for improving AM radio reception and reducing
interference. This article is not intended to supersede those articles but to
give enthusiasts a few ideas on suitable
antenna systems that will achieve good
results in most situations.
The early days
Experimental broadcasting commenced not long after the end of World
War I. Amateurs produced programs
on a nightly or weekly basis and the
general populace was keen to listen to
these new transmissions.
The output power of these early
experimental stations varied considerably but most were well under 100
watts. The radiating systems connected to the transmitters were also quite
varied, some consisting simply of an
inverted “L” receiving type antenna
fed against an earth that consisted of
a metre or so of pipe driven into damp
soil. Others were more ostentatious,
being something like the smaller
broadcasting masts that can be seen
around the countryside today.
Official broadcasting commenced in
Australia in late 1923 (2SB/2BL) and
the power of the stations varied from
around 50W to 5kW (5000 watts). In
most cases, the power ratings were
for input power, not output power, so
a 5kW transmitter may have had an
output power of just 1.5kW.
Fig.1: the earth stake or pipe should be driven a metre or so
into the ground, with a small depression around the top. This
depression is filled with water at regular intervals, to keep
the soil around the earth stake moist. Fig.2 (right) shows how
to install an effective earth through concrete.
84 Silicon Chip
www.siliconchip.com.au
“Egg” insulators will be required at
the support points if you use bare
wire for the antenna. Alternatively,
you can use polyproylene pipe.
and earth system for low sensitivity
receivers such as crystal sets.
Getting a good earth
Fig.3: a “clothesline” type aerial frame can be used in back yards where
space is a problem. It’s not as effective as a long, straight run of wire but is
still quite good.
Gradually the standards for broadcast transmitting stations became
more uniform. The commercial class
B stations that came into being were
rated at an output of 2kW in the country and 5kW watts in the city, whilst
Class A (ABC) stations used upwards
of 10kW (today, the ABC stations run
upwards of 50kW in many instances.)
At the same time, strategically placed
low-power stations (50-500W) were
used to provide signals in areas where
the high power stations were relatively
ineffective (such stations are still used
today).
Because of the low power of early
transmitting stations and the general
insensitivity of receivers up until the
mid-1930s, large outside antennas
were mandatory for reasonable reception during that period. Many different
antenna types were used in an attempt
to get the best performance possible.
Some were a lot of work to install but
may not have worked any better than
much simpler structures. But they did
look commanding!
A receiver using a large earth (mat)
system (much like transmitting installations use) will work a little better
than a receiver using the common,
relatively inefficient receiver earths.
In practice, transmitting stations go
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for the most effi
cient antenna and
earth system that is economically
feasible. That’s because they need to
provide the strongest signal possible
for receivers that are using inefficient
antenna systems, eg, a few metres of
wire.
The reasoning here is that it is
better to have efficient transmitting
installations so that receivers can get
by with convenient low-cost antenna
systems.
Having said that, it is still necessary
to use a relatively efficient antenna
The earth lead can be attached to the
pipe using a metal hose clamp. Wrap
the wire around the clamp several
times to ensure it makes good contact.
In an ideal situation, a 1-metre
length of pipe or metal stake driven
into the ground in a damp location
will provide an adequate earth for
most receivers. This earth should be
as close as practical to the receiver
so that the minimum length of con
necting wire is used back to the earth
terminal of the receiver. This also
helps to reduce the effect of mainsborne interference, particularly in
more sensitive receivers.
In my case, I have often used 19mm
galvanised water pipes as the earth
stake. However, because I am an amateur radio operator, I need a better
earth than a 1-metre length of pipe can
provide. For this reason, I commonly
use up to three 2-metre lengths of pipe
driven into the ground, leaving about
300mm above ground level. The tops
of the pipes/stakes are quite close and
are bonded together but their bottom
ends are quite some distance apart, as
the pipes are driven into the ground
at angles to each other.
In order for the earth to be effective,
the whole length of the pipe/stake and
the earth around it needs to be kept
moist (not just the top few centimetres
of the soil). This is achieved by making
a small depression around the top of
the pipe(s) and a bucket of water then
poured into this depression at regular
intervals.
A single pipe driven at least a metre
into the ground is usually quite adequate as an earth for radio receivers
(providing it is in moist ground). Fig.1
shows the details.
The wire (preferably multi-strand)
March 2003 85
This simple antenna tuner uses a multi-tapped coil wound on a cylindrical
former. It’s tuned by sliding the ferrite rod inside the former and by changing
the coil tapping.
Fig.4: this diagram shows how to add a dial drive system to the antenna tuner
shown in the photo at the top of this page. You can scrounge the parts from a
junk radio or buy them from an electronics retailer.
going from the earth to the receiver
earth terminal should be 1-2.5mm2
in cross-sectional area (equivalent to
the average earth wire on the mains).
It doesn’t have to be insulated but insulated wire is easier to handle if it is
likely to be close to any other metal,
including the Sisalation type material
used in the walls of modern homes.
Don’t let a bare wire brush up against
any metallic object as noise and “crackles” (interference) may be induced into
the wire and thus into the receiver.
Unfortunately, it’s not practical to
solder the end of the wire directly to
the earth stake or pipe. That’s because
the metal mass is too great to allow it to
get to the melting point of solder when
using a soldering iron that’s suitable
for electronic work.
Instead, an electrician’s earth clamp
or a screw type hose clamp can be
used to hold the earth wire firmly
against the pipe. The wire should be
wrapped through the clamp a few
86 Silicon Chip
times to make sure it will not move
once clamped. Before doing this
though, clean the pipe using sandpaper to get rid of any oxidation at the
contact point. Once the earth wire
has been clamped to the pipe, the assembly should be painted to stop any
corrosion.
Alternative methods
Not everyone has an ideal location
to install earth stakes and so other
methods of obtaining an earth must
be used. For example, many premises
today have large areas of concrete
which this can make things quite
difficult.
Drilling a hole through the concrete
to accept an earth pipe is one possibility but make sure that you check
the locations of water, drainage, gas,
telephone and electrical lines before
doing this. You will need a large
masonry bit and a hammer drill for
the job.
Begin by drilling a series of holes
around the circumference of a circle
large enough to accept a 100mm-diameter plastic stormpipe. A cold
chisel and heavy hammer will be
required to break up the pieces but
even so, it usually isn’t easy getting
the pieces out.
In fact, it’s probably a good idea to
send the family out for the day so that
they don’t learn any new words!
The storm piping should protrude
about 200mm above the ground and
can be fixed into position using cement
or silicone sealant. This stormpipe is
then filled with water to keep the earth
pipe moist - see Fig.2.
If metal water pipes are used, it’s
possible to clamp the earth wire onto
these and obtain quite good earthing.
However, earthing to a water pipe
does raise the possibility of circulating
currents through the pipe system and
anything connected to the mains earth.
To overcome any chance of electrolysis (which can cause corrosion of
the pipes), it’s a good idea to install a
capacitor in series with the earth wire
near the radio. This can range in value
from .001µF up to 0.1µF - preferably
one of each in parallel. Note that this is
necessary only if the receiver is mains
operated and has its chassis earthed
through the mains.
Gas pipes are not to be used under
any circumstances.
For people in units, an earth via the
mains may be the only viable alternative. Any piece of electrical equipment
that has a 3-core power lead and has
its metal frame earthed can be used
as the “earth”. All you have to do is
attach a wire from the metal frame
of the earthed appliance to the earth
terminal of the set.
This provides a reasonable earth
but make sure the appliance has a
relatively short lead to the power
point.
Do not attempt to obtain a mains
earth in any other way - the possibility of making a fatal mistake is much
too great.
A good antenna
The old saying of “the higher and
longer the better” when referring to
antennas for crystal sets and other
low-performance sets is still true
today.
If you live out on a farm, erecting
an effective antenna is relatively easy.
A good standby is the old standard
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Provided it’s fitted with a good aerial and earth, this simple “two circuit” crystal
set is quite a good performer.
inverted “L” antenna some 30 metres
long across the top and 13 metres
high. “Higher and longer” will capture
even more signal and possibly also
lightning, so lightning arresters are
desirable on antennas of this size.
Most of us do not have the wide
open spaces to install this type of
broadcast receiving antenna so we
have do the best we can with the
available space. It’s also necessary to
comply with local council bylaws.
In most cases, any antenna that’s
erected will be a compromise between
performance and available space. The
installation must also be safe and must
not be an eyesore to neighbours or
others living on the premises.
It is possible to run an antenna (flat
top section) around the yard, with
one end attached to the chimney (if
the house has one) or to a TV antenna
mast – see Fig.3. Specialist TV antenna
supply shops, such as Lacey’s Australwww.siliconchip.com.au
ia in Frankston, Victoria, have a wide
variety of TV antenna mounting masts,
brackets and other items which can be
used for this job.
A number of the advertisers in
SILICON CHIP, such as Jay
car, Dick
Smith Electronics and Altronics, also
have a range of TV-antenna bits and
pieces. Check out their catalogs and
you should have little difficulty in
choosing the necessary parts to make
your antenna effective, safe and aesthetically pleasing.
Commonly, in the past where there
was little space for an antenna, it was
made to look something like a clothes
line, as shown in Fig.3. This is an effective way of getting a considerable
length of wire up into the air in a confined space. It may not be as effective
as a long, straight run of wire but it is
still quite good.
Regrettably, some people are not
allowed to have any outside antenna
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March 2003 87
Above: this miniature 2-circuit crystal set uses all ferritecored coils and is quite a good performer. Its circuit is
shown in Fig.5, for those who want to make their own.
At left is an old ferrite-cored tuned circuit crystal set,
complete with wooden case and a set of high-impedance
headphones.
system at all (perhaps for “aesthetic”
reasons). However, all is not lost as
outside antenna systems can be erected at night by suitable mechanisms
and pulled down after use. Some
people have even disguised an antenna
mast as a flag pole. A little ingenuity
is sometimes needed here.
If that doesn’t suit, an antenna in
the ceiling space can be much better
than nothing at all. There are a few
provisos, however – the house must
have a gable roof and the roofing must
not be metallic. Nor should there
be any metallic foil underneath the
tiles (metal acts as a shield for radio
signals).
To build a ceiling space antenna, install several folded lengths of insulated
wire near the peak of the roof and bring
one end down to where the receiver is
located. Thirty metres of wire should
give reasonable performance. It cer-
tainly won’t be up to the standard of
a high outside antenna but it’s much
better than nothing.
Antenna wire
An outside antenna lead can be
made from single or multi-strand
copper or galvanised iron wire. It can
be insulated or bare but it must not
too thin, otherwise it will break in
the wind or if a bird flies into it. I use
common gardening tie wire (available
at hardware stores) of around 16 gauge
– it’s cheap and solders quite well.
If you use bare wire, it is necessary
to also use egg insulators (available
from stores selling electric fence components and major electronic stores)
at the points where it is supported,.
Alternatively, the polyproylene pipe
used for garden
ing systems can be
used in some situations.
The down lead to the receiver from
Fig.5: this simple “2-circuit” crystal set uses coils wound on ferrite rods.
The constructional details are in the text.
88 Silicon Chip
the antenna should be run using insulated multi-strand cable, such as one
half of twin-flex cable used on small
electrical appliances.
Any joins in the antenna wire must
be soldered. If they are not soldered,
scratchy noises will be heard in the
radio or variations in level will occur
after only a few weeks. That’s because
the wires oxidise and intermittent
good and bad contact between the
joined wires will occur.
Finally, to alleviate any stress on the
lead in, it should be attached to the
antenna proper in a manner similar
to that shown in one of the photos.
Optimising performance
A normal AM broadcast antenna is
an aperiodic device, meaning that it is
not tuned to any particular frequency.
By contrast, if it were tuned to a specific frequency, the amount of signal
picked up by the antenna would rise
noticeably.
In other words, tuning the antenna
system can greatly improve its performance and this can be done quite
easily.
Fig.4 shows the details. First, obtain 150mm of 20mm-diameter PVC
electrical conduit and wind about 180
turns of 0.5mm (24B&S) enamelled
copper wire onto it, with tappings
made at every 20 turns. The tappings
can be made by inserting a match under a turn, winding some more turns
on and then sliding the match further
along as you progress, keeping each
tapping point proud of the coil.
www.siliconchip.com.au
One end of the finished coil is then
wired in series with the antenna lead,
while a “fly lead” fitted with a small
alligator clip is soldered to the other
end. This fly lead allows sections of
the coil to be shorted out if necessary.
Alternatively, either the antenna
or the receiver aerial lead can be
terminated on one of the terminals
along the coils, as shown in one of the
photographs.
Once the coil is in place, it is then
time to trial the tuner by sliding a
180mm-long (not critical) x 9mm-diameter ferrite rod into the coil and
observing the results. Try differ
ent
coil tappings until you get the nest
performance.
Once the correct tapping is found
for the coil, altering the position of
the ferrite rod in the coil will peak
the perfor
mance on any particular
station. Note that this coil will also
work with the average domestic receiver, although its performance at
the low-frequency end of the dial can
be unpredictable due to the way some
aerial coils were designed. More turns
may be required on the peaking coil
in some cases.
Initially, the rough and ready method described above will give good
results. However, it can all be made
much tidier. Excess turns on the coil
can be removed if desired and a more
sophisticated method of adjusting the
position of the ferrite rod inside the
coil can be constructed – see Fig.4.
Jaycar Electronics can supply suitable ferrite rods (Cat. LF-1012) for an
antenna tuner but loopstick antenna
rods scrounged from defunct sets
are also quite suitable - just remove
the windings from the rod. If the rod
has been broken and you have all the
pieces, it is often possible to effectively
rejoin them provided the breaks are
clean and the two sides of a break
mate without gaps. You can use Aral
dite or some other similar adhesive
for this job.
Crystals sets & antenna tuners
On my own crystal sets, I can
receive just one station without the
antenna tuner. By contrast, if the
antenna tuner is used, I can receive
four stations, including a Melbourne
station some 150km away! The tuning
is also much sharper than normal
and the aerial tapping is quite low
on the coil.
Initially, however, I found that the
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Photo Gallery: Tecnico
Aristocrat Model 651 Radio
Built in 1946, the Tecnico Aristocrat featured a distinctive “mottled”
bakelite cabinet. This cabinet was completely different to those used
by other manufacturers, some of which were still using cabinets from
their pre-war models.
The set pictured here is a 5-valve dual-wave radio with a 20cm (8-inch)
permanent magnet speaker (loudspeakers of this size were rarely seen
in mantel or table radio). A colourful badge and dial glass complete the
attractive presentation.
The set was advertised as being available in various colours and
Tecnico achieved this by painting over the bakelite. This particular
unit has been fully restored by its owner, Maxwell Johnson, Kingston,
Tasmania. (Photo: Ross Johnson).
crystal sets were not performing at all
well, barely receiving the local station
just 20km away. I tested the germanium diodes using the diode tester
function on my digital multimeter
and found that the forward voltage
drop had increased from 0.2V to 0.8V,
while the reverse voltage conduction
point was down to 3V. The diodes were
consigned to the rubbish bin and new
ones installed.
The crystal sets in the photos belong
to members of the local vintage radio
club. Two are very nice looking sets
and perform quite well. The small one
in the jiffy box (130 x 65 x 40mm) is
extremely interesting as it uses ferrite
cores on all coils and a tapped antenna
loading coil (possibly to resonate the
antenna).
I have not heard this latter set in
operation but have heard others claim
that it is a particularly good performer.
As a result, its circuit is enclosed for
readers who may care to experiment
- see Fig.5.
All the coils are wound using 0.5mm
(approx.) enamelled copper wire. L1
and L2 are wound on the same piece
of ferrite rod but at opposite ends of
the rod which is around 50mm long.
Coils L3 and L4 are wound on two
ferrite rods, each around 30mm long.
The amount of coupling between
L2 and L3/L4 is open to experimentation – around 50mm apart should
give good results.
Summary
A high and long antenna coupled
with a good earth system is important if worthwhile results are to be
obtained from crystal sets and other
low-performance sets. In some cases, it may also be necessary to tune
the antenna to the frequency of the
desired radio station (ie, by using an
antenna tuner).
As for crystal sets, the best performing types are those using two tuned
circuits – as in Fig.5. Also, it’s necessary to use a germanium diode for the
detector and a pair of high-impedance
SC
(2-4kΩ) headphones.
March 2003 89
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