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LOOP ANTENNA
AND AMPLIFIER
for long-distance AM radio reception
Design by Branko Justic*
Words by Ross Tester
• Listen in
to AM radio
stations you
only dreamed
existed!
• Separate close
stations
• Suits upper AM
broadcast band
• Small enough for
flats & home units
82 Silicon Chip
siliconchip.com.au
* Oatley Electronics
O
NCE UPON a time, listening
to long-distance radio signals
(whether on the broadcast or
shortwave bands or even the amateur
bands) was a popular hobby.
Hours upon hours were spent, listening for that elusive station . . . the
ability to bring very weak stations “out
of the mud” was the ultimate thrill.
In the 21st century (and at the risk of
earning the ire of diehard shortwave
listeners!) those days have all but
gone. Today, there is little interest in
the big, high-performance communications receivers of last century. A
lot of gear came out of WWII, perhaps
modified but there was an enormous
amount of commercial receiver equipment on the market as well, reflecting
the popularity of “listening”.
Who can forget (if you were around
50 years ago!) the Marconis, Hallicrafters or Gelosos, the Nationals,
Eddystones, Collinses or the build-ityourself Heathkits?
Speaking of build-it-yourself, the
late Ian Pogson described two mighty
“Deltahet” Wadley-Loop communications receivers in “Electronics
Australia” magazine during the 1960s
and 1970s. Later on came the Frogs
(Yaesu’s famous FRG-7) and many
more. Have I missed your favourite?
Sorry!
Those who still indulge in the art
of “listening” are these days just as
likely to use WinRadio in/on their PCs
– which in many ways outperforms
even the best of the communications
receivers of yore.
In fact, the vast majority of receivers
today have little more than the AM
and FM broadcast bands.
The aerial
There are three things which make
a receiver “good”. Two are fixed (at
the whim of the designer or manufacturer).
First is the receiver’s sensitivity,
which is its ability to resolve very weak
stations. Second is the receiver’s selectivity, which is its ability to separate
stations whose transmit frequencies
are very close. Note that NO receiver,
on its own, can separate stations which
are on the same frequency.
However, the third factor, which
can often help a receiver distinguish
between stations on adjacent frequencies (and even sometimes the same
frequency) is the aerial or antenna.
Even though the terms are virtually
siliconchip.com.au
Here’s the top PC board mounted inside one of the low-cost Oatley weatherproof
cases. The two cables entering at left are for the loop antenna above (you can
just see the loops behind the case lid). All four wires in this cable are soldered
to the underside of the board together. The cable entering in the middle is the
downlink – this cable has all four wires individually soldered to the underside
of the PC board .
interchangeable these days, no selfrespecting old-timer would ever call
his aerial an antenna. Antennas were
those small flimsy things designed to
pick up TV!
Unless you have made an in-depth
study of aerial/antenna theory, to most
people (many electronics hobbyists
included) it’s a black art.
Sure, everyone knows aerials/an
tennas are the “inductor” part of a
tuned circuit which, depending on
the antenna length, resonates at a
particular frequency, according to
the formula
1
2π√LC
where L is the inductance in Henries
and C is the capacitance in Farads.
Wanted frequencies (ie, the station you want to listen to) can pass
virtually unhindered but (at least
theoretically) all other frequencies
are rejected.
If you make the frequency of the
tuned circuit variable, then you have
October 2007 83
LOOP
ANTENNA
DOWNLINK
AMPLIFIER
(IC1a)
VARIABLE “CAPACITOR”
(VARICAP DIODES D1,D2)
530
Modern receivers are often quite
reasonable in the selectivity and
sensitivity department, so all that’s
left for us to play with is the antenna.
Even if the receiver has provision for
an external antenna and earth, you
might be quite disappointed with
the performance. That’s because a
random-length antenna is unlikely
to be impedance-matched to the
receiver and unlikely to be resonating anywhere near the required
frequencies.
1600
LOOP AROUND AM RADIO
0-10V OR 0-8V
VARIABLE SUPPLY
(VR1)
PLUGPACK SUPPLY
AND REGULATOR
(REG1)
The loop antenna
A far better approach is to use the
one we’ve gone for here – a loop
antenna with an in-built amplifier.
Moreover, a loop antenna exhibits
reasonably good directivity – if you’re
trying to pick up a distant station and
another station is swamping it, you
can rotate the loop to “null out” the
unwanted one.
Coupling the loop antenna to the
receiver is made simple because no
physical connection is required. A
second, single-turn loop couples the
signal into the radio’s in-built ferrite
rod antenna. The distance between
the loop antenna and the receiver can
usually be as much as you require – up
to several tens of metres, in fact.
We should point out now that this
Fig.1: block diagram of the Active Loop Antenna. The first two blocks are
connected in parallel to form a tuned circuit.
a means of tuning over a specific band
of frequencies.
Well, at least that’s the way it’s supposed to work. Old timers will tell
you they used to use another aerial
formula: “as long and as high as possible”. You’d see many a length of
wire stretched on poles “down the
backyard” – or further. But not everyone these days has the room (or the
neighbours!) to allow this to happen.
You need something smaller!
for those who want to listen to distant,
weak or interference-prone AM radio
stations, particularly those in the upper portion of the band. This tends
to be where the weaker stations are
located – most country commercial
AM stations are about 2kW; some are
even less. Compare that with city commercial stations and the ABC which
can be up to about 50kW!
We mentioned before that the aerial/
antenna (OK, let’s standardise on the
word “antenna”) can make a great
deal of difference to the performance
of a receiver.
Back to the future
This project is specifically intended
D
D
C
C
REG1
78L08, 78L10
OUT
+11-30V
IN
GND
6
15nF
5
7
47k
220k
ANTENNA
LOOP
IC1b
470k
15nF
15nF
220k
2
3
K
K
A
A
4
15nF
D1,D2
KDV149
1
IC1a
56
15nF
B
B
A
A
SC
100 F
OUTPUT
LOOP
(AROUND
RADIO)
56
IC1: BA4560
47k
GND
LOWER BOARD
UPPER BOARD
KDV149
2007
100 F
47k
TUNING
8
DOWNLINK CABLE
100 F
VR1
ACTIVE AM LOOP ANTENNA
K
78L10
BA4560
4
8
A
COM
1
IN
OUT
Fig. 2: the antenna loop picks up radio signals which are amplified and sent to an output loop, which re-radiates
it into an AM radio receiver.
84 Silicon Chip
siliconchip.com.au
A
*
*
B
*
47k
470k
47k
C
IC1
BA4560
A A
© oatleyelectronics.com
*
15nF
15nF
220k
15nF
15nF
220k
K
100 F
56
D
*
+
K
D1 D2
*
15nF
K256B
UPPER BOARD
ENDS OF
ANTENNA
LOOP
CABLE TIES
DOWNLINK
CABLE
Fig.3: component overlays for the top and bottom PC boards,
with their same-size photos alongside. Note that the proto
type top (amplifier) board shown here used a single SR1060
Schottky diode in the tuning circuit whereas the final version
uses two KDV149 varicap diodes in parallel.
+
56
100 F
REG1
*
START OF
COUPLING
LOOP
AROUND
RADIO
100 F
V+
*
*
*
–
11–30V
DC
GND
*
D
+
*B
+
*A
C
*
FINISH OF
COUPLING
LOOP
AROUND RADIO
© oatleyelectronics.com
* ALL SOLDERED TO
COPPER SIDE OF PC BOARD
VR1
47k
K256A
LOWER BOARD
project will NOT work on any AM
radio which does not have an in-built
ferrite rod antenna – this is the only
way the received signal is coupled to
the radio.
Incidentally, if you have a radio
without a ferrite rod antenna but
with antenna and earth connections,
a somewhat similar loop antenna, designed to plug in to such connectors,
was described in the March 2005 issue
of SILICON CHIP.
How it works
Take a look now at the block diagram (Fig.1). It shows the operation
of the loop antenna.
Countless electromagnetic waves
passing through the wire loops – generated by anything from lightning to
electric motors to radio and television
stations – induce tiny electric currents
at those frequencies.
The tuned circuit, consisting of Xframe-mounted coil loops in parallel
with a variable capacitance (we’ll
explain how this is achieved in a moment), effectively filters out almost
all of these currents, except for the
ones which correspond to its resonant
frequency. The resulting narrow band
of signals is then fed into an amplifier,
based on op amp IC1a.
siliconchip.com.au
The amplified signals are then fed
to another coil, this one designed to
wrap around the AM receiver. This
loop re-radiates the signal so that the
ferrite rod aerial coil inside the radio
can receive it again and process the
signal just as it would any other radio
signal it receives.
What we are doing, therefore, is essentially “preconditioning” the signal
so that the radio itself doesn’t have to
try hard to extract the wanted signal.
Remember those three things we
mentioned earlier which determine
a receiver’s performance? Well, this
circuit not only boosts the signal level,
making the receiver more sensitive to
weak signals, but also adds another
stage of filtering, making the receiver
more selective. As a result, the performance must be better – and in fact
can be markedly better!
To keep interference to a minimum,
the X-frame loop antenna itself should
be mounted outside the home, well
away from motors, switches, etc.
Making the tuned
circuit variable
As you probably know, you can
make a tuned circuit’s basic frequency
variable by varying either the inductance or capacitance (remember that
formula above?).
In general, it’s a lot easier to adjust the
capacitance, although many multi-band
radio receivers do change coils (inductance) when switching bands.
We could use a small variable tuning
capacitor but these are not only hard to
get, they’re also getting rather expensive. The miniature ones commonly
Modifications & An Alternative Antenna Configuration
The original circuit developed by Oatley Electronics used a single SR1060 Schottky
diode as the variable capacitance diode and this covered just the 900-1600kHz end of
the broadcast band. This device is shown in the photos but was subsequently replaced
by two KDV149 varicap diodes, enabling the entire broadcast band to be covered.
Note that if you don’t want to build a large wooden antenna mast, you can achieve
similar results by winding 10 turns of wire (spaced about 10mm apart) on a plastic
hobby storage box (or crate) measuring about 350 x 350 x 260mm deep
October 2007 85
10
5 x 4mm
DIAMETER
HOLES
SPACED
10mm
APART
HORIZONTAL
ARM
35
35
720
WATERPROOF
ENCLOSURE
FOR TOP
PC BOARD
MATERIAL: 70 x 20mm
DRESSED PINE OR
SIMILAR
20
70
70
VERTICAL
ARM/POST
5 x 4mm
DIAMETER
HOLES
SPACED
10mm
APART
ALL DIMENSIONS
IN MILLIMETRES
10
Fig.4: here’s how to cut out your timber to make the
frame and “mast” for the Active Loop Antenna. The
two pieces of timber form a cross with the loops of wire
forming a square (turned 45°) through holes drilled
close to its three outer ends and an equivalent distance
down the “post”. A knot in each end of the loop will
keep it nice and taut – just be careful you don’t pull
the cross out of square as you pull the loops through.
The top PC board mounts on the post at a convenient
position under the coil loops with the downlink wires
secured to the post using cable ties. Don’t use a length
of wire as this could constitute a shorted turn around
the wires and effectively kill some or all of the signal.
sold these days are incredibly fiddly to
use and not at all conducive to tracking
down weak radio signals!
But there is another way to obtain
a variable “capacitor”. Many semiconductors exhibit a change in capacitance when the voltage across them is
changed. Varicap diodes are one such
device and in this circuit, we have used
two such diodes in parallel to achieve
86 Silicon Chip
8V or 10V DC by REG1, depending on
the regulator used (either a 78L08 or a
78L10). The resulting rail supplies the
amplifier IC directly and is also fed to a
47kΩ potentiometer (VR1). VR1’s wiper
is in turn connected to the cathodes of
the paralleled varicap diodes (D1 & D2)
via a 220kΩ resistor.
At the top of its travel, the wiper will
have the full 8V or 10V connected to
the varicap diodes, while at the bottom, it will of course be near enough
to 0V.
The antenna coil is coupled into
the varicap diodes diode via a 15nF
capacitor. This prevents the DC voltage which is applied to the varicap
diodes from being shorted to ground
via the quite low resistance of the
antenna coil.
Similarly, the output from the tuned
circuit is coupled to the input of op
amp IC1a via another 15nF capacitor
so it cannot introduce DC into the
amplifier circuit.
Finally, we should point out that
only one of the two op amps in the
BA4560 package is used. The other
has one its inputs connected to the
positive supply and its other input
to its output to ensure that it doesn’t
become unstable.
Construction
ENCLOSURE
FOR TOP
BOARD
MOUNTS
HERE
AS LONG
AS IS
NEEDED
the required tuning range (ie, right
across the AM broadcast band).
Power supply
The supply voltage for this project
can be quite wide – from about 11-30V
DC. A nominal 12V plugpack, for example, will usually give about 16-18V
unloaded and would be fine.
This voltage is regulated to either
There are three parts to the project
– two PC boards plus the “X”-shaped
timber antenna support which houses
the turns of telephone cable forming
the antenna.
On one board, we have the amplifier section and the terminations for
the loop antenna. The second board
carries the power supply plus the connections for the loop to place around
the AM radio receiver. Between the
two boards is the downlink wiring.
Start by making your timber “X”
frame, using Fig.4 as a guide. You
can use just about any scrap timber
that you can find (but we wouldn’t
use Pyneboard or other composites
if the antenna is to be erected out in
the weather). Any timber used should
have generous coatings of paint applied to weatherproof it.
Don’t forget to drill all the holes for
the wire before you glue and screw the
sections together – it’s a lot easier to
drill flat timber!
Wind the five turns for the coil
through the holes, starting with an
outside hole closest to where the top
PC board will be mounted – leave
siliconchip.com.au
Parts List – Active Loop
Antenn
a
yourself about 200mm or
1 PC board, 31 x 94mm
, code OE-K256A
so of cable to work with
1 PC board, 58 x 48mm
, code OE-K256B
past the position where
1 Weatherproof plastic
box (eg Oatley HB4)
the PC board goes. Tie a
1 30 x 54 x 83mm pla
stic box
single-loop knot in the
1 8-pin IC socket
cable as it passes through
the first hole
Semiconductors
Continue to pass the
1 BA4560 dual op am
p (IC1)
wire through the other
1 78L08 or 78L10 volta
ge regulator (REG1)
three outside holes, then
2 KDV149 varicap dio
des (D1,D2)
the next across and so
Capacitors
on until the coil is com3 100μF 16V electroly
tic
plete. As you go, keep
5 15nF disc ceramic
(code 15n or 153 or .01
the turns of the coil nice
5μF)
and taut but not so taut
Resistors (0.25W 5%)
as to pull the timber out
1 470kΩ
(code yellow purple ye
llow gold)
of the “X” shape.
2 220kΩ
(code red red yellow go
ld)
2 47kΩ
When completed,
(code yellow purple ora
nge gold)
2 56Ω
tie a single-loop knot
(code green blue black
gold)
1 47kΩ linear potentio
in the last hole so that
meter (and knob to su
it).
it keeps the wire loops
Miscellaneous (not includ
ed in Oatley Electronics
taut. Again, leave yourkit)
Timber, screws and mo
unting hardware as req
self 200mm or so of cauired
40m of 4-wire telepho
ne cable or equivalent
ble underneath where
the PC board will
mount and then cut
the remainder off. Hang onto that –
you’ll need it shortly for the downlink!
wire and so offers some protection
from, for example, birds sitting on it
PC board construction
or even pecking at it!
It doesn’t matter which board you
If you use telephone cable for the
start with – both are quite simple and
loop antenna wiring, simply connect
should only take half an hour or so to
all four wires in parallel as you solder
complete.
them to the PC board. The same applies
On the loop antenna board, the
for the output loop – the one which
only polarised components are the
goes around your radio. The wires
amplifier IC, the electrolytic capacitor
can be twisted together to make this
alongside it and the varicap diodes.
easier. Note that all connecting wires
Start by installing the smallest
solder to the pads on the underside of
components, ie, the resistors and
the PC board.
non-polarised capacitors, then install
You might be wondering why each
the semiconductors and the electrosingle wire of the telephone cable
lytic capacitor. Don’t worry about
was not connected in series with its
the loop or downlink wiring at the
mate and terminated as such on the
moment.
PC board. Wouldn’t this create a sigOn the power supply board, three
nificantly greater inductance (ie, four
of the five components are polarised
times greater)?
so make sure you get them in the right
Oatley Electronics originally had
way. The potentiometer will only go
exactly the same idea. Unfortunately,
in one way (otherwise the shaft points
when they tried it out, they found that
inwards!).
the capacitance of the closely-spaced
wires within the cable started to create
Loop and downlink wiring
its own problems.
In the prototype, 4-wire telephone
They found that by paralleling all
cable was used because this hapfour wires in the cable, this problem
pened to be on hand – even though
was eliminated. More importantly,
the loop antenna does not use the
they found that the overall perforfour individual wires (however, the
mance of the antenna was better!
downlink does).
In the downlink, all four wires in
Therefore you could just as easily
the telephone cable are used indepenuse single-conductor wire for the loop
dently and are connected to the points
if you wished. Note that telephone
A, B, C & D on the PC boards.
cable is quite a lot tougher than single
With coloured wiring in the cable
siliconchip.com.au
(black, red, blue and white),
it’s not easy to get it wrong!
This downlink wiring can
be quite long – the prototype
had 20m between the two PC
boards and there didn’t appear to be any loss of signal
compared to a 5m separation.
If you need more distance,
give it a go – you have nothing
(except signal!) to lose.
In use
If possible, make a complete
turn around the radio receiver
with the output loop (remember, the radio must have a
ferrite rod for this antenna to
work). How do you know if it
has a ferrite rod antenna? If you
can turn it on and it works without anything connected, it’s a
pretty fair bet that it has one!
Virtually all small AM radios
have a ferrite rod antenna inside.
Tune the radio to the weak station you want to listen to. Now
adjust the potentiometer slowly – at
one point, you should find a significant increase in the level of that station
(or a decrease in any other stations that
are interfering with it).
Also recall what we said before
about the antenna being able to turn
so that it faces the wanted station – by
facing, we mean broadside on, or if
you take a line across the X frame the
wanted station should be perpendicular to that line.
The antenna will work equally well
from both sides. However, if you turn
the antenna through 90°, you should
find that its performances decreases
significantly. Conversely, any other
stations that are now broadside-on
SC
will be much better.
WHERE FROM, HOW MUCH?
This project was designed by Oatley
Electronics who retain the copyright.
A kit for this project (K256), which
includes both PC boards, on-board
components and cases (as listed
above) is available from Oatley
Electronics, for $AU22.50 The kit does
not include the 4-wire telephone cable
nor any timber or mounting hardware.
Contact Oatley Electronics via their
website: www.oatleyelectronics.com
or call (02) 9584 3563.
October 2007 87
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