e01c062.PDF
(
578 KB
)
Pobierz
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
HF
&RADIO
Induction Radio
a nostalgic listening experience
Design by B. Stuurman
This radio is a sight for sore eyes. The magnificent coils, the gleaming valve
and the striking wooden chassis suggest that this is a collector’s item from
the early days of radio. But appearances can be deceptive. This radio can
be constructed easily at home and offers good reception across the
medium-wave band.
In the period between 1930 and 1960
building your own radio receiver
was a very popular pastime. In those
early days of radio you had to make
nearly everything yourself, but after
the increase in broadcasting stations
more of the components could be
found ready-made. Since then, the
construction of a crystal set or sin-
gle-valve receiver became a piece of
cake. You saved your pennies until
you had enough money, bought all
the bits and constructed the receiver
according to one of the many tried
and tested designs.
These days, if you try to build a
simple valve based medium wave
radio you soon hit a brick wall. All
those trusted components of the
past are hardly available anymore.
The supply transformer, output
transformer, medium wave coil, tun-
ing capacitor — try finding them.
The radio described in this article
turns a need into a virtue. It is a
combination of old and new. The
radio is built along an old design,
but using modern components. The
components are widely available
and the construction really is simple.
62
Elektor Electronics
12/2001
HF
&RADIO
The components
important, because the medium
wave band covers 550 kHz to
1600 kHz (545-187 m) and a high
self-capacitance would cut off the
high end of the scale. Decisive in our
choice was that the coils look very
impressive. This gave the radio the
look that we imagined right from the
start.
types rated at 63 V. Capacitor C10 is added
for high-frequency suppression.
Triode V1a functions as a standard grid
detector circuit. The detector circuit (LD +
C3) is connected to the grid via a small
capacitor and the grid is connected to ground
via grid bias resistor R1. The cathode is con-
nected directly to ground.
During reception the tuned circuit con-
nected to the aerial (LA + C1) induces an RF
signal in the detector circuit. Due to the space
charge in the triode the grid potential will be
about 0 V when no signal is received. With an
RF signal present, capacitor C5 will be
charged up slightly during each positive half-
cycle, because the grid functions as a recti-
fier. After several cycles the voltage across C5
will be equal to the peak voltage of the RF
signal and the grid potential follows that of
the RF signal, with a maximum of 0 V. As is
well known, the RF signals in the medium
wave band are amplitude modulated. C5 dis-
charges via the grid bias resistor (R1), caus-
ing the grid potential to follow the envelope
of the RF signal. Hence the anode current
changes in step with the modulation. The
result is a strongly amplified LF signal across
R2, along with the remnants of the RF signal.
This RF component, which is inverted
compared to the RF signal in the detector cir-
cuit, is fed via C4 and P1/P2 to a third coil
(LR). This coil is connected with the windings
opposite to that of the detector circuit,
putting the signals back in phase. This posi-
tive feedback gives rise to amplification,
increasing the Q factor of the detector circuit.
If the damping of the circuit is reduced too
As we said, HV transformers for
valves are no longer easily obtain-
able. But operation is still possible
with an ordinary low voltage trans-
former. Likewise, headphones with
an impedance of 2000
have
become a rarity. But the modern
‘walkman’ style headphone (32
) is
inexpensive and sounds much bet-
ter. In this design the anode voltage
doesn’t have to be high and an ordi-
nary low voltage mains transformer
is used. Due to the use of the low
supply voltage no isolating gap is
required in the transformer.
Tuning capacitors with air as
their dielectric give the best results,
but they are almost impossible to
find. Luckily there are low-profile
tuning capacitors with a plastic
dielectric. It is a pity that these come
with circular plates, since this gives
them a linear capacitance variation.
The kidney shaped plates often
found in the past resulted in a linear
tuning scale over the range of the
band, which was much more pleas-
ing.
The last problem parts are the
coils. After experimentation we
decided on an old but tried type, the
spider’s web coil. This type of coil is
not difficult to make, has a reason-
able Q factor and a low self-capaci-
tance. This capacitance is very
The circuit
For the receiver we’ve chosen an old
fashioned but simple direct conver-
sion design, using an ECC82 double
triode valve (
Figure 1
). This valve
has its heating filaments connected
in series with a tap for 6.3 V opera-
tion. In the series connection a volt-
age of 12.6 V at a current of 150 mA is
required. For the power supply a
transformer (T2) with two secon-
daries of 18 V/133 mA is used. The
filament supply is derived from one
of the secondaries via series resis-
tors (R7/R8), reducing the voltage to
12.6 V. The current through this sec-
ondary is slightly above its rated
value, but since the current through
the other secondary is only a few
mA this doesn’t really matter.
Both secondaries of T2 are con-
nected in series and after rectifica-
tion (D1) and smoothing (C12, R6
and C11) this produces a DC voltage
of about 50 V. The electrolytics can
therefore be standard low voltage
D1
R6
470
Ω
1N4001
A1
T1
R2
C9
T2
V1 = ECC82
500p
C4
R
K1-HS
K2
P1
P2
100p
C2
V1a
V1b
230V
100p
1k
10k
C5
C7
D
R3
10k
C10
C11
C12
100n
470
µ
470
µ
10n
100p
R7
R8
LA
LD
LR
C1
C3
R1
R4
R5
C6
C8
f
500p
500p
100p
100p
f
Gnd
012014 - 11
Figure 1. The ECC82 double triode (V1) is at the heart of the circuit.
12/2001
Elektor Electronics
63
HF
&RADIO
much the circuit will begin to oscil-
late. The receiver then becomes a
transmitter and nearby receivers
tuned to the same station will be
subjected to very irritating whistles.
So you have to take care that this
doesn’t happen. With the correct
adjustment of P1 and P2 the sensi-
tivity increases enormously, but at
the same time the bandwidth of the
signal is reduced. The high frequen-
cies disappear and the signal begins
to sound muffled.
The audio signal is fed via C7 and
R3 to the grid of triode V1b, which is
configured as an LF amplifier. C6, R3
and C8 attenuate the RF compo-
nents considerably, leaving only the
audio signal for the triode to amplify.
The transformed impedance at the
anode
use the recommended components
you’ll find that everything fits per-
fectly and it is just a matter of care-
ful soldering to complete the board.
To keep the PCB to a reasonable
size, two of the mounting holes —
diagonally opposite each other —
are underneath the transformers. On
the component side of the PCB,
these holes have to be countersunk
such that the heads of the M3
mounting screws (5mm long) are
flush with the PCB. Then all compo-
nents can be mounted, apart from
the transformers.
The holes for Gnd, D and R have
1.3 mm PCB terminal pins inserted.
Headphone connector K1 is a stan-
dard 3.5 mm stereo jack socket; the
left and right channels are con-
nected in series giving an imped-
ance of 16
K2
T2
D1
k
R
+
C11
C12
+
R2
C6
R3
K1
V1
is
about
10
kΩ
36/2]. C9 gives a slight
boost at 5 kHz, making the sound
clearer and the cathode bypass
capacitor has been left out to keep
distortion down.
[(220/9)
2
×
. K2 is a PCB terminal
block with the pins 10 mm apart.
Resistors R7 (vertical!) and R8 are
mounted about 5 mm above the
board because they become fairly
hot.
After all the components have
been mounted two standoffs
(M3
ECC 82
T1
Gnd
R1
D
C5
Construction
A simple PCB has been designed for
this radio (
Figure 2
), which contains
nearly all components. When you
10 mm) are screwed to the sol-
der side of the board with counter-
sunk screws. It’s only then that the
×
COMPONENTS LIST
Inductors:
LA,LD,LR = see inset
Resistors:
R1,R4 = 1M
Ω
R2 = 68k
Miscellaneous:
D1 = 1N4001
V1 = ECC82
T1 = mains transformer, secondary
9V 1.5VA, e.g., Gerth # 3109-1,
Display Electronics (Holland) #
02.10.159.091
T2 = mains transformer, secondary
2x18V 4.8 VA, e.g., Gerth #
421.36-2, Display Electronics
(Holland) # 02.10.459.362
K1 = stereo headphones socket, 3.5
mm, PCB mount
K2 = PCB terminal block, lead pitch
10mm
Socket, noval, 9-pin, for ECC82
Binding post (red) for antenna wire
Binding post (black) for earth
connection
2 pointer knobs, 15 x 33 mm, for
6mm spindle
2 pointer knobs, 13 x 20 mm, for
6mm spindle
Mans cord
Headphones, 32
Ω
R3 = 10k
Ω
R5 = 2k
2
R6 = 470
Ω
R7 = 68
1W
R8 = 150
Ω
0.5W
P1 = 1k
linear potentiometer
P2 = 10k
Ω
linear potentiometer
Capacitors:
C1,C3 = tuning capacitor, flat models,
500pF, (e.g., Conrad Electronics #
482323, Display Electronics
(Holland) # 02.84.500p
C2,C4,C5,C6,C8 = 100pF
polystyrene or polypropylene (e.g.,
Conrad Electronics # 458686)
C7 = 10nF, MKT (Siemens), lead pitch
7.5 or 10mm
C9 = 500pF (470pF) polystyrene or
polypropylene (e.g., Conrad
Electronics # 458767)
C10 = 100nF, MKT (Siemens), lead
pitch 7.5 or 10mm
C11,C12 = 470
µ
F 63V
Figure 2. PCB and component layout of the
induction Radio (board not available ready-
made).
64
Elektor Electronics
12/2001
HF
&RADIO
The spider’s web coils
changes side. In this way 53 complete turns should be made.
Since the number of segments is uneven this ‘changing sides’
works through the whole winding. If you keep the
wire at a reasonable tension it will almost slide into
place by itself. After 53 turns the wire should be
cut, leaving a length of 16 cm, and fed through the
hole at B from the rear of the board. The wire
from A is unwound and is given a slight kink such
that it can’t slide back through the hole; the same is
done for the wire from B. The ‘A-wire’ is the start
of the coil; the ‘B-wire’ is the end. The start and
end wires should be on the same side of the board,
otherwise there probably was a mistake with the
number of turns.
We now have three identical coils, each with an
inductance of 200
µ
H, a low self-capacitance and
(in conjunction with the tuning capacitor) a Q fac-
tor of about 60, which isn’t bad.
The coils are mounted such that they are induc-
tively coupled. At the left side is the aerial coil (LA),
10 cm to the right is the detector coil (LD) and
5 cm to the right of that is the reduction coil (LR).
To simplify the construction of the coil assembly a
wooden coil holder can be made. Three pairs of
slats (0.5 x 0.5 x 10 cm) are glued to a wooden
base, with the resulting slots 10 and 5 cm apart.
Each slot should hold a coil firmly in place. The
coils should have the B-wire at the front and facing to the right.
Three connecting rods are put through the central and top holes,
and are held in place with a drop of glue. We’ve used transparent
plastic rods because they look so good (they are available in
shops for model train accessories). Once the glue has dried, the
coil assembly can be turned round and the two rods in the bot-
tom can be put in place. The coil assembly is mounted to the
chassis using two clear Perspex strips (110 x 5 x 10 mm, l x w x
h). The photo shows the construction method very clearly.
To obtain a reasonable Q (quality) factor it is important that air-
cored coils are large and that a thick gauge of wire is used. Since
the medium wave frequencies are relatively low there is no need
to use Litz wire.
To keep the self-capacitance low in an air-cored coil it has to
‘contain as much air as possible’, which is one of the properties of
spider’s web coils (as with honeycomb coils, but these are much
more difficult to wind).
We can use three identical coils for our radio. This is made possi-
ble by varying the separation (and hence the coupling)
between the coils.
For the coils you’ll need a reel of enamelled copper wire
(0.4 mm diameter) and a board of ABS with a thickness
of 1.5 mm. These ABS boards are available in many
model shops.
Construction of the coil formers becomes easy when
you use the template shown here. You’ll need to make
three copies of the template onto tracing paper. The
copies are stuck to the ABS board with double sided
sticky tape or paper glue. In the last case the glue has to
be put onto the paper. The three rectangular coil form-
ers are then cut out of the board using a fretsaw. Then
we make cuts along the 26 lines from the sides to the
central circle. Using a screwdriver with a 2 mm head we
remove the resulting strips along the circle, leaving a coil
former with 13 segments. The paper (and glue or sticky
tape) is then removed and all sawn edges are deburred
and smoothed by sliding a knife along them.
After drilling four holes at the corners and one in the
middle (using a 3 mm drill) and cleaning with water and
soap, the coil formers are ready for use. Two holes have
to be made at positions A and B for the wires to go
through. The wire is pushed through the hole at A from
the rear of the board, leaving a length of about 15 cm
sticking out at the front. This is rolled up and pressed flat
against the centre, keeping it out of the way. The wire at
the rear of the board is then brought forward through
the slit below segment 1 and then back to the rear
through the slit below segment 2. At each slit the wire
4
3
5
2
6
1
B
7
A
13
8
12
9
11
10
012014 - 12
12/2001
Elektor Electronics
65
HF
&RADIO
with the PCB on the right. The front
panel contains the tuning capacitors,
potentiometers and the aerial and
earth connectors.
The aerial coil (LA) and detector
coil (LD) are connected directly to C1
and C3 respectively (A-wires to the
wipers). The wipers are connected
together and to the common ground.
A wire is connected to the wiper of
the detector tuning capacitor, for
connection to the ‘Gnd’ terminal on
the PCB. This wire is soldered with
the B-wire from the reduction coil
(LR) to a terminal socket, which
plugs onto the ‘Gnd’ pin. The con-
nection between the ‘hot’ side of C3
and pin D on the PCB is also made
using a terminal socket. Then the A-
wire from coil LR is connected to P1,
the wipers of P1 and P2 are con-
nected together, and P2 is connected
to pin R on the PCB again using a
terminal socket. And finally the aerial
socket is connected via a 100 pF
capacitor to the ‘hot’ side of C1.
Fig-
ure 4
gives a rear view of the com-
pleted receiver.
For the professional look and for
ease of use we’ve stuck transfers to
the front panel, with a scale for the
tuning capacitors.
Figure 3. Populating the PCB is easy.
transformers can be soldered into place. Two
standoffs (M3 × 10 mm) are then bolted
through the other holes onto the solder side of
the board with nuts and washers. The board
is then supported at all four corners.
Figure 3
shows a photo of the completed PCB.
Now for the rest of the receiver. The coil
assembly is mounted on the chassis and the
tuning capacitors, reduction potentiometers
(P1: fine, P2: coarse) and the sockets for the
aerial and earth are on the front panel.
A good starting point for the chassis is a
beech wood breadboard of about 33
1.2 cm. You need to cut off a 7 cm
piece along its length, leaving
33
13 cm. A laminated beech
wood front panel is screwed
against the cut side of the chassis.
The top corners of the front panel
should be sanded down to give a
rounded appearance. All of the
wood should then be sanded down
thoroughly and given a coating of
clear yacht varnish.
The coil assembly is mounted on
the left-hand side of the chassis,
×
Performance
During the day the author received
(using a washing line on the balcony
as aerial and a radiator as earth)
about six stations at his flat in
Weesp (near Amsterdam in The
Netherlands). The BBC came
through very clear and a station from
Brussels less so. All the Dutch
medium-wave stations came
through loud and clear. Using a mod-
erate damping reduction the result-
ing sound quality is surprisingly
good.
Using the two tuning capacitors,
the volume can be adjusted and the
bandwidth can be adjusted such
that a weaker station becomes per-
fectly audible next to a stronger local
station. In the evening and at night
the number of stations that can be
received increases tremendously.
Yo u will find that it is a special
experience to listen to this receiver.
Twiddling with those knobs brings
back a nostalgic feeling that no mod-
ern radio can provide!
×
20
×
Figure 4. Rear view of the completed receiver.
(012014-1)
66
Elektor Electronics
12/2001
Plik z chomika:
gasma
Inne pliki z tego folderu:
bge.jpg
(26 KB)
detail1.htm
(5 KB)
detail2.htm
(4 KB)
detail3.htm
(4 KB)
detail4.htm
(6 KB)
Inne foldery tego chomika:
1974
1975
1976
1977
1978
Zgłoś jeśli
naruszono regulamin