|
|
This is only a preview of the January 1999 issue of Silicon Chip. You can view 34 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "High Voltage Megohm Tester":
Articles in this series:
Items relevant to "Getting Going With BASIC Stamp":
Items relevant to "A LED Bargraph Ammeter For Your Car":
Items relevant to "Keypad Engine Immobiliser":
Articles in this series:
Articles in this series:
Articles in this series:
Purchase a printed copy of this issue for $10.00. |
VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
Improvements to AM broadcast
band reception; Pt.3
In our final article this month, we look at
making a practical antenna booster for AM
transistor radios. The circuit is basically a
separate broadcast-band tuned circuit.
Last month, mention was made of
the problems that occurred when antenna/earth connections were made
to the cheaper transistor radios. Often, the reception will be made worse
by these connections due to the poor
selectivity of such sets.
So what can be done to make these
sets quite useable with improved
antennas and earths? This was a
problem that exercised my mind for
quite some time. The solution turned
out to be relatively simple and very
effective.
I reasoned that if I could improve
the front-end selectivity of such receivers, their response to shortwave
transmissions would diminish, if not
completely disappear. But how could
this be done without delving into the
internals of the sets?
The answer is to connect the antenna and earth to a separate broadcast-band tuned circuit. By placing
this circuit near the set, sufficient
signal is then inductively coupled
into the receiver’s loop-stick antenna
to give a worthwhile improvement.
The tuned circuit arrangement is
virtually the same as for a crystal set
but without the detector and headphones.
A crystal set coil and tuning capacitor tend to be rather bulky, so a
ferrite loopstick antenna coil and a
small tuning capacitor were wired
You can make a simple antenna booster using a ferrite rod antenna and a
tuning capacitor to tune the AM broadcast band.
88 Silicon Chip
up instead. This was connected to
an antenna and earth and when the
receiver’s loopstick and the booster
were lined up a few centimetres apart,
a significant improvement in the performance was observed.
Measurements confirmed that the
improvement in set perfor
mance,
when used with the booster and a
reasonable antenna/earth system was
of the order of 14-20dB.
Many have been sceptical about the
performance of such a simple device
but I can assure you that it really does
work well. For this reason, I call it
the “AM Radio Reception Booster”.
It can even be demonstrated that
sitting a transistor set with a large
(eg, 200 x 13mm) ferrite rod antenna
alongside a mediocre set with a small
ferrite rod antenna will boost the
performance of the latter (provided
that the two sets are tuned to the
same station. This even applies when
the larger set is turned off. Naturally,
the improvement is nothing like that
obtained with an outside antenna and
earth attached to the booster but it
does prove that sets with bigger rod
antennas tend to be better performers.
The booster can be built into a
small plastic project box. A ferrite
rod antenna (either prewound or one
which you wind the coils yourself), a
tuning capacitor, a knob and a 2-way
screw terminal strip are all the major
parts required.
The circuit of the “deluxe” version
of the booster is shown in Fig.9.
Here’s how to build it.
First, obtain a 100mm length of
9.5mm diameter ferrite rod and wind
on 70 turns of 0.5mm diameter enamelled copper wire towards one end of
the rod. This tuned winding is tapped
at 7 turns from the earthy end.
Next, you need to wind on a bifilar
winding consisting of 15 + 15 turns of
0.5mm enamelled copper wire. This
must be spaced 20mm from the end
of the tuned winding.
To make the bifilar winding, first
put one end of two 500mm pieces
of the wire into a vyce. Place the
other ends into the chuck of a small
hand-drill and rotate the drill whilst
keeping modest tension on the wires,
until the wires are wound together
with a twist every 2-3mm. These two
wires are then wound onto the rod (15
turns) and are connected together so
that they are in series.
The junction of the start of one
winding and the end of the other becomes the centre tap, which may go
to earth in some instances. The start
of each winding is shown by a dot
on the circuit diagram. Nail polish
or other “plastic” glues will hold the
windings in position. You may care
to slip the first and last turns of each
winding under the adjacent one to
make it just that bit firmer.
One of the accompanying photographs shows a couple of variations
of the booster. If you are using a
plastic case, the ferrite rod can either
be glued in position or tied to the lid
using short lengths of spaghetti sleeving (this passes through holes drilled
in the lid). The tuning gang needs
to have a maximum capacitance of
300pF and is attached to the lid using
machine screws.
The commonly available twin-gang
plastic capacitors are quite suitable
for this job, if both sections are
paralleled and the trimmers set at
minimum capacitance. A few more
turns may be required on the tuned
winding if one of these is used, in
which case the antenna tap should
also be moved up the winding. Make
sure that a knob comes with the capacitor otherwise it will be difficult
to find a knob to suit.
If you really want to
keep costs down, you
can make a booster
using the parts from
a defunct AM pocket
portable transistor set.
could also be connected across these
two terminals. However, better results
with a loop may be obtained by using
the “Ant 1” and “Ant 2” terminals
at the top of the diagram. An earth
is optional but in a noisy situation
may give sufficient improvement to
be worthwhile.
With the booster connected to the
antenna and earth, move it close
to a transistor radio and adjust the
tuning knob for an improvement in
the received signal. Initially, the set
and the booster can be close together
while you adjust the tuning. However,
if your antenna system is large, the
amount of signal coupled into the set
from the booster may be enough to
cause overload. If this happens, just
move the booster away from the set.
Make sure that the booster is oriented for best performance - the loop
Testing it
Having assembled the AM Radio
Reception Booster, now is the time
to try it. The deluxe version gives the
user several options for obtaining the
best noise-free reception.
First, the booster may have an ordinary antenna and earth connected
to the terminals shown on the bottom
of the circuit in Fig.9. A loop antenna
Fig.9: the circuit of the AM Radio
Reception Booster.
stick in the receiver and the booster
should both be horizontal.
Although the deluxe version gives
the user a variety of options, it is usually not necessary to go to that amount
of trouble. For example, instead of
winding your own ferrite coil, try using a prewound ferrite loop antenna.
These have four wires coming out of
the windings and the pair with the
greatest resistance, as measured using an ohmmeter, are attached to the
tuning capacitor. The other two wires
go to the antenna system.
Adjust the coil on the rod so that
complete coverage of the broadcast
band is achieved (you may also have
to connect both sections of the tuning
gang in parallel). Note that the perfor
mance of this simple version will not
be quite as good as the deluxe version.
If you really want to keep costs
down, you can make a booster using
the parts from a defunct AM pocket
portable transistor set. Open up the
set, remove the speaker and the battery carrier, and mount a terminal
strip near the ferrite rod antenna.
Next, undo the PC board mounting
screws so that you have access to the
antenna leads where they connect
to the base circuit of the converter
transistor. Unsolder these and connect them to the new terminal strip.
Finally, reassemble the set, connect
the external antenna system to two
terminals on the front (or back) of the
set as shown in the photo, and your
booster is complete.
This is surely one of the most inexpensive methods ever to improve
radio reception. It costs just one
terminal strip and two self-tapping
screws, plus a defunct set that you
SC
already own!
January 1999 89
|
