3.1.2. The Antenna
Extremely important factor for good work of simple radio receivers is
the outside antenna that has to be long enough, and in which voltages
induced by the radio transmitters will be high enough. At first sight, one
can think of using instead some modest antenna made of a piece of wire,
compensating that with supplying the receiver with amplifier strong enough
to give the end result as if much better antenna have been used. That, of
course, is not the case, since every amplifier creates noise that makes the
reception worse, if not impossible. This fact is the cause for a
radio-amateur saying that "Antenna is the best HF amplifier." The external
(outside) antenna is being made of copper wire, thick enough to resist
strong wind conditions. In the sense of mechanical strength, the best thing
is to use the litz wire (cable), i.e. a cable made of huge number of thin
threaded copper wires. There is no need to remove the wire isolation if it
exists since it doesn't represent an obstacle for the electromagnetic waves.
The length of antenna is being determined in accordance with the "TLTB" law
(The Longer, The Better ). The antenna that we have been using for testing
the receivers described herein was 6 metres long (the length of the Radio
Receivers Lab at "Tesla" highschool, where it was spread), but if you are in
position, you should make it even longer (the author has a friend whose
antenna is about 30 metres long). It should be moved away from the sources
of electrical disturbances (public electricity cables, various household
electrical devices, cars, electric motors etc.). Considering this, the best
place for your antenna should be the building roof. The wire can be crossed
between two chimneys (Pic.3.3), between a chimney and some pillar, between
two purposefully built pillars, between two buildings, a building and a pole
in the yard etc. You should, however, always keep in mind that the wire,
however strong it may be, can snap during some big storm and, in case that
happens, under

NO CIRCUMSTANCES it should fall onto electrical network cables, telephone
leads and similar, or to cause some other damage. if your roof cover is not
covered with tin, the attic could also be a good place for the antenna. You
can also stretch it between two stronger laths attached to two opposite
window frames of your apartment. If you have no other options, you can put
your antenna between the walls in your room.
The antenna must be electrically isolated from the carriers being attached
to. In amateur conditions, one can make the isolators of a piece of thick
wall plastic pipe where, acc. to Pic.3.3, an indent should be made with the
round rasp, in order for the wire not to slip away.
The antenna lead is another piece of wire which carries the signals from
antenna into the receiver. It should be isolated and placed in such a way
not to touch the walls, to be as far from metal parts as possible (gutters,
city grounding etc.).
At the end of this chapter, let's just say that in mobile receivers ferrite
antennas are being used, which we are going to talk about later.
3.1.3. The Ground
As all other sorts of ground, the ground for the radio receivers is being
accomplished by connecting the receiver ground (point Z on Pic.3.1) to Earth
over a coper wire. You can live without the ground but the reception is much
better with it though, especially considering simple devices, such as one at
the Pic.3.1-a. Water plumbing is an excellent ground (central heating pipes
are not), but it is most often inappropriate for use. There is no housewife
that would agree to have some dreadful wire stretched across the house, from
bathroom to the living room! House electrical installation's ground is
excellent, but it should be used under NO circumstances, since
life-threatenning danger from electrical shock exists. If you live on a
ground floor, and there's plain soil beneath your window you can make your
own ground by sticking a piece of water plumbing in it, acc. to Pic.3.4. The
pipe should be about 80 cm long, and on its end you should connect the
receiver ground, attaching it with a metal ring and a screw with the nut.

3.1.4. Other Components
a. On Pic.3.1-a with letters A, Z, S1 and S2 the hubs where one can
connect the antenna (A), ground (Z) and the headphones (S1 and S2) are
labelled. Since the cabinet for our radio receiver(s) is being made of
material that is the electrical isolator (plywood, plastics etc.), the
simplest metal hubs can be used, although hubs with isolation plates (for
metal plate mounting) can be found in shops more easily, but are
significantly more expensive.
b. C1 capacitor is the so-called coupled capacitor, through it the signals
from the antenna being led into the oscillatory circuit. Its capacitance
depends upon the length of the antenna, and it lies within the limits of few
pF (antenna longer than 10 m), up to few dozens pF (a couple of metres long
antenna), the optimal value is to be found through experimenting. Every
reception antenna behaves as a voltage generator, having its own internal
resistance and capacitance. Antenna's resistance damps the oscillatory
circuit and reduces its selectivity (which manifests as the "mixing" of
stations) and sensitivity (which exerts as signal strength reduction), and
antenna's capacitance reduces the reception bandwidth. More precise,
antenna's capacitance reduces the upper bound frequency of the reception
bandwith (Pic.3.2), making reception of the stations laying close to this
frequency impossible. Both these features are undesirable and manifest
themselves as less as the capacitance C1 is smaller. On the other hand, the
smaller the capacitance C1, the weaker the signal that goes through it from
the antenna, the reception therefore getting weaker. As you can see, the
compromise solution is a thing to go for, i.e. one must find the capacitance
at which the signals from the antenna won't be much weakened while
simultaneously keeping the selectivity and the bandwidth big enough. You can
start with C1 being about 30 pF. Then,
using C, tune yourself to some radio stations you can receive. If all the
stations that interest you are there, and the strongest one of them still
does not jam the reception of other stations all's well. Try then with some
bigger capacitance for the capacitor C1. The reception will be getting
louder, so do continue increasing C1 as long as it is still possible, by
changing C, to receive all the stations of your interest that can be heard
in your place, without the interference of some strong or local station. If,
however, reception of some nearby station isn't possible, smaller C1 should
be tried out. In this manner the biggest capacitance for C1 should be found,
that allows optimal reception both regarding selectivity and bandwidth. The
simplest solution is using variable capacitor for C1, its capacitance
ranging from few picofarads to few dozens pF, adjusting it to obtain optimal
reception for each station individually. During this, whenever C1 is being
changed, the receiver must be re-tuned to the station using C.
c. The coil is one of the components that cannot be bought, therefore it has
to be manufactured. Its main property is the inductance L. As an example, we
are going to take a look at how to build a coil for the MW receiver with
band range from fd=540 kHz til fg=1620 kHz. The inductance is being
calculated using the Thomson formula (being solved by L):

Where Cx denotes the so-called parasite capacitance. It comprises the
capacitance of the trimmer capacitor (its average value) that is connected
parallel to the variable capacitor C, input capacitance of the next stage of
the receiver (where the signal from the input circuit is being lead),
antenna capacitance, coil capacitance and capacitance of the connections
between the components of the input circuitry. The amount of this
capacitance is not known in advance, therefore must be assumed. Taking that
value, the coil inductance is calculated and the appropriate coil is made,
together with the input circuit. The error being made with the assumption of
the capacitance Cx is then compensated with the abovementioned trimmer
capacitor. In all our projects this capacitor had minimum capacity Cmin=12
pF. We assumed Cx=15 pF, and therefore:

We made this coil, conented it with other components from Pic.3.1 and, after
some experimenting and measurement, came upon the conclusion that its
inductance should be somewhat smaller. We uncoiled a few reels, re-checked
the bandwidth, then uncoiled some more, re-checked again, and after several
tries came up with the solution. With variable capacitor that will be
described in the following chapter, the abovementioned bandwidth is achieved
with the coil of inductance L=330 ěH (microhenries).
The coil body i.e. the body where the coil is being reeled is a
cylindrically shaped isolation material. For this purpose we have been using
the carton core of the household aluminium foil package, its diameter being
3.2 cm. The number of bends required and wire diameter are calculated acc.
to the formulas from Pic.3.5.

In order to use these expressions coil length must be assumed first. If this
length later proves to be incorrect because the wire is too thick or thin,
new length is adopted and the calculation is repeated. Let us assume that
coil length is l=4 cm. The number of reels and coil diameter are:



Since there is no wire of such diameter, we adopt the closest existing
value, d=0.3 mm. In that case the length l will be somewhat bigger, and so
will be the number of reels. After a few iterations in calculus and later
inductance check of the finished coil, we came upon the n=144 reels of
lacquered copper wire (the mark for such wire is CuL), its diameter being
d=0.3 mm.
This coil is shown on Pic.3.6. As you can see, two holes are made in coil
body (with a bodkin) and through them the wire origin is being threaded
twice. After that 90 reels are made, then a leg, then another 55 reels and
finally the wire end is again threaded twice, through the other two holes.
The leg is made by multiple twisting the wire. It is then cut, and from
these new ends about 5 mm of isolation is removed, after which they are
tinned, twisted around each other and finally, soldered (the easiest way to
remove the isolation is by burning it with lighter, then carefully scrapping
it with the pocket knife or similar). Two small pieces of wood are then
glued onto coil's ends. When the coil is being mounted into the box, they
are pasted onto its top panel, as shown in the rightmost part of Pic.3.6-b.
If you are using a coil of different diameter, you should keep in mind that
the necessary inductance for the coil which measures more than 3.2 cm in
diameter will be obtained with number of reels less than 144 and vice versa,
if the coil body is less than 3.2 cm you will need more than 144 reels.
d. Variable capacitor C is hard to find in stores, therby we have been using
in all our receivers the one that we took from a disused commercial pocket
size MW radio receiver, the one shown on Pic.3.7. On Pic.3.7-a you can see
it together with the reel with numbers that represent the frequencies,
divided by 100, on which that receiver was able to be tuned at. On Pic.3.7-b
you can see the front, side and rear views of this capacitor. Electrical
diagram is given on Pic.3.7-c. As one may notice, there are actually two
variable capacitors under the same cover, Co and Ca, and two trimmer
capacitors connected parallel to them, Cto and Cta. The dashed line shows
symbolically that the rotating plates of the variable capacitors are
connected on a common shaft, so that by turning the reel their capacitances
are being changed simultaneously. For our use, all four capacitors are
parallel connected, by joining the legs O and A. The trimmers are set to
minimal capacitance. In such way the variable capacitor is attained with
capacitance ranging from Cmin=12 pF til Cmax=218 pF.
In commercial radios that can receive both stations from AM and FM ranges,
variable capacitor shown on Pic.3.8 is being used. Four variable and four
trimmer capacitors are placed under the same cover. If you wish to use
capacitor like this in the receiver from Pic.3.1 (and in most of the
receivers described in this book), you should then connect in parallel Cto,
Co, Ca and Cta, after which you shall obtain a variable capacitor ranging
from Cmin=16 pF til Cmax=286 pF. Other capacitors from this block are not
being used.
In all input circuits (more about them soon to come), one end of the
variable capacitor is always connected to the device ground. For capacitors
shown on Pics.3.7 and 3.8 that is the middle leg, marked as G.
During the dismounting of the capacitor from the old radio, you should pay
attention not to loose the screw for the reel attachment, and two screws for
mounting the capacitor onto the PCB, since they are very hard to provide
separately.



If the reciever is being put into the box whose front pannel is made of
isolating material not thicker than 1 mm, then one 10 mm hole should be
drilled on it, followed by two 3 mm holes, as shown on Pic.3.9-a. Having
thicker front panel does represent a problem, the shaft of the capacitor
being too short to mounting the reel. In that case you will have to make the
auxiliary plate about 1 mm thick as shown on Pic.3.9-1 and then mount the
capacitor on it , acc. to Pic.3.9-b. On the front panel a round eye should
be made, its diameter being a little bit bigger than the reel. The auxiliary
plate with the capacitor should then be tightened onto this front panel with
two small screws Z1 and Z2, and the reel on the capacitor shaft with the
screw Z3 (While tightening this screw you should hold the reel with your
other hand, and not the capacitor housing). Finally, a button made of a
thick plywood can be glued to the reel. This is not necessary, but gives the
device a more sophisticated looks.
Different variable capacitors than the ones described here can also be used,
for example, an air variable capacitor described in the first issue of
Practical Electronics. The important thing for it is to have a big max/min
capacitance ratio, at least 15, i.e. Cmax/Cmin>15. While connecting the
capacitor, care should be taken to connect its rotor with the ground (as on
Pic.3.1), labeled Z, and its stator with the point 1 of the coil.
e. The diode D, capacitor C2 and headphones' resistance comprise the AM
signal detector, also called the serial diode detector. When the AM signal
of the station the receiver is tuned at is brought on its input, NF signal
is obtained on the output, its shape being the same as the envelope of the
AM signal. An example of this is given on Pic.3.10. When voltage uul is
present on the input of the detector, the voltage uizl appears on its
output. It is useful to notify that on the output, besides the LF voltage
(speak, music etc.), DC voltage Uo is also present.
The detection diode D must be of low-power GERMANIUM type, such as AA112,
AA116, AA121, 1N21, 1N34, 1N54, 1N78 etc.
Product of the capacitance C and resistance R (on Pic.3.1. R is the
headphones resistance) should be approx. equal to 50 ěs (microseconds). That
means that if you're using the bigger resistor (which is advisable, since
the detector then damps less the oscillatory circuit), the capacitor should
then be smaller. For example, if R=500 kŮ then C=100 pF, and if R=10 kŮ, C=5
nF, etc.
f. The headphones are the electro acoustic convertor that transforms
electrical signal into the sound. We have been using old fashioned
electromagnetic headphones with 1.5 kŮ resistance that were serially
connected, giving the total resistance of 3 kŮ. The receiver from Pic.3.1
will be working as better as headphones' resistance is bigger. if you're
using the crystal headphones, parallel to them you should add a resistor of
couple of hundreds of kiloOhms. There's a very simple way of testing the
high resistance headphones: Hold one end of their cable between your fingers
while rubbing the other over the surface of a big metal object, say, the
radiator. If snapping can be heard from them they are, most likely,
satisfactory.
All the components of the receiver from Pic.3.1 should be placed into some
kind of a box. That can be any box made of an isolation material (plastics,
wood etc.), big enough to receive all the components. As an example, a
receiver is shown in scale 1:1 on Pic.3.11, being placed in a box made of
plywood. The top, bottom and side panes are made of plywood that is 5 10 mm
thick. The front and the rear side are being made of some thinner material,
that allows for simple mounting of the variable capacitor. One can notice
straightaway that the box is at least twice as big as it could be. That has
been done for the sake of better visibility, and for the box to be big
enough to accept the devices that will be described later in this book.

Component joining is being done by soldering. Performing soldering onto
the hubs can represent a small problem. In order to complete this operation
successfully, you should turn the box for the part of hub where cables are
being soldered pointing upwards. Put a piece of the tinol wire through the
eye on the hub, put the soldering iron top onto the eye from upwards and
hold it like that a while, for the tinol to melt. Then add some more tinol,
until the eye is completely covered with the solder. After that, push the
wire end into the melted solder while holding it with pencers, until it
hardens and cools itself a little. The pencers remove the heat and prevent
its transfer onto the component that is being soldered.
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