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
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.
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 ìH (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:
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.
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 ìs (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 kÙ then C=100 pF, and if R=10 kÙ, 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 kÙ resistance that were serially connected, giving the total resistance of 3 kÙ. 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.
