Page 86 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Figure 5-3 shows one of the first tries on a low-powered TRF radio. Resistive loads

were tried initially, but because the voltage drop occurred, a higher voltage was
required (e.g., 3 volts).
The TRF radio's block diagram is rather simple, as seen in Figure 5-3. An external
antenna loop is connected to an antenna transformer Tl. The antenna transformer
receives RF energy from the antenna loop with a low-impedance drive at the low
side tap winding of Tl. The secondary winding, or the antenna transformer, is

stepped up and provides the necessary inductance to resonate with the variable
capacitor VCl to form a parallel inductance-capacitance tank circuit (LC circuit).
An amplifier consisting of Ql to Q6 must have a very high input
impedance/resistance so as not to load down the parallel LC circuit (Tl and VC1)
that generally has a high impedance of over 100 kV. For example, the input

impedance/resistance of the ampl!ifier at the base of Ql should be on the order of
500 kV or more.
The voltage gain of the amplifier then delivers sufficient AC voltage levels for
envelope detector Q7. Q7 acts very m!uch like a diode but requires less driving
current from the amplifier's output transistor to perform AM demodulation or
detection to a crystal earphone.

One should note that even though this TRF radio uses an external loop antenna, an
antenna coil such as a ferrite bar antenna coil may be substituted for the antenna
transformer and external loop antenna.
Now let's take a look at another design of a low-power TRF radio, as seen in Figure

5-3. In the figure, the antenna transformer is set nominally for about 330 IJH at its
secondary winding, with the external loop antenna connected to a tap of the
330-IJH secondary winding. Here we use a commonly available oscillator coil
(42IF100) for the antenna transformer. Other commonly available oscillator coils
may be used in place of the 42IF100, such as the 42IFll0 or 42IF300, all available
through Mouser Electronics (www.mouser.com). The 330-IJH inductance is
connected to a 270-pF variable capacitor. In this case, the variable capacitor has

twin sections of 270 pF. Thus only one section of this variable capacitor is used.
The ground terminal of the variable capacitor is always connected to its shaft. In
this way, touching the shaft while tuning has no effect on adding stray capacitance.
The 330-IJH inductance and the 270-pF variable capacitor then allow tuning from

about 535 kHz to about 1,600 kHz. One may notice that the resonant frequency of
an LC tank circuit is proportional to the square root of the inductance or
capacitance. Thus, if 270 pF resonates at about 535 kHz, then at 1,600 kHz, which
is about three times 535 kHz, we would need a capacitance of about 1/3 squared of
270 pF, or 1/9 3 270 pF 5 30 pF.
RF signals developed at the tank circuit at variable VCl typically will be around 20

mV or more with strong stations and much less with weaker signals. Therefore,