Page 103 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Here the secondary winding of Ll is connected to the amplifier output of Q2 to

magnetically induce some of the amplified RF signal back to the antenna coil. The
secondary winding of the coil is not reversible, so positive feedback occurs only
when the windings are connected in the correct manner. If connected in any other
manner, negative feedback occurs, and the gain cannot be increased as desired.
The amount of RF signal fed back to the antenna coil is related to how variable
resistor VRl is set. The lower the resistance set to VRl, the higher is the DC

collector current of Q2, which increases the gain of the Q2 RF amplifier. Conversely,
with VRl set to a maximum resistance value, a minimum DC collector current is set
for Q2, thus setting the gain of the RF amplifier to a minimum. Thus VR1 controls
the amount of positive feedback to the antenna coil. The more positive feedback
sent back to the antenna coil, Ll, the higher is the overall gain of the system. At

some point, the RF amplifier Q1 and Q2 will oscillate when VR1 is set to a gain that
is sufficient to induce oscillation.
In Figure 7-1, Q2 also works as an AM detector by being a power detector. With a
sufficient RF signal level into the base of Q2, power detection or AM demodulation
occurs. The audio transformer Tl then extracts the audio signals (e.g.,
demodulated AM signals).

In operating a regenerative radio, one turns the regeneration control below the
threshold of oscillation and then tunes for the stations desired. Once a station is
tuned in, turn up the regeneration control until gain is increased, but back off the
regeneration as soon as an oscillation is heard (e.g., a whistling or squealing
sound).

Improving Selectivity by Q Multiplication via
Regeneration

The Q determines the selectivity of an inductor capacitor circuit. Selectivity can be

determined by the bandwidth of a tank circuit. The narrower the bandwidth of a
resonant circuit, the higher is the selectivity. One measurement of bandwidth is
determined by tuned frequency divided by Q.
Thus, if a station is tuned to 1,000 kHz and the Q of the antenna coil is 50, the
bandwidth is 1,000 kHz/50 or 20 kHz.
However, AM stations are spaced 9 kHz or 10 kHz apart depending on what part of

the world you are in. Thus a bandwidth of 20 kHz theoretically can receive two
stations that are adjacent to each other. If the antenna coil has a Q of 100, the
bandwidth of the antenna coil variable capacitor tank circuit is 10 kHz, which would
be a minimum requirement to separate channels from each other.

In some cases, a Q of 100 is achievable for a particular antenna coil. But getting a
Q factor of 200 out of an antenna coil is rare. A limitation on the Q of a coil,
antenna coil, or inductor is the internal coil resistance. For example, a typical coil
may have a coil resistance of from less than 1
n