Page 76 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

The RF transformer may be adjusted to allow matching with tuning capacitors with

various values. For example, the RF transformer may be nominally set at 330 ~H to
match with a 270-pF tuning capacitor. But the inductance of the RF transformer
(e.g., part 42IFlOO) may be adjusted to an increased inductance of 480 ~H, which
then matches a 180-pF (or 200-pF) variable capacitor, or decreased to an
inductance of 270 ~H to work with a 330-pF tuning capacitor.
So, using an RF transformer with an external loop antenna allows for flexibility in

designing the TRF radio with different values of variable capacitors. In addition, the
external loop antenna allows for moving it away from the amplifying circuit to avoid
feedback oscillation. In the past when TRF radios were built with multiple RF
stages, care had to be taken to avoid oscillation. If the receiving antenna is located
in the same area as the multiple RF amplifiers and filters, an oscillation can occur.

In the J. W. Miller Model 570 TRF radio from the late 1930s, there are four sections
of variable capacitors to form a four-stage tunable RF circuit. However, the antenna
is a long wire that is external, far away, and shielded from the four-stage RF circuit.
Thus, using an external antenna loop allows for easier design of multiple RF stages
in TRF radios.
In this book, a design using an internal loop antenna coil (e.g., ferrite antenna coil)
whose inductance matches with the tuning capacitor will be shown. Ferrite antenna

coils are used commonly because of their compactness and sensitivity. A quick
comparison between the ferrite antenna coils in Figure 3-1 and the loop antennas
with RF transformers in Figure 3-2 shows that the ferrite antenna coils produce
higher signal levels by at least twofold over the loop antenna-RF transformer

combination. Although the ferrite antenna coil is generally fixed in inductance
value, one can slide the coil to the middle for maximum inductance (e.g., 100
percent = maximum inductance value) to the end for minimum inductance (e.g., 80
to 85 percent of maximum inductance value) when matching with a particular
variable capacitor.
Improving Sensitivity and/or Selectivity via Antenna

Coils or Circuits

To receive more stations, usually a larger-area loop antenna can be made or a
longer ferrite bar or rod can be used for the antenna coil. Also, the Q or quality
factor of the coil determines the gain and selectivity (with selectivity being the
ability to reject an interfering adjacent channel). The Q is a function of the

resistance of the wire used in winding the loop antenna or antenna coil. Generally,
the lower the reSistance, the higher is the Q. Antenna coils are made, for example,
with wire of American Wire Gauge (AWG) numbers that vary from 30 to 22. Litz
wires, which consist of multiple-strand insulated wires, work better (e.g., lower
resistance at high frequencies) than uninsulated stranded or solid conductor wires.

For the amplitude-modulated (AM) band from 535 kHz to 1610 kHz, though, almost
any type of insulated wire of low DC resistance will do.