September 2000 61
By Charles Kitchin, N1TEV
ere’s a low cost, simple-to-build, portable shortwave
receiver. Its design is noncritical and the receiver is
easy to get going. With it, you can receive dozens of
international shortwave broadcast stations at night
even indoorsusing a 39-inch whip antenna. This little radio is
perfect for discovering ham-band QSOs, news, music and all the
other things the shortwave bands have to offer.
Although this little receiver is quite sensitive, it naturally won’t
match the performance of a commercial HF rig, and if you’ve not
used a regen before, you’ll have to practice tuning the radiobut
that’s part of the adventure. Most of today’s experienced
“homebrewers” got their start by building simple, fun circuits just
like this one. You’ll gain experience in winding a coil and fol-
lowing a schematic. As your interest in radio communication
develops, you can build a more complex receiver later.
The little receiver requires only a single hand-wound coil
and consumes just 5 mA from a 9-V battery. At that rate, an
alkaline battery can provide approximately 40 hours of opera-
tion. The sound quality of this receiver is excellent when using
Walkman headphones. The radio can also drive a small speaker.
To simplify construction, a low-cost PC board is available from
FAR circuits.
1
You can house the receiver in a readily available
RadioShack plastic project box.
Circuit Description
Take a look at the schematic in Figure 1. L1 and C1 tune the
input signal from the whip antenna. Regenerative RF ampli-
fier Q1 operates as a grounded-base Hartley oscillator. Its
positive feedback provides a signal amplification of around
100,000. The combination of the very low operating power of
this stage, only 30 µW, with the use of a simple whip antenna
makes this receiver easily portable and prevents it from inter-
fering with other receivers located nearby. Regenerative
receivers are, after all,
oscillators. R2 controls the
amount of positive feed-
back (regeneration).
D1 and C4 comprise a
floating detector that pro-
vides high sensitivity with
little loading of Q1. The rela-
tively low back-resistance of
the 1N34 germanium diode
(don’t use a silicon diode
A Simple Regen Radio
for Beginners
Need a simple, fun projectpossibly for a Scout Radio Merit Badge? This project is
a great way to introduce kids of any age to electronics and shortwave listening.
H
In this version of the receiver, a prototype PC board is used;
it is not directly representative of the currently produced
board, although they are similar. In this view of the receiver,
the antenna has been removed. The
TUNING
capacitor is at
the left. Immediately behind the capacitor is the coil, L1.
Attached between the
TUNING
capacitor and the
VOLUME
control pot immediately beneath you can see D1, C4 and R4
as discussed in the text.
here!) provides the necessary dc return path for the detector.
VOLUME
control R5 sets the level of detected audio driving U1, an
LM386 audio amplifier. C5 provides low-pass filtering that keeps RF
out of the audio amplifier. R4 isolates the low-pass filter from the
detector circuit when the volume control is at the top of its range. The
bottom of the
VOLUME
control, R5, and pin 3 of the LM386 float
above ground so that both inputs of the IC are ac coupled. This allows
the use of a 100-kΩ
VOLUME
control; this high resistance value pre-
vents excessive loading of the detector. D5 protects the receiver from
an incorrectly connected battery.
L1 is wound on a standard 35-mm plastic film can or a 1-inch-
diameter pill bottle. C1 can be any air-dielectric variable capacitor
with a maximum capacitance of 100 to 365 pF. Total frequency
coverage varies with the capacitance value used, but any capacitor
in that range should cover the 40-meter ham band and several
international broadcast bands. If you use a capacitor with a large
capacitance range (such as 10 to 365 pF), you’ll find that selectiv-
1
Notes appear on page 64.
(12 pages)
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