TRF AM Broadcast Receiver Using a Loop Antenna






The TRF radio above operates on the AM broadcast band in the range of 550-1650 Khz.
It's similar to a crystal radio with the addition of a couple buffer stages and
audio amp to drive a speaker. Reception is limited to strong local stations. KFI
(Los Angeles) comes in loud and clear at a range of about 7 miles from the 50KW
station. KNX (1070) and KLAC (570) can be heard at low levels and a few other
stations can be barely heard if I put my ear against the speaker. The receiver uses
a 16 inch square loop antenna wound on a wooden frame (13 turns #18) with a variable
(30-365pF) capacitor. Larger loops would probably work better. Also, performance
might be better if the loop is rotated 45 degrees, so the frame looks like a
cross (+) instead of an X. That way, the top and bottom sides will not be parallel
to the ground, and the antenna will have more height.

The antenna signal is buffered by the first emitter follower stage which presents
about 150K input impedance. The buffer stage avoids losing much voltage from the
antenna when connected to the circuit. The buffered RF voltage at the emitter of
the transistor is rectified by the diode, and the RF component removed by the
capacitor at the base of the second transistor. This leaves only the audio signal
at the base with about 5X higher amplitude at the collector. The 3 remaining
transistors form an audio amp to drive the 8 ohm speaker. The transistors used
are 2N3906 (PNP) and 2N4123 (NPN) however 2N3904 should work as well. The 100K
variable resistor shown is only used to reduce the volume of very strong stations,
such as KFI in my case. The speaker is a 4 inch model with a heavy 4 inch magnet
inside a 6 inch box. Seems to be more efficient than speakers with smaller magnets.
Headphones work better and I can hear 8 stations in the Los Angeles area at low
volume. The complete circuit draws about 10mA from a 9 volt battery.

Loop Antennas:

A loop antenna can greatly improve medium wave reception. Loop antennas are
directional and receive signals along the plane of the windings. The directional
quality improves signal to noise ratio of the desired signal while rejecting
signals perpendicular to the plane of the windings. Larger loops are better than
smaller ones but good results can be obtained from moderate sizes of one or two
feet on a side. The shape doesn't make much difference so the loop can be circular,
rectangular or a triangle shape. The main idea is to cover as much area as possible,
so I imagine a circular loop would be the best. The loop pictured here measures
16 inches on a side and is about 1.5 inches wide. It was wound with 13 turns of #18
copper wire, and has a unloaded Q of about 300 at 600 KHz and a loaded Q of around
80 when connected to the circuit. The loop is tuned with a variable 30-365 pF capacitor
in parallel.

More detailed information on loop antennas can be found at:

 AM Loop Antennas

A calculator for finding the length of wire needed for various
rectangular loops can be found at:

Loop Antenna Calculator - By Bruce Carter 

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