TRF AM Broadcast Receiver Using a Loop Antenna

Published:2012/10/22 21:53:00 Author:muriel | Keyword: TRF AM Broadcast, Receiver, Loop Antenna | From:SeekIC

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 andaudio 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 50KWstation. KNX (1070) and KLAC (570) can be heard at low levels and a few otherstations can be barely heard if I put my ear against the speaker. The receiver usesa 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, performancemight be better if the loop is rotated 45 degrees, so the frame looks like across (+) instead of an X. That way, the top and bottom sides will not be parallelto the ground, and the antenna will have more height.The antenna signal is buffered by the first emitter follower stage which presentsabout 150K input impedance. The buffer stage avoids losing much voltage from theantenna when connected to the circuit. The buffered RF voltage at the emitter ofthe transistor is rectified by the diode, and the RF component removed by thecapacitor at the base of the second transistor. This leaves only the audio signalat the base with about 5X higher amplitude at the collector. The 3 remainingtransistors form an audio amp to drive the 8 ohm speaker. The transistors usedare 2N3906 (PNP) and 2N4123 (NPN) however 2N3904 should work as well. The 100Kvariable 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 magnetinside 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 lowvolume. The complete circuit draws about 10mA from a 9 volt battery.



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