A Shortwave Loop Antenna and its Enhancement by Positive Feedback

Version 1.0.1 2005-12-27
(Version 1.0.0 2003-02-18)

Antenna Circuit


__________________
| |
| | f1: a ferrite rod with 1 cm diameter
| | L2: 2 turns around a 0.5 m x 0.5 m frame
| | L1 and L3: 5 turns around f1 in the same
| L2 | direction as L2
| | L4: 1 turn (or more if no positive feedback
| | is provided)
| | L5: 5 turns (to provide the positive feedback)
| | CV: Variable capacitor 2 - 100 pF
|_______ _______| (=> 4.7 - 12 MHz)
| |
| |
+--L1-+ +-L3--+
==|==============|== f1
|+L5+ +L4------------------ o1
|| | | | To receiver
|| | +-------------------- o2
|| | |
+|--|--CV------+
| | |
| | o5
o3 o4

Amplifier Circuit

This amplifier circuit and the coil L5 which provide the positive feedback to the antenna are optional. It may be, however, very interesting to experiment with the positive feedback and see what happens if the feedback is gradually increased until almost all energy lost from the antenna is replaced by the positive feedback.
 


o4 ---------------+-- 9V T1: A high frequency silicon transistor
| having a dc amp factor of about 50
o3 ---+-----+ | at 3 mA collector current
| | | R1: 51000 ohm
R | | R2: 1000 ohm
1 |c | RV: Variable resistor 0-10000 ohm
| b / C C1: 0.1 µF
o5 ---+---T1 1 The regeneration (positive feedback) is
\ | adjusted by RV and by the position of L5
|e | in relation to the other coils. Take care
| | about the correct polarity of L5 to get
R | positive feedback instead of negative.
2 |
| |
R |
V |
| |
+-----+-- 0

Wikipedia: Regenerative circuit