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The '6 x 6' Loop antenna;

6 turn 6ft square for balcony or garden.

by Graham Maynard                                        graham.maynard1@virgin.net

This is the circuit diagram of the impedance matching preamplifier for a broadband, six foot square, six turn loop antenna which covers 50KHz - 5000KHz.  It is a fully tested and sensitive design that was optimised for MW DXing, but which is still useful for LW and tropical band reception.

One of the problems with a tuned MW loop antenna is that any mixer generated reception pattern drifts with temperature;  ie. a cardioid null will drift away and become more shallow as the phase angle of loop transduced signal drifts.  Generally a tuned loop has compact advantage due to resonance increasing its voltage output;  to equal the signal to noise ratio and output with a broadband antenna both the wound area and amplifier gain must be increased.  The change of transduction phase angle with frequency for a larger low Q antenna is then quite gradual and very much reduced in angular variation versus frequency (as well as via incident bearing), thus not only does a generated (cardioid) pattern usefully hold well over several channels and thereby also reduce adjacent splatter interference (say to null EU signals for TA reception) but it does not drift with day-night temperature change.
 

The loop is of a size convenient for mounting on a balcony or inconspicuously against a garden fence or garage/shed wall away from household interferences. Mine was in amongst and concealed by garden shrubs with the centre tap grounded via a pipe driven into the ground. 

This '6x6' design uses the centre tap of the loop winding as an input phase splitter to balance input/output for low distortion push-pull RF amplification.  The amplifier has a medium input to low impedance output for maximise efficiency, and of course voltage interference fields are picked up equally by both windings halves to at the output transformer. These aspects provide a better signal to noise ratio than possible with a single ended or higher/lower impedance amplifier, thus providing substantial gain over the main LW + MW ranges, see the Bode Plot below.

Please note, that this design was in use long before electronic software became available, and this Bode plot has been derived for the amplifier circuit ONLY.

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