High Performance Tuned MW Ferrite Rod Loop Antennas

Dallas Lankford, 4/4/2012, rev. 4/29/2012

I gave up on small tuned ferrite rod loop antennas for the MW band over 25 years ago for the simple reason that despite my
best efforts, they were insensitive at my location when man made noise was low. Their insensitivity was not severe, and
for urban locations with higher man made noise they were probably satisfactory MW loop antennas, especially when used
inside a house. With improved FET follower and improved Norton transformer feedback amplifiers, I wanted to see if I
could do better.

At first I thought that winding the loop coil on a coil form considerably larger than the ferrite rod was advantageous, but that
turned out not to be the case. It was commonly believed 25 years ago that longer rods gave better signal to noise ratios, but
that also turned out not to be the case. The improvements I have gotten appear to be due entirely to the lower noise of the
J310 FET and the lower noise of the Norton transformer feedback amplifier. The circuit above is not exactly what I
implemented. What I implemented used a 8T:8T at the output of the push-pull J310 FET follower which fed the 11T:8T
input transformer of the Norton. I would have preferred to use a push-pull complementary J310 – J271 FET follower
because of its greater gain than the push-pull J310, but the push-pull J310 – J271 was unstable when attached across the
entire loop coil. Curious.

This is the best I have been able to do so far. I still need to build one of the amplifiers I used 25 years ago to see if he above
is actually an improvement.

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