"H. Dziardziel" wrote in message
...

It's been years so the erudite here will please set me straight.

The coil-antenna combines signal gathering with tuning and
directivity. The ferrite core just increases the antenna-coil
inductance. This results in a smaller coil for tuning frequency
coverage but less antenna (the physical coil proper) for actual
signal reception. It adds to core losses but there is less wire
loss. It makes a much smaller transformer. So, there are
several tradeoffs.

The most efficient AM band ferrite coils for external antenna use
were only about an inch and a half or so as I recall. They are
variable inductance too for tweaking. Some old radios may have
them.


You're right about the ferrite core increasing the inductance of the antenna
coil, however, it does something else as well - it provides a 'low
resistance' (in a magnetic sense) path for the signal's magnetic field to
travel through. Suppose you're facing the transmitter, and suppose the
magnetic field lines from the station's signal are horizontally oriented
(which I think is likely to be the case for a signal traveling along the
ground). This means that a particular field line would approach you from
the right and leave you going to the left (or vice versa since these are
alternating fields - it just depends on the instant when you look). Given
all of this, the MW antenna should be oriented such that the maximum number
of field lines pass through the inside of the coil (which will maximize the
current that gets induced in the coil windings). If the antenna uses a
ferrite bar, this orientation means that the length of the bar is lined up
with the magnetic field lines.

Now for the interesting part ( :-) ). If there were no ferrite bar, the
magnetic field lines would travel in fairly straight paths (in your local
vicinity), and the coil would intercept a cross-section of the field
determined by the cross sectional area of the coil. However, when a ferrite
bar is used, many of the magnetic field lines in the vicinity of the antenna
take a detour so as to pass through the ferrite bar, because travelling
through the ferrite bar is easier than travelling through the air. This has
the effect of increasing the number of magnetic field lines that pass
through the inside of the coil, thereby increasing the antenna's signal
pickup. If the bar is short, only a small number of field lines will decide
it's worth their while to take the detour. If the bar is longer, a larger
number of field lines will end up travelling a shorter distance *through
air* as a result of taking the detour and travelling through the ferrite bar
for its length. So a longer bar does work better because it creates a
bigger distortion in the local magnetic field, and, in effect, catches a
larger number of magnetic field lines and sends them through the center of
the coil.

So there you go - I may have left a few physicists cringing over some of the
terminology, but you get the general idea...

-- Stephen


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