Topband: Why bother with the coax shield loop RX antennas?
kristinn1 at gmail.com
Mon Dec 31 08:28:45 EST 2007
After the the Stew Perry event I need to get some RX improvement on
topband. A small-space candidate is what many refer to as a "magnetic loop"
or "shielded loop" antenna. After reading lots of contradictory material on
the subject, I think W8JI has it right. If I understand correctly this
- does NOT work on the principle of favoring magnetic field over electrical
- does NOT use the outer conductor of the coax for "shielding" parts of the
The antenna consists of basically three components:
a) The outer surface of the coax shield, which acts solely as the antenna.
The antenna currents on the outer shield result in RF voltage at the shield
edges at the gap.
b) The inner surface of the coax shield. The RF voltage excites current
that runs along the inner shield.
c) The center conductor fo the coax. The current on the inner shield
induces current along the center conductor, which resonates with a
So, basically the outer shield surface (a) is the antenna that generates a
voltage, the inner surface (b) is the primary of a transformer, and the
center conductor (c) is the secondary of the transformer, which is
terminated in a capacitor. Thus, a more proper name for this antenna might
be "a coax tranformer loop".
If I understand correctly, the same current runs on all three conductors.
Now, my question is: Why bother with all of this transforming hassle? Why
not use a single wire for the loop, terminate it in a capacitor, tap off the
received energy with a small balun transformer and on to a coax to the RX?
If I would carefully construct this simple wire loop in a symmetrical way,
wouldn't I get the same balance as with the coax loop? Does the coax
configuration offer anything beyond this?
73 - Kristinn, TF3KX
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