Al wrote:
Is there a nominal impedance or approximate impedance for a loop antenna?
For example, a dipole is considered 72-ohm, a bevarage 450-ohm. What is a
nominal loops Z?

I have a 4-1/2 foot diameter loop antenna with 12 turns paralled by a
capacitor for the antenna and one additional turn for the pickup coil. The
loop is for the 200 - 500kHz band. About how much is the impedance of such
an antenna? Thanks.

Al

Hi Al,

You've described what SWLers call a "box loop" antenna. Its notable
characteristics are deep nulls in the radiation pattern that are 180° apart,
and excellent selectivity. The multiturn loop coupled to a loop with much
fewer turn(s) of wire provides the high Q. Treat the two loops as a
transformer. A one-turn secondary might seem to indicate a very low output
impedance but, in theory, the parallel resonant primary winding appears as
an infinite impedance. Practically speaking, it will be some high value.
However, the feedpoint impedance will be a function of the square of the
turns ratio. In this case, 12^2 or 144:1. Not knowing the electrical
characteristics of the primary loop, it would be a crapshoot in determining
the feedpoint impedance.

There are stand-alone antenna analyzers available for a few hundred dollars
US. If you have a general coverage receiver, a noise or antenna bridge may
be usable and cost much less. MFJ (http://www.mfjenterprises.com/) is one
manufacturer. I have used a MFJ202B noise bridge, and find it useful.

Bryan