Nominal Impedance of Loop Antenna
 

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

Nominal Impedance of Loop Antenna
 

"Al" wrote in message
...
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



probably a few tenths of an ohm. a full wave single turn loop is about 100
ohms, but very small loops have very small impedances. the reason for the
pickup coil was probably to act as a transformer to bring the impedance seen
by the receiver to a reasonable level.

Nominal Impedance of Loop Antenna
 

"Al" wrote in
:

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,

What is a nominal loop?

Though there will no doubt be some instant answers to your question, the
information you have provided does not allow calculation, anything else
is a guess.

The impedance (if you need to know it) is probably easiest found by
measurement, and it will probably vary with frequency even though you
resonate it at each frequency (assuming the capacitor you describe is
variable).

Owen

Nominal Impedance of Loop Antenna
 

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

Nominal Impedance of Loop Antenna
 

On 10 Mrz., 21:41, "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

In the legacy Reg Edwards left is a program that will tell you what to
expect from square loops. If you analyze a square loop with the same
loop area as your (presumably round) loop, you should be close. The
loop alone will look like a low resistance in series with the
inductive reactance of the loop; resonated with a capacitor, the
impedance will be much higher. The low resistance will most likely be
strongly dominated by the resistance of the wire from which the loop
is made.

Cheers,
Tom

Nominal Impedance of Loop Antenna
 

Al wrote:
"Is there a nominal impedance for a loop antenna?"

Kraus` 3rd edition of "antennas" is a good reference.

From page 207 we see the radiation resistance is proportional to the
square of the number of turns.
On page 209 Kraus gives a graph of radiation resistance versus loop
size. We know that radiarion resistance is less the smaller the loop
area. From the graph, a one-wavelength circumference loop slightly
exceeds 100 ohms.

Best regards, Richard Harrison, KB5WZI