are AM loop antennas directional or not?

Thread Starter

Vignelli

Joined Nov 13, 2023
20
I always thought AM loop antennas were bi-directional. turn it 180 degrees and you get the same thing.

until last night. I was listening to CJBC Toronto (860 AM) and discovered it wasn't. after some experimenting I determined that winding the turns on the loop toward you or away from you didn't matter, as long as the wire coming from the antenna jack on the stereo (not ground) goes clockwise, the loop receives to the left. counterclockwise and the loop receives to the right. or, swap the ANT and GROUND wires and direction is reversed.

but, WGN Chicago (720 AM) was bi-directional. so was WBAP Fort Worth (820 AM).

and then....

listening to CJBC again tonight, the antenna receives in the opposite direction. the wire coming from the antenna jack had to be wound counterclockwise to receive that station to the LEFT.

scanned a few more stations. WGN and WBAP were still bi-directional, but found a few more faint stations that were directional, and I couldn't tell which "polarity" because I don't know which direction they were from.

are AM loop antennas directional or not? why are WGN and WBAP bi-directional but CJBC is not, and why did it reverse "polarity"? do I need to call them up and tell them to stop cooking croissants with their antenna because it's messing up the signal?
 

MrChips

Joined Oct 2, 2009
30,416
There are a number of things to consider.

Radio signals are transmitted via an EM (electromagnetic) wave which consists of an electric field and a magnetic field. AM on the MW band is vertically polarized in order to broadcast omnidirectionally on a ground wave. The receiver radio‘s ferrite core antenna receives the magnetic wave and is directional. The axis of the ferrite core has to point towards the location of the transmitter for maximum reception. This is independent of the way the coil is wound.

Line of sight along the surface of the earth is about 50 miles or 80 km for signals above 3MHz. MW signals benefit from the ground wave which can travel farther by following the curvature of the earth. Chicago, Illinois is about 440 miles or 700km from Toronto, Ontario. Thus, you are not receiving the ground wave. You are receiving the EM wave after being refracted off the ionosphere. This is a phenomenon known as “skip” by amateur radio operators. This allows radio communications across very long distances, often times after multiple skips. You would receive a stronger signal with a long-wire antenna than with a loop antenna.

Did you notice that you only receive the distant radio stations at night time and not during the day? That is because the conditions for skip to occur improve at night.
 

Janis59

Joined Aug 21, 2017
1,813
Single loop antenna have brilliant sensitivity in the plane and no sensitivity along the plane, however the loop width is rather wide. Vice versa at two-loop or three loop "double quadrat" or "triple quadrat" loop antennas, where ray width may stand rather narrow, ending with 72 element loop Yagi, where the angle is like the angle-seconds wide.
Here is one example of real one-loop antenna pattern. https://i0.wp.com/bestengineeringprojects.com/wp-content/uploads/2021/12/loop-antenna.jpg?ssl=1
 
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