Directionality of ferrite rod antennas...

[Externet]

Hello.
Those 1MHz AM band radios with ferrite rod antennas exhibit directionality of the source/reception as used in radiogoniometer instruments, the sensitive bearing is transversal to the rod. Your face to the image :

Now, I remember tilting an AM radio on its side, its internal rod being then, vertical.

What would you expect from the directionality behavior with a vertical ferrite rod antenna ?

 

[Nigel Goodwin]

Universally 'poor', as the broadcasts are horizontally polarised - far better than the nulls ether end, but no where near as good as the peaks on the sides. 'Could', perhaps, make a reasonable compromise?, but at a 45 degree angle might be better overall?.

Pretty moot point now though, who uses AM radio these days? - even FM is pretty well dying out.

I've not listened to an AM radio this century, and not for a considerable period at the end of the previous one.

 

[danadak]

Amateur Radio guys experiencing a resurgence in AM activity.

The SWLs also active listening to a few AM commercial stations spread around the planet. No where
as many as 50 years ago, but some still there.

 

[MrAl]

Hi,

I listened to AM back when we had a really bad storm and all the electric went out for something like 4 or 5 days. Was listening for information regarding the restoration processes.
That's it though. I don't listen to AM for anything else.
I do listen to FM now and then, the jazz station we have here in Newark, NJ. I am only about 30 miles from that station. Mostly in the car though hardly ever at home.

 

[JimB]

An electromagnetic wave has two components:
An electric field, and a magnetic field.
The two fields are at right angles to each other.
The polarisation of an electromagnetic wave is determined by the direction of the electric field.

The antenna of most low and medium frequency broadcast stations is a vertical mast which is the radiating element of the antenna. The electric field from this type of antenna is in the plane of the antenna.
Hence signals from such stations are vertically polarised.

A ferrite rod antenna responds to the magnetic field component of the electromagnetic wave which is at 90 degrees to the electric component of the wave.
Hence the ferrite rod responds to the signal when it is in the horizontal position.
The radiation pattern of the ferrite rod antenna is shaped like doughnut with the rod passing through the centre hole of the doughnut.
Thus there is a deep null in the response off the ends of the ferrite rod antenna.
This makes such an antenna very useful for direction finding.

When the ferrite rod is rotated into a vertical position, there is another null in the response because the rod is at right angles to the magnetic component of the electromagnetic wave.
A receiver using a vertically mounted ferrite rod antenna will not receive LF and MF broadcast stations very well.

However, the vertically mounted ferrite rod would receive horizontally polarised signals well, for example a signal from a horizontal wire dipole antenna.

I hope that this rambling missive makes sense.

JimB