Hi Mike -

| The receiving (and transmission) patterns of ferrite rods are similar to a
| dipole. In actual fact, the field pattern of a ferrite rod antenna is at 90
| degrees to that of a dipole if both antennae were mounted with their long
| axis parallel to one another. In practice, the influence of nearby buildings
| and structures probably distort the pattern so much that the ferrite antenna
| becomes pretty much omnidirectional.
|
| You are absolutely right about the low impedence of small transmitting
| antennae. Matching 50 ohms to perhaps 1 ohm or less will always be a major
| problem. The low impedences, lead to high currents and high resistive
| losses, which result in the ferrite heating up as well as the
| interconnecting wiring.

There is a difference between ferrite heating because of ohmic losses in the coils, and
ferrite losses. Both will rise temperature in ferrite rod and coil.


|
| The miracle whip type of antenna, uses a toroidal autotransformer
| transformer which happens to be wound on ferrite to reduce the size of the
| unit for portable operation. There are other ways to transform impedence to
| get a match to a 54 inch whip antenna. A decent transmatch ATU will load up
| just as well, but makes for a much bigger unit to carry around.

Where can I see an explanation of the miracle whip antenna? Mechanical construction.


|
| The ferrite rod is used to concentrate lines of flux within the wire coil
| winding around the rod. While this works well in a receive situation, as I
| understand it, the situation is effectively reversed when transmitting. The
| coil of wire concentrates the transmitted signal within the ferrite rod
| core. Because of the way ferrite is made up, it becomes extremely lossy when
| saturated with RF and most of the energy is dissipated as heat. The higher
| the frequency, the more energy is absorbed by the ferrite.
|
| As losses are lower at lower frequencies, I would anticipate that a workable
| transmitting and receiving antenna could be produced for use on the 160 and
| 80 metre bands, with the possibility of being able to use this type of
| antenna on 40 metres. Certainly ferrite can be used to make a receiving
| antenna for use up to 7MHz or so.

Yeah. At about 2MHz there is a first corner frequency for ferrits.


| Having a good earth connection makes a world of difference to the way these
| miniature antennae behave. Water and central heating pipes work okay, but
| you may pick up quite a lot of local electrical noise as well. Be prepared
| to hunt around for a decent quiet earthing point.

Mike, I don't have any earth. That is a portable design.

regards -
Henry