I'm not an engineer, nor even an Amateur, but I can start the answer in a simple fashion.

Essentially, the ratio of the ferrite rod length compared to its diameter determines the nulling quality. The larger the ratio, the better the nulling. A second minor factor is the length of the coil winding compared to the length of the ferrite rod. The effect is small, and generally a winding length of 1/3 to 1/2 of the ferrite rod length improves nulling a small bit by keeping the coil away from the ferrite rod cylinder-ends.

Sometimes one encounters a non-cylinder shape that is wider than thick. One can presume the circumference of the shape as an equivilent circle. Extreme cases exist, like so-called "Antenna Strips": some have widths 5 or 6 times the thickness. The reasoning is to present a large surface area to the signal received (a so-called apparent diameter). An easy test is to hold the radio antenna vertically, and tune in the station. The cylinder will not receive well, if at all. The ferrite antenna strip can be tuned due to the wide width facing perpendicular to the station.

When coupled with good rf circuitry, the front-end of a radio is greatly improved, preserving sensitivity, with a low noise-floor (that enhances the nulling effect).

Paul S. in CT

--- In ultralightdx@yahoogroups.com, "farmerik" <farmerik@...> wrote:
>
> Can any of the engineers here explain why some radios are VERY directional using the internal ferrite loops?
> With my PL-600 I can easily separate stations in different directions by only a fairly small number of degrees, perhaps 15 or 20, The 310 seems good at nulling stations too, and the 911 series are supposed to be very good at that also, as well as a few others. Many other radios cannot do this. I can't completely null out a station that close in direction, but I can make a target much louder than a pest, and switch back and forth between them. What ever causes this would be very good to incorporate in any external antenna designs, if possible. - FARMERIK
>