Many others on this Yahoo Group are more qualified to discuss the theory, but I'll comment from a "hands-on" DXer point of view.
The reception pattern of a small loop is similar to a dipole antenna, as can be seen on this instructional page:
I'll appreciate corrections if I am wrong, but it's my understanding that only a theoretical, infinitely small "point" antenna (isotropic antenna
) is perfectly omni-directional (i.e., no polarization). Any antenna that has any length whatsoever, in any X-Y-Z plane, has some amount of directionality or polarization. It depends greatly on the received frequency too (the Wellbrook and Pixel loops are essentially omni-directional at mid and higher HF frequencies).
When you turn a small loop horizontal, all you a really doing is orienting skyward the direction of greatest nulling. For most DXers there is no benefit to this, but I suppose there are situations where you may want to favor a groundwave signal constantly, and seek to reject interfering skywaves on the same frequency. Perhaps a radio station wanting to do off-air monitoring of their own signal would use a horizontal small loop to reduce night time, skywave interference.
You asked about the strength of the polarization. To my way of thinking, this is another way of asking how many dB of nulling can be expected for a small loop antenna that is broadside to a signal. There are a number of factors to consider such as symmetry of the loop and the steadiness of the offending signal's arrival angle. In actual use of these small loop antennas on a rotor I've measured from 30 dB (common) to 50 dB (infrequent) nulls (on medium wave) when measured with the Perseus SDR's highly linear signal meter. If the loop can be tilted in elevation, it can be possible to null the interfering signal even further. That's why good MW receiving loops like the Radio Plus Quantum Loop (ferrite core) and the Kiwa Loop (air core) antennas can be tilted as well as turned.
Puyallup, WA USA
--- In perseus_SDR@yahoogroups.com, Werner Karn <werner.karn@...> wrote:
> Hi Guy and others,
> what is the reason for this effect ? ( polarisation ? ) and how strong is
> it approximately in average ?