good question

[Andre Kesteloot]

"'Geri' Kinzel, DK8KW" wrote:

> Hello LF group,
>
> I went to Holland recently, and listened to Deutschlandradio on 153 kHz. I
> have already noticed years ago that mediumwave and longwave signals are
> being attenuated or even completely faded out under bridges. Now, listening
> to that radio transmission not too far away from our 136 kHz band I asked
> myself some questions:
>
> Why is the signal faded out, even if the bridge over the highway is only a
> few meters wide? The wavelength is almost 1000 times larger than the
> obstacle. What does this tell us about the radiation pattern of the
> Deutschlandradio? Does it really mean that the waves are coming
> perpendicular from above (which seems to be a common explanation for this
> effect)? Why does this effect only occur on LF and MF, but not on HF (I
> have checked for example transmissions on 14 MHz, where I can see no effect
> at all under bridges, altough the wavelength is much smaller, and the
> obstace compared to the wavelength is much larger)? Could any similar
> vertical obstacle (e.g. a chinmey, etc) also lead to such an attenuation,
> for example if that chimney is in the direction of an LF station I want to
> hear (I have a power plant with a 200m high chinmney only 4 km = 2 x lamda
> away, 270 degress from here ...).
>
> Well, maybe some stuff for discussion ... or maybe a very simple technical
> explanation.
>
> Vy 73
>
> Geri, DK8KW (W1KW)