In message <[email protected]>, 'Geri' Kinzel,
DK8KW <[email protected]> writes

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 ...).

Is this not akin to microwaves in a waveguide, whereby waves longer than
a cutoff wavelength will not propagate? The cut-off frequency is:

 Fco = 2a  where a is the dimension in the direction of the electric
component.

This gives the longest wave that will propagate. If you try and send
longer waves through then only a small 'fringing' effect is observed.

IIRC it is called 'evanescent propagation'.

The long waves will of course propagate round an obstacle by
diffraction, but not necessarily though a hole.

Mike
--
M.J.Powell