When we listen to the radio ?

[mowhoong]

I have always wondered about the need to angle the radio antanna at 45
deg for FM and 90 deg for AM or SW mode.
I would appreciate if someone can kindly explain the rationale to me.
Thanks

Regards

 

[Jamie]

mowhoong wrote:

I have always wondered about the need to angle the radio antanna at 45
deg for FM and 90 deg for AM or SW mode.
I would appreciate if someone can kindly explain the rationale to me.
Thanks

Regards
Waves at some point will end up horizontal if transmitted vertically.

The lower frequencies (SW) bounce off the atmosphere and depending on
the time of day, the point of angle will be different and there for end
up coming down at different places. Hence the term Short wave and
changes angles depending on the charge activity up there..

FM, if we are to assume you are thinking of 88 MHz and up, tend not to
bounce some much and there for, you are more to a direct line of sight
which means the angle of the wave isn't some much flat any more, that
is, if the original signal was transmitted vertical to start with.

If you listen to AM at night time, you can hear stations far away
compared to the day time. This is because of the angle getting broader
due to the charge dissipating up there in the sky. Just think of playing
pool! Which is a reason why many stations by FCC rules needs to lower
their power at night time, so not to interfere with local stations far off.

In the case of a SW Low band station in your local area, it's not
going to matter much how you have the aerials laid out because of the
signal level, but if you were to do a signal check you'd most likely
do better with a vertical aerial in that case.

There are some cases where high frequencies do skip to long distances
and come down, we normally we call that ducting. In that case, the
signal could be unpredictable in its angle. But most I've talked to
seem to think it ends up horizontal.

That is about the simplest way I can explain it.

Jamie