Imagine a powerful torch with a good beam. If you shine the beam out of a
window eventually the beam fades out and you cannot make the beam light up
an object. The same is true for radio waves. The further they have to travel
the weaker they get. Also if you measured the width of the beam at the torch
and then at distance away where you could still see the beam it would be
wider. Also at that point it would be less intense and the light fades as
it gets further away and it is spreading out. Alternatively the beam could
be focused into a narrow beam just like a yagi focuses the beam of radio
signal.
As a generalization radio waves spread out as they leave the aerial.
Let's consider VHF(about 144mhz) and UHF(about 433mhz) frequencies. These
frequencies act more like the torch beam than those at HF because if you
shine a torch at a pillow and have the beam just going over the top the back
of the pillow is in shadow as the light from the beam cannot reach it but
a little of the beam can be seen still at a distance. If this was your radio
signal the pillow would have severely reduced the signal just as it has reduced
the torch beam.
Can radio waves go through a wall?
Now consider shining a torch at a solid wall. Nothing comes out the other
side, but the beam would penetrate a little way into the wall even is only
less than 1mm. This again is much the same as a radio wave but they are a
little better at getting through a wall but the received signal would be
much reduced and some of the signal would be
reflected back.
Can radio waves go through a window?
Yes a torch beam travels right through. What you cannot detect is how much
the beam is reduced. It is much less than when shone at the wall. And so
with radio waves they too can go through glass much more easily. Thus VHF
and UHF can go through windows but the HF might have a bit of trouble
because it wave length is bigger it might have to penetrate the window
and part of an adjacent wall.
We can see from this that a
clear path from one point to another is much easier to get a clear signal
than having a signal say 10 or 100 times stronger and trying to go through
a dense object. This is why satellite communication is so good. There is
a clear path (for all practical purposes) between the aerial and the satellite
and therefore very low power can be used at a very great
distance. Here though the Foundation
Licence will hold you back for as far as I can see you are not allowed to
operate through satellites !!!
A hand held (with you located at ground level) in the clear can have a range
of about 3kms, but with the same output power, a mast mounted aerial will
give the same hand held a range of many many kms. In fact the higher the
mast the better and this is also better than higher power as the height helps
both the transmitted signal and the received signal performance. If you try
using a hand held indoors then your success rate will be much poorer than
using the same unit out doors.
So what are the limits of range for a simple hand held. Well on both VHF
and UHF it is considered that what you can see you can normally reach.
There will be some diffraction/refraction over the horizon but this will
not greatly increase the range under normal circumstances. If you have building
near by then the signal can be bounced off the building and reflected but
this also causes losses in signal strength and therefore cannot be relied
upon to provide access to areas not line of sight. Building
will have a screening effect on radio waves due to
their construction. Those with high metal content will have
a great propensity to screen the signals.
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