Re: FM Stereo DXing - Polarisation?


Leif Asbrink
 

Hi Jose,

Indeed it is difficult with different languages.
I can read your long text, but you use "polarization"
in an undefined way that is unacceptable!

You have to be more verbose. There are two cases:

1) An antenna receiving an incoming wave of
a specific polarization.

2) An antenna receiving a signal transmitted
from an antenna with a specific polarization.

Please rewrite the posting you made explicitly stating
which of the above cases you talk about. They of course
differ by the effects of the path and any possible
phenomenon that could alter polarization (reflection
and/or Faraday rotation)

I think that you will find that your statements below
sometimes refer to case 1 and sometimes to case 2. This
is confusing and misleading. When you specify in each
case what you refer to with the word "polarization" I
think you will write something that I find correct.
The below text is not because it mixes case 1 and 2 in
a non-transparent way.

Regards

Leif







On Fri, 12 Apr 2019 12:55:34 +0200
"jose maria trueba" <josemariatrueba@...> wrote:

Leif,

Many times is difficult to put in words ideas, specially when writting in a
foreign language. I tend to write the same idea with different sentences in
order to avoid confussions but even so miss-interpretations are still
possible like this time.

Let me try again.

Maxwell works are VERY difficult to understand. Their three famous vectors
and formulae are well known but concepts are VERY complex.
Receiving with circular polarization any linear polarization transmission
at any E plane, normally either vertical or horizontal but at any other
angle too, makes us loose exactly 3dB respect to the signal strength that
we receive with the same antenna in linear configuration perfectly aligned
to the received E plane.

Isolation of orthogonal polarizations are at most around 20dB in cross
dipoles, although many times we see lower numbers and some very good
designs can get higher numbers. A perfect antenna would loose an infinite
number of dB, in practice decent antennas can only get a 20dB polariation
isolation or so. But it always are much higher than 3dB.

That is why using circular is good because we loose only 3dB, exactly
three, and we can forget which angle the linear polarizated wave had been
transmittes and at what angle is arriving to our antenna. We can measure
the signal strength arriving at our locations simply adding 3dB.

Using a 45° skewed linear polarization antenna to receive vertical or
horizontal does not result in a 20dB loss or more, nor 3dB loss, nor 0dB
loss, but in unknown number of dB loss. It depends on how imperfect our
antennas are. It is not a good idea. Simple, easy, but no good. We do loose
exactly 3dB when receiving circular, but we do not loose exactly 3dB when
receiving with a 45° skewed linear.

When a radio wave hits a perfect reflector the transmitted polarization
suffers a 90° change, as seen in common satellites parabolic reflectors. We
set a vertical dipole at the focus to receive horizontal polarizartion and
a vertical dipole for horizontal. A ccw circular for cw, and a cw for ccw,
if using circular polarization against a perfect reflector because it acts
same as a mirror.

Reflections, refractions in the propagation paths delays produced by
multipath and polarization distirtions do appear althoug they are nor a
perfect mirror. Circular antennas behave better for multipath problems.
Besides creating multipath, ionizated air layers are not good mirrors but
they do affect polarization somehow.

But when reflections are not that good, angle changes in polarization are
random, in practice impossible to predict, although we could move a linear
antenna to obtain a minimum signal strength see obtained angle and add 90°
to calculate the exact angle.

Similar but different is what we do when aligning skew in a sat dish. But
in this case there are no polarization change due to reflections but due to
our location respect ro the satellite location after some espheric
trigonometric calculations.

Mountains in my area create big problems. Moving an antenna small distance
have strong effects. This is why is good to have two or more antennas in
phase rather than a bigger single one.
Space diversity is welcome.

I have a pair of 4.5m long LFAs separated 3m side by side, to get some
space diversity having two phased beams instead one twice as long of the
same total antenna gain. I can null local noise, and even null a repeater
when working another one i n the same frequency.They are excellent.

As Michael just wrote...All this is very interesting, no doubt.


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