[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: polarization question



Greg,

We commonly think of radio waves reflecting off a surface in the manner of
light.  In fact radio energy induces a weak electrical current in the
reflecting material (why reflections are stronger from good electrical
conductors, i.e. metal).  The radio wave is re-radiated by these electrical
currents in the opposite direction that they arrived from, but the
electrical phase is also reversed 180-degrees.  It is this phase reversal
that causes circular polarity to switch sense.  Linear polarized energy is
also reversed in phase but 180-degrees results in polarity in the same
plane so we do not see the effect.

This is important for those using BBQ dishes with linear polarity feeds
(say a dipole).  The dipole should be aligned in the same direction as the
bars or grids in the dish since we want to maximize the current coupling.
If the dipole were aligned 90-degrees to the grid of the dish it would
perform very poorly.  A circularly poalrized signal has a constantly
rotating electrical field so half the time it would be at right-angles to
the gird in a BBQ dish, thus this is why such a dish suffers a half power
(3 dB) disadvantage with circular polarized signals.

You question regarding refection off other surfaces is a good one.  If they
are planar then probably there will be no polarization shift.  If they are
coarse, rough, or irregular shaped (at a significant part of a wavelength)
then the reflected signal will exhibit multiple phase shifts which may
either shift the alignment or cause the signal to be randomized in
polarity.  Passing thru electrified media (like the ionosphere) can cause
significant phase shifts and resulting polarity changes (most commonly know
as the Faraday Effect).

Much design goes into antenna ranges to eliminate or reduce the effects of
multiple reflection surfaces.  Signals taking more than one path to the
receiving point may combine in certain phase combinations to both affect
polarity and strength of the signal (this is commonly called multi-path
signal).

Using a vertical antenna on HF which normally utilizes ionospheric
propagation can be received by most any antenna (vertical or horizontal)
due to the fact that the signal is subject to polarity shifts in transit.
Whereas, a point-to-point VHF antenna is quite sensitive to being set up
the same polarity as the other antenna since it encounters little to affect
polarization.  Ionospheric effects vary significantly by frequency so at
2.4 GHz, used for AO-40, Faraday effects are minimal.  At 2m and 70 cm they
are significant (one reason we use circular polarization for space
communications).

Characterizing real radio scattering surfaces or media is a complicated as
predicting weather!

73, Ed - AL7EB

At 10:06 PM 5/23/2003 -0700, Greg D. wrote:
>Hi folks,
>
>I should know this, but I'm not sure...
>
>I know a circularly polarized signal switches polarity when it is reflected 
>off, say, a
>dish antenna.  What happens to a linearly polarized signal?  Does it matter 
>if the
>polarization is parallel to the reflecting surface, or cross-ways to it?
>
>I'm wondering if a vertically polarized signal is always that way, or can 
>the
>various reflecting surfaces in our ilves make it go horizontal or someplace 
>in between.
>
>Thanks,
>
>Greg  KO6TH
>
>_________________________________________________________________
>Help STOP SPAM with the new MSN 8 and get 2 months FREE*  
>http://join.msn.com/?page=features/junkmail
>
>----
>Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
>Not an AMSAT member? Join now to support the amateur satellite program!
>To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> 

----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home