"JIMMIE" wrote in message
...
On Oct 5, 2:34 pm, Jim Lux wrote:
david boyd wrote:
Are there any websites that give the polarization for TV signals?

For the US, the FCC database tells the polarization (elliptical is
pretty common.. mostly horizontal, with some vertical component)

For instance, here's the stuff for KCBS-TV in the Los Angeles
area:http://fjallfoss.fcc.gov/cgi-bin/ws....cc/prod/eng_tv....

Horizontal pol

But channel 11,
KTTVhttp://fjallfoss.fcc.gov/cgi-bin/ws.exe/prod/cdbs/pubacc/prod/eng_tv....
has Elliptical..

The search page is
hehttp://fjallfoss.fcc.gov/prod/cdbs/p...d/sta_sear.htm

Concerning a TV station using elliptical polarization is there a
purposeful phase difference between the horizontal and vertical fields
or is it simply just dividing power between horizontal and verical
radiators with no concern as to phase relationship

Jimmie


Probably the latter, as is done around the world nowadays for VHF FM radio,
although I confidently predict someone will contradict me with stuff about
odd-order reflections changing the sense of rotation and stuff like that.
Unless the receiving antennas are circularly polarised that won't count for
much. There's a potential benefit when mobile receivers are in use, in some
cases portable receivers as well..

In the UK the term 'mixed' polarisation is used in Band II and the
broadcasters (mainly via Arqiva) don't pay extra for a specified phase
relationship, but antenna manufacturers like Alan Dick
(http://www.alandick.com/broadcast_an...roduct_004.htm) produce
mixed-polarisation and circularly-polarised versions to suit different
markets, as they would. UK terrestrial television in Bands IV/V is all
plane polarised (H from 50 main stations and V from most of the rest) at the
moment. DVB-H trials in L-Band may have used mixed (i.e. slant or
elliptical) polarisation.

Way back up the 'phone bill I posted in a thread titled 'Circular versus
linear polarisation' a link to:
http://downloads.bbc.co.uk/rd/pubs/reports/1970-35.pdf which you may find
interesting.

Chris

JIMMIE wrote:

Concerning a TV station using elliptical polarization is there a
purposeful phase difference between the horizontal and vertical fields
or is it simply just dividing power between horizontal and verical
radiators with no concern as to phase relationship

Jimmie

If equal horizontal and vertical components are in phase, you have
linear polarization. If they're 90 degrees out of phase, you have
circular polarization. If they're at some other relative phase angle,
you have elliptical polarization. (Actually, linear and circular are two
extreme special cases of elliptical polarization.)

This makes a big difference in how the waves behave. The polarization of
a circularly polarized wave rotates 360 degrees each cycle, while the
polarization of a linearly polarized wave is fixed. If you add equal
vertical and horizontal fields in phase, you get a linearly polarized
wave tilted 45 degrees relative to horizontal or vertical. If you rotate
a linearly polarized receiving antenna in that field, you'll get a
maximum when the antenna is aligned with the field, and zero when it's
at right angles to the field, that is, tilted 45 degrees the other way.
If, on the other hand, you add the linearly polarized vertical field to
a linearly polarized horizontal field that's 90 degrees out of phase,
you get a circularly polarized wave. The signal amplitude won't change
at all as you rotate the linearly polarized receiving antenna. It'll
stay at a constant 3 dB below the best response you got from the
linearly polarized field.

If the H and V fields are added with some other phase angle, you'll get
some variation as you rotate the receiving antenna, but no complete null.

Roy Lewallen, W7EL