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Old 2010-03-21, 10:10 AM   #1
jbkeh
 
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Default Doesn't Polarity Count In Gain Measurement Anymore?

I've noted that most measurements of gain, be they 'raw' or 'net', are based on Total Gain, rather than Horizontal Gain. (Or better yet, separate measurements of both.)

Has polarity become immaterial?

This question was triggered by the Popular Science "V" antenna, where the 'problem' of it having a narrow horizontal beamwidth was 'solved' by rotating it 90 degrees. Since this antenna appears to be highly loaded to one polarity, does not this rotation destroy the antenna's effectiveness?
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Old 2010-03-21, 10:24 AM   #2
holl_ands
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Most antennas have very little response to Vertical Polarization, hence Total Gain = Horizontal Gain.
Some notable exceptions are RabbitEars (at an angle), Monopole Whips and Loops.

In Dual Vee antenna Total Gain = Horiz. Gain when whiskers point to the sides. When it is
rotated onto it's side (the Hi-VHF example), Total Gain = Vertical Gain. There is very little
cross-polarization (mis-matched) response. BTW: SWR is best (<2.5) matched to 150-ohms.

Preparing for deployment of ATSC-M/H (Mobile, Handheld) waveform, more and more stations
are adding another transmitter and antenna in order to transmit both Horiz. and Vertical...
either EP (Elliptical Polarization) or CP (Circular Polarization) when equal power.
In the U.S., they are permitted to transmit the SAME power in Vert. as current Horiz.
Fol. website maintains a Spread Sheet showing EP/CP status for U.S. stations:
http://www.rabbitears.info/ss/

Even if a station is still transmitting Horiz. (only) at the receiver, signficant power may also be
present in the Vert. Polarization....due to reflections. Indoor RabbitEars have exploited this crude
Polarization Diversity for many years.....
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Old 2010-03-21, 11:34 AM   #3
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Quote:
I've noted that most measurements of gain, be they 'raw' or 'net', are based on Total Gain, rather than Horizontal Gain. (Or better yet, separate measurements of both.)

Has polarity become immaterial?
Heck no, heh. Thats why 3D radiation patterns are so nice, they allow for seeing the full picture. An example is when a GH goes over channel 51. The gain dips a lot dead straight ahead, but 20 degrees up its much higher.
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Old 2010-03-21, 07:51 PM   #4
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Quote:
Preparing for deployment of ATSC-M/H (Mobile, Handheld) waveform, more and more stations
are adding another transmitter and antenna in order to transmit both Horiz. and Vertical...
hmm, would it make any sense then to try and exploit that at the receiver end? i.e in a dense urban area, with lots of tall buildings, would seem to me
that a cross polarized receive antenna may be beneficial in combating multipath. Are there any such examples of commercially available cross polarized TV antennas? Ignoring the rabbit ears of course.
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Old 2010-03-21, 08:17 PM   #5
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I believe the diamond shaped centipede type antenna designs can work vertical or horizontal. So, would they be considered cross polarized antennas that would work for TV & mobile ?
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Old 2010-03-22, 03:55 PM   #6
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Centipede-2 Vertical Gain is less than -78 dBi (hit "M" key in PATTERN box).
So it responds exclusively to Horizontal polarization....rotate 90-deg to receive Vertically polarized.
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Old 2010-03-22, 04:05 PM   #7
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Quote:
Originally Posted by majortom View Post
hmm, would it make any sense then to try and exploit that at the receiver end? i.e in a dense urban area, with lots of tall buildings, would seem to me
that a cross polarized receive antenna may be beneficial in combating multipath. Are there any such examples of commercially available cross polarized TV antennas? Ignoring the rabbit ears of course.
Circularly Polarized antennas:

Crossed Yagis (Quarter Wavelength feeder bandwidth problem):
http://sv1bsx.50webs.com/antenna-pol/polarization.html
http://broadcastengineering.com/test...ge-part4-0515/

Helix/Helical (no special feedpoint required)....also search this forum:
http://www.wade-antenna.com/Wade/CircularHelical.htm
http://scholar.lib.vt.edu/theses/ava...ts_etd_CH2.pdf

Spiral (no special feedpoint required), such as W-G SquareShooter:
http://www.wpnc.net/fileadmin/WPNC06...unications.pdf
http://scholar.lib.vt.edu/theses/ava...ll_etd_Ch2.pdf
http://www.ece.uci.edu/rfmems/public...C076-APS03.pdf
http://wbilljohnson.com/zmvantenna/zmvspiral.htm

Quad Zig-Zag LPA:
http://astro.berkeley.edu/~greg/ieee5.pdf
http://ceta.mit.edu/PIER/pier78/20.07090801.Li.SZL.pdf

I posted plans for a (Dual) Wedge Zig-Zag LPA for Hi-VHF here:
http://imageevent.com/holl_ands/zigzaglpa/zigzagnoboom
Quad with Circular Polarization would have another pair of LPA's, except
rotated 90-degrees about the line of fire (X-axis)....and figure out how
best to arrange the feedline(s) and other parameters to minimize SWR.
UHF version would be about 3:1 smaller....incl. wire size....
Fortunately, Zig-Zag LPA's tend to have HIGHER impedances, so it's
easier to simply join the H and V feedlines to match 300-ohms.

Numerous other examples:
http://www.digitalhome.ca/forum/show....php?p=1072535

====================================================
Whether it is worthwhile to employ Circularly Polarized (CP) antennas for reception is still
unresolved....simply because not enough people (or companies) have tried it:

This 2004 "survey" of CP usage (to unspecified Horiz. outdoor & indoor RabbitEars?) found
an overwhelming improvement to ANALOG reception:
http://ieeexplore.ieee.org/Xplore/lo...hDecision=-203

DTV test by WRAL in 1999 showed CP wasn't worth it to EXISTING indoor/outdoor antennas:
http://www.tvtechnology.com/article/67946
http://cat.inist.fr/?aModele=afficheN&cpsidt=1845203

CP (and higher power) helped WFAA in Dallas:
http://www.tvtechnology.com/article/82640

Propagation of Vertical signals are (sometimes?) better through trees:
http://www.tvtechnology.com/article/11500

Note that NONE of these tests tried a CP receive antenna!!!!!

A test showing CP was superior for HANDHELD Mobile TV Reception:
http://broadcastengineering.com/RF/d...olutions-0210/
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