Forums

U.S. Television Polarization

Started by Randy Yates December 3, 2019
Hey Folks,

I did some googling on this and couldn't get a satisfying answer. 

1. What polarization is used to transmit?

2. What polarization is used to receive? 

The answer to question 2 would seem to be horizontal since that is the
orientation of the antennas I've seen. But most broadcast TV antennas
are (I believe) vertical.

If I've got it right, then why do the tx/rx polarizations differ? I
would think this arrangement would lose signal.
-- 
Randy Yates
Embedded Linux Developer
111 West Main Street, Suite 201, Garner, NC 27529
http://www.garnerundergroundinc.com
Randy Yates  <randyy@garnerundergroundinc.com> wrote:

>Hey Folks, > >I did some googling on this and couldn't get a satisfying answer. > >1. What polarization is used to transmit? > >2. What polarization is used to receive?
>The answer to question 2 would seem to be horizontal since that is the >orientation of the antennas I've seen. But most broadcast TV antennas >are (I believe) vertical.
>If I've got it right, then why do the tx/rx polarizations differ? I >would think this arrangement would lose signal.
I was able to google up the folllowing: http://sbe.org/handbook/fundamentals/RF/RF-TV_Antennas.pdf According to this (mostly section 5.2.2) NTSC was originaly horizontally polarized at the transmitter (for about thirty years until the 70's), thus explaining all the horizontal TV receiver antennas. FCC then allowed an equal amount of power to be radiated with vertical polarization, such that stations could double their power and send a circularly polarized signal. Usually right-handed. And the horizontally polarized receivers did not lose signal. How much this changed for the ATSC era, I'm not sure, but probably not much, given the FCC's typically glacial pace of change. Steve
spope384@gmail.com (Steve Pope) writes:

> Randy Yates <randyy@garnerundergroundinc.com> wrote: > >>Hey Folks, >> >>I did some googling on this and couldn't get a satisfying answer. >> >>1. What polarization is used to transmit? >> >>2. What polarization is used to receive? > >>The answer to question 2 would seem to be horizontal since that is the >>orientation of the antennas I've seen. But most broadcast TV antennas >>are (I believe) vertical. > >>If I've got it right, then why do the tx/rx polarizations differ? I >>would think this arrangement would lose signal. > > I was able to google up the folllowing: > > http://sbe.org/handbook/fundamentals/RF/RF-TV_Antennas.pdf > > According to this (mostly section 5.2.2) NTSC was originaly horizontally > polarized at the transmitter (for about thirty years until the 70's), > thus explaining all the horizontal TV receiver antennas. FCC > then allowed an equal amount of power to be radiated with vertical > polarization, such that stations could double their power and > send a circularly polarized signal. Usually right-handed. And the > horizontally polarized receivers did not lose signal. > > How much this changed for the ATSC era, I'm not sure, but probably > not much, given the FCC's typically glacial pace of change. > > Steve
Hey Steve, Great reference - thank you! So circular at the TX and (usually) horizontal at the RX. But as they state, transmitting circular polarization requires double the transmit power, but for horizontally polarized antennas this extra power is not directly utilized. They go into some detail, if I'm reading it right, to explain that two circularly polarized antennas are better for rejecting unwanted reflections because the reflections "have undergone a reversal of the sense of rotation of the polarization ellipse," and so get rejected. But what about plain old horizontally polarized antennas, which it seems the vast majority of receivers use? I see no benefit, and a lot of wasted power! I am also curious to know what TX/RX polarizations are used for U.S. commercial FM broadcast antennas. -- Randy Yates, DSP/Embedded Firmware Developer Digital Signal Labs http://www.digitalsignallabs.com
Randy Yates  <yates@digitalsignallabs.com> wrote:

>spope384@gmail.com (Steve Pope) writes:
>> http://sbe.org/handbook/fundamentals/RF/RF-TV_Antennas.pdf
>Great reference - thank you! > >So circular at the TX and (usually) horizontal at the RX. > >But as they state, transmitting circular polarization requires double >the transmit power, but for horizontally polarized antennas this extra >power is not directly utilized. [..]
>But what about plain old horizontally polarized antennas, which it seems >the vast majority of receivers use? I see no benefit, and a lot of >wasted power!
I think broadcast TV stations in those days commanded as much power as the FCC would allow, measured in megawatts. They were the only game in town. They didn't have to be energy efficient.
>I am also curious to know what TX/RX polarizations are used for U.S. >commercial FM broadcast antennas.
Editing the above URL down to: http://sbe.org/handbook/ leads to the entire online excerpts of this SBE handbook, which doesn't seem to include FM Antennas. However they note that the base publication is Whitaker, _SBE Broadcast Engineering Handbook_ . Looks like it costs about $50. HTH Steve
On 4.12.19 04:28, Randy Yates wrote:
> spope384@gmail.com (Steve Pope) writes: > >> Randy Yates <randyy@garnerundergroundinc.com> wrote: >> >>> Hey Folks, >>> >>> I did some googling on this and couldn't get a satisfying answer. >>> >>> 1. What polarization is used to transmit? >>> >>> 2. What polarization is used to receive? >> >>> The answer to question 2 would seem to be horizontal since that is the >>> orientation of the antennas I've seen. But most broadcast TV antennas >>> are (I believe) vertical. >> >>> If I've got it right, then why do the tx/rx polarizations differ? I >>> would think this arrangement would lose signal. >> >> I was able to google up the folllowing: >> >> http://sbe.org/handbook/fundamentals/RF/RF-TV_Antennas.pdf >> >> According to this (mostly section 5.2.2) NTSC was originaly horizontally >> polarized at the transmitter (for about thirty years until the 70's), >> thus explaining all the horizontal TV receiver antennas. FCC >> then allowed an equal amount of power to be radiated with vertical >> polarization, such that stations could double their power and >> send a circularly polarized signal. Usually right-handed. And the >> horizontally polarized receivers did not lose signal. >> >> How much this changed for the ATSC era, I'm not sure, but probably >> not much, given the FCC's typically glacial pace of change. >> >> Steve > > Hey Steve, > > Great reference - thank you! > > So circular at the TX and (usually) horizontal at the RX. > > But as they state, transmitting circular polarization requires double > the transmit power, but for horizontally polarized antennas this extra > power is not directly utilized. > > They go into some detail, if I'm reading it right, to explain that two > circularly polarized antennas are better for rejecting unwanted > reflections because the reflections "have undergone a reversal of the > sense of rotation of the polarization ellipse," and so get rejected. > > But what about plain old horizontally polarized antennas, which it seems > the vast majority of receivers use? I see no benefit, and a lot of > wasted power! > > I am also curious to know what TX/RX polarizations are used for U.S. > commercial FM broadcast antennas.
To get the advantage of circular polarization, you'll need a circularly polarized receiving antenna e.g. a helical beam antenna (corkscrew with a reflector) or crossed and phased linear antennas. The receiving antenna needs have the same handedness in the twist as the transmitting antenna. -- -TV
On Tuesday, December 3, 2019 at 7:28:50 PM UTC-7, Randy Yates wrote:
> spope384@gmail.com (Steve Pope) writes: > > > Randy Yates <randyy@garnerundergroundinc.com> wrote: > > > >>Hey Folks, > >> > >>I did some googling on this and couldn't get a satisfying answer. > >> > >>1. What polarization is used to transmit? > >> > >>2. What polarization is used to receive? > > > >>The answer to question 2 would seem to be horizontal since that is the > >>orientation of the antennas I've seen. But most broadcast TV antennas > >>are (I believe) vertical. > > > >>If I've got it right, then why do the tx/rx polarizations differ? I > >>would think this arrangement would lose signal. > > > > I was able to google up the folllowing: > > > > http://sbe.org/handbook/fundamentals/RF/RF-TV_Antennas.pdf > > > > According to this (mostly section 5.2.2) NTSC was originaly horizontally > > polarized at the transmitter (for about thirty years until the 70's), > > thus explaining all the horizontal TV receiver antennas. FCC > > then allowed an equal amount of power to be radiated with vertical > > polarization, such that stations could double their power and > > send a circularly polarized signal. Usually right-handed. And the > > horizontally polarized receivers did not lose signal. > > > > How much this changed for the ATSC era, I'm not sure, but probably > > not much, given the FCC's typically glacial pace of change. > > > > Steve > > Hey Steve, > > Great reference - thank you! > > So circular at the TX and (usually) horizontal at the RX. > > But as they state, transmitting circular polarization requires double > the transmit power, but for horizontally polarized antennas this extra > power is not directly utilized. > > They go into some detail, if I'm reading it right, to explain that two > circularly polarized antennas are better for rejecting unwanted > reflections because the reflections "have undergone a reversal of the > sense of rotation of the polarization ellipse," and so get rejected. > > But what about plain old horizontally polarized antennas, which it seems > the vast majority of receivers use? I see no benefit, and a lot of > wasted power! > > I am also curious to know what TX/RX polarizations are used for U.S. > commercial FM broadcast antennas. > -- > Randy Yates, DSP/Embedded Firmware Developer > Digital Signal Labs > http://www.digitalsignallabs.com
I would think circular polarization wouldn't take more power: the transmitter is still only using one polarization; it's just that the polarization angle increases continuously. But then a horizontally-polarized antenna would only be able to receive half the power, so maybe the transmit power has to be doubled so that the horizontally-polarized rx antenna gets enough power? Is that why the TV antennas were polarized, to filter out multipath? That makes sense. I get a visual example of this while fly-fishing; the sunlight reflecting off the creek's surface is polarized, so by wearing polarized glasses and tilting one's head a bit one can minimize the glare and see into the water. These days, though, you can minimize multipath with equalizers (or using OFDM). That makes me wonder why they wouldn't be using polarization multiplexing these days to increase bandwidth. That is what they are doing on fiber now. The newest optical fiber modulation is DP-16QAM, where DP=Dual Polarization. At the beginning of a frame, you look at the pilot signals to do symbol/phase recovery, and in the same manner you do polarization recovery at the same time. I think the hardware just gives you two orthogonal polarizations and then you have to do a mathematical rotation of the correct angle to recover the two original data streams, so as long as the polarization changes slowly enough you can track it.
Hi Randy.
I called my local television station and spoke to an "Engineer." He said their transmitted television signal is "horizontally polarized with a 17-degree tilt."
 
Kevin Neilson  <kevin.neilson@xilinx.com> wrote:

>On Tuesday, December 3, 2019 at 7:28:50 PM UTC-7, Randy Yates wrote:
>> > http://sbe.org/handbook/fundamentals/RF/RF-TV_Antennas.pdf >> > >> > According to this (mostly section 5.2.2) NTSC was originaly horizontally >> > polarized at the transmitter (for about thirty years until the 70's), >> > thus explaining all the horizontal TV receiver antennas. FCC >> > then allowed an equal amount of power to be radiated with vertical >> > polarization, such that stations could double their power and >> > send a circularly polarized signal. Usually right-handed. And the >> > horizontally polarized receivers did not lose signal.
>I would think circular polarization wouldn't take more power: the >transmitter is still only using one polarization; it's just that the >polarization angle increases continuously. But then a >horizontally-polarized antenna would only be able to receive half the >power, so maybe the transmit power has to be doubled so that the >horizontally-polarized rx antenna gets enough power?
I wasn't clear in the above -- it's that the regulator (FCC) permitted a doubling of power if the transmission was converted from horizontal to circular polarization. I would guess that in at least the ~10 or so major markets TV transmitters were at or near their maximum regulatory power limit, and that the shift to circular (with power doubling) did in fact increase the reach to some customers. Steve
On Wednesday, December 4, 2019 at 2:27:03 PM UTC-7, Steve Pope wrote:
> Kevin Neilson <kevin.neilson@xilinx.com> wrote: >=20 > >On Tuesday, December 3, 2019 at 7:28:50 PM UTC-7, Randy Yates wrote: >=20 > >> > http://sbe.org/handbook/fundamentals/RF/RF-TV_Antennas.pdf > >> > > >> > According to this (mostly section 5.2.2) NTSC was originaly horizont=
ally
> >> > polarized at the transmitter (for about thirty years until the 70's)=
,
> >> > thus explaining all the horizontal TV receiver antennas. FCC > >> > then allowed an equal amount of power to be radiated with vertical > >> > polarization, such that stations could double their power and > >> > send a circularly polarized signal. Usually right-handed. And the > >> > horizontally polarized receivers did not lose signal. >=20 > >I would think circular polarization wouldn't take more power: the > >transmitter is still only using one polarization; it's just that the > >polarization angle increases continuously. But then a > >horizontally-polarized antenna would only be able to receive half the > >power, so maybe the transmit power has to be doubled so that the > >horizontally-polarized rx antenna gets enough power? >=20 > I wasn't clear in the above -- it's that the regulator (FCC) permitted > a doubling of power if the transmission was converted from horizontal > to circular polarization. =20 >=20 > I would guess that in at least the ~10 or so major markets TV transmitter=
s=20
> were at or near their maximum regulatory power limit, and that the=20 > shift to circular (with power doubling) did in fact increase the > reach to some customers. >=20 > Steve
Circular polarization sounds better--the reflections can still be filtered = out (since the reflection changes the handedness) and the horizontal orient= ation of the rx antenna doesn't matter. But it would probably be worse if = you are transmitting circularly-polarized signals to a horizontally-polariz= ed antenna, because the horizontally-polarized antenna won't be able to fil= ter out the reflections. This all makes me think of OAM: Orbital Angular Momentum modulation. It w= as a fad for a bit, but I haven't heard much of it for a while.
"Richard (Rick) Lyons" <r.lyons@ieee.org> writes:

> Hi Randy. > I called my local television station and spoke to an "Engineer." He said their transmitted television signal is "horizontally polarized with a 17-degree tilt."
Hey Rick, Really?!? Well that makes more sense! Man, you're too practical! -- Randy Yates, DSP/Embedded Firmware Developer Digital Signal Labs http://www.digitalsignallabs.com