Theory Discussion: Circular Polarization Vs. Vertical Polarization

FM does it with frequency!
Paul678
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Paul678 » Mon Nov 07, 2016 2:59 pm

Dale H. Cook wrote: For example, a stereo multiplex signal has the same transmitter power as a mono signal or even an unmodulated carrier, contrary to what one poster said.
That's true, but a stereo signal will have to turn down it's modulation level to avoid over-deviation, because it's FM Bessel function bandwidth will be larger compared to a mono signal:

http://transmitters.tripod.com/stereo.htm

"A Stereo transmitter has to transmit various parts of the MPX signal and has to be set such that it does not over-modulate on the sum of all the MPX signal. A Mono Transmitter would be set up to allow maximum allowed deviation on the peaks of the L+R signal it is transmitting. The Stereo Transmitter would have to be turned down a bit as it also needs to consider the deviation caused by the L-R signals. Hence a Transmitter signal will travel less distance in Stereo than when it is transmitting in Mono. Put another way, if you need to start transmitting in Stereo, you need a more powerful transmitter.

A Stereo transmission tends to sound noisier that a Mono signal. This is mainly caused by the noise in the L-R channel. As the noise in the 23 KHz to 53 KHz segment is also brought down to the audible 0-15 KHz region by the decoding process, we now have more noise than receiving the same signal in Mono. The decoder circuits in the receiver also contribute extra noise. On top of that, as MPX signals have more bandwidth than a Mono signal, the station has to use less modulation with MPX to remain in the deviation limits. All above tend to increase the noise."

Paul678
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Paul678 » Mon Nov 07, 2016 3:49 pm

TPT wrote: Now back to the LPFM example...the approach is the same. LPFM's are limited to 100 watts at 30 meters AHAAT. While the rules do not expressly state this, the contour distance for an LPFM is 5.6 kilometers, or 3.5 miles for the 60 dbu/1 mv/m contour. LPFM's do not have a "protected contour" as a secondary service. Increase the AHAAT of the antenna --to 60 meters or just under 200 feet--then the LPFM must reduce power to 24 watts to keep the reference contour the same.

So...an LPFM can not operate in any plane with more than 100 watts, and those with high antennas will be licensed at a reduced power, to keep this coverage contour the same.
Yeah, I spoke with a broadcast engineer buddy of mine (different guy) who claims the LPFM rules are 100 Watts at 30 meters, and when you reach the limit of 450 meters AHAAT, at that point it's one Watt! So the FCC rules appear to be not just limits on ERP, but also an anticipation of FM line-of-sight interference, and multi-path fading and interference.

And while he also recommends circular polarized antennas for LPFM, he also understands how costly they can be, and he has set up vertical only antennas for LPFM stations, using a very cost-effective broadband antenna:

http://www.bext.com/broadcast-antennas/mdr1/

So he feels that if your main target audience is driving around in cars, that a vertical-only signal will be sufficient. He only recently spent the extra money to go full circular for his own LPFM station, when he got a permanent antenna location, and realized many of his listeners were listening to his station indoors.

taylorengineer
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by taylorengineer » Mon Nov 07, 2016 3:53 pm

There is a noise penalty when operating in FM stereo as opposed to mono. Yes...a more robust signal is needed for full quieting. But as others have explained, the FCC grants power levels based on separation; you accept the coverage limitations and try to maximize S/N by limiting dynamic range to some extent. (Some stations are a constant +/- 75 and vary no more than a db or two.)

taylorengineer
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by taylorengineer » Mon Nov 07, 2016 3:57 pm

Circular polarization is very important for mobile listening. Not all car antennae are vertical and the penalty is up to 20 db if you're not polarized correctly.
The difference is only a few dollars to get a ring antenna. This is a great illustration of "penny wise, pound foolish." Your antenna IS your signal...this is no place to pinch pennies!

Ray
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Ray » Mon Nov 07, 2016 4:39 pm

Good discussion,

I think polarization mismatch loss is a big issue. This is really a RF link budget problem that is not simple. If the mobile antenna or line-cord antenna is oriented with the E-field orthogonal to the incoming linear pol wave, the pol mismatch loss can be very large (10-20 dB). Multi-path complicates this significantly. A RHCP wave that that is reflected off of a perfect conductor becomes a LHCP wave and vice versa. Using real earth, the system becomes complex and low grazing angles and the reflection coefficient from the ground becomes different between vertical and horizontal pol.


Ray

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Deep Thought
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Deep Thought » Mon Nov 07, 2016 4:43 pm

Slab Bulkhead wrote:I think some of those Moapa stations (the class C Vegas rimshots) are H-Pol just to get the extra antenna gain and get the TPO requirement down. I recall reading that the site they were using had no utilities and was run on diesel generator power, so there would be a definite advantage to getting the TPO down there.
Those transmitter sites exist solely to provide a 60 dBu contour on paper that an on-channel booster can live inside closer to the population. The last thing they want is the main to provide a decent signal into the Las Vegas valley.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

w9wi
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by w9wi » Mon Nov 07, 2016 9:56 pm

I count fifteen FM stations in Nevada with zero vertical power. Ten of them have boosters, usually in either Las Vegas or Reno. That would boost Deep's suggestion, that these transmitters are not designed to reach any audience. They exist solely for legal reasons, to justify boosters that actually reach listeners.

I note that every one of the ten boosters is circular.

The impression there are an unusual number of these horizontal-only stations in Nevada does seem accurate. 14.7% of the FM station license records in the FCC DB for Nevada specify no vertical power. In most other Western states the figure is closer to 5%. (it's 9.5% in Utah - which one might expect given the fair number of boosters serving Salt Lake City)
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Doug Smith W9WI
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Slab Bulkhead
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Slab Bulkhead » Tue Nov 08, 2016 7:54 am

True, I even think some of them have the boosters also licensed as aux facilities or have occasionally had STA's that requested the booster be treated as an aux.

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Dale H. Cook
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Dale H. Cook » Tue Nov 08, 2016 9:06 am

Paul678 wrote:A Stereo transmission tends to sound noisier that a Mono signal.
That depends upon many factors, including the received field intensity, the terrain and its effect upon multipath, the receiver in use and how well it handles multipath in stereo and mono modes, and the polarizations of transmit and receive antennas.

Why are you so hung up on Bessell functions? You sound like a man whose only tool is a hammer and to whom every problem looks like a nail. Have you ever made practical use of a Bessell function in broadcasting, say, by using it to calibrate an FM modulation monitor?
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by rich wood » Tue Nov 08, 2016 9:54 am

Howdy,
Historically most TV stations broadcast in HV only for the the roof top antenna reception. Now with the digital transmission, they are finding circular punches through the ambient noise inside the buildings for the smaller "wall or window" mounted antennas.
I agree that Circular is the way to go with FM radio regardless of the TPO or ERP. The more real estate you can cover the better the chance of the station being received. Multi path in a low power??? I am not sure they push enough signal to create a bounce wave.

Paul678
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Paul678 » Tue Nov 08, 2016 4:42 pm

Dale H. Cook wrote: Why are you so hung up on Bessell functions? You sound like a man whose only tool is a hammer and to whom every problem looks like a nail. Have you ever made practical use of a Bessell function in broadcasting, say, by using it to calibrate an FM modulation monitor?
I'd rather have a hammer, than a poop scooper! :lol:

The Hewlett Packard spectrum analysis application note 150-1 is kind of my Bible for FM, and it emphasizes the Bessel function.
And yes, it definitely covers the carrier nulls, or "carrier zeros" and the corresponding modulation indexes.

But I've never personally owned an FM modulation monitor. Can you recommend a reasonably priced one, with good features?

What do you think about this page:

http://www.thebdr.net/articles/rf/ctl/W ... onitor.pdf

Paul678
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Paul678 » Tue Nov 08, 2016 4:50 pm

w9wi wrote: The impression there are an unusual number of these horizontal-only stations in Nevada does seem accurate. 14.7% of the FM station license records in the FCC DB for Nevada specify no vertical power. In most other Western states the figure is closer to 5%. (it's 9.5% in Utah - which one might expect given the fair number of boosters serving Salt Lake City)
So is this because Nevada is the country's most mountainous state? Or was my first buddy BS-ing me?

If RHCP waves reflecting off perfect conductors become LHCP waves and vice versa, then wouldn't circular polarization multi-path
problems be even worse in the cities, where you have tall metal buildings?

Sounds like my friend was talking out of his you-know-what.....

:wink:

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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by R. Fry » Tue Nov 08, 2016 5:34 pm

... If RHCP waves reflecting off perfect conductors become LHCP waves and vice versa, then wouldn't circular polarization multi-path problems be even worse in the cities, where you have tall metal buildings? ...

Not if an RHCP receive antenna is used, as it tends to reject incoming c-pol waves having the opposite polarization sense.

Ditto for the vice-versa condition.

Theory supports this, as well as the consultant tests made in the 1970s using a mobile, analog receive setup with various c-pol and linear-pol receive antennas when observing the analog c-pol transmissions of WLS-TV (Ch 7, Chicago) at various locations in the Chicago Loop.

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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by w9wi » Tue Nov 08, 2016 10:10 pm

Image
Paul678 wrote:
w9wi wrote: The impression there are an unusual number of these horizontal-only stations in Nevada does seem accurate. 14.7% of the FM station license records in the FCC DB for Nevada specify no vertical power. In most other Western states the figure is closer to 5%. (it's 9.5% in Utah - which one might expect given the fair number of boosters serving Salt Lake City)
So is this because Nevada is the country's most mountainous state? Or was my first buddy BS-ing me?

If RHCP waves reflecting off perfect conductors become LHCP waves and vice versa, then wouldn't circular polarization multi-path
problems be even worse in the cities, where you have tall metal buildings?

Sounds like my friend was talking out of his you-know-what.....

:wink:
Legal reasons. See the image. (assuming it works :) if not, try http://www.w9wi.com/images/KRRN.jpg )

We're looking at KRRN, 92.7MHz in Las Vegas. Legally, KRRN is NOT a Las Vegas station -- it's authorized to serve Moapa Valley, Nevada.

KRRN cannot locate its transmitter any closer to Las Vegas than it has. The lime green circle on the map is the interfering radius of the KRRN transmitter -- the area in which its signal is strong enough to interfere with stations KOMP (92.3MHz) and KYMT. (93.1MHz) The red circle is the area in which KOMP and KYMT are protected from interference. Note that the circles touch but don't overlap. Note the location of Las Vegas on the map -- at the outer edge of the area reached by the KRRN main transmitter. Note that there isn't much population within the green circle, except in the southwestern corner.

The irregular blue area is the area served by KRRN's booster. Note that this encompasses nearly all of the city of Las Vegas -- and not much else. Note mountains northeast of Las Vegas: the signal from the main KRRN transmitter is probably considerably weaker in the city than the green circle suggests. (if it wasn't, the booster would interfere with the main)



My point is.. most of the area covered by the booster is populated. Most of the area covered by the main transmitter isn't. The main transmitter must be located where it is because of FCC regulations. (I'm sure KRRN would rather its transmitter be located on the same mountain as KOMP and KYMT, but the regulations say otherwise) A booster won't be licensed unless it has a main transmitter to boost. (I'm sure KRRN would rather not bother building & operating the main transmitter, but they can't legally operate the booster if they don't)

So the main transmitter exists solely to justify the booster, and will be operated as cheaply as possible. The booster exists to reach Las Vegas, and it's worth investing in a circularly-polarized plant.
--
Doug Smith W9WI
Pleasant View, TN EM66

Ray
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Ray » Wed Nov 09, 2016 7:41 am

If you don't believe that RHCP becomes LHCP when bounced off of a perfect conductor, try configuring the polarization for a prime focus satellite feed for the incoming downlink circular polarization, it will be cross polarized. You have to configure the prime focus feed/polarizer for LHCP to receive a RHCP signal coming off the satellite and vice versa because of the reversal. A Cassegrain antenna gets a double bounce off both the subreflector and the main reflector so the feed polarization can be in sync with the desired incoming polarization. Diffraction effects off of buildings, are not perfect conductors so the axial ratio will be much worse; RHCP does not become a great LHCP signal etc.

Imagine a spinning wave traveling toward the reflector, it hits the reflector and the direction of propagation reverses. The spin rotation direction does not change, but the direction of propagation does causing the pol reversal. We keep helix antennas of both senses for a large frequency range in our antenna lab for polarization validation sense. It is very easy to use the right hand rule with a helix to know which is RHCP and which is LHCP. Every antenna gets a check before it is shipped.

Most people don't use a CP receive antenna, so we rarely see a huge pol mismatch loss with CP (they are almost always some linear antenna with often an arbitrary orientation). Every polarization has an orthogonal sense, it is just that most listeners don't have a CP antenna.

Ray

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