Theory Discussion: Circular Polarization Vs. Vertical Polarization

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

Post by Paul678 » Sat Nov 05, 2016 2:03 pm

Here's what an RF friend of mine thinks:

"Circular polarization is probably not the best idea for FM. Broadcasters use circular polarization because they are limited in the amount of power they can broadcast with. In some states, they are limited to 100kW on an open channel- so they usually broadcast circularly just to have more energy leave the antenna. But if the FCC allowed them to do so, they would much rather broadcast with 200kW in one plane and increase their coverage radius by like 20% (depending on height). Circular broadcasting does little to actually increase the coverage radius. Plus circular polarization is kind of a magnet for multipath problems, which is why almost all TV broadcasters won't broadcast circularly, a few may broadcast elliptically. Similarly, I think in places like Moapa and Mesquite NV (Nevada is the country's most mountainous state), they broadcast single plane for FM because the multipath would beat up their signal as soon as you left town.

Since LPFM has a limited amount of transmitter power, you'd cover way more people with the highest ERP possible versus filling in a few dead spots with circular polarization. Even with a low broadcast height, ordinary obstacle loss isn't such at 100 MHz where a good signal suddenly falls off of the table and requires circular polarization to rescue it. In my experience, FM radio signals are remarkably consistent until they start to get near the 40dBuV level, some 20dB below the FCC's protected standard, assuming no multipath problem. And at that point, 3 dB of extra signal would help a lot more than circular polarization. Circular polarization is fine for unlimited budgets and huge transmitters, but it really isn't what you need in a LPFM situation.


Not quite convinced? The old class D FM station standard was that you can have only up to a 10 watt transmitter, but you can use any gain antenna you want as long as vertical gain did not go above 10 dB (that might cause a weird dead spot close to the antenna). So the vast majority of the old class D's are single plane. The new class D's are limited to 250 watts ERP no matter what, so far more of them use circular polarization."

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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by TPT » Sat Nov 05, 2016 4:52 pm

Hogwash. Your audience today is predominantly mobile. Just try to buy a portable radio. Or find a radio in home that works. Now look at the cars.

--Pickup trucks: Most have a nice vertical whip.
--Small cars: Many have a short whip, raked back at an angle.
--medium sized cars: Hmmm...is it the "shark fin" on back of the roof? Or is it the horizontally polarized window antenna?
--SUV? Who knows?

Now the other part of the premise is that if a station decided to use vertical polarization ONLY, it should be allowed to double power. Nice try, but the FCC's allocation policies are based on contour protection in both planes. LPFM was discussed, this is an allocation scheme based on minimum distance separations for simplicity--see 73.807. The minimum distance table is still based on a contour protection scheme, except that the protected co-channel and adjacent stations are assumed to operate at the maximum contour distance for their class.

So what is the advantage of vertical only? Well, assuming an antenna 100 feet aHAAT, 100 watts, with a typical vertical antenna, the transmitter TPO using 100 feet of 1/2" foam would be about 135 watts. Change to a single bay CP antenna, the TPO for the same installation is 269 watts. But you risk losing have of your mobile audience.

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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Lee_Wheeler » Sat Nov 05, 2016 6:39 pm

LPFMs, like all stations save some really old grandfathered old 10 watt Class D NCEs, are licensed by ERP, not TPO. Other than some NCE's that had channel 6 television interference problems all FM stations are licensed in the horizontal plane with the choice to have power in the vertical plane such that it does not exceed the horizontal ERP. Circular polarization is, for the purposes of licensing, exactly as the same as discreet horizontal and vertical polarization. Any reasonable combination of antenna gain and transmitter power is permissible so as to achieve the authorized ERP as long as the transmitter is type accepted at the requested power level.

For the purposes of licensing on a non directional antenna the actual effects of supporting structures is ignored as is the case with so called "off the shelf" directional antennas which are common on translators. In reality the vertical ERP may in fact exceed the horizontal ERP on some azimuths but, on paper, the vertical portion cannot exceed the horizontal.

LPFMs are not limited to 100 watt transmitter power, they are limited to a maximum of 100 watts ERP.

...Lee

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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by TPT » Sun Nov 06, 2016 8:25 am

Good point. Some of the common vertical antennas are considered "off the shelf" directional, such as the Scala FMV. I've used these for translators, but have never been convinced that they are as directional as claimed in typical installations.

Except in certain installations (think of a vertical at a tower site on the side of a hill overlooking a town) any slight gain in one direction would be a wash compared to the reduced radiation in other directions. As well as the loss in received signal to horizontally or randomly polarized antennas.

And on TPO vs. ERP: Community college installation where the tower was only 40 ft.--mounted on the roof of a first floor part of the school. But TPO ended up being 259 watts. FM/1 SWR CP antenna (less than half gain). Forty feet of 1/2 inch down the tower, then a run of super-flex cable into the second story wall, through a wooden trough along the back of the studio into a closet in the main classroom where the VS-300 resided in a small rack. Works well enough--biggest factor limiting coverage is the location of the college in a bowl outside of town.

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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Deep Thought » Sun Nov 06, 2016 1:10 pm

Your "RF Friend" clearly does not have much if any broadcast experience.

As an aside, those 10 watt class-Ds he cites as proof of his theory used horizontally-polarized antennas because that is what was (relatively) inexpensively available when these were licensed, and is what the equipment companies peddled to the schools. There is also the issue of vertical real estate...many of these were on a pole or short tower on the roof of a school. You could certainly have interwoven four vertical bays between the four horizontal bays most of them used but the cost would have more than doubled along with the maintenance expenses. I suppose he would also argue most of them were mono because it 'got out better'.

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

Post by Kelly » Sun Nov 06, 2016 5:50 pm

Agreed, the claim is total hogwash. I suspect the true motivation by someone trying to license a LPFM CP and insisting on using V-pol only, is purely about antenna cost, not whether it works any better, because it doesn't.
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by ChuckG » Sun Nov 06, 2016 9:11 pm

TPT wrote: But you risk losing have of your mobile audience.
That's really the end of the story right there.
I just took a quick peek at the receive antennas on the cars in my driveway. One vertical whip, one shark fin, one horizontal trace across the top of the rear glass and yet another I cannot find at all. C-pol at least attempts to treat them all equally.

I remember the old H-plane only ERI FM antennas. Collins sold them among others. You could add Vertical bays as an option. 70/30 split IIRC on the one I was familiar with.
Back then being the 1960's, when FM listening was done at home with horizontal plane antennas. Made perfect sense for the Educational 10-watters to run H-only, that's where the listeners were when they were built.
By the 1980's that predominantly H-plane antenna was crippling mobile listeners and it fortunately came down. I'm not sure why anyone would put anything similar back up.
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Paul678 » Mon Nov 07, 2016 1:31 am

Deep Thought wrote:Your "RF Friend" clearly does not have much if any broadcast experience.

As an aside, those 10 watt class-Ds he cites as proof of his theory used horizontally-polarized antennas because that is what was (relatively) inexpensively available when these were licensed, and is what the equipment companies peddled to the schools. There is also the issue of vertical real estate...many of these were on a pole or short tower on the roof of a school. You could certainly have interwoven four vertical bays between the four horizontal bays most of them used but the cost would have more than doubled along with the maintenance expenses. I suppose he would also argue most of them were mono because it 'got out better'.

:roll:
Thanks for the input, everyone. Please forgive my questions: I'm more of a phase-locked-loop, cellular phone GaAs ePHEMT GSM/DCS/PCS power amplifier specialist.

If I understand correctly, the FCCs ERP rules for a service contour put a limit on BOTH the vertical and horizontal planes. So if ERP is your limiting factor, it makes sense that you would rather use 200Watt TPO into a circular polarized antenna, rather than 100Watt TPO into a vertical-only polarized antenna, the former of which would obviously make your signal coverage much more even.

But I think my buddy's point is that if your limit is the transmitter output itself (TPO), like 10 Watts for the old Class D FM standard, then it might be better to focus ALL of your transmitter output power completely into the vertical plane only.

Also, no one has addressed if circular polarization has more multipath problems. Is it true almost all TV broadcasters won't broadcast circularly? Is it true in mountainous Nevada, they broadcast single plane for FM because of multipath problems?

Mono signals do not require the transmitter to modulate the 19kHz pilot tone, or the dual sidebands of the L-R signal on either side of the 38kHz suppressed sub-carrier. Mono signals only have to modulate the L+R mono baseband, so by the FM Bessel functions, the bandwidth is much smaller. Consequently, the part 15 people certainly agree that mono signals have a greater LISTENABLE range than a stereo signal:

http://www.part15.us/forum/part15-forum ... vs-mono-fm

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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Dale H. Cook » Mon Nov 07, 2016 4:45 am

Note that the Part 15 site that you cited is rife with technical errors. A number of posts in the thread that you cited show a lack of technical knowledge. 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. Furthermore, the description of the coverage in the first post of the thread that you cited indicates that the poster almost certainly is in violation of Part 15, and other posts in that thread demonstrate a failure to understand the Part 15 FM rules and regulations.

If you are interested in technically sound Part 15 information I would avoid that site - much of what is found there is either utter hogwash or concerns operation in violation of Part 15. HobbyBroadcaster.net is the only technically sound Part 15 site. It is run by a long-time broadcast engineer and frequented by a few other experienced broadcast engineers.
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by TPT » Mon Nov 07, 2016 7:06 am

But back to your mention of 100 watts--that's the ERP limit, not TPO limit--on low power FM stations. And, like higher power stations, the ERP limit is coupled with an antenna height above average limit that sets the real limit of the class: the coverage contour. Look at Section 73.211, which sets both a power limit and CONTOUR Limit for the different classes of stations. E.G. Class A--6 kw at 100 meters & a contour of 28 KM.

Elsewhere in the code the AHAAT is defined as the average of the average height of eight radials--that is , you take the average height of the antenna above terrain in each of 8 compass directions (N, NE,E,SE--etc.) and average those 8 averages. In the old days I would tape together several topos, draw the radials out, and count the elevations every so many feet on the map. Computers do the work now.

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.

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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Kelly » Mon Nov 07, 2016 8:25 am

Paul678 wrote:
Deep Thought wrote:
Also, no one has addressed if circular polarization has more multipath problems. Is it true almost all TV broadcasters won't broadcast circularly? Is it true in mountainous Nevada, they broadcast single plane for FM because of multipath problems?
From the beginning the reason most TV transmit antennas used horizontal polarization only because horizontal was the standard for 99.9% of the TV receive antennas. (Remember TV antennas atop most homes in America?) Coincidentally, much of the reflections causing multipath in the world are from horizontally-oriented surfaces.

Many DTV stations have now included either true circular polarity or have added parasitic elements to their antennas to provide elliptical or some vertical component to not only reduce horizontal multipath, but also in anticipation of mobile reception of DTV signals.
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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by taylorengineer » Mon Nov 07, 2016 12:27 pm

The reason television stations have historically not used circular polarization is because most tv viewing is done in a stationary environment and does not suffer from multipath in the same way as mobile radio listeners. Radio listening is a constantly changing environment making mitigation difficult.

When receive and transmit antennae are not aligned in the same plane there is a 20 db penalty. It used to be car antennae were vertical whips and you could get away with vertical only but today's auto may have an antenna embedded in the glass and it may be horizontal, vertical, or a combination. You never know exactly what you're going to get as there is no standard for automotive antennae.

I seem to remember research from years back showing better building penetration with circular polarization. Quite a few television stations now use elliptical polarization and some are full circular. The circular television station in my market has the best picture and is easiest to pick up both in a mobile environment and when using rabbit ears.

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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Shane » Mon Nov 07, 2016 12:30 pm

So if ERP is your limiting factor,
STOP right there. There is no "if." ERP is your limiting factor. The old 10 watt licenses are no longer being issued; haven't been for some time now. So you can't use that as any justification.

The calculation sort of works backward: your TPO; given your antenna gain (which can be a positive or negative number), your transmission line loss, the loss in any kind of filters and/or combining networks; is determined based on what ERP you are licensed to produce.

Real life example without the actual math: a 250 watt translator is licensed at 244 feet. Taking into account all of the above factors, the transmitter actually has to put out 880-odd watts to get 250 watts ERP from the antenna. A combiner was recently added along with a new antenna to accommodate another translator at the same site. Before that, the TPO needed to be 655 watts.

Given the height limitations of an LPFM, transmission line loss is less of a factor because the transmission line will most likely be shorter than the above example. There is hardly ever a combiner with an LPFM. There may or may not be a filter.

As to mono vs. stereo on FM, while you might be able to "fool" a given receiver to appear to give you more coverage area, some radios exhibit audio distortion in the presence of multipath on a mono signal even where the signal is strong. The radio in my car does not handle mono FM well; my wife's car radio has no such problem.

There's plenty of misinformation on the internet. On this particular forum live an experienced, knowledgeable group of folks, who will now point out wherever my above dissertation is wrong. It is better to listen to good advice, than to restate the same question hoping the answer you want will come out sooner or later.

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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by Slab Bulkhead » Mon Nov 07, 2016 1:02 pm

Paul678 wrote:Here's what an RF friend of mine thinks:

"Circular polarization is probably not the best idea for FM. Broadcasters use circular polarization because they are limited in the amount of power they can broadcast with. In some states, they are limited to 100kW on an open channel- so they usually broadcast circularly just to have more energy leave the antenna. But if the FCC allowed them to do so, they would much rather broadcast with 200kW in one plane and increase their coverage radius by like 20% (depending on height). Circular broadcasting does little to actually increase the coverage radius. Plus circular polarization is kind of a magnet for multipath problems, which is why almost all TV broadcasters won't broadcast circularly, a few may broadcast elliptically. Similarly, I think in places like Moapa and Mesquite NV (Nevada is the country's most mountainous state), they broadcast single plane for FM because the multipath would beat up their signal as soon as you left town.
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.

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Re: Theory Discussion: Circular Polarization Vs. Vertical Polarization

Post by NECRAT » Mon Nov 07, 2016 1:05 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.
All the stations in Potosi Mountain are circular.
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