Vertical or Circular polarization FM Broadcast

FM does it with frequency!
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nobby6
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Vertical or Circular polarization FM Broadcast

Post by nobby6 » Mon Apr 24, 2017 11:05 pm

OK, so, I know the pro's are horizontal and vertical paths with CP which is supposed to help with hilly terrain, and city blocks full of skyscrapers and whatever else the marketing dept comes up with to help sell the things.

The cons is you lose half your power from what I see, typically -3db, so needing about 2 or more (depending upon manufacturer gain loss claims) of these things to get back to unity, else you are having to increase substantially transmitter power to recoup your losses.

In a LPFM station - 100watts ERP, to cover an area of no more than 10 miles in any direction from the comms 20 meter tower on a 28 meter hill, flat terrain otherwise, is there any problem with using a standalone single side mounted dipole which holds unity and emits a true 100w ERP?

My background is two way, and we all know those vertical dipole arrays or colinear based repeater TX antennas send a saturated signal no matter the angle of the unit or units antenna, we all know those vertical rabbits ears for horizontally polarized TV signals work great, so why is there such hype about circular polarization over vertical for FM broadcast, if you want to get farther as some marketing claims, why not use normal dipoles in phase to get extra gain, it kind of seems its twice the effort, and twice the TX power and your still playing catchup to plain SMDs.

Maybe I'm missing something in my calculations?

TPT
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Re: Vertical or Circular polarization FM Broadcast

Post by TPT » Tue Apr 25, 2017 6:25 am

Unless you are streaming, you have next to no home listening. Go ask your friends what kind of table radio they use.

So your audience is the car radio audience. Look at what passes for antennas on most cars. My company van--a 2005 Ford Econoline--has a vertical whip antenna. But my business partner's Honda has a "shark fin" type antenna; the company Ford Fusion the same, my Malibu a rear window antenna.
Smaller cars like the Ford Focus I used to have --have a whip, but it is at an odd angle off the back of the roof.

Common denominator? All these antennas are randomly polarized. With a horizontal antenna--essentially my rear window antenna in the Malibu, you have up to 20 db loss over a vertical antenna. Less loss with the randomly polarized shark fins--but the antennas are too small to be very efficient, usually there is an RF preamp to compensate. So to reach these mobile listeners you need circular polarization.

Understand, too, that there is a considerable difference between a narrow band FM radio used for business and public safety communications and the typical wide band FM radio in most cars. Also understand--especially in pool-table country--you can have co-channel high power stations rolling in on your low power FM station. I can remember listening to a low-power in Defiance, Ohio that used a vertical antenna for a while. One morning--a 1/2 mile away from his site--I was hearing a co-channel from Dayton on my car radio completely wiping out the local signal.

Lee_Wheeler
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Re: Vertical or Circular polarization FM Broadcast

Post by Lee_Wheeler » Tue Apr 25, 2017 6:45 am

It will work either way but the rabbit ears analogy falls apart a bit in that the listener, the end user, rarely has any control over the orientation of the antenna at any given time. The nice thing about C-pol is that it is effectively in all planes all the time, not just H and not just V. It gives your signal the best chance of being received.

The cost difference between low end C-pol and V-pol antennas isn't much but the transmitters seem to have inconvenient break points of 100W, 250W, 500W, 1 kW. With the coax losses chances are you won't make 100 W ERP with a C-Pol and a 250 Watt transmitter but the cost difference between a 250 W and a 500 W isn't typically huge. Once you get to 1 Kw the price jump is fairly steep since they are often combined modules and the cost of the combiner is relatively high when compared to the cost of the modules.

...Lee

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Deep Thought
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Re: Vertical or Circular polarization FM Broadcast

Post by Deep Thought » Tue Apr 25, 2017 8:27 am

I recently helped a 250 watt translator diagnose and fix a problem they were having. The station had installed a vertical-polarized log periodic antenna for transmit and was suffering from close-in co-channel interference on some car radios from a class-A station located more than 40 miles away. Longley-Rice said the FX should have had more than 20 dB higher signal than the A over the area of concern. Guess what happens to that calculation when you correct for cross-polarization losses?

They installed another in H polarization and the interference was gone.

The advantage of H&V or circular polarization is you essentially "get" to transmit twice the power, which helps with the power density at all antenna orientations as noted above. If you're too cheap to take advantage of that, especially for an LPFM where proper antennas are available for well under $1000, your loss. The transmitter requirements are rarely above 300 watts TPO, much less if you don't cheap out on the transmission line by trying to use 100 feet of RG-8.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

TPT
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Re: Vertical or Circular polarization FM Broadcast

Post by TPT » Tue Apr 25, 2017 1:18 pm

20 meter tower (65 feet) plus an extra 5 feet to play with: 70 feet of 1/2 foam coax (Andrew LDF-4 used to calculate) efficiency 90%.
SWR single bay CP antenna, gain .441. TPO needed: 258 watts.

Nautel VS300, type accepted for LPFM service, 330 watts maximum power. Can be configured with built in Orban processor, comes standard with internal stereo generator and RDS.

Incidentally, have almost exactly that setup at a local community college. Same antenna. They have 40' of tower on-campus atop a building, 1/2 inch to the base of the tower. However I used 15 feet of superflex to get into the building & along a trough built through the studio to a closet where the transmitter is located, thus 259 watts to overcome the excess loss in the superflex.

nobby6
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Re: Vertical or Circular polarization FM Broadcast

Post by nobby6 » Wed Apr 26, 2017 5:17 am

Deep Thought wrote:
Tue Apr 25, 2017 8:27 am
I recently helped a 250 watt translator diagnose and fix a problem they were having. The station had installed a vertical-polarized log periodic antenna for transmit and was suffering from close-in co-channel interference on some car radios from a class-A station located more than 40 miles away. Longley-Rice said the FX should have had more than 20 dB higher signal than the A over the area of concern. Guess what happens to that calculation when you correct for cross-polarization losses?

They installed another in H polarization and the interference was gone.

The advantage of H&V or circular polarization is you essentially "get" to transmit twice the power, which helps with the power density at all antenna orientations as noted above. If you're too cheap to take advantage of that, especially for an LPFM where proper antennas are available for well under $1000, your loss. The transmitter requirements are rarely above 300 watts TPO, much less if you don't cheap out on the transmission line by trying to use 100 feet of RG-8.
Thanks, I am just trying to figure out if it is truly worth the effort, 200 watts out just to get 100watts ERP, and no we didnt skimp on cable, we use heliax, co-channel wont be much a problem, we are not in USA, our local regulator is very strict about co channel use, over 100 miles LPFM, or greater if one is higher power and/or if flat terrain, closer to 2 or 3 hundred.

I gather your LPFM translator only used one bay?

nobby6
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Re: Vertical or Circular polarization FM Broadcast

Post by nobby6 » Wed Apr 26, 2017 5:27 am

TPT wrote:
Tue Apr 25, 2017 6:25 am
Unless you are streaming, you have next to no home listening. Go ask your friends what kind of table radio they use.

So your audience is the car radio audience. Look at what passes for antennas on most cars. My company van--a 2005 Ford Econoline--has a vertical whip antenna. But my business partner's Honda has a "shark fin" type antenna; the company Ford Fusion the same, my Malibu a rear window antenna.
Smaller cars like the Ford Focus I used to have --have a whip, but it is at an odd angle off the back of the roof.

Common denominator? All these antennas are randomly polarized. With a horizontal antenna--essentially my rear window antenna in the Malibu, you have up to 20 db loss over a vertical antenna. Less loss with the randomly polarized shark fins--but the antennas are too small to be very efficient, usually there is an RF preamp to compensate. So to reach these mobile listeners you need circular polarization.

Understand, too, that there is a considerable difference between a narrow band FM radio used for business and public safety communications and the typical wide band FM radio in most cars. Also understand--especially in pool-table country--you can have co-channel high power stations rolling in on your low power FM station. I can remember listening to a low-power in Defiance, Ohio that used a vertical antenna for a while. One morning--a 1/2 mile away from his site--I was hearing a co-channel from Dayton on my car radio completely wiping out the local signal.
most cabins /residences are equipped with small 2 in 1 radio/cd players with telescopic, most cars have vertical guard mounts, many use the FM on their mobiles, I know the fins and slants you speak of, the ford focus and BMW's, and the verticals on Toyota hiluxes and so on.

We have to replace the gear in-place now due to its age and lightning damage - currently its using an emergency backup - a diamond discone, its coverage is pretty freaking good, but the porcupine adds excess wind loading to top of tower, not to mention is a bigger target than our lightning rod :)

nobby6
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Re: Vertical or Circular polarization FM Broadcast

Post by nobby6 » Wed Apr 26, 2017 5:30 am

Lee_Wheeler wrote:
Tue Apr 25, 2017 6:45 am
It will work either way but the rabbit ears analogy falls apart a bit in that the listener, the end user, rarely has any control over the orientation of the antenna at any given time.
...Lee
Thats my point though, the user had vertical rabbit ears for a horizontal polarized signal and works, the TV translator is pure horizontal, not vertical, and is also 100 watts.


Strange I recall when i lived near a big city, some stations operating at 12KW could be heard on my portable radio sitting on my desk, some other stations with 96kw, whos transmitters were only about 150 meters away from those transmitters using CP, couldnt be picked up :)

W2XJ
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Re: Vertical or Circular polarization FM Broadcast

Post by W2XJ » Sat Aug 26, 2017 4:03 pm

Actual field studies show horizontal polarization is the best option. If you want to burn up the extra power, then go CP. I could write volumes on this but to keep it short I'll just share this story.

I had the opportunity to put the first 2 legal private stations on the Eiffle tower in the late 1980s. The tower was configured with two master antennas, one horizontal and one vertical. This was done to ease combiner design. Adjacent chahhels were on different polarizations. We are given the choice of some frequencies, some on the horizontal and some on vertical. I had already read and studied the very scientifically conducted German and Canadian field tests comparing horizontal, vertical and CP. Based on that data I strongly recomended a horizontal frequency. My French counterpart strongly disagreed and push pushed for and got vertical frequencies. Fast forward and the stations were signed on and immediately there were reception complaints. We did some drive around Paris and quicly noticed the superiority of the horizontally polarized signals, which is primarily due to greatly increased multipath on the vertical signals. I should mention that all the transmitters at the Eiffle Tower were identical 10 KW Thompsons so the only discrepancy was the polarization. We also did some DX runs out of town and quickly confirmed that vertically polarized signals were more rapidly attenuated than horizontal.

The myth about vertical auto antennas is just that. In the real world, signals arrive at the receiver randomly polarized. Even the metal body of the vehicle contributes to this.

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Deep Thought
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Re: Vertical or Circular polarization FM Broadcast

Post by Deep Thought » Sat Aug 26, 2017 4:15 pm

One corroborating incident does not prove a theory. I can point to at least three stations whose effective auto coverage area more than doubled when they switched from H to CP. The other issue with that argument is "{i}n the real world, signals arrive at the receiver randomly polarized." That is true of both H- and V-only polarized signals. The advantage of CP antennas is the higher effective power density off the antenna and having horizontally- and vertically-polarized components.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

W2XJ
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Re: Vertical or Circular polarization FM Broadcast

Post by W2XJ » Sat Aug 26, 2017 5:11 pm

All good except we really weren't discussing CPole. That is a whole different discussion. CPole is not always the magic bullet some think, though. For LPFM I think it is a waste but that said, I have built CP LPFM because 300 Watts is about the lowest power decent approved TX available and the extra electricity at that power is negligable. I just keep the customer happy.

W2XJ
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Re: Vertical or Circular polarization FM Broadcast

Post by W2XJ » Sat Aug 26, 2017 5:23 pm

I should clarify. I wasnt making a case regarding CPole. I was making a case AGAINST vertical polarization. The choice should really be between Hpole and CPole although there is something to be said about slant as well.

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