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vertical vs circular polarization

K

karasak

We must replace old 2 panel antenna (with 1 diple panel) system with new one (30 years old) , so one FM specialist told us that we could get better results with circular polarized antenna (2x RVR ACP1). We have only 700w transmitter power and like to cover 2 cities about 15 miles far away. Our transmitter is about 400 metes above sea and that two cities are 100m above sea. What is your opinion about that.
Kreso
 
go with the 2 bay CP antenna.Don't know alot about the RVR brand.Probably a good antenna.Here is the USA most engs really like the ERI brand.I;ve used about all of them,ERI,Shively,SWR,PSI,but still like the ERI roto-tiller.
 
karasak said:
We must replace old 2 panel antenna (with 1 diple panel) system with new one (30 years old) , so one FM specialist told us that we could get better results with circular polarized antenna (2x RVR ACP1). We have only 700w transmitter power and like to cover 2 cities about 15 miles far away
Click to expand...
________

Note that a typical circularly polarized panel/bay/tier/array has about 1/2 the power gain of a similar one using the same number and arrangement of linearly polarized radiators.

If you are licensed to produce a certain radiated power in the polarization plane of your current antenna, then for circular polarization you will need to increase the transmitter output power by something like 2 X in order to maintain your licensed, radiated power in that plane.

More detail and analysis of your situation including knowledge of the directions to your two cities, and the terrain profiles along those paths would be needed to give a better opinion.

Maybe one of the antenna manufacturers would be willing to do this analysis for you.

//
 
Vertical: works well with car radio antennas, but not with randomly-polarized antennas (e.g.: line cord antennas on table radios, etc.)

C.P.: more field for a given ERP, but, as noted, you get half the gain for a given number of bays compared with a vertical antenna system.

If I read your post right, though, you would be going from one vertical bay to two C.P. bays, which would probably give you the same ERP (essentially unity gain). I did a real rough calculation, assuming that you would have a path 300 meters above average terrain with about 650 watts ERP (bit of line loss cranked in)--this would indicate that the signal about 28 km. away would be 60 dbu. Generally 70 dbu is desired for a good in-home (as opposed to in auto) signal. But not knowing your local terrain makes this nothing more than a sophisticated wild-a** guess.
 
R. Fry said:
Note that a typical circularly polarized panel/bay/tier/array has about 1/2 the power gain of a similar one using the same number and arrangement of linearly polarized radiators.

If you are licensed to produce a certain radiated power in the polarization plane of your current antenna, then for circular polarization you will need to increase the transmitter output power by something like 2 X in order to maintain your licensed, radiated power in that plane.
Click to expand...

Unfortunately the peculiarities of Croatian Telecom Agency (Kreso being from Croatia) for reasons only known to them, specify that the allowed ERP they give you is a sum of both planes! Meaning that if you switch from linear polarization to circular, you will not be allowed to double your transmitter power. That should be kept in mind when advising. Most of the Croatian stations figured out they would be better off with full power in one plane (usually vertical), but I'd also like to hear opinions on this issue...

I don't know where the station's TX site is exactly, so I took a wild guess with this map http://tinyurl.com/5oaohk I'm sure Kreso will be able to give more accurate position. The two cities I presume they would like to reach are Zagreb (Croatian's capital) and Karlovac.


Regards,
Goran Tomas
(from Zagreb, now in London)
 
At that point, I would stick with vertical, since you are not that close to either city. Even in Croatia--or perhaps especially in Croatia, I would suspect that many of the radio listeners are in their cars,--and car radio antennas are vertically polarized.

If you were sitting over town with your tower, going CP would make sense. You would loose some fringe area listeners who would find they couldn't carry the station as far as the could before as they drove out of the center city. But you would add listeners using the typical table radios with line cord antennas or just a piece of wire hooked to a stereo tuner.

But in your situation, the best you can hope for is to reach the mobile audience in your two distant cities. Moreover, you have to fight the terrain. Even if you are on a nice high hill, more than a few kilometers out, there will be blockages. CP would help if you could keep the same ERP, but becomes a liability if you must cut back power. Again, at 15 miles, at your power, there will be enough blockage because of terrain that you will really have little more than a mobile audience.
 
Usually CP is THE PREFERRED choice over VP.Does your license state an ERP?But in your case VP may be the best way.Get the best VP you can afford.i'm amazed how much broadcasters will spend on rf gear,but scrimp on the antenna..Replaced brand X with a ERI at a friends station last year.He could not believe the difference in coverage.I don't work for ERI,but i will say they know antenna systems and how to maximize them.
 
I changed the antenna of my translator from a CP omni (10 watts ERP) to a vertical-only directional array last week. This allowed up to a 10 dB power increase in some directions, but I was limited to approximately a 3 dB increase towards a co-channel station north of the site.

I drove out that northern radial the day after the new antenna was installed and the mobile coverage was noticeably better. In the 3 dB "notch", the overall increase in strength seemed more like a 6 dB improvement. Perhaps the pattern of the old antenna wasn't all that close to omni, but I was favorably impressed with the change -- we certainly didn't lose any perceived coverage by going vertical. However, multipath increased in some shadowed areas.

It's unfortunate that the Croatian government doesn't allow TPO to be doubled if a station goes from linear to circular polarization. Is there anything the broadcasters could do to get that rule changed?

It makes about as much sense as the former FCC rule that required AM transmitters to operate in discrete steps of 0.25, 0.5, 1, 5, 10 kw, etc. regardless of antenna efficiency. I remember a 1 kW station on 1490 which was required to waste about 20% of the transmitter's power output in a resistor because the tower was too tall. Bureaucracy at its worst!
 
Not. Apart from some unusual installations in the big cities--community antennas on a tall tower, or mountain-top site, the typical FM installation is stacked circularly-polarized antennas, either top (pole mounted) or side mounted.

Here in the east (I am near Pittsburgh~Columbus, Ohio), maximum powers are lower because of population densities (hence more stations). Class A stations--limited to a 1 mv/m--60 dbu coverage contour of 28 km., typically use a three to four bay CP antenna with transmitters running 3500 to 5,000 watts. Class B-1 stations--39 km/57 dbu coverage area, and Class B stations will normally not use more than six bay CP antennas.

In the west, maximums go up, to as much as 100 kw at 2000 meters above an averaged terrain, so some installations may use large antenna installations, perhaps 12 bays, in order to work out a practical balance between transmitter power and antenna gain. At some of these older installations you may find, instead of a massive CP antenna, stacked vertical and horizontal elements.

But the "typical" installation for most of the country is a 3 to 6 bay C.P. antenna; ERI, Jampro, and Shively common brands.
 
Question for Karasak and Goran:

If I am understanding Croatian regulations correctly, your ERP is calculated as the sum of both polarizations?
This sounds similar to the calculation used in the U.S. to determine RF hazard (where the antenna is mounted on a low tower) as opposed to licensed power/coverage limits. In the U.S., licensed operating constants are determined in the commercial bands by contour and distance separations. That is, if the site meets distance separation criteria, then you can propose any combination of height/antenna gain/transmitter power needed to achieve an averaged contour distance for the class of the station in the horizontal, or horizontal and vertical plane of the antenna. Hence the popularity of CP antennas, since there is no coverage penalty for installing one.

Croatian regulations appear not to encourage C.P use. In other words, if you have a licensed ERP of 1,000 watts vertical, then switch to a C.P. antenna, Croatian regulations would require transmitter power to be limited to produce an ERP of 500 watts horizontal, 500 watts vertical. As I mention above, this may be advantageous if the antenna was on a hill with line of site to all of Zagreb. But 15 miles out, even on a mountain, the vertical antenna may be the best bet for reaching the biggest audience.
 
Studio1 said:
Out of curiosity, how popular (or not) is slant [45°] polarisation in the US?
Click to expand...


Not very. The original Alford master antenna on the Empire State Building in New York (still in place as an auxiliary) uses slanted dipoles wrapped around the building in two layers above and below the 102nd floor observation deck, but its V/h ratio is slightly less than 1. There are some pictures near the bottom of this page:

http://www.necrat.com/empirefm.html


I recall seeing an ERI slanted-dipole antenna at WTOS atop Sugarloaf Mountain in Maine (a high peak not far south of the Canadian border). Judging from the scraps of fiberglass and twisted metal at the tower base, WTOS first attempted to use a conventional ring-stub CP antenna in radomes, but it couldn't withstand winter icing conditions, so the dipoles were substituted. This has since been replaced with a "rototiller".



There are also some older directional antennas with separately-fed vertical and horizontal dipoles; WPHH in Hartford is one example:

http://www.necrat.com/wmrq_pro.html
 
For 1 city Karlovac we have clear Los but for Zagreb we can see only half of the city and on that side we have strong station only 200kHz below our frequency. Yes, in Croatia seems to me that ERP is calculated from H+V polarization. So you suggest that we stay vertical?
 
That would seem to be the best route for your situation. One consideration--all vertical antennas (and for that matter c.p. antennas if mounted on a wide structure) tend to be directional. How directional depends on the antenna design, and the type of structure the antenna is mounted on. Note the horizontal pattern shown in the RVR literature for your present antenna, which uses a screen reflector. I would suspect that the vertical pattern is similar. I've used Scala-Kathrein's FMV vertical antenna for low power FM translators, which appears somewhat omni-directional if mounted on a skinny pole. That antenna is just a dipole on a horizontal pole-- if mounted according the Scala's spec 's it is supposed to be a cardioid in the vertical plane & our FCc recognizes it as such.

Don't know the geography that well, but if you are equidistant from Zagreb and Karlovac, that would leave me to believe that your signal is aimed toward one city and away from the other. With a station in Zagreb only 200 KHZ away, I suspect that your station may have been designed originally to protect that Zagreb station.
 
TomT said:
In the west, maximums go up, to as much as 100 kw at 2000 meters above an averaged terrain, so some installations may use large antenna installations, perhaps 12 bays, in order to work out a practical balance between transmitter power and antenna gain. At some of these older installations you may find, instead of a massive CP antenna, stacked vertical and horizontal elements.

But the "typical" installation for most of the country is a 3 to 6 bay C.P. antenna; ERI, Jampro, and Shively common brands.
Click to expand...

You got your terms mixed there Tom.....max for a Class C is 100KW at 2000ft, not METERS :) Any FM higher than 2000ft must reduce to keep its contours at the same range.
 
TomT said:
Croatian regulations appear not to encourage C.P use. In other words, if you have a licensed ERP of 1,000 watts vertical, then switch to a C.P. antenna, Croatian regulations would require transmitter power to be limited to produce an ERP of 500 watts horizontal, 500 watts vertical.
Click to expand...

Yes, that's correct.

As I mention above, this may be advantageous if the antenna was on a hill with line of site to all of Zagreb. But 15 miles out, even on a mountain, the vertical antenna may be the best bet for reaching the biggest audience.
Click to expand...

That seems to be general consensus among broadcasters here and why most of the stations use vertical polarization. Plus the fact that the prevailing majority of listeners listen to radio in their cars.

I wish the Croatian Telecom Agency had flexible policy as FCC does, regarding contour being your only parameter...


Regards,
Goran Tomas
 
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