Is FM field strength really 54 dBu at the 54 dBu FCC contour?

[briankay]

I've always wondered how the FCC calculate that the 54 dBu contour for class B stations. Because when I am at the 54 dBu contour (65km/40 miles) from the class B transmitter (flat Chicago terrain, LOS to the Sears tower), I don't believe I get a signal level of 54 dBu (more like 35 to 45 dBu range) even though my car antenna base is 5 feet from the ground on the roof center.

Does the FCC data assume the antenna is at 20 feet or so above the ground?

Would any one have data on the typical signal levels 4 to 5 feet above the ground (at the 54 dBu contour)? This would be more useful information for the vast majority of listeners. Most mobile FM radios should not blend to mono when the signal is around the 54 dBu mark (in non multipath conditions) according to their spec, so I believe the reason my receive has blended to mono 5 miles inside the 54 dBu contour is because the signal level, in reality, is much lower than the FCC specified levels (due to being closer to the ground)?

 

[rfry]

Does the FCC data assume the antenna is at 20 feet or so above the ground?

It is the field intensity expected 30 feet above a relatively smooth terrain path at 50% of locations, 50% of the time.

Would any one have data on the typical signal levels 4 to 5 feet above the ground (at the 54 dBu contour)?

A VHF field near the earth varies approximately in a linear manner with elevation. So a 54 dBµV/m (0.5 mV/m) field at an elevation of 30 feet would become 5/30 * .5 = an 0.083 mV/m field at an elevation of 5 feet.

RF

 

[TomT]

"A VHF field near the earth varies approximately in a linear manner with elevation. So a 54 dBµV/m (0.5 mV/m) field at an elevation of 30 feet would become 5/30 * .5 = an 0.083 mV/m field at an elevation of 5 feet."

At 50% of the locations, 50% of the time.

 

[rfry]

At 50% of the locations, 50% of the time.

A small point here, but field intensity values expressed in "dBu" do not automatically imply any statistical probability. A 54 dBµV/m field is always/everywhere a 54 dBµV/m field.

If referring to the units and convention used by the FCC when estimating FM coverage then the correct form to express that is 54 dBu F(50,50) field.

So if a 54 dBµV/m VHF field exists 30 feet above the earth at some set of geographic coordinates, then the net field 5 feet above the earth at those same coordinates will be about 38.4 dBµV/m (0.083 mV/m) -- with no probability about it.

//

 

[briankay]

Thank you so much for the answers. This is exactly what I wanted to know and confirmed my intuition on the signal level.

It would be nice if the data for 1 meter (3ft) above the ground, for example was readily made available by the FCC and others, to accommodate the vast majority of people that don't have their antenna at 30 feet. So at 3 feet the 54 dBu contour is 34 dBu (.05 mV/m) give or take. It would be nice if most car stereos would stay in stereo at the 34 to 38 dBu level, but I guess most people would prefer the mono over the stereo signal becoming noisy. Every car Pioneer Supertuner (II, III, IIID) I've tried always blended to near mono before the 54 dBu contour was reached even in non-multipath rural areas. Funny thing is the factory stereo in our Toyota Sienna retains a fairly good mostly stereo image 5 miles beyond the 54 dBu contour whilst retaining a relatively noise free reception. I wish any aftermarket head unit did that. And I did try all these Pioneer head-units both with/without the antenna booster with a full rooftop 31" antenna. So I don't know why the Toyota head unit does such a better job of eeking the stereo out of the weaker FM signals.

 

[ChiefEngineer]

Thank you so much for the answers. This is exactly what I wanted to know and confirmed my intuition on the signal level.
It would be nice if most car stereos would stay in stereo at the 34 to 38 dBu level, but I guess most people would prefer the mono over the stereo signal becoming noisy. Every car Pioneer Supertuner (II, III, IIID) I've tried always blended to near mono before the 54 dBu contour was reached even in non-multipath rural areas. Funny thing is the factory stereo in our Toyota Sienna retains a fairly good mostly stereo image 5 miles beyond the 54 dBu contour whilst retaining a relatively noise free reception. I wish any aftermarket head unit did that. And I did try all these Pioneer head-units both with/without the antenna booster with a full rooftop 31" antenna. So I don't know why the Toyota head unit does such a better job of eeking the stereo out of the weaker FM signals.

Consider the Chevy Venture (or bettter yet don't) as it uses a window antenna. We used to tape these things to the window in the 1970's and they were junk then. They are lucky to receive FM within the 60dbu. If you add a cell phone charger of dc converter don't expect to hear noise free into the 70dbu.

 

[CaptBob92]

Be aware that in real life the contour level varies quite a bit from day to day. I conducted some field readings on an FM station that had just put a directional antenna in place to prove it was working properly. As I remember in order to confirm the signal was accurate I had to take readings on other stations in the area at each test point to determine if the contours were changing from my station or if it was a general probagation change I was seeing. It was not a ducting issue as it was in Winter in the Midwest.
My point is even with all the calculations etc you are still working with a medium that varies alot at those frequencies hense the time element in the discription of a contour.

 

[OKCRadioGuy]

http://lrcov.crc.ca/main/ Poke in 2.1dbi and put your erp in. It's a pretty cool coverage predictor of what you will REALLY have.

 

[Gatekeeper007]

Some people will say it's not possible but I have seen it heard it tested it but never had the chance to check it out. Back in the early 1970's abuddy of mine said he had a car radio that beat anything he had seen for being able receive FM radio stations a long way off and it could seperate stations right next to each other with no bleed over, yes I said no bleed over. When he told me this I thought he was pulling my leg as I had just spent good money for a Poineer Super Tuner, one of the first which he said his radio would out do after hearing mine. Of course we had to test them at many different locations around the lake Okeechobee, FL area with some very weak radio station signals on the same day,time and location. Both antennas were about the same height and size and without a dought he won hands down. His radio was an $89.00 unit from Sears which made me wonder why it worked so well, it should not have. Before I could test things and see what might be going on a leaky fuel pump caused his car to burn to the ground. Then years later 2002 a fellow who worked for me bought a used firebird with electric up/down antenna and had a Pioneer radio intalled. He mentioned to me he could hear WLRE radio a lot further than he could in his truck so one day we were on our way to Columbia in his car I tried our station and sure enough I could listen to it all the way to the Columbia out skirts, I couldn't believe it. I tried mnay different stations as we drove back home and found out that his radio could receive stations 75 miles or more away, great seperation and range like you wouldn't believe. So I talked him into letting me try another radio in his car and test his radio in mine. His radio worked in my truck about like mine had worked but my radio worked great in his car seperation like you have never seen and range of 75 miles or more. I really wanted to know what caused thiseffect but again never got the chance for in the next few days his daughter came down from up north wrecked her car while she was here and took his car back home with her to keep. I still wish I knew what caused this.

 

[OKCRadioGuy]

It was probably a Pioneer Supertuner IV. The new Sony XDRF1HD or XDR-S10HDiP will outperform it these days. I've tried them side-by-side. That HD tuner is the best unmodified consumer radio ever built for DXing. It's a DSP radio from hell.

If you have a wealth of money and just want to play, here's the best http://www.broadcastwarehouse.com/bw-broadcast/rx1--fm-receiver/1392/product or http://www.broadcastwarehouse.com/bw-broadcast/rbrx1-fm-re-broadcast-receiver-/1393/product It's the Sony-type DSP radio with the ablity to adjust the IF window yourself. I'd love to have one of them, but it's too expensive for personal use.

 

[BobOnTheJob]

No FM station is perfectly omni-directional in it's pattern. At the very best, you might get within 2-3db of the same value in all directions, and only then if the station is using a panel antenna with parts of the antenna on all 3 sides of the tower. In almost all cases, you'll see different horizontal vs vertical field strengths at any given azimuth from the transmitter. Very deep nulls (especially in the vertical signal) are common in the direction behind the transmitting antenna. Think of a balloon...push one side in & another side goes out. Wise stations put a lot of thought into their antenna installations to make certain that pushed in area (called a null or shadow) lands where the population is at a minimum. Likewise, there can easily be areas with several db of boost...the station will make every attempt to put that boosted signal area where the population lives. My point being that the 54dbu you are "promised" could easily be between upper 40's & upper 50's. In some cases, nulls of 20db or more can happen with lesser (but still large) boosts also possible. What you see on paper will not always be what you get, even at 30' 50% of the time.

 

[rfry]

No FM station is perfectly omni-directional in it's pattern. ... In almost all cases, you'll see different horizontal vs vertical field strengths at any given azimuth from the transmitter.

Paper 6 at http://rfry.org goes into some detail about this, with NEC calculations of the H- and V-pol azimuth and elevation patterns of a 2-bay sidemounted FM antenna on tower sections of two different face widths.

RF