Antenna Patterns

[pianoplayer88key]

And I can't overemphasize the importance of understanding that in the real world, RF transmission systems in close proximity interact with each other. While most of that RF goes out into free space, some of it ends up being received by other transmitting antennas nearby, and those antennas can in turn serve as unintentional re-radiators, or as receiving antennas that conduct that RF down their own transmission lines and back toward the transmitters at the end of those lines.

Oh, ok.  I was thinking it was just my <$100 portable radios that were Schieße, and that >$100k professional broadcast equipment would handle it sin problemo. Guess they're not to that level of engineering performance yet.

 

[Schroedingers Cat]

You cannot change the Laws of Physics, Jim. I see many politicians, marketing people, etc. who think they can, but they can't. There are ways to circumvent some of them artificially, but you can't have two currents of the same frequency in the same tower without them just adding vectorially to a resultant current, even if you could prevent the power from getting into the other transmitter.

 

[OKCRadioGuy]

So to be clear, the FCC is only worried about horizontal directionization, not vertical? So... Is this an exploit for those of us having to mess with directional FMs?

 

[rfry]

So to be clear, the FCC is only worried about horizontal directionization, not vertical? So... Is this an exploit for those of us having to mess with directional FMs?

IIRC, sometimes the FCC also has defined the vertical plane radiation pattern permitted for an NCE FM station, in a theoretical attempt to minimize close-in interference to an (analog) TV station on Channel 6.

 

[rfry]

Oh, ok. I was thinking it was just my <$100 portable radios that were Schieße, and that >$100k professional broadcast equipment would handle it sin problemo. Guess they're not to that level of engineering performance yet.

Sorry, but it is unlikely that such "thinking" reasonably and scientifically can be expected to lead to the conclusion you posted.

 

[Schroedingers Cat]

Let's be clear that there is a difference between the vertical radiation pattern and the vertically polarized radiation pattern, which is specified in the horizontal plane. You can't exceed the maximum ERP in either the vertically polarized or horizontally polarized pattern, which by geometry and symmetry are usually different. Only a theoretical horizontally polarized magnetic dipole combined with a vertically polarized electric dipole fed equally 90 degrees out of phase in free space with no feed line could produce a truly circular pattern which is also circularly polarized. This is as impossible in practice as an isotropic point source antenna.

Using noise cancelling techniques to cancel out the signal from the other AM transmitter, both the carrier and modulation 180 degrees out of phase, it MIGHT be possible to feed the two separate signals into the antenna. But once the signals are in the antenna elements, they would just add the carriers and modulation vectorially, resulting in combined modulation and a single directional pattern, not a separate pattern for the two signals. That is my analysis anyway.

 

[MarioMania]

Thanks for the info

Can anyone explain this too me

I'm in Vallejo...at night

KYAA 1200 10,000 Watts - Soquel, CA -- I barely pick it up at my house or at my bros

KJDJ 1030 700 watts - San Luis Obispo, CA -- I could get it I have to null it out to get KTWO

 

[gar hi]

It looks like KYAA has a null in your direction.

http://radio-locator.com/cgi-bin/pat?call=KYAA&service=AM&status=L&hours=N

Hey, since KYAA is so weak where you are, you should listen for WOAI from San Antonio, Texas.

They are talk and news. That would be a good catch!

The reason you hear KJDJ is because it's non directional.

 

[MarioMania]

Can you explain to me about non directional & nulling in my direction

 

[davideduardo]

Can you explain to me about non directional & nulling in my direction

Non-directional: sends equal amount of signal in all directions. Thing of a light bulb in the very center of the ceiling of a room, sending equal amounts of light to every part of the room.

Directional: sends more signal to some places than others. Think of track lights, aimed at only a portion of a room, leaving other parts less illuminated.

Directional stations usually send less signal in certain directions so as to not interfere with stations in other cities that share the same frequency or stations on nearby frequencies. In a few cases, stations are directional to send more signal over the city they want to be best heard in, and less signal to areas that are not part of the market.

Go to
www.radio-locator.com
and put in the call letters of all the local AM stations you know, and look at their coverage patterns.

 

[rfry]

To carry on with with DavidEduardo's post, the patterns on Radio Locator also show the effects of earth conductivity on the radiation pattern "launched" by an AM broadcast station. This can sometimes produce unexpected results in the shapes of their coverage areas.

The link next below shows the radiation patterns of all of the 850 kHz stations in the US. The coverage area contours for KOA, Denver might be expected to be perfect circles, because KOA is an omnidirectional station.

http://i62.photobucket.com/albums/h85/rfry-100/850_kHz_Patterns.jpg

But earth conductivity is quite a bit higher to the east of Denver than to the west, so their groundwave coverage contours are far from being truly circular (link below).

http://www.radio-locator.com/cgi-bin/pat?call=KOA&service=AM&status=L&hours=U

 

[gar hi]

Radio Locator is a very valuable tool, especially when it comes to daytime AM DXing but I've found from my own experience that their coverage distances can sometimes be conservative in the daytime, at least when it comes to the limit of their expected coverage area and some stations can be heard for a ways outside the 'fringe' line.

A good example is KFI.

Here's the daytime coverage map for KFI.

http://www.radio-locator.com/cgi-bin/pat?call=KFI&service=AM&status=L&hours=D

And here's a prety good signal for KFI midday up in Santa Cruz 300 miles away. The signal would be easily listenable if not for the splatter from the local station on the adjacent frequency.

http://www.youtube.com/watch?v=yNzZw2aS_34


Here's the coverage map for WZAB which is down in the Miami area.


http://www.radio-locator.com/cgi-bin/pat?call=WZAB&service=AM&status=L&hours=D

But I could still hear WZAB midday up in Daytona Beach when I was listening for WCBS. The signal would have been even stronger but I had it mostly nulled to see if I could pick up any of WCBS which had to be one of the two stations on that frequency.


http://www.youtube.com/watch?v=yoNF4v6i5b0

 

[rfry]

Radio Locator is a very valuable tool, especially when it comes to daytime AM DXing but I've found from my own experience that their coverage distances can sometimes be conservative in the daytime, at least when it comes to the limit of their expected coverage area and some stations can be heard for a ways outside the 'fringe' line.

The "fringe" line (the blue trace) on Radio Locator daytime maps for each AM broadcast station shows the calculated 150 µV/m contour of that station -- including its licensed radiation pattern along each compass bearing, and the effects of earth conductivity in those directions.

In the absence of co- and adjacent-channel interference and local r-f noise, the daytime groundwave of such stations might be received well beyond the fringe contour shown on Radio Locator, when using a sensitive AM broadcast receive system which includes a highly directional receive antenna.

 

[gar hi]

OK, I thought the fringe line meant the greatest distance the station could be heard with a listenable or audible signal on the average radio.

Thanks for explaining this in more detail.

 

[MarioMania]

I think 1310 RD is weaker where I am..

Now all I hear is nothing at night around 10 pm..it's at 5000 watts in Oakland

Did they change or something??

 

[jd]

Did they change or something??

Well, yes...yes, they did. It seems that KMKY was having some issues with modification of their site, which mainly involves shortening their towers. This has necessitated the occasional use of one tower at a time, non-directional, with reduced power of no more than 1,250 watts during the day (1/4 of their licensed 5,000 watts).

Here's the FCC's reply concerning KMKY's request for special temporary authority while the work is being performed: http://licensing.fcc.gov/cgi-bin/prod/cdbs/forms/prod/getimportletter_exh.cgi?import_letter_id=31054

 

[MarioMania]

Oh ok..I thought they went off the air for a few hours at night

I hope they fix it soon

 

[davideduardo]

OK, I thought the fringe line meant the greatest distance the station could be heard with a listenable or audible signal on the average radio.

The radio-locator maps are clearly marked as being for entertainment purposes only. While they are a great source for approximate and very visual information, a caveat is in order:

The usable coverage of an AM station for at home and at work listening is somewhere inside the innermost red contour. That's because in todays urban environments, it takes a walloping signal to overcome computer an microprocessor noise, CFLs and flourescents, dimmers and power line noise and all manner of other man made noise.

For in-home and at-work listening, where around 70% of radio listening takes place, a 10 mV/m signal is needed... if not more. In the car, in cities, a 5 mV/m at minimum is needed inside urban areas. Analysis of ratings participations shows that over 90% of listening conforms to these "rules."

Those other contours on radio-locator may be fine for radio fans and DXers, but the average listener will not put up with such weak signals.

Same goes for FM... 95% of listening takes place in the 65 dbu contour, just inside the red contour on radio-locator.

 

[pianoplayer88key]

So a 10 mV/m signal is sufficient to overcome noise from a computer? I'm still getting considerable noise with my radio on the desk here, even on my strong locals. (The strongest one, KFMB, is 7.3 miles distant at a heading of 320.69° (reverse 140.65°). The augmented field toward me is about 1995 mV/m @ 1 km, and the ground conductivity, per the M3 map, is supposedly about "8".) As I understand it, most listeners prefer something like a 120 dB signal-to-noise ratio (or whatever it is you have in a situation like.... you're sitting in an anechoic chamber with your radio's volume set so the program is almost deafening, then if the station went unmodulated for a while and someone else switched the radio off, you wouldn't hear when they do it - there'd be no difference in what you hear.)

I can put up with some broadband / white noise, but heterodynes, squeals, etc. are not fun to listen to. In fact, right now I'd like to be listening to KMIK, but my computer monitor is putting a harmonic of its current horizontal scan rate (105.2 kHz) just 2 kHz below, blocking KMIK's signal completely. If I go in the other room or outside, though, it's often quite listenable with little or no noise on peak propagation. Also I checked a few other harmonics of my monitor's scan frequencies (changing the refresh rate and resolution a few times along the way), and several of them indicate up around 87 to 94 dBµ on my Tecsun PL-606. There are other things I often get almost comparable levels of interference from, but I often spend more time at my computer than near those other interfering culprits. So how much of a signal would it take to overcome that level of QRM with a quality that I suspect most people prefer?

If my 120 dB SNR guess isn't accurate, what quality signal are you referring to that most people listen to? Also what field would you say would be necessary in a rural area away, from noisemaking devices, to provide a "usable" signal?

Also, this is what I got from KOGO last time I was in the Pauma Valley area northeast of Escondido.
http://www.youtube.com/watch?v=gRB-64G4iws#t=0m26s
I thought that 600 AM wasn't supposed to be audible in inland North County, which was why they duplicated on 95.7 FM. Seems that I was able to get AM 600 just fine, though. (I didn't try 95.7 FM, though.)
Other than the fact that I used a narrow bandwidth setting on my radio, would you still consider the signal quality after 0:26 unusable?

 

[rfry]

So a 10 mV/m signal is sufficient to overcome noise from a computer? I'm still getting considerable noise with my radio on the desk here, even on my strong locals. (The strongest one, KFMB, is 7.3 miles distant at a heading of 320.69° (reverse 140.65°). The augmented field toward me is about 1995 mV/m @ 1 km, and the ground conductivity, per the M3 map, is supposedly about "8".)

For those conditions the FCC groundwave propagation chart shows a field of about 130 mV/m at 7.3 miles. A local noise source likely would be way out of compliance with Part 15 if it caused serious interference to a 130 mV/m MW signal.

As I understand it, most listeners prefer something like a 120 dB signal-to-noise ratio ...in an anechoic chamber with your radio's volume set so the program is almost deafening...

Few people listen to audio sources in that environment. A more typical listening situation has an acoustic noise floor that would permit maybe a 50 dB SNR without damage to the listener's ears.

But in any case -- the SNR of many program sources transmitted by AM and FM broadcast stations is in the region of about 65 dB below 100% modulation, when measured at the output of the transmitter.