How does it work on AM/FM/TV??
like KCBS 740 beems it signal down North and South, But not East
If all the Antenna's are up ward
like KCBS 740 beems it signal down North and South, But not East
If all the Antenna's are up ward
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Antenna Patterns
- Thread starter MarioMania
- Start date
MarioMania said:
If you ever did those experiments in school where you poured metal filings on a paper and moved one or more magnets under them. According to the magnetic fields of the magnet or magnets, the metal filings moved and aligned.
Radio signals are electromagnetic fields, and they can be shaped to go in some directions and not in others.
On AM, this is done with multiple towers which, without going into phase and current explanations, push and pull the signal into different shapes. The more different directions the antenna systems pull in (nulls) and push out (lobes) generally the more towers needed to achieve the result. KCBS, located at the north end of the bay, pushes most of the signal south by southeast, but very little towards Canada (at night), which it protects.
The initial adjustment of a complex directional may take months. Some never get fully adjusted... this is complex engineering complicated by the environment with things like hills, mountains, buildings and power lines making adjustment very difficult some times.
On FM, directional operation is usually achieved by the use of little tuned sticks and rods called parasitic elements placed around the bays of an FM antenna to nudge the signal in and out.
This is a necessarily simplified explanation, of course.
I never knew that FM stations could be directional because the few stations I've looked at on Radio Locator all showed patterns of exact circles.
But I know how you know your stuff when it comes to this kind of thing, so it's interesting to learn something I never knew about.
As far as AM directional stations go, I'm only a couple miles from 970 WFLA and 1250 WHNZ which both share the same towers.
The multiple tower complex is quite a sight to drive by up close.
But I know how you know your stuff when it comes to this kind of thing, so it's interesting to learn something I never knew about.
As far as AM directional stations go, I'm only a couple miles from 970 WFLA and 1250 WHNZ which both share the same towers.
The multiple tower complex is quite a sight to drive by up close.
gar fla said:
Look at the pattern of KKLA in Los Angeles, 99.5. That is one of the more extreme ones.
Interestingly, stations sharing multiple towers can have entirely different patterns, based on the amount and phase of energy sent to each tower. Multiple stations can be fed to one or more towers, as it is just a matter of each sending the power to the tower or towers, and filtering so the other station does not sent its power back into the transmitter of the other station.
I'll try to make a stab at simplifying this as best I can without a diagram and without complications. Take a pencil and paper a draw a vertical line for the first antenna. Draw a sine wave from the bottom of antenna to the right. Try and put in a couple of cycles of the sine wave. That is the wave from the first antenna. The wave will go up then down and then back up again. Where the wave goes from - (below the horizontal) to + (above the horizontal) put a another vertical line. That is the second antenna. Now draw a sine wave looking exactly like the first going off to the right. You should see that they both peak at the same place. That means that most the radio signal will be going in the direction off to the right.
Next repeat the drawing with the second antenna at the point where the since wave goes from + (above the line) to - (below the line). Draw a sine wave identical to the first starting at that antenna. You should see that when the first one is positive, the other is negative and so forth. In that case little or no radio signal will go to the right since they cancel each other out.
That is the basic principle of dierctional attennas. It gets a LOT LOT more complicated (see the notes above on phasing) and multi-tower arrays but it is all on the same basic principal.
Next repeat the drawing with the second antenna at the point where the since wave goes from + (above the line) to - (below the line). Draw a sine wave identical to the first starting at that antenna. You should see that when the first one is positive, the other is negative and so forth. In that case little or no radio signal will go to the right since they cancel each other out.
That is the basic principle of dierctional attennas. It gets a LOT LOT more complicated (see the notes above on phasing) and multi-tower arrays but it is all on the same basic principal.
The link below leads to an example of the physical and electrical parameters used to create a directional radiation pattern for AM broadcast stations.
The net radiation pattern is a function of the number of towers and their physical orientation with respect to each other, along with the relative power and phase applied to each tower compared to a "reference tower."
Many different patterns are possible depending on the number/location of towers, and the power/phase each one uses.
The usual purpose for using a directional AM broadcast array is to minimize interference to co- and adjacent-channel stations.
http://i62.photobucket.com/albums/h85/rfry-100/DAPatternExample.gif
RF
The net radiation pattern is a function of the number of towers and their physical orientation with respect to each other, along with the relative power and phase applied to each tower compared to a "reference tower."
Many different patterns are possible depending on the number/location of towers, and the power/phase each one uses.
The usual purpose for using a directional AM broadcast array is to minimize interference to co- and adjacent-channel stations.
http://i62.photobucket.com/albums/h85/rfry-100/DAPatternExample.gif
RF
@ David Gleason :
Since I dabble only very occasionally into FM DX -- usually finding it a real treat when I do, though -- I have a question about directional FMers.
True that FM directionals do not benefit much from the power they DON'T send toward a protected station? There are AM patterns galore where a 5000-watt regional has deep nulls in one direction but the 5000 watts goes in some other direction. Sort of like a long tube ballon pinched in the middle ; there's still 5000 watts of air in the balloon.
But all the FM directionals I've seen (again, only casually) seem to consist of the same rough circle but with a rounded chunk taken out of it in the direction of the protected station. Sorta like a big cookie with an erosion-type nibble on that one side. Or if you will, like the start of a lunar eclipse when the disc begins to get eaten away. An FMer I saw, which formerly was directional, got to open up their signal full omni when an adjacent station changed their frequency. But in all the other directions, the newly omni'ed signal contours didn't change at all.
Since I dabble only very occasionally into FM DX -- usually finding it a real treat when I do, though -- I have a question about directional FMers.
True that FM directionals do not benefit much from the power they DON'T send toward a protected station? There are AM patterns galore where a 5000-watt regional has deep nulls in one direction but the 5000 watts goes in some other direction. Sort of like a long tube ballon pinched in the middle ; there's still 5000 watts of air in the balloon.
But all the FM directionals I've seen (again, only casually) seem to consist of the same rough circle but with a rounded chunk taken out of it in the direction of the protected station. Sorta like a big cookie with an erosion-type nibble on that one side. Or if you will, like the start of a lunar eclipse when the disc begins to get eaten away. An FMer I saw, which formerly was directional, got to open up their signal full omni when an adjacent station changed their frequency. But in all the other directions, the newly omni'ed signal contours didn't change at all.
You've got it right, Steve. A directional for an FM can only subtract coverage from the class maximum. Canada, which has adopted FM directional antennas very eagerly in the last decade or so, understands this clearly: a directional FM's license there lists both average ERP and maximum ERP. A "class B" FM station there will show up as something like "17 kW average, 50 kW maximum."
US FM directional licenses list only the maximum power, so a class B directional FM in the US will show "50 kW" ERP, but will be putting out significantly less - but never more - in some directions.
An AM license, by contrast, shows what amounts to "average" power. A 50 kW station with a non-directional antenna (think KNBR) is putting out the equivalent of 50 kW (give or take antenna efficiency, which is a big variable) in all directions. But a 50 kW station with a directional antenna (think KGO or KCBS) is putting out much less than the equivalent of 50 kW in some directions - and thus pushes the rest of that power out in other directions. (That's the "balloon" of which Steve speaks.)
US FM directional licenses list only the maximum power, so a class B directional FM in the US will show "50 kW" ERP, but will be putting out significantly less - but never more - in some directions.
An AM license, by contrast, shows what amounts to "average" power. A 50 kW station with a non-directional antenna (think KNBR) is putting out the equivalent of 50 kW (give or take antenna efficiency, which is a big variable) in all directions. But a 50 kW station with a directional antenna (think KGO or KCBS) is putting out much less than the equivalent of 50 kW in some directions - and thus pushes the rest of that power out in other directions. (That's the "balloon" of which Steve speaks.)
DavidEduardo said:
Something I've been wondering about re: multi-tower diplexing stations... Let's say you had a transmitter site way out in the sticks (where except for the transmitter site itself and any road passing through, the nearest development of any kind is at least 5-7 miles away - it's all nature within that radius) that's about 40-50 miles away (conductivity averaging at least 8-10 or so) from each of two 70-100k-plus-population metro areas in opposite directions, or three metro areas directionally separated by something like 120°, for example.
Would it be theoretically possible to have two (or three) 50kW stations diplex into the same site that are on first or second adjacents, with very different directional antenna patterns (for example where one's field might be 8,000 mV/m @ 1 km, the other(s) might be 0.01 mV/m @ 1 km)? Would it even be possible to have co-channel diplexers if their directional patterns are severe enough so that once they get outside that 5-7 mile no-development-in-here-except-transmitter-site radius, the protections are back to normal? Or is it impossible to diplex co-channel stations, even with severe directional patterns out in the sticks?
No, you can't diplex co-channel stations, or even stations on closely adjacent channels. Even if you're creating different patterns out of the antenna, you're putting RF from two transmitters into common towers, and you need to be able to install filtering to prevent station A from feeding RF back into station B's transmitter, and vice versa. Just because they're going into the towers with different phasing doesn't mean you can then filter what comes back down the transmission line on the same frequency on which you're trying to transmit.
Conventional wisdom says you need at least a 10% difference in frequency to make a diplexed AM antenna system work. The tightest one I know about, in practice, is the 1290/1340 (and 1490) in Santa Barbara, and it comes at a price: the very sharp filtering needed to make 1290 and 1340 work also significantly cuts into the audio bandwidth of both stations. That site "works" only to the extent that both 1290 and 1340 can stay on the air in a market where land-usage issues might otherwise prevent them from finding transmitter sites.
Conventional wisdom says you need at least a 10% difference in frequency to make a diplexed AM antenna system work. The tightest one I know about, in practice, is the 1290/1340 (and 1490) in Santa Barbara, and it comes at a price: the very sharp filtering needed to make 1290 and 1340 work also significantly cuts into the audio bandwidth of both stations. That site "works" only to the extent that both 1290 and 1340 can stay on the air in a market where land-usage issues might otherwise prevent them from finding transmitter sites.
R. Fry said:
In many instances, particularly outside the US, directionals are used to tailor a signal to a market.
The first one I saw was HJED in Cali, Colombia, with 50 kw on 820 into two towers spaced and fed so that the signal went up and down the Cauca River valley and was not wasted on the mountain areas on either side. That way they covered the most useful population areas.
I built my first directional in Quito, Ecos de la Montaña on 660, with a two tower array intended to send the signal up and down the Andes, a fairly synergistic market area with several major cities. I had no phaser, just two electrically equal coax lines leading to each tower producing a flattened figure 8, with maybe 500 watts to the sides and about 20 kw off each end (approximate; we never measured field strength and went on our overall satisfaction with the design and the resultant signal).
Then, more recently, I was involved with Radio 10 in Buenos Aires, 100 kw on 710. There was a feeling that the 100 kw would still not be enough to give a clear noise free signal in the central city part of the market, so a parasitic tower was used to nudge about the equivalent of 150 kw towards downtown... it worked, and the station has been the #1 AM for nearly 15 years, often #1 in the total market, too.
Scott Fybush said:
Hmm... What about FM or TV, though - stations that while they may use common towers, use separate antennas? For example, let's say you had a fairly high mountain (possibly in a mountain range in real life, but in the examples below there are few mountains nearby greater than 10% higher than the plains floors relative to the peak) between a few major cities.
---- T---
--- / | \
---- / - ---
--- / / \ \
---- / / \ ---
--- / | \ \
---- / | | ---
--- / / | \
---- / - | ---
--- / / \ \
---- / / \ ---
--- / / \ \
---- / / --- --- #
--- / / \ # ## #
# # / -- Summit Elev 22,440 ft AMSL \ ## ### ## ##
# # # # # # # / Tower Height 1980 ft AGL ---------------------------------------------------
# ## #### ### # # ### ### / City A distance 80 mi City C dist 40 mi
------------------------------------------------------------------ plains elev 1000 ft AMSL
City A dist 65 mi plains elev ~ sea level City B dist 15 mi
### ##
#### City E
### ###
####
|
| # #
City D | # ### City F
# # | ###
# ## | - #
# \ | - # ## # #
## \ - - # ### #
#### / \ - # # ### ## ## ### City H
# ## ----------------------| ^ |----------------------------------#-#---##---#------------------------------- ### # ###
#### \ / ----------- # #### # ## # # ##
City M / - \ ---------- ###### # # # ## ## #
- | \ # ### # City G dist = 200 mi
/ | \ # # ###
- | \
/ ## ## \
- # ## # \
/ # # \
- City K ### #
# # # / | #######
# ## # - | ## #### # City I
# ## | ### ## ##### #
## #### | # ## ## ###
### # # |
# |
City L |
|
|
|
## ###
## # #### City J
# # ## <--- dist = 70 mi
# ### # ##
# ####### ## #
In the above examples, the two mountain tops are different scenarios. Also the examples aren't really to scale.
City A and City B are approximately the same elevation (about 4 miles below the summit), but different distances, so they'd be served with different beam tilts in the same direction. City C is the opposite direction.
In the multi-azimuth examples, the mountain peak is about 26,400 ft AMSL, and the tower height is about 1980 ft AGL.
City G is about -240 ft AMSL, and City H is about 10,560 ft AMSL.
City K is about 3,000 ft AMSL at a distance of about 30 mi, and City J is about 500 ft AMSL at approximately 80 mi.
The #'s approximate the locations of each city. Population density of about 75% of the area between one city to another is comparable to that of north central Nevada, for example, and each city itself could have a population of 50k or more with an average land area of about 10 to 50 sq miles or so. Population within 5 miles of the transmitter site, except station employees, is 0.
The rest can be approximated however you wish. BTW I'm wondering if I'm being a bit conservative on the 200-mile distance from the 2nd mountain to City H, considering the elevation extremes in the above example, and the fact that KVYB 103.3 Santa Barbara gets reception 200 miles away with much flatter elevation differences relatively?
In the above examples, on FM stations, how closely could they be spaced? Could they possibly all be co-channel, considering they're all aimed different directions from different bays on the same tower? Assume each station is very directional, with a horizontal pattern similar to that of an AM station that has 8 or more towers and concentrates it on one long narrow lobe (think 1190 KFXR Dallas, TX's night pattern, for example). Also would it be possible to have comparably narrow beamwidths?
Also what about narrow radiation angles (or whatever the term is) for AM stations? For example a Franklin has less skywave and consequently more groundwave for a given power than, say, a 1/4-wave antenna. In looking at a Wikipedia article I also saw circle group aerials mentioned, where the article claims very flat radiation patterns could be realized. I was wondering ... if you combined the various techniques (a circle group aerial where each tower was a Franklin), what might the radiation efficiency be in mV/m @ 1 km for 1 kW? Would 1-2 V/m or more @ 1 km @ 1 kW be impossible?
Also in the realm closer to existing reality, on an AM station what would be the performance difference between a 1/2-wave radiator over a ground, vs. a sectionalized antenna with two 1/4-wave elements insulated and fed at the center (or 1/4-wave and 1/8+1/8-wave respectively)?
pianoplayer88key said:
Unlikely, as that combination isn't much less than the elevation AMSL of Mt Everest.
So probably not worth pursuing the rest of the coverage conditions described in the post, if someone was so inclined.
Just as with medium wave, each FM transmitter needs to be isolated from other systems physically nearby, in order for their r-f intermodulation products to meet the FCC limit of -80 dBc (5 kW and above). That pretty much eliminates using co-channel, co-located systems. Frequency separations of at least 600 kHz might be possible depending on the cross-coupling of the antenna systems, using suitable r-f filters between the tx outputs and the antenna inputs at the site.
Some who knows more about it may want to chime in on the major AM non-directional diplex site in Honolulu where the bandwidth on these AM stations
is pretty bad, I recall when working for Cecil Heftel who owned KGMB now KSSK on 590. Until KISS-108 Heftel Broadcasting bought exclusively RCA, when visiting
KISS for the first time he saw the new Harris MW-5A and commented that this was the first non RCA he ever owned. They had just bought a couple of new
RCA BTA-5L'S Ampliphase transmitters at KGMB and that did not like the diplex situation at all. When these transmitters were carefully tuned into a perfect load
none sounded better due to their even order harmonics that are very sweet, under less than ideal conditions...... frightening.
is pretty bad, I recall when working for Cecil Heftel who owned KGMB now KSSK on 590. Until KISS-108 Heftel Broadcasting bought exclusively RCA, when visiting
KISS for the first time he saw the new Harris MW-5A and commented that this was the first non RCA he ever owned. They had just bought a couple of new
RCA BTA-5L'S Ampliphase transmitters at KGMB and that did not like the diplex situation at all. When these transmitters were carefully tuned into a perfect load
none sounded better due to their even order harmonics that are very sweet, under less than ideal conditions...... frightening.
To add what David has commented on regarding MW stations using directional antenna arrays for increased signal levels in target area, many of the international MW broadcasters have been doing this for years.
The VOA Poro, Philippines and Okinawa MW stations were built in the early 1950s, and while both had non-directional capabilities designed into the array, the normal operating pattern was one of the several selectable patterns available.
Poro had a four-tower array, Okinawa had a six-tower array.
Okinawa was closed in the mid-1970s, and the old Poro MW site was closed a few years back, after it was replaced by a five tower array and new transmitter building about a half mile away. The IBB Kuwait 1548 KHz facility has seven towers in that array, although not all of the seven towers are used in each of the four selectable patterns.
The VOA Poro, Philippines and Okinawa MW stations were built in the early 1950s, and while both had non-directional capabilities designed into the array, the normal operating pattern was one of the several selectable patterns available.
Poro had a four-tower array, Okinawa had a six-tower array.
Okinawa was closed in the mid-1970s, and the old Poro MW site was closed a few years back, after it was replaced by a five tower array and new transmitter building about a half mile away. The IBB Kuwait 1548 KHz facility has seven towers in that array, although not all of the seven towers are used in each of the four selectable patterns.
I want to come back to pianoplayer's mountaintop scenario for a moment, because there are a few other factors at play beyond RFry's well-reasoned response.
For full-power FM stations, the FCC limits the amount of directionality an antenna system can employ. It's 73.316(b)(2):
In practice, then, you can't license an FM antenna for a full-powered station in the US that has a deep enough null to do what pianoplayer describes.
You can, however, license much more directional antennas for translators, and you can certainly have mountaintop sites that serve different areas with different directional beams coming from separate antennas on the same tower.
Even so...those antennas all need to be fed from the same transmitter if they're on the same frequency (or close to it). As RFry points out, you have a real-world issue of isolation between transmitters at the same site to prevent intermodulation. There's no way you can isolate two signals on the same frequency at the same site - in the real world, there's just too much RF coming back down the transmission line from the antenna.
It doesn't have to be 600 kHz separation these days; there are plenty of sites with a full-power signal separated by only 400 kHz from a co-located transmitter, and everyone gets along. But co-channel doesn't work.
(Now...if instead of putting those antennas all the way at the peak of your hypothetical mountain, you were instead to put them somewhere down the slope, thus using the mountain itself to provide physical shielding between antennas, then yes, one can imagine co-channel facilities at surprisingly close distances, though current FCC rules would prevent that for full-power signals.)
For full-power FM stations, the FCC limits the amount of directionality an antenna system can employ. It's 73.316(b)(2):
In practice, then, you can't license an FM antenna for a full-powered station in the US that has a deep enough null to do what pianoplayer describes.
You can, however, license much more directional antennas for translators, and you can certainly have mountaintop sites that serve different areas with different directional beams coming from separate antennas on the same tower.
Even so...those antennas all need to be fed from the same transmitter if they're on the same frequency (or close to it). As RFry points out, you have a real-world issue of isolation between transmitters at the same site to prevent intermodulation. There's no way you can isolate two signals on the same frequency at the same site - in the real world, there's just too much RF coming back down the transmission line from the antenna.
It doesn't have to be 600 kHz separation these days; there are plenty of sites with a full-power signal separated by only 400 kHz from a co-located transmitter, and everyone gets along. But co-channel doesn't work.
(Now...if instead of putting those antennas all the way at the peak of your hypothetical mountain, you were instead to put them somewhere down the slope, thus using the mountain itself to provide physical shielding between antennas, then yes, one can imagine co-channel facilities at surprisingly close distances, though current FCC rules would prevent that for full-power signals.)
I don't know if anyone has mentioned this, but there is also a 15 dB (31.6:1 ERP ratio) maximum to minimum directionality ratio for FM stations. It's 20 dB in Canada (100:1 ERP ratio) under treaty as I recall.
Scott, I was thinking that the only thing the co-channel co-located stations would have in common is the fact that they're mounted on the same tower. Otherwise they'd be feeding different antenna bays through different transmission lines, etc. Your post does explain it some, though, and the part about putting them down the mountain a little to take advantage of the mountain, while it defeats my idea of having the GPS coordinates be the same on the stations, is an interesting idea. (Also I can understand not being able to broadcast two different co-channel stations from the same antenna, even if it was fed with different transmission lines, but it might be interesting if it could be done.)
As for your first comment, are you referring to having a deep enough null on the horizontal plane to serve a city 75 miles away while having no detectable signal in another city 15 miles away and and 30° off beam, or are you referring to using different radiation angles to pass over a city 10 miles away (close to the mountain) but serve a city 50 miles away on the same horizontal bearing? (BTW I'm talking about a hypothetical mountain at least 25,000 feet AMSL, where the plains areas are near sea level and can be clearly seen from the mountaintop in all directions.)
As for your first comment, are you referring to having a deep enough null on the horizontal plane to serve a city 75 miles away while having no detectable signal in another city 15 miles away and and 30° off beam, or are you referring to using different radiation angles to pass over a city 10 miles away (close to the mountain) but serve a city 50 miles away on the same horizontal bearing? (BTW I'm talking about a hypothetical mountain at least 25,000 feet AMSL, where the plains areas are near sea level and can be clearly seen from the mountaintop in all directions.)
From the FCC's perspective, directionality of an FM antenna is entirely in the horizontal plane. (Well, almost - some FM and TV stations with antennas at extremely high elevations are allowed to use either electronic or mechanical "beam tilt," usually of no more than 2 or 3 degrees, to ensure service to communities at lower elevations by avoiding the sort of "pass over" effect of which you speak. Such antenna systems are not considered "directional," however.)
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.
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.
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