Alright...you engineering profs, tell me all about it. In terms easily understood, exactly how does an AM pattern work. How are they set up, how do you get the tower to send signal in one direction and not the other, and what exactly does the tuning cabinet do? No one has effectively explained this in a manner easily understood. Wanna give it a try? Thanks!
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AM PATTERNS QUESTION
- Thread starter Steve Eberhart
- Start date
In brief, it's the physical property of wave action that makes a directional pattern work for an AM station.
Let's begin by taking the simplest form of directional antenna, two towers spaced one wavelength apart, with the signal arriving at both towers at the same instant.
In areas where the signals from the two towers arrive at a receiver with the same intensity at the same instant, the amount of signal entering the receiver is twice the amount which would result from a single tower at the same power level.
As the receiver is moved from one place to another, the strength of each tower's signal, and the timing by which the signal from each tower reaches the receiver will vary. These are called power differential and phase differential. Eventually, places will be found where the signals from the towers arrive at total phase differential (180 degrees) with respect to each other, cancelling the station's entire signal, or in engineer's terms, creating a "null". These nulls tend to occur in bands, at predictable intervals and directions from the antenna system.
The nulls of a given directional antenna system can be modified by four characteristics, usually set up in its design:
1) Position of the towers - Straight-line, triangle, parallellogram, etc.
2) Spacing between the towers
3) Varying the amount of signal radiated by each tower
4) Introducing a slight phase delay in the signal's arrival to each tower
Except for the very simplest of designs, the actual design of a directional tower system requires someone who knows much higher math, or a VERY good computer system.
The tuning cabinet matches the impedance of the feed line from the transmitter or phasor cabinet to the impedance of the antenna, thus ensuring that the maximum amount of signal passes from one to the other. It has no direct relationship to a directional antenna system, other than being the point from which tower currents are read, which ensures that the required ratios of signal are being fed to each tower.
Well, I can't guarantee to be 100% understandable in this explanation. In 1981, I was working for a station with a tight directional pattern. Up until that point, we had a full staff of engineers while the announcing staff worked from the studio at a remote site and didn't have to even have third tickets. Then, the FCC relaxed the rules on directional stations, and the owners moved the studio out to the transmitter boonies so they could fire the engineers. Fortunately, I had a third ticket but had never run any tight 4-tower directionals. The learning curve on that system was something else!
Later....
Matt Smith
WGSR-TV...glad TV has ONE tower!
The true explaination of directional AM arrays requires Calculus. With that being said, here's somewhat of a simpler explaination of how the beast works. If you were out on a lake and had two pebbles you precisily dropped at the same time, with the same force spaced, let's say a half wave apart from each other, you'd end up with a figure 8. This is a very common pattern of many AM directionals. Adding force on either side would yeild a distortion moreso in one way, causing that lobe to be bigger. If you change the spacing, that affects the pattern also. Change the timing (phase), you'll push it out one end. If you were to have pebbles you dropped with more force on the first one, less on the center one, and even less on the first one with the more force one hitting first, then the middle one, then the most powerful one hitting last, you'd have a classic 3 tower directional array with a the major lobe firing right out the front end. Once again, the amount of force (current) on each pebble (tower) you apply, and the distance between the pebble drops (tower locations) affect the waves (pattern) that is visiable with water, and obviously not visable, but measurable with R.F. Computer modeling has made all the figuring much easier, but keep in mind when most directional AMs were put into operation, they didn't even have a calculator. In some ways the concept is very simple, but the math used to figure and install a system is mind-boggling, especially in the 30s, 40s, and 50s.
If you can find a copy of it, the NAB put out a VERY good book on AM directionals back in the late 70s-early 80s. It's a paperback book. There is also a larger book that's a hardback that's even harder to find, but it's more difficult to understand, and I wouldn't recommend even bothering to try to find it.
As time rolls on, less and less people actually have enough training or ability to install, engineer, or even maintain directional arrays. There's only a handful of EEs that still know how to design a system or make major modifications to it. Our industry could be in a great deal of trouble soon in AM radio. The beancounters have no clue of what awaits them in 15 years or less IMHO.
If you can find a copy of it, the NAB put out a VERY good book on AM directionals back in the late 70s-early 80s. It's a paperback book. There is also a larger book that's a hardback that's even harder to find, but it's more difficult to understand, and I wouldn't recommend even bothering to try to find it.
As time rolls on, less and less people actually have enough training or ability to install, engineer, or even maintain directional arrays. There's only a handful of EEs that still know how to design a system or make major modifications to it. Our industry could be in a great deal of trouble soon in AM radio. The beancounters have no clue of what awaits them in 15 years or less IMHO.
OK, short version:
consider two towers, spaced one-half wavelength apart and fed in-phase (neither tower is phase-shifted in relation to the other)
at any point equidistant to the towers (on a line midway between them and to either side) the energy from the two will arrive at the same time, adding together
now consider a point on a line that runs thru both towers
energy from the far tower starts out toward the observer, but because it travels one-half wavelength further, it is exactly 180 degrees out of phase (opposite polarity) when it reaches the closer tower
thus on the line THRU the towers, there is a null in both directions (equal and opposite cancel)
plotted from above, the pattern would look like a figure '8', with the two towers in the 'notches' on the sides of the '8'
now take the same two towers but at quarter-wave spacing and shift phase to one by 90 degrees in relation to the other
in line with the towers, in one direction you will find the radiated energy adding (it starts off 90 out of phase, but by the time it travels the half-wave (90 degrees) to the second tower it is back in-phase), in the other direction the radiation is 180 out of phase and cancels
this is the classic 'cardioid' pattern, like a very blunt valentine heart
this is the basis for AM directionals: spacing and phasing, plus power ratio to control 'depth of null'
three things to remember:
1) relative phasing controls direction of nulls
2) power ratio controls 'depth' of nulls (in first example above, in theory, 1:1 ratio would give a null so deep that you couldn't measure radiation on the equidistant-from-towers line; in practice, re-radiation from guy wires and such makes that an impossibility)
3) for an in-line array, maximum number of nulls is 2N-2, and are symetrical about the line of the towers
simplest way to plot it is vector addition, especially for non-in-line arrays
the phasing cabinet is 'where the magic starts', in that it contains components to shift phases and divide power
hope this helps !
it is a fascinating subject, one that I love to deal with (KHMO 6-tower monster built in 1948, 6 Blaw-Knox 229 ft self-supporters)
consider two towers, spaced one-half wavelength apart and fed in-phase (neither tower is phase-shifted in relation to the other)
at any point equidistant to the towers (on a line midway between them and to either side) the energy from the two will arrive at the same time, adding together
now consider a point on a line that runs thru both towers
energy from the far tower starts out toward the observer, but because it travels one-half wavelength further, it is exactly 180 degrees out of phase (opposite polarity) when it reaches the closer tower
thus on the line THRU the towers, there is a null in both directions (equal and opposite cancel)
plotted from above, the pattern would look like a figure '8', with the two towers in the 'notches' on the sides of the '8'
now take the same two towers but at quarter-wave spacing and shift phase to one by 90 degrees in relation to the other
in line with the towers, in one direction you will find the radiated energy adding (it starts off 90 out of phase, but by the time it travels the half-wave (90 degrees) to the second tower it is back in-phase), in the other direction the radiation is 180 out of phase and cancels
this is the classic 'cardioid' pattern, like a very blunt valentine heart
this is the basis for AM directionals: spacing and phasing, plus power ratio to control 'depth of null'
three things to remember:
1) relative phasing controls direction of nulls
2) power ratio controls 'depth' of nulls (in first example above, in theory, 1:1 ratio would give a null so deep that you couldn't measure radiation on the equidistant-from-towers line; in practice, re-radiation from guy wires and such makes that an impossibility)
3) for an in-line array, maximum number of nulls is 2N-2, and are symetrical about the line of the towers
simplest way to plot it is vector addition, especially for non-in-line arrays
the phasing cabinet is 'where the magic starts', in that it contains components to shift phases and divide power
hope this helps !
it is a fascinating subject, one that I love to deal with (KHMO 6-tower monster built in 1948, 6 Blaw-Knox 229 ft self-supporters)
A
Andy_Brown
Guest
"Alright...you engineering profs, tell me all about it. In terms easily understood, exactly how does an AM pattern work. How are they set up, how do you get the tower to send signal in one direction and not the other, and what exactly does the tuning cabinet do? No one has effectively explained this in a manner easily understood. Wanna give it a try? Thanks!"
rule #1 Never ask an engineer for a simple explanation of a complex subject
rule #2 Simple explanations to some complex subjects are not tenable
rule #3 You can't change rule #2
rule #4 Break rule #1 at your own risk
"How does an AM pattern work:"
What everyone is trying to explain in simple terms is called "constructive" and "destructive" interference. Here is a simple explanation: http://www.glenbrook.k12.il.us/gbssci/phys/CLass/waves/u10l3c.html
So, in lay terms, as detailed by the other posts which are mostly accurate, by varying the amount and phase (timing) of the RF to multiple towers, the ground waves will propagate and interact with each other ... sometimes adding and becoming larger, sometimes subtracting and becoming smaller.
"How do you get the tower to send signal in one direction and not the other"
By putting up another tower. Just the differences in transmission line length by itself will create a directional pattern. Add to that reactive (capacitive and inductive) losses introduced at the tower base, and you have a D.A.
"What exactly does the tuning cabinet do?"
As previously explained, it matches the impedance of the towers + their transmission lines to the common point, which is generally maintained to be electrically 50 Ohms with no reactive component (aka 50 + j0). This is where TPO (transmitter power output) is measured. So, the transmitter feeds the common point which in turn feeds the phasor network which has an output for each tower. It can have bearing on phase relationships, so it is considered part of the directional hardware. The phasor cabinet is often where the fine adjustments to the behavior of the array are made. The tuning box at the base of the tower is another matching network where the tower's impedance is matched to the phasor cabinet output for that tower, plus the transmission line effects are taken into consideration. Tower power is measured at the tower base in the tuning cabinet, although operators (spelled computer) usually measure samples taken inductively off the radiating tower and fed back to the phase monitor on a separate coax. These samples can be calibrated and compared with the sample from the other towers to make sure the pattern is operating correctly. In the old days, once a month you had to go out to monitoring points and measure the FI (field intensity) to further support that, although these days I don't even know what's required.
AM radio will probably disappear altogether, since the frequency is so valuable for it's propagation characteristics. It will take some time, but eventually the channels will be widened and modulated digitally and the bandwidth will be awarded to some emerging technology that may not even be thought of yet. With the consolidation of ownership ever growing, less and less companies would be effected by the cessation of AM broadcasting, and they'll agree to it probably in exchange for the rights to broadcast the emerging technology it is replaced by, whatever that is.
rule #1 Never ask an engineer for a simple explanation of a complex subject
rule #2 Simple explanations to some complex subjects are not tenable
rule #3 You can't change rule #2
rule #4 Break rule #1 at your own risk
"How does an AM pattern work:"
What everyone is trying to explain in simple terms is called "constructive" and "destructive" interference. Here is a simple explanation: http://www.glenbrook.k12.il.us/gbssci/phys/CLass/waves/u10l3c.html
So, in lay terms, as detailed by the other posts which are mostly accurate, by varying the amount and phase (timing) of the RF to multiple towers, the ground waves will propagate and interact with each other ... sometimes adding and becoming larger, sometimes subtracting and becoming smaller.
"How do you get the tower to send signal in one direction and not the other"
By putting up another tower. Just the differences in transmission line length by itself will create a directional pattern. Add to that reactive (capacitive and inductive) losses introduced at the tower base, and you have a D.A.
"What exactly does the tuning cabinet do?"
As previously explained, it matches the impedance of the towers + their transmission lines to the common point, which is generally maintained to be electrically 50 Ohms with no reactive component (aka 50 + j0). This is where TPO (transmitter power output) is measured. So, the transmitter feeds the common point which in turn feeds the phasor network which has an output for each tower. It can have bearing on phase relationships, so it is considered part of the directional hardware. The phasor cabinet is often where the fine adjustments to the behavior of the array are made. The tuning box at the base of the tower is another matching network where the tower's impedance is matched to the phasor cabinet output for that tower, plus the transmission line effects are taken into consideration. Tower power is measured at the tower base in the tuning cabinet, although operators (spelled computer) usually measure samples taken inductively off the radiating tower and fed back to the phase monitor on a separate coax. These samples can be calibrated and compared with the sample from the other towers to make sure the pattern is operating correctly. In the old days, once a month you had to go out to monitoring points and measure the FI (field intensity) to further support that, although these days I don't even know what's required.
AM radio will probably disappear altogether, since the frequency is so valuable for it's propagation characteristics. It will take some time, but eventually the channels will be widened and modulated digitally and the bandwidth will be awarded to some emerging technology that may not even be thought of yet. With the consolidation of ownership ever growing, less and less companies would be effected by the cessation of AM broadcasting, and they'll agree to it probably in exchange for the rights to broadcast the emerging technology it is replaced by, whatever that is.
Simple answer...
Its trigonometry (I don't think its Calculus...I don't understand Calculus) ...add sine waves from the various towers together at the location of the receiver.
Sine waves from the different towers add in some directions and cancel in others due to the received phase difference at the listener's location.
Relative power in each tower determines the depth of the nulls.
Two towers at 100% power received 180 degrees out of phase cancels completely (theoretically a null)
One tower at 100% power and another at 50% power (still 180 degrees out of phase) only cancels half (theoretically a minima)
Manipulate all of the above by number of towers, orientation, relative phase, and relative power to create the coverage/protection required.
Add tower height to the formula for all the same possibilities at different elevations above the horizon (skywave).
Oh! That tuning cabinet...it adjust the relative phase and power ratio fed to each tower to "tweak" the real world pattern to match the theoretical one designed above. That antenna monitor is the measuring device.
Its trigonometry (I don't think its Calculus...I don't understand Calculus) ...add sine waves from the various towers together at the location of the receiver.
Sine waves from the different towers add in some directions and cancel in others due to the received phase difference at the listener's location.
Relative power in each tower determines the depth of the nulls.
Two towers at 100% power received 180 degrees out of phase cancels completely (theoretically a null)
One tower at 100% power and another at 50% power (still 180 degrees out of phase) only cancels half (theoretically a minima)
Manipulate all of the above by number of towers, orientation, relative phase, and relative power to create the coverage/protection required.
Add tower height to the formula for all the same possibilities at different elevations above the horizon (skywave).
Oh! That tuning cabinet...it adjust the relative phase and power ratio fed to each tower to "tweak" the real world pattern to match the theoretical one designed above. That antenna monitor is the measuring device.
As long as we're not being serious...
Mirrors angled to reflecting the smoke differently.
radiosaur said:Magic.
Mirrors angled to reflecting the smoke differently.
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