Cancellation Zone ?

[Steve Green NEPA]

Perhaps there is someone aboard here who can explain (in small-word layman terms :- ) what that means.

And how it's generally measured ....... if it is measurable. Does it, say, vary from the upper dial AM from the lower AM dial? Or from an omni to a directional station ?

Despite what I've read on this forum about the blowtorches with stingy signals and the improbable low-power stations that cook, I honestly can say I never heard the term before in over 50 years of AM DXing. I think I can make a stab at what the 'cancellation zone' would be, and maybe even why, but it probably would be very sophomoric. Someone here could explain the rudiments of what it is, for non-techies such as I, just in case I have to explain it to someone one day .

-- Rod Serling voice --
So, for my perusal, evaluation and edification (on the signpost up ahead) : What is The Cancellation Zone?

 

[Schroedingers Cat]

It could refer to two things.

1) If the Omni or equal height directional towers are above 1/2 wavelength, and the groundwave is low enough, there will be a zone where the current on two parts of the tower cancel (actually the integration of all the moment vectors at infinitely small distances along the tower equals zero), and the vertical radiation characteristic is (near) zero (the theoretical). At the distance corresponding to reflection of the skywave at this angle, there will be near zero skywave. R. Fry's graphs show that this is closer to 20 dB, so a 50 kW station is radiating the equivalent of 0.5 kW at that angle and corresponding distance. But if there is very little groundwave, all you will hear the is the nearly completely cancelling, phase distorted skywave.

2) The place where the groundwave and skywave are the same strength, and as the phase changes with propagation, the signal is theoretically zero. Usually you hear a weak phase distorted signal and not a complete lack of signal. The distance depends on many factors. Tower electrical height, actual conductivity at actual conditions (which varies with the time of year, temperature, soil wetness, etc.) along the groundwave path, and skywave conditions. Any combination which produces equal groundwave and skywave will produce this type of intermittent cancellation.

 

[fybush]

Skywave goes up. Skywave comes back down, over a fairly long path that goes up to the ionosphere and back down at an angle.

Groundwave goes straight out, at nearly the speed of light over a straight line from the transmitter along the ground to the listener.

If the listener is at a spot close enough for the groundwave to still be receivable but far enough for the first hop of the skywave to also be coming back down, the two signal paths of different lengths can cause those two signals to be received out of phase and thus to cancel each other out. That's the cancellation zone.

The nature of the cancellation is that it's highly variable - the skywave fades in and out as it beats against the groundwave; the extent of the skywave depends on the vertical radiation angle from the antenna and any given night's propagation; the extent of the groundwave depends on ground conductivity, which itself varies with the weather, and so on.

In general (but very much in general), cancellation will happen closer in at higher frequencies and somewhat farther out at lower frequencies. The zone will look more like a circle for a non-DA station (give or take variability in ground conductivity), less like a circle for a DA station (give or take the real-world differences between the pattern a given DA puts out via groundwave and at higher angles).

 

[rfry]

Perhaps there is someone aboard here who can explain (in small-word layman terms :- ) what that means.

The "cancellation zone" is the range of distances from a transmit antenna system where the skywave and groundwave signals it radiates produce roughly the same field intensity at receiver locations.

For MW AM broadcast stations and depending mostly on propagation conditions for the nighttime skywave (also on many other factors) -- these two radiated components either may cancel or strengthen each other in this distance range over fairly short intervals of time.

This can cause objectionable, periodic, nighttime fading and audio distortion in consumer-level AM receivers located there, even though they provide acceptable reception of that station during daylight hours.

 

[schmave]

I'm no engineer, and all I can add comes from observations over the years.
Here in Ohio where I live, we experience the effect of various cancellation zones, largely caused by our ground conductivity. WTAM from Cleveland, 140 miles northeast, often is unlistenable at night because of all the fading. WLW, 90 miles southwest, puts a decent signal over our metro with very little if any cancellation, but just beyond Columbus to the north and east is where that ring of cancellation starts, and it can go 40-50 miles. It's not like you can drive 10 miles and you're into territory where the reception is pure skywave.
It's no coincidence that the conductivity between here and Cincinnati is MUCH better than between here and Cleveland. Same for Pittsburgh. I've heard cancellation less than 40 miles from KDKA's tower at night, and it still comes in horribly here at all hours, about 190 miles from the stick.
WSB starts to cancel at about that far because of the bad conductivity down in the Atlanta area. Conversely, the low-dial Chicago stations (WSCR, WGN and WBBM specifically) can last well more than 100 miles before any skywave starts to interfere.

 

[rfry]

I'm no engineer, and all I can add comes from observations over the years. (etc)

And cogent they are, schmave.

But just to elaborate, the nighttime skywave signals of WJR on 760 kHz and WSB on 750 kHz can be close to equal here in west central Illinois, even though neither of them has daytime groundwave coverage here.

A point to ponder is that earth conductivity at/near the WJR antenna system is quite a bit better than at/near the WSB antenna system.

 

[w9wi]

Perhaps there is someone aboard here who can explain (in small-word layman terms :- ) what that means.

And how it's generally measured ....... if it is measurable. Does it, say, vary from the upper dial AM from the lower AM dial? Or from an omni to a directional station ?

Despite what I've read on this forum about the blowtorches with stingy signals and the improbable low-power stations that cook, I honestly can say I never heard the term before in over 50 years of AM DXing. I think I can make a stab at what the 'cancellation zone' would be, and maybe even why, but it probably would be very sophomoric. Someone here could explain the rudiments of what it is, for non-techies such as I, just in case I have to explain it to someone one day .

-- Rod Serling voice --
So, for my perusal, evaluation and edification (on the signpost up ahead) : What is The Cancellation Zone?

I'm seeing a lot of valid explanations but kinda fear they're all in engineer-ese

While listening at night to a station maybe 80-100-150 miles away, have you ever heard the signal get badly distorted for a few seconds & then spontaneously clear up? Or, simply take a deep fade?

What's going on, is that you're receiving that station via two different paths. One path goes directly along the ground from the station's tower to your radio. The other one takes a side trip through the atmosphere.

And the two signals interfere with each other -- really, the station is interfering with itself.



There is a narrow range of distances in which this happens. If the station is too far away, (maybe more than 200 miles) you don't get any ground wave. If it's too close (maybe 40-50 miles) there's no "skywave". You have to have both for the station to interfere with itself.

It's called "cancellation" because if the difference in the length of the ground path, and that of the sky path, is just right, then the skywave cancels out some of the groundwave, or vice-versa.

=====

I suppose you could measure it with a field strength meter, watching the change in strength of the carrier signal. Although the level of fading and cancellation can vary widely within the AM channel, so often there isn't one specific amount of cancellation. IMHO it's more common on higher frequencies, where the ground wave doesn't get out as far and the skywave is stronger.

 

[Steve Green NEPA]

Great stuff, folks.
Turns out I had been kind of stranded somewhere between Scott Fybush's great 'DX-for-Dummies-in-NEPA' take and Cat's technical explanation.

And as I thought, the effect is as variable as there are variables.

Thanks all!

* * * * * * *

@W9Wi : They hardly play music on AM anymore, but I do remember hearing that effect you described, listening on Long Island, on both WKBW and WDRC.
'DRC was closer but slightly directional away from Long Island, while the farther-off 'KB sent a lot of signal east at us.
I used a pretty good radio for sound then, and the fading effect, iIrc, began as a crashing mess, then faded into what sounded like the bass control was cranked. After an interval (the 'cancellation'), the signal would return as though the treble control had been cranked. Eventually the mix became 'normal' once more, for a listenable while.
Sometimes the effect would be reversed ; treble fade and bass return.

 

[schmave]

I'm no engineer, and all I can add comes from observations over the years. (etc)

And cogent they are, schmave.

But just to elaborate, the nighttime skywave signals of WJR on 760 kHz and WSB on 750 kHz can be close to equal here in west central Illinois, even though neither of them has daytime groundwave coverage here.

A point to ponder is that earth conductivity at/near the WJR antenna system is quite a bit better than at/near the WSB antenna system.

I do my best! Didn't want to derail the topic either.
Now that said, what's the method to figure out how large a cancellation zone can be (if there is one)? I mentioned 40-50 miles in my experience with WLW. With WOWO back when they were on 50K at night, I remember it used to fade badly here in Columbus at night and we couldn't get it really well on U.S. 33 heading northwest until around Bellefontaine, maybe 100 miles from Fort Wayne and 50 miles from there. It was like breaking through a wall, and all of a sudden the reception improved dramatically.
The Chicago stations start cancelling at night just west of Fort Wayne, and by the time you hit the Ohio line 20-30 miles east, the skywaves are terrific.

 

[Schroedingers Cat]

The new high power Class Bs in Michigan demonstrate the cancellation zone quite dramatically. Stations such as WWJ and WFDF have a lot of fading in an area about 60 to 150 miles from the transmitters, and WXYT being on a higher frequency, from about 40 to 100 miles. The effects are made worse by shorter towers than a Class A would have, and because the design of the arrays have a broadside component which has higher angle skywave than an endfire linear array design. This was used to maximize power used, and maximize minor lobe directions that an endfire array would sacrifice. This has to do with the array RSS and RSS/RMS ratios, which theoretically affect the minimum null depth. Endfire arrays build up large fields in the center towers, which increases the RSS computation, and theoretically cause array instability. Once they get to 200 miles away, the new arrays blast in at above calculated NIFs much of the time, giving them de facto skywave service, which often exceeds Class A facilities, in the array maximum directions.

To get an idea what the skywave and groundwave are at some (x,y,z,t) location and time, measure the groundwave in the daytime at various times of the year, adjust for night pattern inverse field, and then measure the skywave by taking the difference between the maximum and minimum field strength on at least several nights. The skywave is then half the difference between the maximum and minimum field strength at any given short delta t (dt). People used to do these measurements back in the 1920s to 1940s, but now there is little interest in it except DXers.

 

[rfry]

... what's the method to figure out how large a cancellation zone can be (if there is one)?

Too many variables for an ~accurate method to estimate the width of the cancellation zone, IMO.

But below is a link to a graphic from F. E. Terman, showing how the distance to the center of the nighttime fading zone is related to the frequency and the radiator height.

Earth conductivity is also a factor here, but Terman didn't identify the conductivity value he used to generate these curves. From related graphics I think Terman used 100 km as the height of the reflecting E-layer

http://i62.photobucket.com/albums/h85/rfry-100/Terman_Fig_58_zpsd87fdce5.jpg

 

[cyberdad]

Real world examples involving the four Chicago 50kw blowtorches 670, 720, 780, 890...

In most directions, the cancellation zone for these is centered about 150-200 miles out. Variations occur based on frequency, atmospheric conditions, and ground conductivity. Generally speaking, the cancellation zone is "closer in" to the east and northeast due to the ground conductivity in southwest Michigan not being as good as in other directions. The signal may still be listenable to some ears in the cancellation zone, but fading is common and typically varies a little each night. As one moves away from the transmitter and gets away from the cancellation zone, the signal improves.

Where I went to college in southeast Iowa, WLS (about 220 miles away) with a so-so daytime signal, but was worse at night. Top 40 fans (like myself) frequently flipped over to KAAY, KOMA, or XERF once the sun went down. All had better nighttime signals than WLS. But if you went 60 miles east OR west, WLS had as good or better signal than any of the aforementioned.

 

[KR4BD]

My experience has shown that the higher the frequency, the cancellation zone moves in closer to the transmitter. A good example of this is WCKY-1530 licensed to Cincinnati. Last month, I was listening to one of the NCAA basketball games on WCKY around 9 PM. I was driving from Cincinnati to Lexington along I-75. The WCKY transmitter is very close to the intersecition of I-75 and I-275 in Northern KY, just a bit SW of Cincinnat itself. I had a very strong signal until I got to just a few miles NORTH of the Dry Ridge Exit. This is a distance of only 35 miles and the signal became extremely distorted with cancellation at this point and remained that way all the way to Lexington (another 40 miles or so). The distortion was not due to being in a null in WCKY's pattern as their null is to the west of Cincinnati, not to the South. Also, Last Week, I was in Dalton, GA (also on I-75) and the WCKY signal was like a solid local inside a hotel room as a result of the STRONG skywave and ZERO groundwave at that location.

WLW at 700 starts to "cancel" at night just as you get to the northern edge of Lexington, a distance of about 100 miles from their transmitter which is located NE of Cinci at Mason, OH. The WLW cancellation is usually not too severe in Lexington where WCKY is up and down constantly all night long with the skywave and groundwave fighting each other.

 

[Schroedingers Cat]

Cancellation zones are best found empirically by repeatedly driving the signal through the area of the zone. You can then notice subtle differences from night to night and season to season. The more directions and times you drive the signal, the better. But it's usually the well protected high power Class B or Class A stations where you can find them. If the NIF is so high that the cancellation zone is buried in interference, you may not notice it. It is important to log your results if you are going to accurately find them. Most people notice and file it away without writing it down. That may work when you're young, but it's better to keep closer track. Maybe use a recorder and say what mile markers on an expressway that you noticed what conditions.

 

[trusty]

Of course, there's the well-known story about WSM having to shorten their tower to move their cancellation zone further out because it was falling on populated areas like Knoxville, Chattanooga and Huntsville, so it may have some variables for each station, but not unpredictable. (Google "wsm cancellation chattanooga tower")

Is there any site that has maps showing cancellation zones for the major stations? Any site that comes close?

 

[davideduardo]

Of course, there's the well-known story about WSM having to shorten their tower to move their cancellation zone further out because it was falling on populated areas like Knoxville, Chattanooga and Huntsville, so it may have some variables for each station, but not unpredictable. (Google "wsm cancellation chattanooga tower")

Since WSM's groundwave is barely 0.5 mV/m at Chattanooga and much less than that at Knoxville, I'm surprised that they considered this to be skywave/groundwave cancellation.

But in reading the Wikipedia article about WSM, I see that it is filled with hyperbole and exaggeration about the coverage, so the comment about cancellation in areas with no usable groundwave coverage pretty much fit the tone of the whole article.

 

[ftballfan]

WTMJ is a strange example. Grand Rapids appears to fall in the cancellation zone, while Manistee has a VERY strong groundwave signal.

 

[schmave]

My experience has shown that the higher the frequency, the cancellation zone moves in closer to the transmitter. A good example of this is WCKY-1530 licensed to Cincinnati. Last month, I was listening to one of the NCAA basketball games on WCKY around 9 PM. I was driving from Cincinnati to Lexington along I-75. The WCKY transmitter is very close to the intersecition of I-75 and I-275 in Northern KY, just a bit SW of Cincinnat itself. I had a very strong signal until I got to just a few miles NORTH of the Dry Ridge Exit. This is a distance of only 35 miles and the signal became extremely distorted with cancellation at this point and remained that way all the way to Lexington (another 40 miles or so). The distortion was not due to being in a null in WCKY's pattern as their null is to the west of Cincinnati, not to the South. Also, Last Week, I was in Dalton, GA (also on I-75) and the WCKY signal was like a solid local inside a hotel room as a result of the STRONG skywave and ZERO groundwave at that location.

WLW at 700 starts to "cancel" at night just as you get to the northern edge of Lexington, a distance of about 100 miles from their transmitter which is located NE of Cinci at Mason, OH. The WLW cancellation is usually not too severe in Lexington where WCKY is up and down constantly all night long with the skywave and groundwave fighting each other.

I've heard WCKY chewing into itself as close in as Kings Island. Other posters (I know BobOnTheJob has, maybe others) have reported that happening in Hamilton, only 25 miles north of the tower. Yeah that signal is up at 1530, but with that ground conductivity I was shocked.
Even though WCKY's groundwave is bad in Columbus, it generally does not put a good skywave signal in here at night. It's there, but usually not at blaster status.

 

[ftballfan]

My experience has shown that the higher the frequency, the cancellation zone moves in closer to the transmitter. A good example of this is WCKY-1530 licensed to Cincinnati. Last month, I was listening to one of the NCAA basketball games on WCKY around 9 PM. I was driving from Cincinnati to Lexington along I-75. The WCKY transmitter is very close to the intersecition of I-75 and I-275 in Northern KY, just a bit SW of Cincinnat itself. I had a very strong signal until I got to just a few miles NORTH of the Dry Ridge Exit. This is a distance of only 35 miles and the signal became extremely distorted with cancellation at this point and remained that way all the way to Lexington (another 40 miles or so). The distortion was not due to being in a null in WCKY's pattern as their null is to the west of Cincinnati, not to the South. Also, Last Week, I was in Dalton, GA (also on I-75) and the WCKY signal was like a solid local inside a hotel room as a result of the STRONG skywave and ZERO groundwave at that location.

WLW at 700 starts to "cancel" at night just as you get to the northern edge of Lexington, a distance of about 100 miles from their transmitter which is located NE of Cinci at Mason, OH. The WLW cancellation is usually not too severe in Lexington where WCKY is up and down constantly all night long with the skywave and groundwave fighting each other.

I've heard WCKY chewing into itself as close in as Kings Island. Other posters (I know BobOnTheJob has, maybe others) have reported that happening in Hamilton, only 25 miles north of the tower. Yeah that signal is up at 1530, but with that ground conductivity I was shocked.
Even though WCKY's groundwave is bad in Columbus, it generally does not put a good skywave signal in here at night. It's there, but usually not at blaster status.

WCKY has a strong skywave signal in Manistee, MI