AM Groundwave/Skywave Cancellation

[trusty]

Is there a set formula to determine the distance from the antenna where the groundwave and skywave interfere with each other? Also, does it involve the frequency?

For instance, at 12 midnight, does the signal of a 50kw-U at 650khz have groundwave/skywave cancellation point at the same distance as the signal of a 50kw-U at 1550khz?

If not, does antenna height (and/or other factors) make a difference?

 

[davideduardo]

Is there a set formula to determine the distance from the antenna where the groundwave and skywave interfere with each other? Also, does it involve the frequency?

For instance, at 12 midnight, does the signal of a 50kw-U at 650khz have groundwave/skywave cancellation point at the same distance as the signal of a 50kw-U at 1550khz?

If not, does antenna height (and/or other factors) make a difference?

The wavelength height of the tower is the most critical factor,a s that determines the angle of radiation.

 

[thathoustonradiogeek]

Is there a set formula to determine the distance from the antenna where the groundwave and skywave interfere with each other? Also, does it involve the frequency?

For instance, at 12 midnight, does the signal of a 50kw-U at 650khz have groundwave/skywave cancellation point at the same distance as the signal of a 50kw-U at 1550khz?

If not, does antenna height (and/or other factors) make a difference?

The wavelength height of the tower is the most critical factor,a s that determines the angle of radiation.

Is there somewhere online where one can look up the relation between antenna height and frequency?

example: for 550 Khz use a x feet tall antenna.

 

[w9wi]

Is there somewhere online where one can look up the relation between antenna height and frequency?

example: for 550 Khz use a x feet tall antenna.

FCC regulation 73.190, "Engineering charts and related formulas", see Figure 7. You can look this up on www.fcc.gov, find Rules and Regulations on the left side. The FCC has granted waivers to this chart but usually only smaller stations have applied. (I believe almost all 50kw stations have towers compliant with this chart)

Parts (b) and (c) of this regulation are the official formulas for the 50% and 10% skywave field strengths; you could compare them to the groundwave values in 73.184 to find the interference zone, but the groundwave tables aren't published. I'll bet they're on the web somewhere...

 

[trusty]

I have read where both WSM and WLW, in the early years, had to adjust the height of their towers to avoid groundwave/skywave cancellation over metropolitan areas 100-200 miles out. I was wondering if there was a formula where you could take the antenna height, frequency, and possible other factors to determine the "cancellation distance" from the station's tower.

For example, can you take the antenna height and frequency, etc. of WLS, apply the info to a formula, and be able to draw a theoretical circle around Chicago where the cancellation would be the strongest?

 

[slim101]

I know that WOAI in San Antonio also adjusted tower height at one time in order to move the skywave/groundwave cancellation point away from Austin (about 80-100 miles away).

For frequency to tower ht. / wavelength try:
http://www.fcc.gov/mb/audio/bickel/amwave.html

 

[K4WRF]

The WLW and WSM antennas (Blaw-Knox dual cantilevers) were both adjusted in height to reduce close in fades. As you increase a vertical antenna past 180 degrees in electrical length, a secondary lobe appears at about 57 degrees theta or elevation. This was in part a result of the large cross sectional area at the waist where the guy wires attach. The width of both towers is 35 feet at this point and this makes the antenna seem electrically longer and a second current moment begins to develop on the tower. I understand this was confirmed with aircraft measurements over the respective antennas. In recent years, we have been able to model the WSM antenna using NEC method of moments and found that the adjustments made in 1939 were correct. Other stations such as WOAI and WWL used sectionalized antennas at one time to accomplish the same goal.

Fade wall prediction is dependant on many factors including frequency, antenna length, soil conductivity, power, summer/winter seasonal effect on soil and the big variable, aurora conditions. It may be easier to develop a table based on actual experience rather that a simplified formula. An excellent treatment of medium frequencies relating to antennas and their behavior can be found in many publications by the late Carl Smith of Smith Electronics and Cleveland Institute of Electronics. Some of these writings were contained in the NAB Broadcast Engineering Handbooks, but you have to go back about twenty-five years. Of course if they allow AM IBOC at night, the point will be moot.

w

 

[Tom Wells]

As the previous post related, so many factors influence the zone of cancellation that a formula would always miss something.
As a practical matter, 50 kw AMs seem to have a "donut" zone of cancellation that runs from 200 miles to 300 miles out, at least in the midwest where good ground conductivity is the norm.
The lower the frequency is, the farther out this zone lies. In the 40 meter (7mhz) range this "skip" zone starts within 50 miles, and runs to about 125 miles.
Before WLW shortened their antenna to reduce fading, the 500-kw experiment was directed toward the same end, reducing fade for the regional audience, and more power was found NOT to be the answer.

 

[Grrrradio]

As the previous post related, so many factors influence the zone of cancellation that a formula would always miss something.

CORN!

I kid you not -- Iowan observers (I think they might have even been on this board) claim that the distance to the cancellation zone of 1040 WHO-Des Moines jumps by 50 miles after the fall harvest is done!