AM Tower on a Hill

[Carmine5]

Here is a partly hypothetical question: Could an AM antenna be an effective radiator built on the slope of a hill? In this case, the ground radials would follow the hill's slope. Or would grading the land flat be better?

And on a more bizarre note: could the hill act as an effective signal deflector/absorber, turning a single antenna system into a DA, particularly if the hill (or foothill in this case) is significantly taller then the antenna?

C5

 

[OKCRadioGuy]

In general you'd be better off finding some nice riverbottom instead. Hills are normally rockier and therefore have worse ground conductivity.

 

[LA_Guy]

Hills that are TOO high (think more then 1500 feet taller then the market), can produce HORRIBLE AM signals by virtue of their height affecting the groundwave. As an example, look at the former KROQ 1500. They put the 7 tower array on a 3800 foot hill (Verdugo Peak) and its 10 kW did worse then when they ran 1 kW on 1490 with a ground mounted antenna.

 

[davideduardo]

Hills that are TOO high (think more then 1500 feet taller then the market), can produce HORRIBLE AM signals by virtue of their height affecting the groundwave. As an example, look at the former KROQ 1500. They put the 7 tower array on a 3800 foot hill (Verdugo Peak) and its 10 kW did worse then when they ran 1 kW on 1490 with a ground mounted antenna.

It's not as much the height of the hill as it is the conductivity; Verdugo, like the surrounding hills, is sold rock and has poor conductivity. I saw a 100 kw AM built on a fairly high hill outside Caracas, and since the hill was mostly earth, it really talked well.

I built a number of AMs at altitudes of between 9600 and 10200 feet AMSL to serve the Quito market, and all but one was on a hill... since the hills had deep, loamy organic soil that maintained humidity year-round, the facilities talked very well. One, my only directional, was on a hillside with an incline of about 35 degrees, and it worked very well, too. It all depends on the conductivity, again.

Verdugo is about 2600 feet above sea level, and about 1800 feet above either surrounding Glendale or Burbank.

 

[Play Freebird]

Upstate NY (central and western part of the state) has a few hilltop AM sites and most of them perform OK. In general, the soil is glacial till and a lot of these sites remain swampy through most of the year. On the other hand, the mountains of upstate PA are very rocky and would not be the best choice to site AM facilities.

As long as the soil in the near field of the antenna has good conductivity, and the path towards the desired market also passes over good ground, I wouldn't expect a problem.

 

[Carmine5]

In general you'd be better off finding some nice riverbottom instead. Hills are normally rockier and therefore have worse ground conductivity.

It was just a crazy idea.

Someone was offering me hillside land for next to nothing and I was thinking that maybe I could grade it enough to erect a tower but let the radials follow the contour of the hill. The transmitter shack would be at the base of the foothill and easy to get to.

Also, I was thinking the hill might offer natural protection to a co-channel thereby eliminating the need for a second tower. I hadn't checked the conductivity of the area or of the hill itself nor did I check the condition of the soil. It may be too rocky.

I've also never seen an AM tower site such as what I'm thinking. Undoubtedly, there is a good reason for that so I'll just forget it.

C5

 

[stormy01]

Hills that are TOO high (think more then 1500 feet taller then the market), can produce HORRIBLE AM signals by virtue of their height affecting the groundwave. As an example, look at the former KROQ 1500. They put the 7 tower array on a 3800 foot hill (Verdugo Peak) and its 10 kW did worse then when they ran 1 kW on 1490 with a ground mounted antenna.

It's not as much the height of the hill as it is the conductivity; Verdugo, like the surrounding hills, is sold rock and has poor conductivity. I saw a 100 kw AM built on a fairly high hill outside Caracas, and since the hill was mostly earth, it really talked well.

I built a number of AMs at altitudes of between 9600 and 10200 feet AMSL to serve the Quito market, and all but one was on a hill... since the hills had deep, loamy organic soil that maintained humidity year-round, the facilities talked very well. One, my only directional, was on a hillside with an incline of about 35 degrees, and it worked very well, too. It all depends on the conductivity, again.

Verdugo is about 2600 feet above sea level, and about 1800 feet above either surrounding Glendale or Burbank.

Since AM/MW towers are vertical radiators, is the signal vertically polarized to some extent? (I know the effect of polarization is not as pronounced as on higher frequencies like shortwave, FM or TV... there one can observe this effect by moving a receiving antenna around)

 

[DanStrassberg]

It was just a crazy idea.

Someone was offering me hillside land for next to nothing and I was thinking that maybe I could grade it enough to erect a tower but let the radials follow the contour of the hill. The transmitter shack would be at the base of the foothill and easy to get to.

Also, I was thinking the hill might offer natural protection to a co-channel thereby eliminating the need for a second tower. I hadn't checked the conductivity of the area or of the hill itself nor did I check the condition of the soil. It may be too rocky.

The three-tower array of WDEV 550 Waterbury VT is on a hillside. Gets out pretty well (for VT), but after all, it's on 550. Also, it has been on that same hill for decades, indicating that the ownership is not displeased. I have no knowledge of the conductivity there, but are there any places in VT with good conductivity (besides the middle of Lake Champlain just east of the New York line, which runs through the lake)? The WDEV array is the only hillside AM DA I can remember seeing.

As for directional effects from the hill, I guess there could be some. At least that's what the consulting engineer on a recently built Utah AM thinks. He discusses this in his request for a modification of CP, which appeared recently in CDBS. I can't recall the station, but IIRC it is on 1060, though I could be wrong.

 

[rrsounds]

Since AM/MW towers are vertical radiators, is the signal vertically polarized to some extent? (I know the effect of polarization is not as pronounced as on higher frequencies like shortwave, FM or TV... there one can observe this effect by moving a receiving antenna around)

Here's something you can try: Rotate an AM radio with a ferrite 'bar' antenna 90 degrees (vertical) from its usual horizontal position. Note the quality of reception.

Kind Regards,
David

 

[stormy01]

Guess I can get better answers from IEEE, ARRL or some other reputable source....Thx...

 

[amfmsw]

I know of two stations EXACTLY like that. 1260Kc WFBS Berwick, PA and 1370Kc WPAZ Pottstown, PA. Your main concern is soil erosion that may expose radials.

 

[Carmine5]

WFBS is very similar to what I was thinking, although the incline of the property I'm being offered is a little steeper than the Berwick tower site. I hadn't thought about soil erosion. Good point.

C5

 

[Play Freebird]

Guess I can get better answers from IEEE, ARRL or some other reputable source....Thx...

I just checked the field strength of our station with a FIM-21 outside the building, about 1/2 wavelength from the center of the two tower array in the direction of the neighboring station we're required to protect. With the loop oriented vertically and rotated for maximum signal, I measured about 3.5 volts/meter. Tilting the loop back 90 degrees while keeping the same azimuth reduced this by 21 dB, and I could obtain 30 dB reduction by tilting it back about 95 degrees.

So when the signal is launched, it appears to have very little horizontal polarization -- however, out at greater distances, reradiation from nearby power lines, etc. might introduce a slight horizontal component to the groundwave.

According to the textbooks, skywave signals tend to have variable elliptical polarization, but I would expect them to favor the vertical component most of the time.

 

[stormy01]

I just checked the field strength of our station with a FIM-21 outside the building, about 1/2 wavelength from the center of the two tower array in the direction of the neighboring station we're required to protect. With the loop oriented vertically and rotated for maximum signal, I measured about 3.5 volts/meter. Tilting the loop back 90 degrees while keeping the same azimuth reduced this by 21 dB, and I could obtain 30 dB reduction by tilting it back about 95 degrees.

So when the signal is launched, it appears to have very little horizontal polarization -- however, out at greater distances, reradiation from nearby power lines, etc. might introduce a slight horizontal component to the groundwave.

According to the textbooks, skywave signals tend to have variable elliptical polarization, but I would expect them to favor the vertical component most of the time.
[/quote]

Thank You Play Freebird for a great answer and taking the time to investigate, I appreciate it!

 

[DanStrassberg]

I just checked the field strength of our station with a FIM-21 outside the building, about 1/2 wavelength from the center of the two tower array in the direction of the neighboring station we're required to protect. With the loop oriented vertically and rotated for maximum signal, I measured about 3.5 volts/meter. Tilting the loop back 90 degrees while keeping the same azimuth reduced this by 21 dB, and I could obtain 30 dB reduction by tilting it back about 95 degrees.

Would you expect the tower height or heights to affect the results? If so. what is or are the tower height or heights in degrees? I wouldn't expect the tower height to be much of a factor, but then, if the two towers are equal in height, I would have expected the minimum field strength to be measured with the plane of the loop horizontal and not 5 degrees off of horizontal.

 

[Play Freebird]

Would you expect the tower height or heights to affect the results? If so. what is or are the tower height or heights in degrees? I wouldn't expect the tower height to be much of a factor, but then, if the two towers are equal in height, I would have expected the minimum field strength to be measured with the plane of the loop horizontal and not 5 degrees off of horizontal.

The two radiators are indeed unequal in height. The one closest to the point where I took the measurement is 73 degrees and the other is 63 degrees, so that may explain why the null wasn't observed when the loop was exactly horizontal.

 

[stormy01]

According to the textbooks, skywave signals tend to have variable elliptical polarization, but I would expect them to favor the vertical component most of the time.

Could you tell us what the textbooks are? I wouldn't mind reading up on that. Obtaining an FIM-21 is another story.

 

[Play Freebird]

Could you tell us what the textbooks are? I wouldn't mind reading up on that. Obtaining an FIM-21 is another story.

If you search under "skywave polarization" in Google, a few excerpts of these books should turn up.