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AM signal directionality affected by rivers nearby?

T

Tom Wells

I live about 300 feet from a point where an East-West portion of a river flows over a small waterfall.
At that point is a turning basin in a much larger section of river that runs fairly North-South.
This makes a T- shaped junction. The land here is almost completely flat.
I am in the "corner" defined by 2 branches, one running to the west and one to the south.
Seems that my Pt 15 signal reach is lesser in ANY direction that involves crossing the river but is enhanced toward the south or west.
It's just a few blocks anyway, but there is decidedly more signal thataway.
When crossing the two closest briges, each about 1 block away, there is at least 3-5 db change in signal, far side to near side.
One bridge on each branch, and same effect noted both places.
Nothng about my installation is intended to create a lopsided pattern, and I can think of nothing in the house to
direct, reflect and phasor the radiation. Just across the street to the east is a solid metal light pole in the park, about 100 feet tall, 50 feet away. I wouldn't think the spacing would be far enough to affect direction, and if anything the best signal is to the south, not E or W.
The only other thing in common with this is that the signal is least in the dierction with a "free and clear" view, no buildings to
absorb in that direction for a block, while the directions with best coverage is toward highest building density.

Has anyone noted coverage of an AM where a non-d pattern showed evidence of a riverbed's conductivity skewing the pattern?
I know ground conductivity can change abruptly in some areas, but Chicago is basically on a big mud flat with no major
geographic features to create wierd conductivity pockets.
Is the fully wet soil of the riverbed defining a ground conductivity reflector of some sort?
 
Tom Wells said:
I live about 300 feet from a point where an East-West portion of a river flows over a small waterfall.
At that point is a turning basin in a much larger section of river that runs fairly North-South.
This makes a T- shaped junction. The land here is almost completely flat.
I am in the "corner" defined by 2 branches, one running to the west and one to the south.
Seems that my Pt 15 signal reach is lesser in ANY direction that involves crossing the river but is enhanced toward the south or west.
It's just a few blocks anyway, but there is decidedly more signal thataway.
When crossing the two closest briges, each about 1 block away, there is at least 3-5 db change in signal, far side to near side.
One bridge on each branch, and same effect noted both places.
Nothng about my installation is intended to create a lopsided pattern, and I can think of nothing in the house to
direct, reflect and phasor the radiation. Just across the street to the east is a solid metal light pole in the park, about 100 feet tall, 50 feet away. I wouldn't think the spacing would be far enough to affect direction, and if anything the best signal is to the south, not E or W.
The only other thing in common with this is that the signal is least in the dierction with a "free and clear" view, no buildings to
absorb in that direction for a block, while the directions with best coverage is toward highest building density.

Has anyone noted coverage of an AM where a non-d pattern showed evidence of a riverbed's conductivity skewing the pattern?
I know ground conductivity can change abruptly in some areas, but Chicago is basically on a big mud flat with no major
geographic features to create wierd conductivity pockets.
Is the fully wet soil of the riverbed defining a ground conductivity reflector of some sort?
Click to expand...

I'm not the best guy here to answer you, but I'm pretty sure that unless your transmitting antenna's length is a significant portion of a wavelength, then all bets are off as to what may or may not influence its directivity.

That being said, any nearby metal object that is significantly longer and in the same plane as your transmitting antenna, and no more distant than a wavelength or two, may indeed influence the direction of signal.

Water is only a decent conductor when it is impure; salt water being particularly good at creating useful ground conductivity, typical fresh river/lake water not being so influential. At extremely low power levels, I would think trees and buildings would have as much if not more effect on coverage than a river, but I could be wrong.

As for your question regarding ground conductivity in and around Chicago: the lay of the land does not necessarily indicate the ground conductivity. Florida:, flat, lots of ground water, but poor ground conductivity; Nebraska: flat, relatively dry soil (however the renowned Ogalalla aquifer is underneath), but excellent ground conductivity.

I don't mean to be a pessimist, heck I had a little "radio station" when I was younger, and tried a LOT of things to make it better! But realistically, extremely low power with extremely short antennas is really just a toy, and the same way that I wouldn't expect my N scale model trains to be able to usefully haul enough coal to run a power plant (unless it, too, is only a model), a legal Part 15 radio station, even when idealized, simply can't be expected to provide much in the way of predictable and reliable service. And because the signal level is (or SHOULD be) extremely low, it can be affected overtly by otherwise benign influences.

Kind Regards,
David
 
Tom Wells said:
... Just across the street to the east is a solid metal light pole in the park, about 100 feet tall, 50 feet away. I wouldn't think the spacing would be far enough to affect direction, and if anything the best signal is to the south, not E or W. ...
Click to expand...

Below are the results of a NEC simulation showing how the parasitic radiation from the light pole affects the azimuth pattern for the arrangement above, with no other re-radiators.

The net radiation pattern has about a 1 dB front to back ratio, so something else must account for the 3-5 dB signal changes you mentioned -- possibly localized re-radiation and/or shielding effects from those two bridges?

Probably the fact that the original radiator is only 3 meters high is not a factor. Once the signal is "launched," the radiated, far-field wave is no different in nature than if it had originated from even a 1/4-wave vertical.

But for typical conditions, the average radiated field from a functionally compliant Part 15 AM is so low after about 1/2 mile that a further reduction of only a few dB can make a big difference to the noise in the receiver. Such reductions are normal due to random path obstructions in urban areas.

NEC RESULTS

1650 kHz 3 meter monopole 3/27/2010 6:12:14 AM

--------------- ANTENNA DESCRIPTION ---------------

Frequency = 1.65 MHz
Wire Loss: Copper -- Resistivity = 1.74E-08 ohm-m, Rel. Perm. = 1

--------------- WIRES ---------------

No. End 1 Coord. (ft) End 2 Coord. (ft) Dia (in) Segs Insulation
Conn. X Y Z Conn. X Y Z Diel C Thk(in)
1 GND 0, 0, 0 0, 0, 9.8425 0.5 11 1 0
2 GND 0, 50, 0 0, 50, 100 12 9 1 0

Total Segments: 20

-------------- SOURCES --------------

No. Specified Pos. Actual Pos. Rel Amplitude Phase Type
Wire # % From E1 % From E1 Seg (V/A) (deg.)
1 1 0.00 4.55 1 1 0 I

-------------- LOADS (R + jX Type) --------------

No. Specified Pos. Actual Pos. R X
Wire # % From E1 % From E1 Seg (ohms) (ohms)
1 1 0.00 4.55 1 20 0
2 1 7.00 4.55 1 5 3385
3 2 0.00 5.56 1 10 0

No transmission lines specified

No transformers specified

No L Networks specified

Ground type is Perfect


1650 kHz 3 meter monopole 3/27/2010 6:18:56 AM

--------------- FAR FIELD PATTERN DATA ---------------

Frequency = 1.65 MHz

Azimuth Pattern Elevation angle = 0 deg.
Bear V dBi H dBi Total dBi
0 -19.05 -99.99 -19.05
10 -19.06 -99.99 -19.06
20 -19.10 -99.99 -19.10
30 -19.15 -99.99 -19.15
40 -19.22 -99.99 -19.22
50 -19.31 -99.99 -19.31
60 -19.40 -99.99 -19.40
70 -19.51 -99.99 -19.51
80 -19.61 -99.99 -19.61
90 -19.71 -99.99 -19.71
100 -19.79 -99.99 -19.79
110 -19.87 -99.99 -19.87
120 -19.92 -99.99 -19.92
130 -19.97 -99.99 -19.97
140 -20.00 -99.99 -20.00
150 -20.01 -99.99 -20.01
160 -20.02 -99.99 -20.02
170 -20.03 -99.99 -20.03
180 -20.03 -99.99 -20.03

(190 to 360 degrees is a mirror of the above)

RF
 
Thanks for the quick response. I'm off to Wisconsin for the weekend. I'll be pondereing this.
It really is as if the signal is reflected outward equally from the angle defined by this "corner".
In exact opposition to building density...so something's up...re-radiation from powerlines? I do note odd little nulls in places that make it seem like it's ground wave meeting powerline radiation and cancelling, rather than absorption of anything nearby.
 
A directional in Boise that's been around for nearly 60 years has a large irrigation canal .2 miles away. It was claimed by the engineers the monitor points would change predictably if the canal was full or empty. Even the flood irrigation of the pasture around it didn’t make as much of a difference as the canal being full.
 
I'm thinking the pole is the culpert even though it doesn't calculate out because it might be 1) closer to a quarter wave ( you didn't specify your frequency but 100 feet is approching a quarter wave at the high end of the band) than you realize 2) seperation is more or less 50 feet and the combination of the raltionship between it and your antenna cause a null or reradiate. I've found that objects such as poles, verticle wires etc cause the most problems verses short steel structures like bridges.

Also, I'd check the power grid and telco lines in the area to see if they are reradiating assuming they arn't all underground. Lastly, i'm sure conductivity of the land must change depending on the flow in the river, dryness of the land etc. Good luck
 
Several years ago I was working on a proof for an AM station in the Midwest. The transmitter site was about six miles east of the Missouri River. I was making field measurements along one of the radials in the main lobe of the daytime pattern. This particular radial crossed the river at about the nine-mile point from the transmitter. I observed that the signal level dropped about 14 dB, from the east bank of the river to the west bank.

This was a fairly well-known phenomenon for AM stations in this particular market. The AM stations with transmitters on the east side of the river all had relatively weak signals in the city on the west side of the river. I don't know if there was a proven scientific explanation for what we observed, but I remember hearing a theory that the river tended to carry downstream all of the minerals and other stuff that enhanced conductivity, so that the riverbed turned into a "conductivity break" that blocked groundwave signals.
 
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