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Part 15 AM Pattern Circularity (nearby homes)

R

rfry

It seems intuitive that nearby houses must affect the radiation pattern of a Part 15 AM setup. But the change to the radiation pattern is very small, because the wavelengths used in the AM broadcast band are much greater than the dimensions of the vertical conductors in a typical house. The amount of power that they can extract and re-radiate from a passing radio wave is extremely small.

Localized effects may be present immediately around such conductors, but the net effect on the radiation pattern well away the houses is negligible.

Trees are much poorer conductors than the wires in a home, and have almost no affect on the coverage of AM broadcast stations (of whatever radiated power).

The NEC-2D study linked below provides some insight on this subject.

http://i62.photobucket.com/albums/h85/rfry-100/Part_15_AM_Circularity.gif
 
I find that I see pattern/field distortions very near the all-metal, grounded (obviously) light poles as used in Chicago,
and underneath/running "alongside" the overhead power wires.
( not the mention that "nearby river/waterfall" effect)

It has made me wonder about bypassing the incoming lines for RF, hot and neutral, to return these currents to the common
point, as would be done with radials or counterpoise. Might it help the neighbors hear with less 60hz intermod?
 
R. Fry said:
It seems intuitive that nearby houses must affect the radiation pattern of a Part 15 AM setup. But the change to the radiation pattern is very small, because the wavelengths used in the AM broadcast band are much greater than the dimensions of the vertical conductors in a typical house. The amount of power that they can extract and re-radiate from a passing radio wave is extremely small.

Localized effects may be present immediately around such conductors, but the net effect on the radiation pattern well away the houses is negligible.

Trees are much poorer conductors than the wires in a home, and have almost no affect on the coverage of AM broadcast stations (of whatever radiated power).

The NEC-2D study linked below provides some insight on this subject.

http://i62.photobucket.com/albums/h85/rfry-100/Part_15_AM_Circularity.gif
Click to expand...

I was thinking/wondering... What would happen if you erected a certain size (1/4 or 1/8 wave for example) metal tower at a certain distance (maybe 1/4 or 1/8 wavelength) from the part 15 ~2.9-meter radiator, so that it functioned parasitically? Or would that have any effect on the part 15 coverage, radiation pattern, etc?
 
Yes, such a parasitic will affect the pattern circularity of a 3-m monopole.

The link below leads to a plot comparing the h-plane pattern of a 3-m monopole alone (blue trace) to the net pattern of the 3-m monopole with a 1/4-wave vertical parasitic located about 1/4-wave away (red trace). The parasitic is connected at its base to an earth ground through 10 ohms, as might be the case when using an 8 ft buried, vertical grounding rod.

The pattern with the parasitic has about 3 dB more peak gain, but in two directions also has about 7 dB less gain than the 3-m monopole alone. In fact the system with the parasitic exceeds the performance of the monopole alone only over a 148 degree span. In all other directions the 3-m monopole without the parasitic has better performance.

http://i62.photobucket.com/albums/h85/rfry-100/Part15AMParasitic.gif
 
Tom Wells said:
It has made me wonder about bypassing the incoming lines for RF, hot and neutral, to return these currents to the common point, as would be done with radials or counterpoise. Might it help the neighbors hear with less 60hz intermod?
Click to expand...

Probably even if the connection of Part 15 AM system hardware to the AC lines added zero r-f current on those lines it would make little difference to the localized effects near the AC lines, because they are still receiving and re-radiating r-f energy directly from the fields of the transmit antenna.

Could the 60 Hz be coming from poorly-filtered DC supplies in the transmitter, or picked up by the program source of program line?

Hmmm, my 1000th post on R-I.
 
R. Fry said:
Tom Wells said:
It has made me wonder about bypassing the incoming lines for RF, hot and neutral, to return these currents to the common point, as would be done with radials or counterpoise. Might it help the neighbors hear with less 60hz intermod?
Click to expand...

Probably even if the connection of Part 15 AM system hardware to the AC lines added zero r-f current on those lines it would make little difference to the localized effects near the AC lines, because they are still receiving and re-radiating r-f energy directly from the fields of the transmit antenna.

Could the 60 Hz be coming from poorly-filtered DC supplies in the transmitter, or picked up by the program source of program line?

Hmmm, my 1000th post on R-I.
Click to expand...

Not a chance on poor bypassing. It's way beyond sufficient...and the AC is fed through a Daitron ultra-isolation transformer.
No hum at all on program here, but I'm thinking more about someone 200-300 feet away, who would likely have
RF on the incoming AC, and listening on a modern radio that does NOT have both sides of the AC line bypassed
to the DC common in the radio. I don't know if anyone does hear any hum ...I'm just extrapolating, based on hearing
hum in a few areas right under the overhead AC lines, and having heard hum on some AM MW signals, sometimes, some radios.

It is most likely picked up, and re-radiated, as you state.
 
Here's a thought... What about a proper, Part 15 legal "Rangemaster" type transmitter, adjusted according to manufacturer specs, and mounted at the top of a metal structure that is 1/4 wave tall, thus resonant at the operating frequency? How about a 1/2 wave structure?

Structures: Cell monopole, water tower, park ranger tower, billboard, etc.
 
WPHA said:
What about a proper, Part 15 legal "Rangemaster" type transmitter, adjusted according to manufacturer specs, and mounted at the top of a metal structure that is 1/4 wave tall, thus resonant at the operating frequency? How about a 1/2 wave structure?
Click to expand...

Using a "Rangemaster" or any other transmitter certified for unlicensed use under FCC Part 15 does not automatically mean that it may be installed/used in the configurations quoted above, and remain compliant with Part 15.

Such transmitters are designed to operate under Part 15.219 of the FCC Rules, and 15.219(b) requires an antenna no longer than 3 meters in total length.

If the ground terminal of the transmitter is connected to the top of a conductor of any length (height), that conductor becomes part of the antenna system, and can radiate more than a 3-meter whip considered to be "the antenna." Therefore such a system will not be functionally compliant with Part 15 even though an FCC-certified transmitter is used.

The link below has information showing this...

http://i62.photobucket.com/albums/h85/rfry-100/AM_System_Comparison.gif
 
We're obviously treading into "grey areas" here, because such a system could be claimed to be LITERALLY compliant with the LETTER of the Law, but not the SPIRIT of it. That's the only point I was addressing. I am well aware of the Laws of Physics that govern such things. :) I'm just raising a point of discussion that others may be trying to address.

As long as the transmitter's combined "Ground Lead" and antenna length are less than 10 feet, it can be claimed to be "TECHNICALLY compliant". The structure upon which it is mounted, is merely a support structure. The structure is grounded to comply with safety and electrical Codes.

The possibility that said structure happens to be resonant at Fo is "technically" not the Rangemaster owner's fault. (Even if it IS deliberate, I'm only talking about the LETTER of the Law, here.)

Let's say that the transmitter is mounted to a pipe on the roof of a building, because the owner wanted to get it outdoors and "up high". They have NO IDEA that this pipe happens to be 1/4 wavelength at 1.7 Mhz, they just know that it is a good Earth Ground, so electrical and safety Codes have been satisfied. I have heard stories of RESPONSIBLE owners of such such operations having them inspected by the FCC, found TECHNICALLY compliant, and left alone.

(Key: RESPONSIBLE owners, IE: people with the ability to communicate intelligently, a modest level of competence, and demonstrating a good level of responsibility regarding use of Public Airwaves, etc.)

What I am curious about, is what kind of actual, useful RANGE can such a setup provide? Specifically, what is the radius of the .1mv/m contour over, say, 4mS ground conductivity? Is there a chart for that? :) RF current through the radiating structure doesn't answer this question. :)

BTW, in case anyone is wondering, no, I am not looking to buy a Rangemaster (or equivalent) transmitter. I am still out of work, and cannot afford such nice toys. :'(
 
WPHA said:
As long as the transmitter's combined "Ground Lead" and antenna length are less than 10 feet, it can be claimed to be "TECHNICALLY compliant"....Let's say that the transmitter is mounted to a pipe on the roof of a building, because the owner wanted to get it outdoors and "up high". They have NO IDEA that this pipe happens to be 1/4 wavelength at 1.7 Mhz, they just know that it is a good Earth Ground, ...
Click to expand...

The pipe doesn't have to be 1/4-wave high in order to function as an antenna. That is because "a good Earth Ground" does not exist at the top of a conductor of any length or construction even though it is connected to an earth ground at its base.

The top of that conductor may be nearly at earth potential for direct current, but that is not true for radio-frequency current. The r-f current flowing in the short "Ground Lead" at the transmitter continues to flow along the pipe until it reaches a true earth ground (something buried in the earth, typically). That r-f current produces radiation -- as is shown in the graphic link I posted earlier in this thread.

A real-world example to demonstrate this point can be found by Googling for KENC NOUO. In this case the operator believed that the top of his 40-foot metal tower was an r-f ground, but the FCC didn't agree.

What I am curious about, is what kind of actual, useful RANGE can such a setup provide? Specifically, what is the radius of the .1mv/m contour over, say, 4mS ground conductivity? Is there a chart for that?
Click to expand...

Here is a link to a graphic showing the coverage radius of a functionally compliant Part 15 AM setup and three other configurations.

http://i62.photobucket.com/albums/h85/rfry-100/150_microvolt_per_meterRadius_Part_.gif
 
tfcwings said:
R. Fry said:
It seems intuitive that nearby houses must affect the radiation pattern of a Part 15 AM setup. But the change to the radiation pattern is very small, because the wavelengths used in the AM broadcast band are much greater than the dimensions of the vertical conductors in a typical house. The amount of power that they can extract and re-radiate from a passing radio wave is extremely small.

Localized effects may be present immediately around such conductors, but the net effect on the radiation pattern well away the houses is negligible.

Trees are much poorer conductors than the wires in a home, and have almost no affect on the coverage of AM broadcast stations (of whatever radiated power).

The NEC-2D study linked below provides some insight on this subject.

http://i62.photobucket.com/albums/h85/rfry-100/Part_15_AM_Circularity.gif
Click to expand...

I was thinking/wondering... What would happen if you erected a certain size (1/4 or 1/8 wave for example) metal tower at a certain distance (maybe 1/4 or 1/8 wavelength) from the part 15 ~2.9-meter radiator, so that it functioned parasitically? Or would that have any effect on the part 15 coverage, radiation pattern, etc?
Click to expand...
I've seen this carried to the extreme...a known 100mw AM transmitter fed to a 10' section of tower that was mounted a couple of inches from a 60' or so tower with a base insulator and a coil across the base insulator that was adjusted for resonance at the operating frequency. It was slightly audible at 7 miles during the day & covered up the night time skip at the high end of the dial for about 1 mile (this was decades before the expanded band existed). They never did have the FCC involved so it's still unknown whether that 'grey area' would have flown or not. I suppose as long as the term in Part 15 is 'antenna' rather than 'antenna system', it could have been argued.
 
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