Part 15 AM Real-World Coverage (Four Systems)

[rfry]

The link below leads to an analysis of the distance to the 150 microvolt/meter groundwave field intensity contour of four different "Part 15" AM systems over a real earth path of 5 mS/m conductivity and a dielectric constant of 13.

These earth parameters are somewhat below their average value in the continental US, so as to partially compensate for added path losses common in residential urban areas.

The effect of filtering or shielding the ground conductor is shown for two elevated systems.

The elevated systems are compared to a functionally compliant system installed with its base a few inches above the earth.

Hope this will be useful...

http://i62.photobucket.com/albums/h85/rfry-100/150_microvolt_per_meterRadius_Part_.gif

RF

 

[LibertyNT]

would be great if the Transmitter could actually push out 35mW ;D

I Like The Red Line the best.
But why is the purple different if its the same set up as the red?

 

[rfry]

But why is the purple different if its the same set up as the red?

The configuration producing the purple trace is not the same as the one producing the red trace.

The table shows that the ground conductor for the purple system is "shielded," while for the red system it is not.

The net conclusion is that "shielding" the ground conductor has relatively little affect on system performance.

RF

 

[rfry]

After further thought, I added a new column at the right side of the table in the document linked above in this thread.

It shows a comment about the functional compliance of each system there to Part 15.219(b).

Clicking on the original link above will bring up the revised page.

RF

 

[rfry]

To expand on the above comments, the distance to the 150 microvolt/meter radius for the specified, functionally compliant system1 when operating on 540 kHz is 0.15 miles.

That 0.15 mile distance is only 38% of the distance produced when that same transmit system is operating at 1700 kHz.

The much lower radiation efficiency of this system when operating on 540 kHz is responsible for the reduction in distance to the 150 microvolt/meter radius. Even though groundwave propagation losses on 540 kHz are lower than on 1700 kHz, that does not nearly compensate for the reduction in radiated power when operating on 540 kHz.

Both of the above analyses were made for a ground conductivity of 5 mS/m. Changing the conductivity to 30 mS/m while leaving the other parameters the same gives the following distances to the 150 microvolt/meter radius:

• 540 kHz = 0.15 miles (the same for 540 kHz as when the ground conductivity is 5 mS/m)
• 1700 kHz = 0.47 miles (an 18% increase for 1700 kHz as when the ground conductivity is 5 mS/m)

The reason that there is no significant change in the 150 microvolt/meter radius for the 540 kHz system is that the propagation loss related to ground conductivity is negligible over such a short path on this low frequency -- which is not true for 1700 kHz.

So the bottom line here is that use of the higher AM broadcast frequencies is preferable for Part 15 AM setups wanting the greatest coverage areas, even though the opposite is true for licensed AM broadcast stations, which use antennas with about equal radiation efficiencies regardless of frequency.

An observer on another website has noted that, at the same, nearby physical distances from the transmit antenna (less than 300 feet), it could be possible that operation on 540 kHz produces higher field intensities than when operating on 1700 kHz. This will be a function of the distance to the near-field radius, which is greater for lower frequencies, and the effective radiated power from the two systems.

Whether or not those distances are sufficient to include the highest number of potential listeners to a Part 15 AM station would depend on local conditions.

1 the green trace in the link shown in the first post in this thread

//