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Protected Contour for X-Band AM's

A

audioguy

I've got a question for you gents: As you know, most Part 15 AM's operate in the expanded band above 1600 kHz. What is the protected contour for the 10kW-D, 1kW-N expanded band stations? I assume that they are protected to some contour during the daytime, such as the 0.1 mV/m contour. If that is the case, is it based on an ideal prediction or on actual measurements? And what protection are they entitled to at night? I assume that they have no protected skywave service, since I doubt that 1 kW would be deemed sufficient for that.

I think it's important to know these things, since the fundamental obligation of a Part 15 operator is to "first, do no harm". In my area, there is only one somewhat local x-band station, and it isn't even listenable at night. The other frequencies are just a jumble after dark, but I want to know what the FCC considers their protected coverage area to be.
 
All X-banders are licensed as class B stations and are protected just as any other class B station would be, under 47 CFR 73.37 - co-channel stations can't have overlap between the protected station's 0.5 mV/m and the interfering station's 0.025 mV/m, or vice versa; first-adjacent protection between the protected station's 0.5 mV/m and the interfering station's 0.25 mV/m and vice versa; second-adjacent 5 mV/m to 5 mV/m and third adjacent 25 to 25. No skywave protection, per se, for class B signals, though 73.182 prescribes a certain degree of nighttime service for class B signals and creates the infamous "ratchet rule". 73.37(e) provides some special rules for the X-banders that were allocated as part of the initial petition process back in the early nineties - they don't have to comply with 73.37(a), but they're not making any more of those, anyway.
 
Thank you Scott; that is very helpful! It would appear that for the typical Part 15 operation it would be sufficient to stay at least 5 miles or so outside of a station's 0.5 mV/m contour for co-channel or first adjacent channel operation. Due to the extremely low power of Part 15 stations, it does not appear that the second or third adjacent channels would be much of an issue, since any interference created would be within a few meters of the transmitter (and therefore, most likely on the operator's property). Interference from a licensed station is likely to be the main concern in most cases.

Does anyone know of another commonly available source of coverage maps besides radio-locator? Their maps are helpful, but fairly coarse.
 
audioguy said:
It would appear that for the typical Part 15 operation it would be sufficient to stay at least 5 miles or so outside of a station's 0.5 mV/m contour for co-channel or first adjacent channel operation.
Click to expand...

But the field of a licensed AM station 5 miles past its 0.5 mV/m contour is not greatly lower than that, and probably would otherwise be listenable close to a proposed Part 15 AM, if the Part 15 wasn't there (see below, based on the FCC's MW propagation curves).

So it is likely that operating the Part 15 AM station there would interfere with the licensed station, in practical terms, because the field from the Part 15 likely is more than 5% that of the licensed station for a radius of 1,000 meters or so from the Part 15 transmit antenna.

An interfering field from an unlicensed station could extend quite a bit further than that if the system is installed on an elevated mount with a long, radiating path to r-f ground (see http://boards.radio-info.com/smf/index.php/topic,117278.0.html ).

The other side of this is that the field of a co-channel licensed station would cause so much interference to a Part 15 AM setup 5 miles beyond the licensed station's 0.5 mV/m contour that the relatively interference-free coverage area of the Part 15 would be very small.

Data:
Frequency = 1650 kHz
Power = 10.00 kW
Radiation at 1 mile = 600.8 mV/m
Conductivity = 8.0 mS/m

Results:
Contour level Distance to contour

0.500 mV/m 47.4 miles
0.400 mV/m 52.4 miles
0.300 mV/m 59.5 miles
0.200 mV/m 70.6 miles

RF
 
The question here was somewhat hypothetical; as I indicated in my original post, a Part 15 operation is generally going to suffer much more interference from a licensed station than it would cause. However, we do have to be concerned about protecting the licensed station from interference within its protected contour, so it is important to know what that is.

The calculations you showed are instructive, but they assume a constant ground conductivity value of 8 mS/m. I looked at the patterns of several dozen expanded band stations on radio-locator, and despite the fact that all but a few x-band stations use non-directional antennas, the contours shown are anything but circular. Some of these patterns vary by a factor of 2 or more in distance depending on the direction. I assume that this is due to variations in ground conductivity rather than local obstructions distorting the antenna pattern. I don't know how accurate the radio-locator plots are, but unfortunately you can't get this information from the FCC Media Bureau web site because it isn't provided for non-directional stations. Bottom line: it's important to know what the actual contours are, and not just base your calculations on transmitter power and antenna efficiency and some assumed ground conductivity value.

The 5-mile separation was a minimum value, and was based on the fact that many (if not most) Part 15 operators use indoor table top transmitters that are designed for "broadcasting" to radios inside the home and barely reach 100 meters at the limits of audibility. My own Part 15 setup, which uses an outdoor Rangemaster transmitter, is completely inaudible at a distance of less than 5 miles using the most sensitive receiver I have connected to a large loop antenna.
 
audioguy said:
The calculations you showed are instructive, but they assume a constant ground conductivity value of 8 mS/m. I looked at the patterns of several dozen expanded band stations on radio-locator, and despite the fact that all but a few x-band stations use non-directional antennas, the contours shown are anything but circular. Some of these patterns vary by a factor of 2 or more in distance depending on the direction. I assume that this is due to variations in ground conductivity rather than local obstructions distorting the antenna pattern. I don't know how accurate the radio-locator plots are, ...
Click to expand...

The Radio-Locator coverage contours are calculated using the FCC propagation curves for that frequency, and for the conductivities in that geographic area together with the ERP from the licensed radiation pattern(s) of the AM station. If the coverage contours are not circular for a licensed non-directional station it is because of varying earth conductivities around the site.

Here is an example of an X-band station in Kansas City, MO that shows quite circular contours http://www.radio-locator.com/cgi-bin/pat?call=KXTR&service=AM&status=L&hours=D . The slight increase in range to the west is the result of a land area with 30 mS/m conductivity, where the rest of the coverage area has 15 mS/m (per the FCC'S M3 chart).

RF
 
A real problem for low power unlicensed AM is that you have no rights. If some DX listener is trying to receive a station 2,000
miles away, he can ask you to leave the frequency. If you refuse, he can involve the FCC.

This is why a new licensed low power AM service would be good. You could have more power and be protected by the FCC.
 
Dutchman,

Without question, if anyone ever complained of interference to a licensed station, I would promptly cease. I enjoy DX'ing as much as anyone! But this has never happened to me. Actually, I don't find the x-band very useful for DX'ing any more. The channels are now all duplicated, and in my area they are just a jumble of interference and IBOC hash at night. You might be able to catch an ID, but you cannot actually "listen" to a program any more.

Personally, I enjoy Part 15 operation as it is-- a hobby. I am not trying to provide a "broadcast" service. If it were licensed, I could not justify the time and expense required to operate in that manner.
 
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