Carrier Current Transmitters - Help?

[spinjector]

Hi all,

I was wondering something about the "carrier current" broadcast technique. 20-odd years ago I remember a community college I went to had a CC station. At the time I thought it was a silly concept because no one listened to it, except the odd secretary or two, and I think the guys in the auto-tech shop... lol

Does it still exist?

Does it require a license?

How reliable is it?

Has anyone ever installed it in a football stadium?

 

[rfry]

The Rules from Part 15 paragraph 15.221 apply (see below).

An average AM broadcast receiver would have to be located very close to the power line in order to receive a useful signal from a legal (compliant) carrier current system.

The 15 uV/m field given in 15.221(a) is below the noise level in most receiving locations outside of a shielded room, and for an operating frequency of 1600 kHz the distance to that legal field intensity from a carrier current system is about 100 feet. An average receiver probably would need to be located within 25 feet of the power line in order to receive a fairly noise-free signal (assuming the 60 Hz power line hum was controlled).

Sorry that this situation probably is not very good news, but it is based on FCC and physical realities.

RF
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§ 15.221 Operation in the band 525–1705 kHz.

(a) Carrier current systems and transmitters employing a leaky coaxial cable as the radiating antenna may operate in the band 525–1705 kHz provided the field strength levels of the radiated emissions do not exceed 15 uV/m, as measured at a distance of 47,715/(frequency in kHz) meters (equivalent to Lambda/2Pi) from the electric power line or the coaxial cable, respectively. The field strength levels of emissions outside this band shall not exceed the general radiated emission limits in §15.209.

(b) As an alternative to the provisions in paragraph (a) of this section, intentional radiators used for the operation of an AM broadcast station on a college or university campus or on the campus of any other education institution may comply with the following:

(1) On the campus, the field strength of emissions appearing outside of this frequency band shall not exceed the general radiated emission limits shown in §15.209 as measured from the radiating source. There is no limit on the field strength of emissions appearing within this frequency band, except that the provisions of §15.5 continue to comply.

(2) At the perimeter of the campus, the field strength of any emissions, including those within the frequency band 525–1705 kHz, shall not exceed the general radiated emission in §15.209.

(3) The conducted limits specified in §15.207 apply to the radio frequency voltage on the public utility power lines outside of the campus. Due to the large number of radio frequency devices which may be used on the campus, contributing to the conducted emissions, as an alternative to measuring conducted emissions outside of the campus, it is acceptable to demonstrate compliance with this provision by measuring each individual intentional radiator employed in the system at the point where it connects to the AC power lines.

(c) A grant of equipment authorization is not required for intentional radiators operated under the provisions of this section. In lieu thereof, the intentional radiator shall be verified for compliance with the regulations in accordance with subpart J of part 2 of this chapter. This data shall be kept on file at the location of the studio, office or control room associated with the transmitting equipment. In some cases, this may correspond to the location of the transmitting equipment.

(d) For the band 535–1705 kHz, the frequency of operation shall be chosen such that operation is not within the protected field strength contours of licensed AM stations.

 

[vacuum tube]

The info provided by R FRY is spot on, but keep in mind that at 530 khz the coverage is three times that at 1600 khz. That's why most carrier current operations are at the low end of the band.
If you are operating under part 15.209 with an intentional radiator it is usallly better to be at the top end of the band as the 3 meter antenna is more efficient than at the lower end.

I don't know if you could get an STA from the FCC for a special event low power station on AM or FM that would give you better coverage. Not likely though.

 

[rfry]

... keep in mind that at 530 khz the coverage is three times that at 1600 khz. That's why most carrier current operations are at the low end of the band.

That is about true for the distance to the (unusable) 15 uV/m field intensity contour, but the useful coverage distance normally is not 3X greater at 530 kHz because the ambient r-f noise in the AM broadcast band usually is worse at the lower frequencies.
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[ToddJenkins]

"Leaky Coax" AM, or Synchronized Low Power AM Transmitters seem to be the current favorites for Event Microcasting. A new thing that I've seen is Low Power Event Transmitters. With FM built into iPods and newer cell phones, this trumps the older AM systems. Canada is a little more lenient with transmitter power at events. Here in the states, these FM units must be 'daisy-chained' to provide clear coverage of a stadium. (Unless there's a policy that now allows FM on leaky coax.)

Can someone shed more light on this?

 

[RadioMan203]

Here in the states, these FM units must be 'daisy-chained' to provide clear coverage of a stadium. (Unless there's a policy that now allows FM on leaky coax.)

As far as leaky coax on FM, I installed this with our large high school campus. LPB had been doing this sort of setup until they went belly-up. While not officially documented, LPB claimed that the FCC would accept the 15.239 field strength measured at 3 meters from the building's exterior. I picked up a Potomac FIM-71 so I could check and cert the system for these readings and it can't be heard off campus. As far as event radio goes I've only heard of leaky coax FM but have never witnessed an actual even with it in use.

 

[rfry]

"Leaky Coax" AM, or Synchronized Low Power AM Transmitters seem to be the current favorites for Event Microcasting.

Unlicensed systems using leaky coax are covered by the same FCC Rules/restrictions as those using carrier current, as follows (text attributes are mine):

§ 15.221 Operation in the band 525–1705 kHz.

(a) Carrier current systems and transmitters employing
a leaky coaxial cable as the radiating antenna (etc)

So other things equal, the practical limits for the coverage radius/area of both types of systems will be the same.

As for using multiple, synchronized AM transmitters, the thread at http://boards.radio-info.com/smf/index.php?topic=175694.0 might be of interest.

Here in the states, ...FM units must be 'daisy-chained' to provide clear coverage of a stadium. (Unless there's a policy that now allows FM on leaky coax.)

AFAIK, using unlicensed, synchronized ("daisy-chained") FM transmitters is not addressed in Part 15, although the hardware cost of doing so would be prohibitive considering how many such systems, each otherwise compliant with Part 15, it would take to synchronize so as to provide useful, continuous coverage of a stadium or equivalent area.

RF

 

[spinjector]

For FM, what's the max-watts or ERP..? The last experience I had with an unlicensed FM tx was a Mister Microphone... ;D

Is it the same deal with CC that you can't hear it outside the "campus" in question?

The area I'm looking at, with the stadium in the middle, is about 3/4 mile in diameter, if you count all the parking lots and the field house(s). But there are also the concession stands and restrooms down in the bowels of the stadium.

Any suggestions on how I might cover this legally (and economically) unlicensed..?

What about licensed? Since the coverage area is tiny, would I have to go through all the usual hoops of licensing a real radio station with towers and all? Or is there some other process for this "microcasting" thing..?

In the end, it doesn't matter to me if it's AM or FM...

 

[rfry]

For FM, what's the max-watts or ERP..? The last experience I had with an unlicensed FM tx was a Mister Microphone.

Here is a quote from Part 15 Sec 15.239, which covers unlicensed intentional radiators in the FM broadcast band:

§ 15.239 Operation in the band 88–108 MHz.

(a) Emissions from the intentional radiator shall be confined within a band 200 kHz wide centered on the operating frequency. The 200 kHz band shall lie wholly within the frequency range of 88–108 MHz.

(b) The field strength of any emissions within the permitted 200 kHz band shall not exceed 250 microvolts/meter at 3 meters. The emission limit in this paragraph is based on measurement instrumentation employing an average detector. The provisions in §15.35 for limiting peak emissions apply.

(c) The field strength of any emissions radiated on any frequency outside of the specified 200 kHz band shall not exceed the general radiated emission limits in §15.209.

The amount of transmitter power that can be radiated by a matched 1/2-wave dipole to just meet the 250 uV/m limit in 15.239(b) is extremely small -- about 11.4 nanowatts (0.000 000 011 4.. watts). The useful coverage radius provided by that signal to a typical Walkman or boombox won't be more than a few hundred feet.

Is it the same deal with CC that you can't hear it outside the "campus" in question

There is no provision in Part 15 to use FM for unlicensed "campus broadcasting," as there is for AM.

The area I'm looking at, with the stadium in the middle, is about 3/4 mile in diameter, if you count all the parking lots and the field house(s). But there are also the concession stands and restrooms down in the bowels of the stadium. Any suggestions on how I might cover this legally (and economically) unlicensed..?

For a temporary setup, the best chance of doing this in functional compliance with Part 15 would be on AM, using an FCC-certified transmitter driving a "whip" antenna a few inches shorter than 3 meters, with a short ground lead (a few inches long) attached directly to the top of a buried ground rod. This setup (if well tuned/optimized, which isn't all that easy to do) could provide a reasonably strong signal to average AM receivers within a few tenths of a mile, but unless it was installed on the playing field of the stadium probably would not well serve receivers used by all of the spectators seated in the stadium.

What about licensed? Since the coverage area is tiny, would I have to go through all the usual hoops of licensing a real radio station with towers and all? Or is there some other process for this "microcasting" thing..?

Sorry, but AFAIK the FCC has no provision for unlicensed "microcasting" such as desired, but some people have unlicensed AM/FM setups that they use for that purpose. However some of those setups may not functionally meet Part 15, and their operators are subject to an FCC citation, if so.
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