Saturday, August 12, 2006

Rule 15 Subpart C - Intentional Radiators

The satellite radios that include FM modulators must be tested under Rule 15, Subpart C as intentional radiators. The relevant part to satellite radio starts on page 78. Section 15.239, Operation in the band 88 - 108 MHz, is the most applicable section to the FM modulators, although there are other important applicable sections.

Section 15.239:

Section 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 Section 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 Section 15.209.

(d) A custom built telemetry intentional radiator operating in the frequency band of 88-108 MHz and used for experimentation by an educational institute need not be certified provided the device complies with the standards in this Part and the educational institution notifies the Engineer in Charge of the local FCC office, in writing, in advance of operation, providing the following information:

(1) The dates and places where the device will be operated;

(2) The purpose for which the device will be used;

(3) A description of the device, including the operating frequency, RF power output, and antenna; and,

(4) A statement that the device complies with the technical provisions of this Part.


Section 15.201 under Subpart C is the one that got XM, which lead to the clarification by the FCC posted earlier.

(c) For devices such as perimeter protection systems which, in accordance with Section 15.31(d), are required to be measured at the installation site, each application for certification must be accompanied by a statement indicating that the system has been tested at three installations and found to comply at each installation. Until such time as certification is granted, a given installation of a system that was measured for the submission for certification will be considered to be in compliance with the provisions of this Chapter, including the marketing regulations in Subpart I of Part 2, if tests at that installation show the system to be in compliance with the relevant technical requirements. Similarly, where measurements must be performed on site for equipment subject to verification, a given installation that has been verified to demonstrate compliance with the applicable standards will be considered to be in compliance with the provisions of this Chapter, including the marketing regulations in Subpart I of Part 2.


Since XM uses the vehicle wiring [the XM radio antenna (leaky coax) and the cigarette power adaptor], it also falls under Section 15.31(d). Section 15.31 deals with measurement standards.


(d) Field strength measurements shall be made, to the extent possible, on an open field site. Test sites other than open field sites may be employed if they are properly calibrated so that the measurement results correspond to what would be obtained from an open field site. In the case of equipment for which measurements can be performed only at the installation site, such as perimeter protection systems, carrier current systems, and systems employing a "leaky" coaxial cable as an antenna, measurements for verification or for obtaining a grant of equipment authorization shall be performed at a minimum of three installations that can be demonstrated to be representative of typical installation sites.


In the conference call the quarter before last, XM indicated that the FCC changed the rules on them when the FCC issued the clarification. It is not entirely true. The rules have always been there; however, I can understand how they might have been ignorant of this additional testing. Ignorance is no excuse for the TCB they were using. That's their job to know this.

As one can see, the testing procedure is rather nebulous. There in lies the problem. XM's TCB may test it one way and the FCC might test it another and come to different conclusions. Obviously, it is XM's desire to push to the limit as close as possible to have the most functional FM modulator. Until the FCC and XM can work this out, it will continue to be a problem for XM and probably WNC. I doubt that WNC is out of the woods yet as Sirius has claimed. If they want a functional FM modulator, they will have to work it out too. Ki Ryung took the easy way out. They will have radios to sell, but the FM modulators aren't going to work well. Many will be returned, resulting in higher churn. Pay me now or pay me later. A wired modulator is the way to go. If you can't do that, it is probably best to stay away from the radios such as those made by Ki Ryung.

XM's technique and now WNC's is to use a leaky coax. Why? The other radios use only an internal FM loop antenna. Some of XM's radios use both, but the newer models such as the Roady XT do not have an internal loop antenna. Car radios have shielding around them to protect them from interference as well as to prevent them from interfering with other electronics, so an antenna near the radio doesn't do that much good. One still needs to get the signal to the antenna. The vehicle itself can act like a shield. A home radio typically doesn't have that much shielding, so it isn't as much of a problem. Using a leaky coax gets the signal out there where it can be received by the vehicle's car radio antenna. That's why putting the satellite radio antenna as close as possible to the car antenna works best. XM specifically mentions this in the Roady 2 and MyFi manuals. The leaky coax also puts the signal out there where it can be received by other car radios. This is why the FCC is so concerned. If it can be received by your car radio, then there is a good chance it can be received by someone else's, especially if the radio station signal is weak.

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