Saturday, December 05, 2009

Getting along with others: Primary and secondary allocations

There is a general rule of radio operation that says that a station is entitled to operate free of harmful interference from other stations in the same or other services, and at the same time all stations are obliged to operate so that they does not interfere with other stations in the same or other services.  Stations that suffer harmful interference from other stations are generally entitled to relief from such interference; that is, the national administration responsible for supervising the interfering station is generally obliged, under law or treaty, to take steps to cure the interference, if the interfering station doesn't do so first.  However, this rule is a very general one, and (like most rules) is subject to some exceptions.  This post is about one of those exceptions: the system of primary and secondary allocations.

I've mentioned the ITU a few times now, and that one of their major responsibilities is frequency management.  One of the things that the ITU, and also national regulators such as the FCC, do in furtherance of this is sometimes allocate a band to more than one use.  When they do so, they will typically designate one of those uses as "primary" and the other(s) as "secondary".  When a band has been designated with both a primary allocation and a secondary allocation (either by the ITU or by the FCC), this modifies the general rule regarding cross-service interference.  Specifically, a station in a service which has a secondary allocation in a particular band has no right of protection against interference, harmful or not, from stations in a service which has a primary allocation in that band.  Such stations, more succinctly referred to as "secondary stations" in §2.105(c)(2), must at all times yield to primary stations and take steps to avoid interference.

Many of the bands allocated to amateurs are also allocated to other services.  For example, the entire 70cm UHF band (420-450 MHz, in the United States) is a secondary allocation for amateurs; the primary allocation is federal and the principal user is the military, who mainly uses this band for various radar systems, most notably the PAVE PAWS early warning system deployed at Beale AFB and at Cape Cod AFS (which leads to limitations on the use of the 70cm band for hams in proximity to either of these sites).  Another band for which the amateur service is a secondary allocation is the 60 meter HF band.  The 60 meter band is channelized and is the only band on which an amateur may not transmit Morse code.  It is shared with the federal government (the primary user) and amateurs must immediately cease operation if interference is caused with a federal station.

The situation with the 30 meter band is even stranger.  While 30 meters is primarily allocated to hams by the FCC, that allocation is only valid within the United States and its outlying insular regions.  Outside that area, the 10100–10150 kHz band is allocated primary to the fixed service, and a footnote (footnote US247) to the Table of Frequency Allocations (§2.106) states that "[t]he band 10100–10150 kHz is allocated to the fixed service on a primary basis outside the United States and its insular areas. Transmissions from stations in the amateur service shall not cause harmful interference to this fixed service use and stations in the amateur service shall make all necessary adjustments (including termination of transmission) if harmful interference is caused."  Because 30 meters has worldwide propagation (and in fact is one of the most interesting bands there is in terms of propagation), interference is pretty easy here, which is probably part of why hams in the US are limited to 200 watts in this band (instead of the more usual 1500), and why this band is the only amateur band with no voice privileges anywhere within the band.  (I've yet to find out the specific, historical reason why this band has this rather unusual noninterference restriction and to what purpose the other fixed stations in this band used it or if they're still there.  If anyone knows, please do let me know.)

This post has been brought to you by pool questions T1C09, G1A14, and G1A15.  Section references above are to Title 47 of the Code of Federal Regulations, browsable via the GPO Access eCFR service.