FOR IMMEDIATE RELEASE

Southern California Adopts New 430-440MHz Sub-Band Plan

August 15, 1995 -- At a meeting in Torrance CA, on August 15, 1995, representatives of all Southern California 430-440 MHz band users met and discussed proposals for digital occupancy of portions of the 430-440 MHz sub-band. This meeting was sponsored by the Southern California Repeater and Remote Base Association, and the Southern California Digital Communications Council. These representatives discussed the proposals and the present occupancy, evaluated alternatives, and then unanimously adopted a new sub-band plan for digital operations in the 431 and 439 MHz segments.

CFR 47 parts 97.201 (b) and 97.205 (b) prohibit operation of Auxiliary or Repeater stations in the 431-433 MHz and 435-438 MHz segments. Recent changes in the definition of a repeater in part 97.3 (a) (37) have been interpreted to mean that store and forward or digipeater operations are not a repeater because the retransmission of the received signal does not occur “simultaneously.” This interpretation appears to allow “digipeater store and forward” operation within these sub-bands as long as the operation does not “simultaneously” re-transmit its received signal. Transmitting or receiving the signal on another frequency may be legal within these sub-bands as long as it is not done “simultaneously.” Use of separate transmit and receive frequencies (by a “digipeater” with at least one frequency within these segments) has not been addressed by the rules themselves nor by recent rule interpretations. The definition 97.3 (a) (7) of an Auxiliary station now specifically excludes “message forwarding” stations, thereby allowing these stations access to these sub-bands.

The 431-433 MHz sub-band has traditionally been reserved for weak signal operations. The existing sub-band is sufficiently wide to protect the very sensitive receivers used there from sideband noise and overload interference from adjacent operations. The representatives of the weak signal community carefully analyzed the interference potentials of the various modes and types of operations proposed. They determined, with certain simple limitations on “high level” activity, that a portion of 431 MHz could be used by the digital community without harming weak signal operations. These same limitations on “high level” or “communications site” uses serve to protect the fixed point to point relay operations on the adjacent lower frequencies.

The 435-438 MHz sub band is occupied by satellite operations and amateur television operations. The TV operations already have numerous local and high level receivers occupying this sub band and the adjacent segments. The TV “channel” in the band plan extends from 433.0 to 438.9 MHz. Normal TV receivers are actually somewhat wider than this “channel.” A TV receiver will “see” a signal anywhere within its passband, resulting in interference. TV transmitters are usually low power, and this power is distributed through the entire 6 MHz bandwidth resulting in a very low power density. These facts render digital operation (or other transmissions) anywhere within the TV receiver passband quite impractical.

The Digital representatives indicated the strong desire of the digital community to be able to operate without regular frequency coordination.  After much lengthy discussion, all the representatives agreed that this would be possible if certain basic limitations were observed by operators using either the new 431 or the existing 439 digital band segments. These limitations are simple and basic. The power and antenna height limits are high enough to allow most amateurs to operate successfully and without the bother of going through the frequency coordination process. These limitations are designed to protect the weak signal operations and the point to point operations that are adjacent to the digital segments. Operations outside of these limitations are possible but are subject to careful technical review through the frequency coordination process to assure minimization of interference.

Amateurs needing to operate stations that fall outside of these limitations must apply for case by case frequency coordination (just as does any repeater or auxiliary station). Amateurs who need to operate from an elevated location and/or with substantial power have a very high potential to create interference to weak signal and point to point operations. These amateurs must go through the frequency coordination process where a technical analysis of their exact proposed operation can be made. This analysis will determine if the operation can take place without causing interference and, if so, what additional limitations may be needed. These determinations are made on a strictly technical basis. Final coordination may require “on air” tests and measurement of sideband noise by those subject to the interference.

Operation in these band segments from a “communications site” is most likely to cause interference. Transmissions from such elevated locations will usually be very strong at weak signal receivers located quite some distance from the site. These strong signals are likely to contain sufficient levels of sideband noise to render the weak signal receivers useless. These transmitters are also likely to be co-located with point to point receivers or transmitters. This co-location will result in either the digital receiver or the point to point receiver being effectively “dead” while the other station is transmitting. While it may be possible to technically work out a scheme to satisfactorily alleviate this interference, it will likely be neither simple nor inexpensive. Because of these considerations, operation on these band segments from these sites is unlikely to be successful.

431 AND 439 MHz DIGITAL BAND SEGMENTS

Any operation at a communications site requires case by case frequency coordination.
Maximum transmitter power of 25 watts, and maximum Effective Radiated Power of 100 watts.
Vertical polarization only*
Ground locations 100 feet AGL maximum**

*Vertical antenna polarization is required to minimize coupling to Horizontal weak signal stations

**Antenna height 100 ft maximum, single radial height above terrain. Operation above 100 ft requires case by case frequency coordination.

***Carrier frequencies must be chosen so as to confine the occupied bandwidth to be within the segment limits.

Communications site:  Commercial or multiple use locations, usually elevated (high level), with multiple transmitters and receivers and users. These sites are often, but not always, commercial facilities.

Ground location: Any location where the antenna is 100 feet or less above the lowest terrain to be found in any direction. Such measurement is usually made by determining the single direction from the site where the terrain is the lowest and taking the ground elevation at 2, 4, 6, 8 and 10 miles from the site and averaging those values. This is intended to require operations on the side of a hill or other advantageous terrain to be subject to frequency coordination in order to minimize interference.

Bandwidth:  97.3 (a) (8) “Bandwidth. The width of a frequency band outside of which the mean power of the transmitted signal is attenuated at least 26 dB below the mean power of the transmitted signal within the band.”

97.3 (a) (8) does not address the sideband noise generated by most transmitters. These effects are considered in the power and antenna height basic limits above, and will be considered in the technical analysis during the frequency coordination process.

For More Information Contact:

Southern California Repeater and Remote Base Association
P.O. Box 5967
Pasadena, CA.  91117-5967
Internet: admin@scrrba.org

Last modified: April 12, 2003