By Tom O'Hara W6ORG
This paper was originally written in Dec. 1990 for the Repeater Coordinators Newsletter.
When the 23 cm ARRL band plan was formulated in the early 80's, FM ATV was being experimented with by just a handful in the USA. It was determined that even though the occupied bandwidth would exceed the 10 MHz wideband experimental slot centered at 1265 by a little for FM ATV simplex (it would stay inside if no sound subcarrier is used), there was not enough FM ATV activity at that time to warrant presenting two differing bandplans to allow for both ATV AM or FM repeaters.
In all but the most populated areas of the county, the possibility of interference from non-coordinated FM ATV on the 23 cm band will not occur for many years. But now is the time to consider and discuss local standards to minimize future hassles.
If we use the modulation standards listed in Table 1, page 20-12 in the 1990 ARRL Handbook, the occupied bandwidth will be a little over 17 MHz given 4 MHz video deviation and 4.5 MHz sound (20 MHz if 6 MHz sound). One FM ATV channel will replace two of the AM ATV channels. There is no room for a combined 12 MHz slot above 1270 given existing FM voice repeaters, but the two AM ATV channels below 1260 do make it possible if there are no existing links or digital systems in place, or so few that they are willing to move..
The question then becomes what are the practical frequencies to move the FM voice link and digital channels to. The links and digital systems are some times duplex so they need the same input/output separation minimums as a repeater does. That says there needs to be some room on each side of the FM ATV channel rather than placing the ATV at the edge of 1240 or 1260 MHz.
Present FM voice gear which is also used for digital systems have programmable offset capability. Many FM voice systems use back to back synthesized mobile transceivers also, so there is no practical equipment degree of difficulty for a specific offset or frequency. There is one difficulty for a FM ATV repeater input, however, if the link transmitter is located at the same site it can capture the ATV receiver, and vice-versa.
I suggest centering the FM ATV carrier at 1252 MHz. The links would then occupy 1240-1242 input pairing with 1258-1260. Which would be the link input or output at a location would depend on whether the ATV system at 1252 was an input or output. The adjacent 1240-1242 would be the same in this case as the ATV system. Digital systems at 1242-1246 would also follow this input or output logic paired with 1297-1300. By keeping all receivers at a site regardless of mode close in frequency to each other, it allows the filters in the antenna lines to have the most rejection to the transmitters.
The frequency could just as easily be 1248 with the digital segment above the ATV segment, but 1252 is just closest to the British 1255 MHz frequency that many use now. Also the link segment at 1258-1260 is unchanged..
Each area will have to get together with their local Frequency Coordinator when the first 23 cm ATV repeater is about to be built to determine whether to stay with the ARRL band plan in this segment for 2 AM systems, or modify it for an FM system.
Tom O'Hara, W6ORG
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