Kevin Custer wrote: > Jeff DePolo wrote: > >> To get the RF phase accuracy you're implying that is required would mean >> that everything in the RF path would have to guarantee that phase >> relationship. That means the same length RF interconnect cables inside the >> cabinet, same RF feedline length (or full-wavelength multiples thereof), >> same antenna type, etc. Even if you could guarantee that kind of accuracy >> at the time of installation, thermal effects would quickly throw it way off >> (cables expanding/contracting with temperature for example). Not to mention >> the propagation delay will vary a whole lot with temperature, humidity, etc. >> Just not gotta happen.... >> > > > And in practice, we know it isn't going to happen, so we purposely set > the frequency difference between transmitters that overlap in coverage > to about 20 Hz. This negates most negative effects of two transmitters > being real close but not dead on frequency or angle. > Transmitters that are very close to one another will cause PL tone > decoders in the users radio to not reliably open, not to mention to > weird audio artifacts that are produced/heard. At 20 Hertz, or so, > CTCSS tone decoders work fine and radios' speakers don't emit a horrible > beat note. > > This makes sense at VHF, where the standing nulls can be pretty big, but not as much sense at UHF where the nulls are smaller and there's more multipath to fill them in.
Remember that for mobile users, you don't need any offsets because the different relative Doppler shifts cause exactly the same offsetting to happen for you... tens of Hz at reasonable vehicle speeds when one transmitter is ahead and the other behind. Matthew Kaufman
