First: They both need to launch the audio waveform at exactly the same time T=0 in same phase. This also means that there may need to be bulk delay to ensure that the most distant (with respect to the IP back haul network) transmitter site, receives the entire data payload before it is transmitted. I am just learning about the RTCM so I believe that bulk delay is inherent to the process.
Assuming both sites are exactly synced T=0, then where the wave fronts meet midway will be in exactly sync even though 35.1us (6.5 miles x 5.4 us = 35.1 us) has passed due to propagation delay.
That's not correct (the part about "the wave fronts"). Simulcast audio/PL launch delay only guarantees AF synchronicity to the extent practical/possible (e.g. within a few microseconds of each other), but certainly not at the same RF phase. A microsecond at 150 MHz is 150 cycles.
The wave fronts will also form a pair of theoretical opposing hyperbolic curves that meet at the center of the two sites like this: x1 )( x2 with the x representing each site.
Again, you can't look at it in the RF domain; AF sync is what you're working hard to achieve; you'll never get it at RF. Also keep in mind that sync at the midpoint isn't necessarily the goal. In a perfect world with no terrain, identical antenna patterns, identical ERP, etc. it might be, but not in the real world. You need to do coverage models for each of the sites (using actual ERP's and antenna patterns), see where the most-problematic areas are (i.e. the areas where capture effect isn't going to hide potential sins), and make decisions as to where to optimize and where to tolerate degradation, and then set delays accordingly.
1) Don't worry about frequency offset until you are sure the timing and other parameters are correct.
Agree. Until the delays are correct (and again, that doesn't necessarily mean equal at the midpoint), there's no sense in adding another variable to the equation. Offsetting carrier frequency only helps mitigate cancellation nulls, it does nothing to mitigate delay errors. --- Jeff WN3A --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus