CS Forums $upporter
- Mar 27, 2019
In the late 70's when low band was still in heavy commercial use, I experimented with deviation on a school bus system. Back then, as now, everybody complained about coverage, but they either didn't have the money, or didn't want to spend the money, to improve their coverage. I was just supposed to wave my hands, tighten a few screws, do some very precise alignments, and magically coverage would suddenly explode to magnificent distances with perfect clarity. Except it never did.
We were allowed +/-5kc, but I noticed these Mocom 10 mobiles accepted almost double that without distortion from my service monitor. Motracs and Mocom 70s would only go to about 8kc, and since those were in this system as well, that set the high water mark for deviation. On their next PM, I proceeded to bump up the deviation on every radio to about six and a half knowing that dull mic elements, and user mic techniques, would ultimately keep most radios well within the legal limits.
But OMG, what a wonderful change it made for the customer. It got me huge accolades. Their supv called my boss to tell him how great I was blah blah blah, the system never sounded better blah blah blah thank you thank you thank you blah blah blah. Okay. I kept that one under my hat, but quietly started bumping up all the cop shops, taxi cabs, and every simplex low bander I had, and got similar results.
I settled on 6.5kc as my limit knowing that (A) we were out in the country with no adjacent channel users, (B) back then most systems used auto-puke, not PL, so they were essentially CSQ with no cochannel users, and (C) they were all low to the ground systems surrounded by the forests of the Michigan UP and the Great Lakes. If I'm not hearing anybody else out there, nobody is hearing them. Let these people get that extra couple miles out of their radios.
When I moved south to Detroit where systems were moving to UHF repeaters in dense RF environments with multiple cochannel and adjacent channel users, I had to tame my game, and come up with another plan. That's when I started making my own RC time constant compressor circuits to flatten out the dynamic range and get it to its legal max. Motorola was going in the same direction with their IDC circuits in the Micor and the Maxar which were just coming out. OTOH, GE was staying au natural, and many many Mastr, Mastr II, Mastr II Pro mobiles became hosts to my mods. Fun times.
RFI: They appear to be trying to fix a symptom rather than the root cause. I believe they have convinced themselves that this is a major technical issue and not something simple. Any differing opinions are not met with enthusiasm as I found out.
The possibility of the multipath argument fails as the unit is mobile thus the mobile to site path would be constantly changing. Add to this other units occasionally in the same area dont have the issue. On a technical note looking at the RF spectrum and the incoming signal would show the level drop or even the cyclic phase cancellation if that was the issue. For it to affect just the detected audio would require a set of circumstances that become mathematically impossible as you pointed out.
There is nothing locally that could be a cause of RFI. I did do a drive by with my test hardware and the input to the repeater range is clean with the nearest in use freq being 945Khz and about 7 miles (a farm) away.
Undoubtedly the statute of limitations have run out!
Major Edwin Howard Armstrong would applaud your efforts in making FM work the way he intended it to be!
You can never have enough FM with FM.
Actually I did something similar to my narrow band ICOM IC-4008A FRS radios. I found there was a sweet spot in the OAB bandwidth and exploited it by bumping them up closer to 4 KHz so they would work spiffy with my Saber GMRS radios.
Got the trifecta today! Yes this actually ran a 4500 mobile A+ lead to the battery... holy shat batman I can't believe this didn't melt.