Field trip to Motorola headquarters
The Sears Tower Skydeck with an FRS walkie talkie and ham radio walkie talkie can yield 50 km line of sight. With standard 4/3 refraction, and especially under conditions where one can see a mirage of Sears Tower, the RF connection across the lake is possible.
Using Radio Mobile Deluxe, I predict on 2 meters (144 MHz) a signal strength of -87 dBm with 70% spot reliability. 70 cm (440 MHz) is predicted to be -94 dBm. In pure free space (outer space) we expect a tripling of frequency to increase path loss by
20*log10(3) = 9.5 [dB]
Yet here we see only a 7 dB penalty for tripling frequency. This is because the Longley-Rice model inside Radio Mobile Deluxe takes into account that (for this path) at 144 MHz the Fresnel zone clearance is only 0.2 and at 440 MHz the Fresnel zone clearance is 0.3. Fresnel zone clearance relates to Huygens’ principle: at each point in space, we may imagine secondary reradiators. Thus for a wave front encountering an obstacle, we can model the “bending” around an object into a “shadow”. This is how we have radio coverage in urban areas, and why mobile radio high gain antennas are worse than low gain antennas in urban areas.
Motorola headquarters tour
The Motorola Museum includes an old Centracom console (complete with OOPS code), and old handsets and dual TX-RX units from WWII era or thereabouts.
We saw an RF development lab where they test radios in real world conditions. They had a spectrum analyzer hooked to a broadband (discone?) antenna. In the 100-1000 MHz range, the quietest spectral locations were the amateur radio bands, they looked as if there’d been notch filters inserted in line. Of course that wasn’t the case, it was the relative daytime quiet of the VHF/UHF ham bands compared to the commercial and government radio traffic. Yes, military aviation UHF was also a bit quiet.
We also saw a CAD lab where the engineers had quad monitors. With perhaps $1200 in two dual-monitor video cards and $1000 per monitor, and probably a $2000 desktop PC otherwise, that’s about $8400 per workstation in computing hardware. The productivity gains are compensating for that initial expense.
We also saw the dispatch center where Motorola handles field support calls from contracted end users and support staff. This might include Motient, large cities and government agencies. There is a backup center with undisclosed location and different employees as it would be vital in the case of a big crisis. The wall of the two story room was a giant screen, like one sees for NASA Mission Control, viewable from any of the perhaps three dozen workstations facing it.