## Field trip to Motorola headquarters

I have been bouncing back and forth to Schaumburg, since Motorola contracts require particular training for servicing and upgrading PSAPs. It’s a great place to spend the summer as it’s just a short trip down to the Loop, which is improving. If you get out early enough check out the Sears Tower Skydeck. I took an FRS walkie talkie and ham radio walkie talkie with me–50 km range is no problem due to line of site. You can make your own radio “pile-up” with so many people calling for you on the ham radio or FRS from Sears Tower.

## Ham radio maximum distance reached from Sears Tower Skydeck

It’s been known for a few decades, particularly since the Sears Tower was built that one can see the tallest bits of the Chicago skyline from the bluffs south of St. Joseph, MI. Constricting myself to HT-HT DX (no yagis, repeaters, mobiles), just 5 watt walkie-talkies between the Sears Towers and them. Assuming Warren Dunes State Park has a bluff 240 meters AMSL, and the Sears Tower Skydeck is 412 meters AMSL, is communication possible? Yes, even with standard 43 refraction. Under conditions where one can see a mirage of Sears Tower, the RF connection is even more likely.

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 for mobile radios, high gain antennas are worse than low gain antennas in urban areas!