Friday, July 22, 2011

The Field Strength Machine

What do professional broadcasters and the FCC use to measure the field strength of a mediumwave station? Let me assure you they don't use charts and graphs like we hobbyists do. Let's find out.

What appears to be the premium device on the market today for field strength measuring is the Potomac Instruments PI-4100 Medium Wave Field Strength Meter, available from Potomac Instruments of Frederick, MD, and shown in the photo just above. It is Potomac's "third generation of precision survey instrumentation intended for the direct measurement of electromagnetic field strength in the 520 KHz to 5.1 MHz frequency spectrum."

This meter has a laboratory quality radio frequency voltmeter, a calibrated, Balanced Loop antenna, an internal GPS receiver, an internal calibration source, and data acquisition hardware and software. The unit weighs in at about 5.5 lbs. It is the successor to the previous industry standard, the FIM-41 meter, also by Potomac.

Also included in the 4100 is a spectrum display. Sensitivity is phenomenal, from 22 µV/m (microvolts per meter) to 50,000 mV/m (millivolts per meter). And there is no "meter" as with old units; everything is indicated digitally and calibrated automatically. The on-board GPS indicates your current coordinates as well as the distance and bearing to the station. It even includes a "wet" compass. For more information, see the Radio World product survey.

Oh, what I wouldn't give to spend a day with one of these. What is the price, you ask?

A cool $14,975. You can pick up a used one for about $13,000. Or, you might be able to buy a second-hand FIM-41 for about $5,000, or rent such an older unit for $595 per week. I can only dream.

The older, Potomac Industries FIM-41 unit:

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