Measuring the RF field strength of a single transmitter co-located with a host of similar transmitters/emitters is by no means simple. The daunting task has resisted the best efforts of system operators since 2000 when the FCC announced expanding safety rules for those working near RF transmitters. For accurate measurements, each transmitter must be measured individually while the others are turned off, much to the chagrin of silenced system operators. To solve this difficult problem, Narda Safety Test Solutions (Hauppauge, NY) has introduced the SRM-3000 selective radiation meter, a battery-powered instrument about the size of a paperback book that can measure the separate power levels of multiple signals without interrupting service. Within a few seconds, it measures all the emitters at a single site in three axes and displays the power levels in a list along with the percentage they contribute to the total field strength allowed by an applicable radiation protection standard such as ANSI/IEEE C95.1-1999 or FCC regulated limits. The total field strength from all emitters is displayed as well.
The SRM-3000 (see figure) is a significant departure from the company's earlier radiation monitors. Earlier units are broadband instruments designed to show the total field strength of a co-located site over a range of frequencies, but cannot identify individual emitters at specific frequencies and calculate their strength as a percentage of the strength allowed by a standard. The lack of equipment that can perform these functions has made compliance with FCC guidelines very difficult because the only alternative is to either measure individual transmitters (at the expense of the remaining system operators) or to calculate the contribution of each signal manually.
Owing to the general laxity with which its rule change was received, the FCC has indicated that it will be far more vigilant in the future about ensuring compliance with the law. The FCC uses the "all transmitters off" method, guaranteed to get the message across loud and clear: Upon determining that there is a "hot" area near a given site, the agency simply informs the operators that they must at a specific time turn off all their transmitters at the site. The FCC then asks that they be turned back on one by one, while the field strength of each one is measured. Adding up the total provides the total EM field at the site. If it's greater than 100 percent of the applicable protection standard, all transmitters contributing more than 5 percent of the total at the offending frequency must work to reduce the field strength or be fined or both.
In July 2002, FCC agents gave evidence of their vigilance at one of the most conspicuous transmitter sites in the US: the peak of Mt. Wilson north of Los Angeles, where most of Los Angeles County's FM and TV broadcasters transmit their signals. All transmitters were shut down, and then individually returned to service in a very short time after measurements were made, upsetting broadcasters and their customers. Had they been armed with the SRM-3000, the FCC could have performed their tests far less painfully.
The SRM-3000 essentially combines the measurement and analysis functions of a high-performance spectrum analyzer (see table) with the computational power of a Windows CE-based personal computer (PC). The primary probe employed by the SRM-3000 uses three, orthogonally mounted isotropic monopole antennas. The antennas are automatically switched at high speed, and the received signals are sent at their original frequency to the instrument via a special ferrite-shielded cable if the probe is mounted on a tripod or directly if the probe is attached to the instrument.
The signals then are processed by the spectrum analyzer that forms the foundation of the SRM-3000 (in addition to the probe technology). Algorithms in the Windows CE-based instrument calculate the required values, and the results are displayed on the 4.7 × 3-in (11.9 × 7.6-cm) 480 × 320-pixel monochrome display. The choice of monochrome was not made to reduce cost but because even the best color liquid-crystal-display (LCD) screens are not as viewable in direct sunlight as backlit monochrome types.
The SRM-3000 has three main operating modes: safety evaluation, spectrum analysis, and time analysis, and the results obtained from all three can be displayed in either tabular form or as a graphic, either one of which can be exported to popular programs such as Microsoft Word or Excel. The safety evaluation mode is the most interesting because here is where the tabular listing of signals appears by name (such as a call sign or service type), along with the field strength of each one as a percentage of a specific standard, and the percentage of the standard reached when signals at the site are combined. This mode allows the signals at a site to be sorted out by frequency, type of service, and measured field strength along with their relation to standard-imposed limits.
There are two other modes of operation: spectrum analysis and time analysis. In spectrum-analysis mode, the SRM-3000 displays the measured spectrum with resolution bandwidth and other parameters selected by the user. In the time-analysis mode, the user selects a center frequency and the resolution bandwidth corresponding to the bandwidth of the channel to be monitored. The display shows a current field strength value along with a plot of field strength over time. In every mode, the SRM-3000 can show field strength in V/m, A/m, W/m2, or mW/cm2.
The SRM-3000 measures 9.5 × 5.5 × 2.3 in. (24.13 ×13.9 × 5.8 cm) and weighs 4.2 lb. including rechargeable lithium-ion (LiIon) batteries. It will operate from 3 to 4 hr. on a charge, and can operate from 120 to 240 VAC via a supplied combination charger/converter. The instrument is heavily shielded, with RF immunity of 500 V/m.
The instrument has enough memory to store 20 user-defined test setups and 500 spectral plots, and can update its standards information via software and its RS-232C port. In addition to its 80-MHz-to-3-GHz triaxial probe, the company also offers a 30-MHz-to-3-GHz dipole E-field probe that can be mounted on a tripod for precision single-axis measurements. The instrument is supplied with a calibration report, RS-232 cable, configuration software, the charger/converter, and a battery pack. P&A: $15,000; stock. Narda Safety Test Solutions, 435 Moreland Rd., Hauppauge, NY 11788; (631) 231-1700, FAX: (631) 231-1711, e-mail: [email protected], Internet: www.narda-sts.com.
