22
4.2.5.4 DC field strength output: A dc voltage output proportional to dBuV/m is available at
the center pin of the RF In BNC jack. With a high-resistance load the voltage is approximately 10
mV/dBu, or 1V for 100 dBuV/m, with a maximum range of 0.2 to 2.0 V. The source resistance is 10,000
Ohms. A 100ua full scale analog meter could be connected between this point and ground for remote
reading. This output can be used while an external RF input signal is connected by feeding the signal to a
T junction through a coupling capacitor.
4.2.5.5 Temperature effects: The low-temperature limit for display operation is -20C/-4F, but
measurements become difficult as the temperature approaches these values because the display response
time becomes very slow. Battery performance is reduced as well. A good cold-weather strategy is to
keep the 4100 in a warm environment as long as possible and to transport it an insulated carrier so its
exposure to cold is minimized. The high-temperature limit is +50C/122F, also set by the display. In
warm weather it is advisable to minimize the time the unit spends in a closed vehicle in the sun, or the
time spent in direct sunlight. The temperature shown on the display is the processor temperature, which
increases to several degrees above ambient with operation.
4.2.5.6 Measuring pure DRM transmissions: In pure DRM transmissions the transmitted
power is uniformly distributed throughout the assigned signal bandwidth, which may be 4.5, 5, 9, 10, 19
or 20 kHz. Since the 4100 measures the power in a 1.0 kHz bandwidth, the total power received is
greater than the measured power in a 1.0 kHz bandwidth by a factor equal to the signal bandwidth in kHz.
Thus the observed field strength in dBuV/m in the 4100 must have added to it a correction quantity
10logB (where B is the DRM signal bandwidth in kHz) to obtain the total received field strength value to
be displayed. Voltage unit values for display are calculated from the dBuV/m values. This correction is
done automatically in the 4100 (for operating software versions 2.3.06 and higher) when a station is
selected from the Tx select list which has the appropriate modulation entry (see Sec. 4.2.2.1, p.19 ). The
correction values are: 4.5&5, 6.8dB/2.19; 9&10, 9.8 dB/3.1; 19&20, 13.0 dB/4.4. Simulcast DRM
transmissions with full carrier do not require this correction and are treated the same as AM signals.
4.2.5.7 Measuring HD Radio All-Digital transmissions: For this signal the carrier but only
part of the sideband power are within the 4100’s 1 kHz bandwidth. A correction must therefore be added
to the measured value to obtain the total field strength. This correction has been calculated to be 3.9 dB
for dBuV/m units or a factor of 1.57 for voltage units. In PI 4100s with software version 2.3.07 or higher
this correction is added automatically if the station data has been entered using Tx add (see Sec. 4.2.2.1,
p. 19) with modulation type HD-ad.
4.2.5.8 Low measuring limit: In the absence of significant ambient RF noise the 4100's field
strength noise floor is typically 10 - 14 μV/m (20 - 23 dBuV/m) and the field strength of AM and
Simulcast DRM signals as low as 25 μV/m (28 dBuV/m) can be measured with approximately ±5 per
cent error. If the noise indication exceeds 14 μV/m at a no-signal frequency near the signal to be
measured, because of ambient RF noise, the low limit for field strength accuracy is correspondingly
higher. For pure DRM signals the low limit for measurements is higher because only the signal within a
1.0 kHz band is measured; the full signal is greater as explained in Sec.4.2.5.6 above. Based on
measurements of a simulated pure DRM signal with a typical PI4100, the low limit for accuracy is, for
bandwidths of 4.5 and 5 kHz, 45 μV/m (33 dBμV/m); for 9 and 10 kHz, 63 μV/m (36 dBμV/m), and for
19 and 20 kHz, 89 μV/m (39 dBµV/m).
4.2.5.9 High measuring limit: The 4100's autoranging input attenuator allows measurement of
very high AM field strengths, up to and beyond the recommended limits for worker exposure, 600 V/m at
or below 1 MHz and (600/f in MHz) above 1 MHz. Potomac Instruments strongly recommends that 4100
users stay within these limits. In high field work, near an antenna tower, electric field strength readings
may not be accurate because the 4100's loop antenna responds to the magnetic field and is calibrated to
read electric field on the assumption that the ratio of electric to magnetic fields is 377, as it is in free space
or the antenna far field. This ratio may or may not hold in the near field.
To Next Page
Loading...
