RF monitor

see also:

• Evaluating compliance with FCC guidelines for human exposure to radio frequency electromagnetic fields.

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23aug06: Test monitor which 430 Mhz transmitter.

• lf: line feed transmitter
• gr: gregorian dome transmitter
• az (xx): the azimuth is  the physical location of the transmitter. 180 (xx) is always on the far side, 0(xx) is always on the near side.

The rf monitor output goes  from 0 to 1 and is linear in power. The value 1. corresponds to a power level that was 50% of the maximum allowable value (weighted by frequency range).  So a value of .1 would be .05 time times the maximum allowable value.
NOTE: later it was found that the meter is using the wrong radiation profile (see below). It uses a maximum values that is 5 times too high. Given a value on the plots, you should multiply it by 2.5 (5/2) to get the fraction of the maximum allowed value. A value of .1 would be .25 of the maximum allowed value.

The first set of plots shows the  rf monitor power versus time (.ps) (.pdf):

• Top: The rfmonitor power versus time 4:00 to 14:00.
• Black was taken on 23aug06 (the day of the test).
• Green was taken on 22aug06 (the previous day when the transmitter was not on).
• The red vertical lines show the time when the xmter was on during 23aug06.
• On both days the reading is quiet until around 6 am. It rises and begins to oscillate (with a period of about 45 minutes).
• The level rises to about twice the morning value at 12:00. It comes back down to the nighttime value around 14:00.
• Middle: rfmonitor power vs time 10:00 to 14:00 xmiter on.
• This is a blowup of the 23aug06 data during the xmiter tests.
• The black data is the rf monitor.
• The green data was taken during the first test (lf, za=19.5, far side)
• The blue data was taken during the 2nd test (lf, za=19.5, near side)
• The purple data was taken during the 3rd test (lf, za=15, near side)
• The light blue data was taken during the 4th test (gr, za=19.5 near side).
• The red solid line  shows the transmitter power. It came on about 11 am. It went off about 13.25. The dashed red line is a tx power of 2 MW.
• The rf monitor level did start to rise when the xmter came on. It went back down before the xmter went off (it did start to move in azimuth). This bump may or may not be caused by the xmter.
• Bottom: azimuth vs hour of day.
• The azimuth position (dome side) during the tests. The colored areas show when the tests occurred.

• Top: test 1. Linefeed far side, 19.5 za. You can see an increase in the power level when the azimuth is at 187. This is when the line feed is aligned 180 degrees from the radiation monitor. The increase is about .02/.10=20%. The absolute value of the increase is .02*50%= .01 of the maximum allowable value.
• 2nd test 2: linefeed near side, za = 19.5. There is no noticeable azimuth dependence of the power.
• 3rd test 3:  Linefeed at near side, za=15. No azimuth dependence of the power.
• 4th test 4: dome at near side, za=19.5. No azimuth dependence of the power.

Glitches in the rf monitor:

Summary:

• The measured power value of the rf monitor changes by a factor of two day to night. The levels peak around noon. The device output may be changing with temperature.
• We saw a small change in the power level versus azimuth when the dome was on the far side and at 19.5 degrees. We did not see any azimuth dependence of the power when the transmitters were on the near side.
• There was an increase in the power level when the transmitter started to come on. The change was a slow rise (10 to 15 minutes). There were no sudden jumps when we increased the transmitter power (usually this took a minute or two).
• The rf monitor is seeing glitches that go full scale every 46 to 48 seconds.  They stay high for 1 or 2 seconds and then undershoot for 3 or 4 seconds. They were not there in the lab or when the device was covered with a mesh. We need to determine whether this is real or a glitch in the device. The alarm in the control room is not being triggered by these glitches so the alarm must be disabled.
• The power levels measured by the rf monitor do not agree with the hand held rf monitor. The hand held monitor read a maximum value of .12 mW/cm^2. This is 41 % of the maximum allowed value at 430 Mhz (using .29 mW/cm^2).  The maximum value read by the rf Monitor was .13 *(50% of max allowable Value) = 6.5%  . So the two devices differ by a factor of ..41/.065=  6.3.  This discrepancy  is probably coming from the standard being used by the fixed rf monitor : fcc 1997 occupation/controlled.
• For uncontrolled access areas the 430 Mhz limit is 430/1500=.29 mW/cm^2
• For controlled access areas the 430 Mhz limit is 430/300 = 1.43 mW/cm^2
• The ratio of these values is 5 which agrees with the measured difference above.
• Taking the above into consideration, the power levels on the plot should be multiplied by 2.5 to get the fraction of allowable radiation.
• We need to switch the rf Monitor to use the uncontrolled/general population access rf limits for the visitors center overlook.

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