Not enough information for a definitive answer. You don't state if these are true ambient rf signals leaking in from outside, or whether they simply represent your meter's noise floor adjusted for antenna factor.
In either case, if an ambient is 6 dB below the signal to be measured, it contributes 1 dB to the signal amplitude. This may or may not be a problem, depending on how close the signal is to your limit. If the ambient you describe is more than 6 dB below the limits you need to measure to, I would say you are okay. Although if I had gone to the expense of buying a SAC, I would want the best performance possible. The numbers you give sound like noise floors adjusted for antenna factor. If that is the case and you want lower numbers, get a pre-amplifier. Here's the math: kTB in a 1 Hz bandwidth at room temperature is -67 dBuV/Hz. Using a CISPR bandwidth of 120 kHz adds 51 dB for a perfect noise floor of -16 dBuV. A typical spectrum analyzer noise figure is 30 dB. That gets you to 14 dBuV. Adding biconical and log-periodic antenna factors to these numbers yields the kinds of numbers you quoted. A pre-amp of the ubiquitous HP8447 variety has a noise figure of around 8 dB and 26 dB gain (going on memory here). That would reduce your measurement system noise figure from the spectrum analyzer's 30 dB to around 10 dB. This 20 dB reduction in noise figure shows up as a 20 dB reduction in noise floor. The HP 8447 (or equivalent) either new or used is a bargain, especially compared to the price of all the stuff you already have. From: [email protected] List-Post: [email protected] Date: Sun, 14 Mar 2004 08:44:34 +0530 To: [email protected] Cc: Ken Javor <[email protected]>, [email protected], [email protected], "T.Sato" <[email protected]> Subject: Noise level inside an fully anechoic chamber Dear Experts, I am conducting pre-scans in a 3-meter fully anechoic chamber. It is noticed that even when no equipment is present inside the chamber the field strength is about 20 to 25 db micro volts per meter in the frequency range 30 to 350 MHz, about 27 to 32 db micro volts per meter in the frequency range 350 to 600 MHz and about 33 to 39 db micro volts per meter in the frequency range 600 to 1000 MHz. What is the level of ambient noise permissible in fully anechoic chambers? Which section of FCC deals with measurement site qualifications? Sincerely K.Balasubramanian Sr.Engineer, Hardware.
