KM3W USE CARE WHEN USING ANY SPECTRUM ANALYZER ON AN EXTERNAL ANTENNA ESPECIALLY AT A COMMUNICATIONS SITE. Don't use HT's or other transmitters in the immediate vicinity of the SA or any antenna attached to it. 1. With a 20 dB attenuator on the input to the DSA815, connect a small, 220 MHz whip antenna to your DSA815. This should protect the SA input from most signals on a transmit site providing you are NOT in the immediate vicinity of a transmit antenna (say, less than 10 feet away). 2. Search the spectrum from 0-1500 MHz to see if there are any signals above -30 dBm. Broadcast (FM, TV) and Trunked Radio systems will almost always be transmitting if they are present. Each of these can radiate noise given their high power levels. You might want to observe signal levels for a half hour or so by using the Trace Max Hold setting to catch anything that might only transmit once in a while just to make certain that there are no signals that might be too high in level hitting the Spectrum Analyzer. 3. Then and only then you can remove the external 20 dB attenuator, tune to your desired band, say 222-225 MHz and narrow up the Receive BW to 3 kHz. This should show a noise level around -83 dBm(18 uV). 4. Assuming that you saw no really strong signals (i.e. above -30 dBm), you should be able to go to the second AMPT screen and punch up the internal RF Preamp. You should notice the noise floor drop to about -107 dBm(1uV). (Note: I am assuming that Rigol has designed their internal preamp to compress at less than their +20dBm maximum level. Thus if there are no signals above -10dBm getting into the SA with the preamp on, its output level shouldn't exceed +10dBm.) If the noise level is any higher than this -107 dBm, you can assume that external noise is the culprit. Example: At my urban (San Jose) location on 220 MHz my background noise is below this level when connected to a 4 element, horizontal beam antenna. However if I point the antenna in certain directions where there are power poles and/or cross-connections between power lines, I do obtain excess noise. It isn't as bad on 220 MHz as it is on 144 MHz, but it is there. I am not quite certain what you mean by doing 'a NF test and found 6.7 dB of noise'. If this is the Noise Figure of the receiver, you probably have a system Noise Figure closer to 9-10 dB when duplexer and cable losses are also considered. Furthermore the impact of your own transmitter's noise output to the receiver's input depends entirely on the TX's initial output level at the RX frequency as well as the actual notch depth provided by the duplexer between the TX and RX. If you do find a noise level that is excessive you may wish to obtain a 222 MHz beam antenna to use to identify specific noise sources. I find that a well designed 5-element beam will have about 10 dBi of gain, can be handheld, and has a beam width that will allow locating noise sources. If a noise source is on your tower site the beam might even allow determining the vertical angle to the offending source assuming you have room enough to get far enough from the tower to get a reasonable look angle up and down the tower. Sort of a long response, I realize, but this kind of situation is more and more common these days and I've only scratched the surface of providing an adequate answer. Jim E WB6JNN

The manual can be downloaded from rigolna.com at: www.rigolna.com/products/spectrum-analyzers/dsa800/

I'm not certain I understand what you mean by SNR (Signal to Noise Ratio). SNR of a given signal? SNR of a particular measurement? Noise Figure of the DSA815? Please clarify.

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