PhilB wondered about the comparison of one day to another, so that's what was just collected on my notepad... today's noon hour reading to compare with yesterday.
Time of today's test = 12:40 Noon. Temp = 63-degrees, sunny, there's been no precip. Very dry.
From position at NE corner of driveway at street, TECSUN PL-310 shows 57/25, with null 47/24.
Yesterday at 12:45 Noon 56/25 and null 47/23.
Today's transmitter power to RF final = 120mW.
Yesterday transmitter showed 123mW.
Today's spectrum analyzer showed 42dB?.
Yesterday's s.a. = 44dB?.
And I have uncovered another variable, remember I found the desk lamp was changing the spectrum reading when adjusted... well I wondered whether the cables from the volt meter had an effect, and the answer is yes.
There are two sets of test points on the AMT5000 for measuring Voltage and milliAmperage, and I have been leaving the probes plugged into whichever was most recently measured.
If today the probes are attached to millAmps, I take that reading first, leaving the probes attached to Voltage, which is the first reading the next time.
When I noticed that today's spectrum reading was two notches less than yesterday, a reversed the probes at the transmitter, and the analyzer went up to 44dB?, even though it's 35' away!
Laboratory precision can be sneaky.
I will remove the probes entirely the next time. Oh, that's right. There might not be a next time, I think I've done everything.
Things sometimes get confused when operator's manuals are translated and perhaps this is the case. Whether or not the readings are actually dBm the relative readings should be OK.
Your observation about things which affect the signal reaching the spectrum analyzer illustrate the difficulty in making reliable RF measurements. When I tried to assess the field strength using an indoor antenna I experienced variability in readings so I elected to use an outdoor antenna which is not subject to the same variables as an indoor antenna. There are other things which appear to affect the readings but not nearly as much as with the indoor antenna. Just one more thing to ponder and worry about.
Neil
Rich's views and description of the ATTEN spectrum analyzer are correct, so far as I am able to verify, and like so many other "fairly priced" tools one may accumulate in the practice of Part 15 radio, it provides a very general reflection of RF activity without laboratory precision. We can observe our signal on the screen without truly knowing its specific and absolute properties.
I think some improvement on the antenna situation could possibly be made with the addition of a professional measurement antenna, at a cost around $300.
Sometimes knowing other, outside, factors can improve one's confidence on what is seen on the screen. For example, the C.Crane FM transmitter provides a certified signal with its native antenna, and thereby provides an RF image on the analyzer which may be considered "typical" of a certified FM transmitter. By placing another FM certified transmitter, the Wholehouse 2.0, the same distance from the spectrum analyzer, the signal strength is very similar, demonstrating that one certified FM transmitter matches another.
..if with zero input attenuation your analyzer reads 42 dB as in your Post 61 of thread, this would indicate an input power that is 80 - 42 = 38 dB below 13 mW, which is 0.002 mW, approx.
My error, because 13 dBm is not 13 mW, it is 20 mW.
So the correct value is 80 - 42 = 38 dB below 20 mW, which is 0.003 mW, approx.
Sorry.
...the ATTEN ... provides a very general reflection of RF activity without laboratory precision.
The specifications for the ATTEN AT5005 actually are quite good, and it would make rather accurate measurements within those limits. But apparently the instructions provided for it are not very complete as to the real values in dBm or mW that are shown on the display.
Below is a table with the powers measured by the AT5005 at different peak values on the display scale.
Carl Blare on February 10, 2013 - 20:01: The dBm measurements are taken from the screen of a spectrum analyzer, which receives its signal from an antenna 13" in length and located 10-feet from the test antenna. (etc)
Carl - A few thoughts about the use/capabilities of your AT5005 spectrum analyzer.
The specifications for it (below) show that the maximum safe input power applied with zero input attenuation is +10 dBm, which is 10 mW.
From looking at a picture of an AT5005 showing the display of a spectrum, there appears to be no electronic calibration of amplitudes shown in the display, itself.
With zero input attenuation, a +10 dBm signal at the input will produce a display with a peak amplitude that is 3 dB below the top horizontal line of the display. That would be 1-1/2 minor divisions on the vertical axis.
Edited 9/15/2013: So with no input attenuation selected on the front panel of the analyzer (via the push buttons), a signal just reaching the top horizontal line of your display does not show that the input is 80 dBm, but 13 dBm -- which is 3 dB more input power than the unit is rated for. Signals that do not reach the top horizontal line with no input attenuation are LESS than 13 dBm by the reading in decibels on the scale of the display. So, Carl, if with zero input attenuation your analyzer reads 42 dB as in your Post 61 of thread, this would indicate an input power that is 80 - 42 = 38 dB below 13 mW, which is 0.002 mW , approx.
But all of this applies to conducted (closed circuit) signals in a 50 ohm environment. The calibration of the display in dBm will not be accurate when the r-f input source is an antenna, unless its "antenna factor" is known at the measured frequency, and its feedpoint impedance is matched to the 50 ohm value needed by the spectrum analyzer.
The analyzer may show a display when connected to an uncalibrated antenna, and the displayed values may be useful for relative measurements. But it isn't possible to make accurate measurements of those received signals, in absolute terms (dBm).
Hopefully this information about the AT5005 is accurate, even though I don't have one myself.
_____________
Company: Shenzhen Atten Electronics Co., Ltd.
Product Name: Spectrum Analyzer
Model No: AT5005
SPECIFICATIONS:
Frequency range: 0.15 to 500MHz
Center frequency display accuracy: +/-100kHz
Marker accuracy: 0.1% span +100kHz
Resolution of frequency display: 100k(4.5digit LED)
Frequency scanwidth accuracy: +/-10%
Frequency stablility: Better than 150kHz/hour
If Bandwidth (-3dB): 400kHz and 20kHz
Video-Filter (ON): 4kHz
Sweep rate: 43Hz
Amplitude range: -100dBm to +13dBm
Screen display range: 80dB(10dB/div. )
Reference level: -27dBm to +13dBm (in 10dB steps)
Reference level accuracy: +/-2dB
Average noise level: -90dBm(20kHz Bandwidth)
Distortion: <-55dBc of 2nd and 3rd harmonic
Third order intermod: -70dBc (two signals>3MHz apart)
Sensitivity: Better than -90dBm
Log scale fidelity: +/-2dB (without attn. ) 250MHz
Input attenuator accuracy: +/-1dB/10dB
Max. Input level: +10dBm, +/-25V DC (0dB attn. ) +20dBm (40dB attn. )
Frequency scanwidth: 100kHz/div. To 50MHz/div, in 1-2-5 steps and 0 Hz/div. (Zero Scan)