From Dave G3YMC
Below is a corrected version of the mail I sent earlier. My signal
generator is calibrated in dB of attenuation relative to +13dBm, and I had
incorrectly converted my figures into dBm. The new figures are more
reasonable, and show how easy it is to get confused when dealing with dBm
(even from someone who should know better!). The new figures are rather
more realistic, but still indicate that we should not rely on S-meters for
quantitative measurements.
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I have calibrated my receive set up against a quality signal generator,
calibrated in dBm into 50 ohms. I won't bore you with the actual figures,
but they show that the use of S meter figures is totally meaningless. The
s-meter for my FT101ZD is designed to measure the signal at its own
receiver input - this is meaningless when there is a receive converter at
the front end with its own gain, which in my case is appreciable. I also
do not know the input impedance of my receiver - the figures are for a
nominal 50ohms but would be double for an infinite input impedance.
The important figures are:
DCF39 (875km) 45uV into 50 ohms (-74dBm)
Noise level (typical) 1.2uV into 50 ohms (-105dBm)
S9 4.5uV
1 S point is typically around 2dB, although surprisingly S9 to S9+10
measures as 10dB.
point taken, S meters are meaningless.
Unlike some I get very little variation in the strength of DCF39 - it is
always S9+18 on my meter with a variation of no more than 1dB or so. I
never ever see the 20 to 30 dB variations some mention. Reception is
normally on my transmit loop.
Thanks to Geri DK8KW for his dB conversion chart. However I would request
that he changes the colours on his page. White text on a black background
looks fine on a monitor, but most of us would like to print it out. I had
to edit the page before printing so that I could see the white ink on my
white paper....
73s Dave G3YMC
[email protected]
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