At 8/2/2004 08:27 PM, you wrote:
>--- In [email protected], "Jeff DePolo" <[EMAIL PROTECTED]>
>wrote:
>"...I have quiet 440 sites that will break squelch and decode PL at
>0.03 uV (Angle Linear PHEMT's doing the job)..."
>__________________________________________________
>______________________
>
>I think if I was getting a receiver threshold reading 5 dB below kTB -
>I'd think about getting my test equipment properly calibrated. [or
>invest in that company that's making preamps with a negative noise
>figure!]
Yes that is a bit optimistic, but possible if T (antenna noise temperature)
< 295 K. At microwave frequencies, antenna noise temperatures can easily
be < 20 K.
I'm looking at a chart from Kraus' "Radio Astronomy". At 440 MHz galactic
noise can fall anywhere from 20 K to 400 K. If 1/2 your antenna's
radiation pattern is falling above the horizon, it's possible to get T < 295 K.
The noise temperature of a 50 ohm signal generator will always be ambient
(~295 K, UNLESS you use a cooled attenuator on the output - yes I've done
this), so if you ever open a NBFM receiver with less than -132 dBm straight
out of the sig. gen. then something's wrong. But the antenna is not a 50
ohm resistor; it's an efficient (we hope) coax-to-outside-world coupler, so
the above doesn't necessarily apply in that case.
Bob NO6B
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