On 23/02/2023 02:02, henry.powell...@gmail.com wrote:
I wanna make this operation in far-field so i calculated from (2D^2)/λ
and made this distance almost 2meter. My horns are good at 4GHz i am
working at this freq. I adjust RX and TX gain with range operator in
GNU, I mean for the observation I am changing gain. To see the signal,
I need to make 70dB gain so i set 30dB RX gain and 40dB TX gain. I
raised gains 60dB for observation but nothing change. By the way, if
tx gain is at 40db and rx gain is at 30db, the signal level is -75dB.
Thanks for the advice. I will try attenuator.
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The absolute maximum power output of the USRP B200 at 4GHz is probably
no better than +7dBm, and the gain control range
of TX is around 80dB. So at a TX gain setting of 40dB, you're at
about -33dBm coming out of the TX horn.
For RX the maximum gain setting is about 72dB as I recall. So, you'll
have about 40dB attenuation in place between the
first and 2nd gain stages in the RX signal chain. With such a short
path, this won't matter much, except your noise figure
will be quite poor.
Further, the numbers coming out of Gnu Radio are *utterly
uncalibrated*. The hardware and signal processing chain
will generally keep the power relationships linear, but you have only
the vaguest idea of what a signal displaying as -75dB
in a Gnu Radio FFT actually *means* with respect to actual received
power levels. What you do know is that two signals
that differ by (let's say) 10dB at the antenna terminals will be
displayed as 10dB different in an FFT display. That will
be true as long as you stay away from the "edges" -- you want to be
well above the system noise temperature at the
bottom end, and well below the point where analog or sampling
non-linearities creep in.
Further things to consider. Is your VNA a swept, analog, VNA, or
something more sophisticated and digital? If it's
digital, and using an FFT for spectral display, is it using the same
windowing and scaling functions as Gnu Radio?
Likely not.
That you're measuring real-world EM properties of a pair of antennas,
and finding that two different instruments return
slightly different results is not, in fact, that surprising.
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