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E5071C intrinsic high reflection (S11) at low freqs (300 kHz)?

Question asked by isend on Jul 28, 2015
Latest reply on Aug 7, 2015 by Laowai
I measured S21 and S11 of a 50-ohm regular coaxial cable (in good condition) with its two ends respectively connected to the two ports of my E5071C-2K5 ENA (2-port,  300 kHz–20 GHz) (i.e. a "thru" situation; calibration off; power = -55 dBm, but the result would be similar with power = 0 dBm). The S21 result looks normal, but the S11 is anomalously high at low freqs (-15 dB at 300 kHz, and reduces to ~ -30 dB at and above 3 MHz. See image below).

My colleague owns a 4-port 20-GHz E5071C, and found similar S11 behavior. I used to have an E5071C-285 (2-port, 100 kHz–8.5 GHz), but it did *not* show anomalously high S11 at low freqs.

Is it normal for 20-GHz E5071C to have high S11 at low freqs intrinsically (i.e. calibration off)? Is there an explanation for this high S11?

(P.S. "Enhanced Response Calibration" cannot avoid high S11 at low freqs, but full "2-port Calibration" can.)

!http://3.bp.blogspot.com/-PEHUOGDy99g/VbdONPYq0cI/AAAAAAAAHy8/gLmsKb6QVAk/s600/S11.png!

Edited by: isend on Jul 28, 2015 3:48 AM

Edited by: isend on Jul 28, 2015 3:49 AM  

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