The typical spectrum analyzer / EMI receiver input only looks like 50 Ω with
some input attenuation. Typically, they specify vswr in a 50 Ω system with 10
dB internal attenuation selected.
--
Ken Javor
(256) 650-5261
From: Richard Nute
Reply-To:
Date: Saturday, July 15, 2023 at 4:28
This discussion not only applies to EMI testing, it also applies to
high-frequency and fast risetime pulse voltage measurements. A 6 dB (2x
voltage attenuation) or 10 dB (~3x voltage attenuation) in a 50-ohm system
which would otherwise be subject to reflections due to impedance
See previous response. A transmission line needs a good match at each end to
control vswr-related reflection uncertainty. Assuming both antenna and EMI
receiver provide that, no extra attenuation is required.
--
Ken Javor
(256) 650-5261
From: Paolo Roncone
Date: Saturday, July 15,
Receiver vswr is specified with 10 dB attenuation because the mixer itself is
not 50 Ω.
Antenna vswr is usually best in the middle of the usable frequency range.
Worst case at the low end for EMI antennas used down to 30 MHz, because except
for half-wave dipoles, they are electrically
Hi Ken and Tom
Thanks for your inputs. Yes, the 6 dB pad is at the receiver input, in
addition or not (depending on the signal level) to the built-in attenuator
of the receiver.
If the same 6dB pad is put at the antenna output instead of receiver input,
it's at the other end of the cable
Hi Ken
Thanks for your feedback. Why should the VSWR be specified with 10 dB
attenuation? That would alter (for the better) the specified VSWR itself. I
checked th
e data sheets of 3 antennas that I'm using (a hybrid bilog, a log-periodic
and a horn) and in all of them a "typical" VSWR is
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