Unfortunately, this method doesn't work in general. Take, for example, a source
and load, both mismatched to (say) a 50 ohm system. One could measure the output
power of the source delivered to a 50 ohm load (power meter). One could then
drive the load with a 50 ohm source (signal generator, with output power equal
to that measured in the previous step) and measure the load response. Then the
source and load could be connected together. The response at the load could
indicate lower, higher, or exactly the same power as measured with the 50 ohm
power meter, depending on the relative phase of the mismatch between the source
and load, and the electrical length of the transmission line between them.
In the case where the response was the same, a zero electrical length 10 dB
attenuator could be inserted, and the response at the load would be exactly 10
dB lower, even though both the source and the load are mismatched.
The mismatch error is bounded by 20log(1 ± |rho1*rho2|). Doing the experiment
can only set a lower bound on |rho1*rho2|, where rho1 is the reflection
coefficient of the source, and rho2 is the reflection coefficient of the load (0
<= |rho| <= 1, any phase angle). And only the magnitude of the product of the
two rho values is know, not the individual rho values.
Don Borowski
Schweitzer Engineering Labs
Pullman, WA
Ken Javor <ken.ja...@emccompliance.com> on 09/26/2002 06:56:44 AM
Please respond to Ken Javor <ken.ja...@emccompliance.com>
To: "Gordon,Ian" <ian.gor...@edwards.boc.com>, "'IEEE EMC & SAFETY PSTC'"
<emc-p...@majordomo.ieee.org>
cc: (bcc: Don Borowski/SEL)
Subject: Re: MEASURING VSWR WITHOUT A DIRECTIONAL COUPLER
I am putting this out not as the best solution necessarily, but as a
solution. It will be interesting to get several different techniques and
see whose is most accurate, fastest, requires least equipment.
I would install a 10 dB pad at each discontinuity (consecutively, not
simultaneously) and measure the difference each time. If you measure
precisely 10 dB less, there was no vswr. Anything different from 10 dB
means there was a reflection. There is a way to get from the reflected
amount to what the vswr actually is, but in your case you don't need a vswr
reading as much as you want to bound the errors.
The biggest mismatch is from 30 - 80 MHz between the antenna and the
attached coax. Another source of reflections is if no internal attenuation
is selected and the spectrum analyzer mixer is a poor match to 50 Ohms. HP
used to warn about this.
Along those lines I recall Don White (does anyone hear from him anymore?)
critiquing a paper at the 1989 EMC Symposium in Denver. The paper was
something about how accurate this facility was in making RE measurements.
Don asked the speaker if they used a pad to match the antennas to the coax
when making NSA measurements, and the speaker said of course. Then he asked
if they took the pads out in order to get usable sensitivity for RE testing.
Again the answer was in the affirmative.
Don didn't have to say another word. The sneer on his face said it all.
on 9/26/02 6:01 AM, Gordon,Ian at ian.gor...@edwards.boc.com wrote:
>
>
> Everybody
> I need to assess the uncertainty of radiated emission and immunity
> measurements and thus need to measure the mismatch between components in the
> systems. The frequency range of interest is 30MHz-1GHz)
> Is there a way in which I can measure the VSWR of a
> cable-antenna-receiver/amplifier combination without having to purchase a
> directional coupler or network analyser?
> The equipment I have at my disposal is: two power amplifiers (150kHz-30MHz &
> 80MHz-1GHz), a spectrum analyser with tracking generator (9kHz-2.6GHz),
> signal generator (150kHz-1GHz), RF power meter (9kHz-1GHz) and a receiver
> (9kHz-2.6GHz).
> Any suggestions would be appreciated.
> Thanks
> Ian Gordon
>
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--
Ken Javor
EMC Compliance
Huntsville, Alabama
256/650-5261
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