Jim Lux wrote:
>>For the record (and correcting an error in the 4th edition of ON4UN's
>>'Low Band DXing') the N2PK VNA does not have a broadband detector, and
>>does not suffer from those well-known limitations.
>
>I should clarify.. my comment about broadband detectors is just for
>various and sundry bridge/3 meter/6 port network schemes.
>
>And, for what it's worth, one could always use a tuned detector with
>any of those schemes.
>
>
>>It uses a narrowband measurement method that continues to give accurate
>>antenna impedance readings in the presence of strong off-frequency
>>signals. If the interfering signal is stronger than the threshold of
>>detector burnout (above 10mW), the VNA can be physically protected by a
>>band-reject filter without any significant loss of measurement accuracy.
>
>I presume one would redo the cal with the filter in place?
>
That's correct.
Like a professional VNA, the N2PK does not rely only on the quality of
its construction and components. It also minimizes its internal errors
by being calibrated with an open-circuit, then with a short-circuit, and
finally with a 50R standard load. The results of this "cal" procedure
are then used to correct all the measurements it makes.
Compared with amateur-market instruments that rely on build quality
alone, the use of calibrations takes the N2PK into the professional
league for accuracy.
Repeating the cal with a BCB protection filter in place is effectively
making that filter part of the calibrated instrument. Its varying loss
and impedance-transforming characteristics are all "calibrated out" so
they don't affect the results.
Another neat trick with an open-short-load calibrated VNA is to keep the
instrument indoors, and do the o-s-l swapping at the far end of the
coax, out by the antenna. Again, this makes any arbitrary length of
feedline into a part of the calibrated instrument, so that you can make
all your measurements remotely.
--
73 from Ian GM3SEK
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