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Re: [TowerTalk] impedance meter recommendations pse

Subject: Re: [TowerTalk] impedance meter recommendations pse
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Fri, 29 Dec 2006 21:31:22 +0000
List-post: <>
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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