[Amps] close to off topic - how to read antenna R + jX ??

[Ian White G3SEK]

Bill L. Fuqua wrote:
>
>The HP vector voltmeters are nice but often the probes have been blown 
>away due to application of either DC or excessive signal. Also, 
>adapters are very hard to find for them. The probes themselves have a 
>sharp tips that are easily broken off and delicate matched hotcarrier 
>diodes inside.  But if you can find one that works they are great! I 
>have one myself.
>
I used to have one too, but let mine go after building N2PK's 
50kHz-60MHz Vector Network Analyser. This is professional-quality VNA 
that amateurs can build for about $200 in parts (though it's definitely 
not a beginner's project).

The N2PK VNA is built on one small circuit board that plugs into your 
PC, and the entire thing is software-driven. The feature that makes the 
N2PK VNA far more accurate than the older generation of instruments is 
error correction.  Before you make impedance measurements, you have to 
go through a little routine of 'open-short-load' calibration. This 
assesses the inaccuracies of your measurement setup - and then the 
computer subtracts them.

This error correction feature puts the N2PK VNA in the professional 
league for accuracy. None of the older generation of manual instruments 
has this (unless you grind through the math yourself, for every 
measurement you make) and instruments like the MFJ-259B and Autek don't 
have it either.

Nothing's perfect, of course, and G3RZP makes a very valid point about 
the inaccuracy of '50-ohm' instruments for  very high, very low and/or 
very reactive impedances. But you can always build yourself a different 
test fixture that is better optimized for the range of impedances you're 
trying to measure. Professionally, Peter himself does exactly that.

The N2PK documentation gives you a very good start, with three 
alternative impedance test fixtures: a general-purpose bridge for 
impedances centered around 50 ohms, and two more fixtures optimized for 
high Z and for low Z.

N2PK has produced a large number of documents about all aspects of his 
VNA. Also there is a world-wide User Group where you can get information 
and advice.
http://users.adelphia.net/~n2pk/VNA/VNAarch.html


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek