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

[David Kirkby]

Ian White G3SEK wrote:

>
>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.
>
Something I have done professionally is to use a lock-in amplifier. 
These are usually used for extracting small signals from the noise. Some 
of the better instruments can measure signals that are 100dB below the 
noise!! They measures magnitude and phase, although most only extend to 
a few hundred kHz. Standford Research makes one that goes to 200MHz, but 
it uses the same principle as I built one many years ago - basically mix 
down to a lower frequency, and make the measurements there with a 
low-frequency locking.

I was never interested in measuring absolute phase, but changes of 
phase. Stabilities of the order of 0.1 degrees over many hours was 
possible at 200MHz.

I don;t know what used lockins sell for now, but they *might* be yet 
another option. Most of the modern instruments use digital signal 
processors, but analogue instruments exist too.

BTW, fancy a laugh??

I used the system that measured phase changes to monitor blood 
oxygenation of babies during labor - with some very willing mothers. 
Since I'm not a medic, but a PhD scientist, I worked with an obstetrician.

The instrument was large, about 2m tall in a 19" rack and put out quite 
a bit of heat, due to the two lockin amplifiers,  2 low phase noise sig 
gens, 4 lasers, computer etc. The lady having her baby (who was a ballet 
dancer) said:

"That instrument is making this delivery room very warm."

to which the obstetrician replied,

"No, it does not, since there are fans in there to keep it cool."

The women in labor then replied:

"Yes, but the fans only move the hot air from one place to another."

He started to argue, but I managed to stop him before he dug himself 
into an even bigger hole!

-- 
Dr. David Kirkby, 
G8WRB

Please check out http://www.g8wrb.org/ 
of if you live in Essex http://www.southminster-branch-line.org.uk/