LF: LF Antena impedance and matching

[Andy Talbot <andy.g4jnt@gmail.com>]

There was a recent set of postings looking at separating part of the loading coil into a large outside part, followed by feeder and a smaller variometer indoors.   I ran into serious problems once trying this, analysed it at the time and realised why then put the idea aside

 

After rehousing my 500kHz variometer I thought I'd actually make some proper measurements to show the drastic effect of what one would normally call a short and insignificant bit of feeder.  Using the recently acquired DG8SAQ Network Analyser I measured the impedance of the Tee antenna plus series loading coil at the base of the coil, resonated for 503kHz, then connected the 12m of UR43 feeder that comes from the shed where the beacon Tx sits and measured the new impedance.

 

Have a look at  http://www.g4jnt.com/500kHz_Tee_Impedances.pdf which shows the real and imaginary components of the impedances at both spots.  

 

At the base of the antenna shown in the upper trace exactly as you'd expect, a linearly increasing reactance that passes though zero at 503kHz in series with 26 ohms resistive - near perfect. (See marker number 2, which has missed precise resonance slightly, 24 -j16 ohms,  or slightly capacitive)

 

BUT, at the end of the feed a different story, See the lower trace.   That same point, now becomes 24 ohms resistive in series with -j8 ohms.  The feed has introduced a small amount of series inductance lowering the "missed resonance" term to -j8 ohms (which is good).  No great difference otherwise and if you hit that point to start with, all would probably be OK and you'd miss the next bit of fun completely.

 

Look what happens a bit higher up at 532kHz  (Marker 3).   There is another very fast moving  hi-Q resonance - where the imaginary part becomes zero and at that point the series resistance is very high,   974 ohms here. 

 

IF, a variometer were being used at the end of the feeder, and phase measurement on a scope with a bridge being used to set up the antenna, it would be easy to hit the upper resonance and assume it was the correct one - except resistance wouldn't make sense.   Which is where I had snags years ago trying just that, and nearly popped the transmitter.    The reason is quite obvious if you look at the maths, but its too tedious to repeat here :-(

 

But more importantly, the fast moving high Q blip, which is not very far out of band just could give rise to some unexpected instabilities in power amplifiers.   Something to watch-out for.   I also suspect the link tapping, beloved of so many, could give a dual resonance pair - but never tried it.   I don't trust link coupling - its too diffciult to analyse.

 

I'd thoroughly recommend the DG8SAQ network analyser to anyone who wants to make serious RF measurements.    Its a 2-port analyser, that goes from near DC to  500MHz at full spec, and on up to 1.3GHz with only slowly degrading performance.   It is steets-ahead in capability, accuracy and just overall niceness of the other more well-advertised and publicly demonstrated  similar offerings.

 

Andy

www.g4jnt.com