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Date: Mon, 17 Jan 2011 15:15:35 +0000
Message-ID: <AANLkTi=LvH+=FbTmozrN6aK4scH2_p3yoHNB9BF3FMhb@mail.gmail.com>
From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
DomainKey-Status: good (testing) 
Subject: LF: LF Antena impedance and matching
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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

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<div>There was a recent set of postings looking at separating part of=A0th=
e loading coil into a large outside part, followed by=A0feeder and a small=
er variometer indoors.=A0=A0 I ran into serious problems once trying this,=
 analysed it at the time and realised why then put=A0the idea=A0aside</div=
>

<div>=A0</div>
<div>After rehousing my 500kHz variometer I thought I&#39;d actually make=
 some proper measurements to show the drastic effect of what one would nor=
mally call a short and insignificant bit of feeder.=A0 Using the recently=
 acquired DG8SAQ Network Analyser I measured the impedance of the Tee ante=
nna 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=A0and measured the new impedance.</div>

<div>=A0</div>
<div>Have a look at=A0 <a href=3D"http://www.g4jnt.com/500kHz_Tee_Impedanc=
es.pdf">http://www.g4jnt.com/500kHz_Tee_Impedances.pdf</a>=A0which shows=
 the real and imaginary components of the impedances at both spots.=A0=A0=
 </div>
<div>=A0</div>
<div>At the base of the antenna=A0shown in the upper trace=A0exactly as yo=
u&#39;d expect, a linearly increasing reactance that passes though zero at=
 503kHz in series with 26 ohms resistive - near perfect.=A0(See marker num=
ber 2, which has missed precise resonance slightly, 24 -j16 ohms,=A0 or sl=
ightly capacitive)</div>

<div>=A0</div>
<div>BUT, at the end of the feed a different story, See the lower trace.=
=A0=A0 That same point, now becomes 24 ohms resistive in series with -j8=
 ohms.=A0 The feed has introduced a small amount of series inductance=A0lo=
wering the &quot;missed resonance&quot; term to -j8 ohms (which is good).=
 =A0No great difference otherwise and if you hit that point to start with,=
 all would probably be OK and you&#39;d miss the next bit of fun completel=
y.</div>

<div>=A0</div>
<div>Look what happens a bit higher up at 532kHz=A0 (Marker 3).=A0=A0 Ther=
e is another very fast moving =A0hi-Q resonance - where=A0the imaginary pa=
rt becomes zero=A0and at that point the series resistance is very high,=A0=
 =A0974 ohms here.=A0 </div>

<div>=A0</div>
<div>IF,=A0a variometer were being used at the end of the feeder, and phas=
e 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&#39;t make sense.=A0=A0 Which is where I=
 had snags years ago trying just that, and nearly popped the transmitter.=
=A0=A0=A0 The=A0reason is quite obvious if you look at the maths, but its=
 too tedious to repeat here :-(</div>

<div>=A0</div>
<div>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.=A0=A0 Something to watch-out for.=A0=A0 I also suspect=
 the link tapping, beloved of so many, could give a dual resonance pair -=
 but never tried it.=A0=A0 I don&#39;t trust link coupling - its too diffc=
iult to analyse.</div>

<div>=A0</div>
<div>I&#39;d thoroughly recommend the DG8SAQ network analyser to anyone wh=
o wants to make serious RF measurements.=A0=A0=A0 Its a 2-port analyser,=
 that goes=A0from near DC to=A0 500MHz at full spec, and on up to 1.3GHz=
 with only slowly degrading performance.=A0=A0 It is steets-ahead in capab=
ility, accuracy and just overall niceness=A0of the other more well-adverti=
sed and publicly demonstrated =A0similar offerings.</div>

<div>=A0</div>
<div>Andy</div>
<div><a href=3D"http://www.g4jnt.com/">www.g4jnt.com</a></div>
<div>=A0</div>
<div>=A0</div>

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