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From: "Dick Rollema" <d.w.rollema@freeler.nl>
To: "LF-Group" <rsgb_lf_group@blacksheep.org>
Subject: LF: More on coil current at PA0SE
Date: Wed, 9 May 2001 14:41:28 +0200
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<DIV><STRONG>To All from PA0SE</STRONG></DIV>
<DIV>&nbsp;</DIV>
<DIV><STRONG>A.&nbsp; I have modelled my aerial using Antenna Optimizer by 
K6STI. </STRONG></DIV>
<DIV><STRONG>This also provides the current distribution on the 
aerial.</STRONG></DIV>
<DIV><STRONG>The coil is modelled&nbsp; as a lumped inductance.</STRONG></DIV>
<DIV><STRONG>As&nbsp; expected the current&nbsp;above and below the coil is 
shown to be the same.</STRONG></DIV>
<DIV><STRONG>Now the books&nbsp;have it that a coil with a length of L metres 
radiates like a straight wire of the same length.&nbsp; To find the radiation 
from my 58cm high coil I looked at the current distribution on the first 58cm 
of&nbsp;aerial wire immediately above the coil, where the current that enters at 
the bottom end&nbsp;is the same as in the coil .&nbsp;AO indicates that the 
current at the top end of that piece of wire is 1.55% lower than at the bottom 
end.&nbsp;So that is&nbsp;current lost by radiation.</STRONG></DIV>
<DIV><STRONG>I measured a difference of 10% between currents at the top and 
bottom of my coil. So most of the current lost must be due to capacitance to 
surrounding objects.</STRONG>&nbsp;&nbsp;</DIV>
<DIV>&nbsp;</DIV>
<DIV><STRONG>B.&nbsp; I also modelled Steve's 12m high&nbsp;vertical.&nbsp;The 
program assumes that the aerial&nbsp;is over perfect earth. </STRONG><STRONG>The 
real earth may be some distance below the surface so the actual aerial may be 
longer than 12 m. (Remember Jim's aerial on a hill at Puckeridge that radiated 
better than expected from its physical length?)</STRONG></DIV>
<DIV><STRONG>The program shows that a coil of 12.87mH will be needed&nbsp;for 
resonance.&nbsp;</STRONG></DIV>
<DIV><STRONG>For the difference in current between 2.2A at the bottom&nbsp;and 
1.8A at the top of the coil&nbsp;to be entirely caused by radiation the coil 
must be 1.85m high.&nbsp;&nbsp;&nbsp;Looking at the picture at Steve's website 
this is not the case. So some current must be escaping through capacitance to 
the surrounding.</STRONG></DIV>
<DIV>&nbsp;</DIV>
<DIV><STRONG>C.</STRONG><STRONG>&nbsp; I entirely agree with those who state 
that a difference in current between the two ends of a coil can only be caused 
by radiation and capacitance to surrounding objects.</STRONG></DIV>
<DIV><STRONG>Any other effects, like loss in the coil, distributed capacitance 
etc. remain internal to the coil and cannot cause a difference between current 
flowing in and out.</STRONG></DIV>
<DIV>&nbsp;</DIV>
<DIV><STRONG>73, Dick, PA0SE</STRONG>&nbsp;</DIV></BODY></HTML>