From: "James Moritz" <J.R.Moritz@herts.ac.uk>
To: rsgb_lf_group@blacksheep.org
Date: Mon, 5 Jan 2004 17:45:45 -0000
Organization: University of Hertfordshire
Message-ID: <000001c3d3b3$bee87d20$fce8c593@rsch15>
MIME-Version: 1.0
Importance: Normal
In-reply-to: <000201c3d2f3$c4fb5f40$61e4fc3e@l8p8y6>
Subject: RE: LF: Re: "T" versus "L"aerial
Content-Type: text/html; charset=windows-1252
Sender: <owner-rsgb_lf_group@blacksheep.org>
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Content-transfer-encoding: 8bit

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns="http://www.w3.org/TR/REC-html40">

<head>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">


<meta name=ProgId content=Word.Document>
<meta name=Generator content="Microsoft Word 10">
<meta name=Originator content="Microsoft Word 10">
<link rel=File-List href="cid:filelist.xml@01C3D3B3.BE8E4F30">
<!--[if gte mso 9]><xml>
 <o:OfficeDocumentSettings>
  <o:DoNotRelyOnCSS/>
 </o:OfficeDocumentSettings>
</xml><![endif]--><!--[if gte mso 9]><xml>
 <w:WordDocument>
  <w:SpellingState>Clean</w:SpellingState>
  <w:GrammarState>Clean</w:GrammarState>
  <w:DocumentKind>DocumentEmail</w:DocumentKind>
  <w:EnvelopeVis/>
  <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel>
 </w:WordDocument>
</xml><![endif]-->
<style>
<!--
 /* Style Definitions */
 p.MsoNormal, li.MsoNormal, div.MsoNormal
	{mso-style-parent:"";
	margin:0cm;
	margin-bottom:.0001pt;
	mso-pagination:widow-orphan;
	font-size:12.0pt;
	font-family:"Times New Roman";
	mso-fareast-font-family:"Times New Roman";}
a:link, span.MsoHyperlink
	{color:blue;
	text-decoration:underline;
	text-underline:single;}
a:visited, span.MsoHyperlinkFollowed
	{color:blue;
	text-decoration:underline;
	text-underline:single;}
span.EmailStyle18
	{mso-style-type:personal-reply;
	mso-style-noshow:yes;
	mso-ansi-font-size:10.0pt;
	mso-bidi-font-size:10.0pt;
	font-family:Arial;
	mso-ascii-font-family:Arial;
	mso-hansi-font-family:Arial;
	mso-bidi-font-family:Arial;
	color:navy;}
span.SpellE
	{mso-style-name:"";
	mso-spl-e:yes;}
span.GramE
	{mso-style-name:"";
	mso-gram-e:yes;}
@page Section1
	{size:595.3pt 841.9pt;
	margin:72.0pt 90.0pt 72.0pt 90.0pt;
	mso-header-margin:35.4pt;
	mso-footer-margin:35.4pt;
	mso-paper-source:0;}
div.Section1
	{page:Section1;}
-->
</style>
<!--[if gte mso 10]>
<style>
 /* Style Definitions */ 
 table.MsoNormalTable
	{mso-style-name:"Table Normal";
	mso-tstyle-rowband-size:0;
	mso-tstyle-colband-size:0;
	mso-style-noshow:yes;
	mso-style-parent:"";
	mso-padding-alt:0cm 5.4pt 0cm 5.4pt;
	mso-para-margin:0cm;
	mso-para-margin-bottom:.0001pt;
	mso-pagination:widow-orphan;
	font-size:10.0pt;
	font-family:"Times New Roman";}
</style>
<![endif]--><!--[if gte mso 9]><xml>
 <o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
 <o:shapelayout v:ext="edit">
  <o:idmap v:ext="edit" data="1" />
 </o:shapelayout></xml><![endif]-->
</head>

<body bgcolor=white lang=EN-GB link=blue vlink=blue style='tab-interval:36.0pt'>

<div class=Section1>

<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Dear Gary, LF Group,<o:p></o:p></span></font></p>

<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p>&nbsp;</o:p></span></font></p>

<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Looking at your diagram, I would avoid
having a <span class=SpellE>downlead</span> near the central mast for two reasons;
firstly, unless the mast is well insulated from ground, the capacitance between
the metal part of the mast and the <span class=SpellE><span class=GramE>uplead</span></span><span
class=GramE><span style='mso-spacerun:yes'>  </span>will</span> result in a significant
proportion of the current going &#8220;up&#8221; the <span class=SpellE>uplead</span>
flowing back &#8220;down&#8221; the mast, resulting in partial cancellation of
the radiated signal. This implies a larger antenna current at the feed point
required to obtain the same radiated signal, increasing the losses in the
loading coil and other conductors. If you do go for this arrangement, make sure
the mast has a low-resistance direct connection to the ground point of the
antenna feed, in order to minimise the losses caused by these circulating
currents. Secondly, there will be a high voltage stress between the <span
class=SpellE>uplead</span> (carrying the full antenna voltage<span class=GramE>)
<span style='mso-spacerun:yes'> </span>and</span> the mast (more or less at
ground potential), increasing the likelihood of problems with corona.<o:p></o:p></span></font></p>

<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p>&nbsp;</o:p></span></font></p>

<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>I think the optimum arrangement would be
to put the loading coil on the garage roof (but spaced from it, especially if
made of corrugated iron or other <span class=SpellE>lossy</span> materials),
close to the sloping end of the wires, to which it could then be connected with
a short <span class=SpellE>downlead</span>. This minimises the amount of high
voltage conductors close to ground level, which have minimal contribution to
the radiated signal, but do lead to increased loss. The fibreglass top section
of the mast will maintain good separation and insulation between the main mast
and the antenna wires &#8211; when using a fibreglass pole in a similar way to
support my <span class=GramE>antenna,</span> I found it was necessary to add a
corona ring around the fibreglass tube to prevent burning where the wire joined
the mast. As far as the arrangement of the top loading wires goes, the main objective
is to maximise the effective height of the antenna. If the antenna was a simple
vertical 14m high, the effective height would be 7m. Adding loading wires above
the 7m level will lead to an increased effective height, whilst adding wire
below the 7m level will reduce the effective height compared to the simple
vertical. I&#8217;m not sure if there is any one &#8220;optimum&#8221;
configuration &#8211; it would depend on all the geometrical restrictions in
the particular situation &#8211; but as a general rule, for an antenna with a
single mast like this it would certainly seem sensible to maximise the amount
of wire above ˝ the mast height, and eliminate as much as possible below that. This
would also keep the high voltage wires well clear of the aluminium supporting
poles. However, some compromise may be required here, since trimming the
loading wires too much would reduce the overall capacitance to the point that
it is difficult to make a big enough loading coil to resonate the antenna that
does not have excessively high losses.<o:p></o:p></span></font></p>

<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p>&nbsp;</o:p></span></font></p>

<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Cheers, Jim Moritz<o:p></o:p></span></font></p>

<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>73 de M0BMU<o:p></o:p></span></font></p>

<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p>&nbsp;</o:p></span></font></p>

<p class=MsoNormal style='margin-left:36.0pt'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'><o:p>&nbsp;</o:p></span></font></p>

</div>

</body>

</html>