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From: "james moritz" <j.r.moritz@herts.ac.uk>
To: <rsgb_lf_group@blacksheep.org>
Date: Thu, 20 Apr 2006 14:38:51 +0100
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Subject: LF: RE: Top load coil at ground level?
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Dear Dick, LF Group,

=20

I am puzzled as to how the top-loaded vertical antenna plus stub is =
supposed
to operate. In Fig 1, the elevated loading coil is in series between the
vertical element and the top loading, which is perfectly reasonable - =
but I
can't see how the stub in Fig 2 could produce an equivalent series =
inductive
reactance, especially since one terminal of the stub is unconnected.

=20

I can see that Dex's antenna works (I presume the arrangement in Section
2.12 of ON7YD's antennas page), but I don't think it is equivalent to an
elevated loading coil. The vertical wire from the hot end of the loading
coil to the top loading section will be at high voltage along its whole
length, leading to much the same displacement currents flowing to ground =
and
in the mast, and therefore the same current distribution, as would occur
with a conventional inverted-L with a ground-level loading coil. This
contrasts with a top-loaded vertical with elevated loading coil at the =
top
of the vertical section, where all the vertical part of the antenna is =
at
low RF potential, and only the top loading section carries a high RF
voltage. The main effect of using the mast and the additional vertical =
wire
from the top of the mast to the cold end of the loading coil as part of =
the
antenna system would seem to be to add their distributed inductance to =
the
cold end of the loading coil. In other words, you would achieve an =
almost
identical result by grounding the cold end of the loading coil to the =
bottom
of the mast, and adding some extra turns to the cold end of the loading
coil.

=20

Cheers, Jim Moritz

73 de M0BMU

=20


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<p class=3DMsoNormal><font size=3D2 color=3Dnavy face=3DArial><span =
style=3D'font-size:
10.0pt;font-family:Arial;color:navy'>Dear Dick, LF =
Group,</span></font></p>

<p class=3DMsoNormal><font size=3D2 color=3Dnavy face=3DArial><span =
style=3D'font-size:
10.0pt;font-family:Arial;color:navy'>&nbsp;</span></font></p>

<p class=3DMsoNormal><font size=3D2 color=3Dnavy face=3DArial><span =
style=3D'font-size:
10.0pt;font-family:Arial;color:navy'>I am puzzled as to how the =
top-loaded
vertical antenna plus stub is supposed to operate. In Fig 1, the =
elevated
loading coil is in series between the vertical element and the top =
loading,
which is perfectly reasonable &#8211; but I can&#8217;t see how the stub =
in Fig
2 could produce an equivalent series inductive reactance, especially =
since one
terminal of the stub is unconnected.</span></font></p>

<p class=3DMsoNormal><font size=3D2 color=3Dnavy face=3DArial><span =
style=3D'font-size:
10.0pt;font-family:Arial;color:navy'>&nbsp;</span></font></p>

<p class=3DMsoNormal><font size=3D2 color=3Dnavy face=3DArial><span =
style=3D'font-size:
10.0pt;font-family:Arial;color:navy'>I can see that Dex&#8217;s antenna =
works (I
presume the arrangement in Section 2.12 of ON7YD&#8217;s antennas page), =
but I
don&#8217;t think it is equivalent to an elevated loading coil. The =
vertical
wire from the hot end of the loading coil to the top loading section =
will be at
high voltage along its whole length, leading to much the same =
displacement currents
flowing to ground and in the mast, and therefore the same current =
distribution,
as would occur with a conventional inverted-L with a ground-level =
loading coil.
This contrasts with a top-loaded vertical with elevated loading coil at =
the top
of the vertical section, where all the vertical part of the antenna is =
at low
RF potential, and only the top loading section carries a high RF =
voltage. The main
effect of using the mast and the additional vertical wire from the top =
of the
mast to the cold end of the loading coil as part of the antenna system =
would
seem to be to add their distributed inductance to the cold end of the =
loading
coil. In other words, you would achieve an almost identical result by =
grounding
the cold end of the loading coil to the bottom of the mast, and adding =
some
extra turns to the cold end of the loading coil.</span></font></p>

<p class=3DMsoNormal><font size=3D2 color=3Dnavy face=3DArial><span =
style=3D'font-size:
10.0pt;font-family:Arial;color:navy'>&nbsp;</span></font></p>

<p class=3DMsoNormal><font size=3D2 color=3Dnavy face=3DArial><span =
style=3D'font-size:
10.0pt;font-family:Arial;color:navy'>Cheers, Jim =
Moritz</span></font></p>

<p class=3DMsoNormal><font size=3D2 color=3Dnavy face=3DArial><span =
style=3D'font-size:
10.0pt;font-family:Arial;color:navy'>73 de M0BMU</span></font></p>

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style=3D'font-size:
12.0pt'>&nbsp;</span></font></p>

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