This is a multi-part message in MIME format.
------=_NextPart_000_0002_01C1C5B0.3FC14030
Content-Type: text/plain;
charset="us-ascii"
Content-Transfer-Encoding: 7bit
I respectfully disagree. The ground screen will reduce
losses in the radials. The radial currents produce
ground losses for the same reason that a low dipole
heats the ground. The fact that the currents in the
radials cancel out in the far field doesn't help, because
the ground is in the near field.
I did some experiments over my 230 foot diameter
wire grid ground screen on 40 meters. I found that
with skimpy radials on a 1/4 wave vertical, the drive
impedance could be as high as 60 ohms, vs 36 ohms
for 60 or 120 radials. HOWEVER, the field strength
measured 800 feet away was not affected at all by
the number of radials (to within .1 dB measurement
resolution). This amazing result could only be due
to the presence of the ground screen. (Note, the source
was adjusted for constant input power into the antenna,
so the changing impedance was accounted for).
Cebik's work is all simulation, which has its limitations
in modeling ground.
Rick Karlquist N6RK
richard@karlquist.com
www.n6rk.com
www.karlquist.com
-----Original Message-----
From: towertalk-admin@contesting.com
[mailto:towertalk-admin@contesting.com]On Behalf Of K4IA@aol.com
Sent: Thursday, March 07, 2002 5:42 AM
To: K4BEV@aol.com
Cc: towertalk@contesting.com
Subject: Re: [Towertalk] Grounding an Elevated vertical
With all due respect to those who have suggested you create a ground
screen in addition to the elevated radials, I think that is a mistake.
Your elevated radials are your RF ground and make up the second half of
the antenna. If you add another RF ground 10 feet below I think you will
throw the antenna off without gaining anything. For one, the 10 feet of
wire to the earth will act as part of your antenna system. Elevated radials
require fewer radial wires and work just fine. They don't need any help.
If you feel compelled to improve your
RF ground add 2 more elevated radials. Cebik seems to think 4-8 elevated
radials slightly less than 1/4 wavelength long is all you need. Check out
his articles at http://www.cebik.com/gp.html and
http://www.cebik.com/gup27.html
Radio K4IA
Craig Buck
Fredericksburg, Virginia USA
QRP ARCI #2550 FISTS #6702 CC 788 Diamond #64
K1 #470 K2 #2460
----------------------------------------------
For cheap long distance, 800#s and more
Tune to http://www.ld.net/?bucksavers
4.9 cents/min - no monthly fees
------=_NextPart_000_0002_01C1C5B0.3FC14030
Content-Type: text/html;
charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=3DContent-Type content=3D"text/html; =
charset=3Dus-ascii">
<META content=3D"MSHTML 5.50.4522.1800" name=3DGENERATOR></HEAD>
<BODY>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>I=20
respectfully disagree. The ground screen will =
reduce</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>losses=20
in the radials. The radial currents produce</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>ground=20
losses for the same reason that a low dipole</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>heats=20
the ground. The fact that the currents in the</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>radials cancel out in the far field doesn't help,=20
because</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>the=20
ground is in the near field.</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>I did=20
some experiments over my 230 foot diameter</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>wire=20
grid ground screen on 40 meters. I found that</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>with=20
skimpy radials on a 1/4 wave vertical, the drive</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>impedance could be as high as 60 ohms, vs 36 =
ohms</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>for 60=20
or 120 radials. HOWEVER, the field strength</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>measured 800 feet away was not affected at all =
by</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>the=20
number of radials (to within .1 dB measurement</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>resolution). This amazing result could only be=20
due</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>to the=20
presence </FONT></SPAN><SPAN class=3D799410416-07032002><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>of the ground screen. (Note, the=20
source</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>was=20
adjusted for constant input power into the antenna,</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>so the=20
changing impedance was accounted for).</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>Cebik's work is all simulation, which has its=20
limitations</FONT></SPAN></DIV>
<DIV><SPAN class=3D799410416-07032002><FONT face=3DArial color=3D#0000ff =
size=3D2>in=20
modeling ground.</FONT></SPAN></DIV>
<DIV> </DIV>
<P><FONT size=3D2>Rick Karlquist =20
N6RK<BR>richard@karlquist.com<BR>www.n6rk.com<BR>www.karlquist.com =
</FONT></P>
<BLOCKQUOTE=20
style=3D"PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #0000ff 2px =
solid">
<DIV class=3DOutlookMessageHeader dir=3Dltr align=3Dleft><FONT =
face=3DTahoma=20
size=3D2>-----Original Message-----<BR><B>From:</B>=20
towertalk-admin@contesting.com =
[mailto:towertalk-admin@contesting.com]<B>On=20
Behalf Of </B>K4IA@aol.com<BR><B>Sent:</B> Thursday, March 07, 2002 =
5:42=20
AM<BR><B>To:</B> K4BEV@aol.com<BR><B>Cc:</B>=20
towertalk@contesting.com<BR><B>Subject:</B> Re: [Towertalk] Grounding =
an=20
Elevated vertical<BR><BR></FONT></DIV><FONT =
face=3Darial,helvetica><FONT=20
size=3D2>With all due respect to those who have suggested you create a =
ground=20
screen in addition to the elevated radials, I think that is a=20
mistake.<BR><BR>Your elevated radials are your RF ground and make up =
the=20
second half of the antenna. If you add another RF ground 10 feet =
below I=20
think you will throw the antenna off without gaining anything. =
For one,=20
the 10 feet of wire to the earth will act as part of your antenna=20
system. Elevated radials require fewer radial wires and work =
just=20
fine. They don't need any help. If you feel compelled to =
improve=20
your <BR>RF ground add 2 more elevated radials. Cebik seems to think =
4-8=20
elevated radials slightly less than 1/4 wavelength long is all you =
need. =20
Check out his articles at <A=20
href=3D"http://www.cebik.com/gp.html";>http://www.cebik.com/gp.html</A> =
and <A=20
=
href=3D"http://www.cebik.com/gup27.html";>http://www.cebik.com/gup27.html<=
/A><BR><BR>Radio=20
K4IA<BR>Craig Buck<BR>Fredericksburg, Virginia USA<BR>QRP ARCI =
#2550 =20
FISTS #6702 CC 788 Diamond #64<BR>K1 #470 K2 #2460=20
<BR>----------------------------------------------<BR>For cheap long =
distance,=20
800#s and more<BR><BR>Tune to <A=20
=
href=3D"http://www.ld.net/?bucksavers";>http://www.ld.net/?bucksavers</A><=
BR>4.9=20
cents/min - no monthly fees</FONT> </FONT></BLOCKQUOTE></BODY></HTML>
------=_NextPart_000_0002_01C1C5B0.3FC14030--
|