X-Virus-Scanned: clean according to Sophos on Logan.com X-SpamCatcher-Score: 2 [X] Return-Path: Sender: To: lml@lancaironline.net Date: Sun, 18 Mar 2007 10:15:42 -0400 Message-ID: X-Original-Return-Path: Received: from smtp104.sbc.mail.mud.yahoo.com ([68.142.198.203] verified) by logan.com (CommuniGate Pro SMTP 5.1.7) with SMTP id 1927121 for lml@lancaironline.net; Sat, 17 Mar 2007 23:32:14 -0400 Received-SPF: none receiver=logan.com; client-ip=68.142.198.203; envelope-from=elippse@sbcglobal.net Received: (qmail 22886 invoked from network); 18 Mar 2007 03:31:30 -0000 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=sbcglobal.net; h=Received:X-YMail-OSG:Message-ID:From:To:Cc:Subject:Date:MIME-Version:Content-Type:X-Priority:X-MSMail-Priority:X-Mailer:X-MIMEOLE; b=5UE7/mGb18oydGw5HnzZyQMvO/Z/Ip/RKRtP9seaLjj14qqsoFjXxaTUc2l+ErAHGR2GCMLTWoRBZXYKchawWoc7YMy6Ju/YLw+lIUHQwCDh9Hw0jatYDMeRKBfyT6q41uqxKZ8RfhiCmBy7kRDjCa9DVn5T70Q7AgjzFSsfGqE= ; Received: from unknown (HELO Computerroom) (elippse@sbcglobal.net@75.15.131.34 with login) by smtp104.sbc.mail.mud.yahoo.com with SMTP; 18 Mar 2007 03:31:30 -0000 X-YMail-OSG: lyQ.6h0VM1lcPxkskD1mZXJtmif3j9TZdhEzH4R4ogCktJFlzR84iNzRGbOv.Mova6m0jy0L23FrB6cX4oDZW9o0BVGAaiUIfBgtNp39V6K.LBWZ4Ax2XA-- X-Original-Message-ID: <000601c7690d$ebc70ad0$22830f4b@Computerroom> From: "Paul Lipps" X-Original-To: "Marv Kaye" X-Original-Cc: "Patrick Panzera" Subject: Counterpoise revisited X-Original-Date: Sat, 17 Mar 2007 20:31:29 -0700 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0003_01C768D3.3E394FC0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.3028 X-MIMEOLE: Produced By Microsoft MimeOLE V6.00.2900.3028 This is a multi-part message in MIME format. ------=_NextPart_000_0003_01C768D3.3E394FC0 Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable The way to get better performance out of your = monopole-over-conductor (ground-plane) antenna is to understand how it = operates. Here's my attempt, and I hope it's not too feeble. Picture a = wooden pencil, the kind with an eraser, standing on-end, eraser up, in = the center of a round mirror which has a diameter of two pencil lengths. = Now let's look at this from a distance, looking down at a 45 deg angle = to the pencil. The distance must be enough so that the image we see is = coming at us as close to a plane-wave as possible, where all parts of = the image are at the same angle as received by the eye. At least ten = mirror-diameters will be fairly good. What we will see is a pencil with = an eraser on each end that has a total length of 1.4 pencil-lengths. = Note that the eraser on the lower half is right at the forward edge of = the mirror. The image of the pencil has become inverted and added-on to = the bottom of the real pencil. Now let's drop our line-of-sight to = where we are looking down at 30 deg. Now we see a pencil whose = upper-half is 87% of a pencil-length long, but the lower image is only a = partial pencil; the upper portion has dropped off the edge of the = mirror. If we made the mirror 73% larger, we would just be able to see = the eraser on the bottom. Basically this is the same as the monopole, the pencil, over a = ground-plane, the mirror. The analysis of the monopole speaks about the = "image" antenna, the one that forms the bottom portion to a plane wave = arriving from a distant source. We can say that the incident RF wave = bounces or reflects off the ground plane up into the monopole to give us = twice as much energy. Just as in our light reflection, the RF wave, in = reflecting off the ground plane, has its RF voltage inverted so that it = adds to the direct monopole reception. And just as when the mirror = wasn't long enough initially to show us the whole lower-half of the = image at lower angles, so, too, does our reception drop off at lower = angles when the ground-plane is short. There is less energy being = reflected up into the antenna element. You can see that to get full = rececption at very shallow angles requires a very long ground plane. And = where does most of our reception come from? Very shallow angles! That is = why the dipole is so much better than a monopole with an = abbreviated-length conductor below it. The only thing that prevents the = monopole from performing too badly is that most of the antenna's = current, which is what causes the radiation, is in the feed-end; maximum = at the feed, zero at the tip. So even the abbreviated image is of the = stronger radiating portion. Now I spoke of the signal bouncing or reflecting off the ground = plane. It doesn't! What actually takes place is that the incident = electromagnetic wave induces a current in the ground plane which then = re-radiates the signal. If the ground plane is not highly conductive, = then the re-radiated signal will not be as strong. This current that is = induced is only in a very, very thin layer of the ground plane in what = is called skin-effect. For copper at 20C it is 2.61 inches / F^1/2 , or = 20.6 nano-inches, 2.06E-08" at 127 MHz! So even 0.001 copper will give = an excellent ground-plane at our Localizer, VOR, and Comm frequencies; = it will be almost 50,000 times as thick as the necessary thickness. But = I can't imagine that a paint highly-loaded with conductive material = would be able to have the required conductivity in such a thin layer. It = may look highly conductive in thicker layers, but that's not what = counts. BTW; the antenna transmits just as well as it receives with the = wave going in the opposite direction; an antenna is considered a = reciprocal radiator. That is, as long as the transmitted power doesn't = get the ground-plane too hot where its conductivity decreases or it = melts! ------=_NextPart_000_0003_01C768D3.3E394FC0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable
    The way to get better = performance out=20 of your monopole-over-conductor (ground-plane) antenna is to = understand how=20 it operates. Here's my attempt, and I hope it's not too feeble. Picture = a wooden=20 pencil, the kind with an eraser, standing on-end, eraser up, in the = center=20 of a round mirror which has a diameter of two pencil lengths. Now let's = look at=20 this from a distance, looking down at a 45 deg angle to = the=20 pencil. The distance must be enough so that the image we see is coming = at us as=20 close to a plane-wave as possible, where all parts of the image are at = the same=20 angle as received by the eye. At least ten mirror-diameters will be = fairly=20 good. What we will see is a pencil with an eraser on each end that = has a=20 total length of 1.4 pencil-lengths. Note that the eraser on the lower = half is=20 right at the forward edge of the mirror. The image of the pencil = has become=20 inverted and added-on to the bottom of the real = pencil.  Now=20 let's drop our line-of-sight  to where we are looking down at 30 = deg. Now=20 we see a pencil whose upper-half is 87% of a pencil-length long, = but the=20 lower image is only a partial pencil; the upper portion has dropped off = the edge=20 of the mirror. If we made the mirror 73% larger, we would just be able = to see=20 the eraser on the bottom.
    Basically this is the same as = the=20 monopole, the pencil, over a ground-plane, the mirror. The analysis of = the=20 monopole speaks about the "image" antenna, the one that forms the bottom = portion=20 to a plane wave arriving from a distant source. We can say that the = incident RF=20 wave bounces or reflects off the ground plane up into the monopole to = give us=20 twice as much energy. Just as in our light reflection, the RF wave, in=20 reflecting off the ground plane, has its RF voltage inverted so that it = adds to=20 the direct monopole reception. And just as when the mirror wasn't long = enough=20 initially to show us the whole lower-half of the image at lower angles, = so, too,=20 does our reception drop off at lower angles when the ground-plane is = short.=20 There is less energy being reflected up into the antenna = element. You=20 can see that to get full rececption at very shallow angles requires a = very long=20 ground plane. And where does most of our reception come from? Very = shallow=20 angles! That is why the dipole is so much better than a monopole with an = abbreviated-length conductor below it. The only thing that prevents the = monopole=20 from performing too badly is that most of the antenna's current, which = is what=20 causes the radiation, is in the feed-end; maximum at the feed, zero at = the=20 tip. So even the abbreviated image is of the stronger radiating=20 portion.
    Now I spoke of the signal = bouncing or=20 reflecting off the ground plane. It doesn't! What actually takes place = is that=20 the incident electromagnetic wave induces a current in the ground plane = which=20 then re-radiates the signal. If the ground plane is not highly = conductive, then=20 the re-radiated signal will not be as strong. This current that is = induced is=20 only in a very, very thin layer of the ground plane in what is called=20 skin-effect. For copper at 20C it is 2.61 inches / F^1/2 ,=20 or 20.6 nano-inches, 2.06E-08" at 127 MHz! So even 0.001 copper will = give an=20 excellent ground-plane at our Localizer, VOR, and Comm frequencies; = it will=20 be almost 50,000 times as thick as the necessary thickness. But I can't = imagine=20 that a paint highly-loaded with conductive material would be able = to have=20 the required conductivity in such a thin layer. It may look highly = conductive in=20 thicker layers, but that's not what counts.
    BTW; the = antenna  transmits=20 just as well as it receives with the wave going in the opposite = direction;=20 an antenna is considered a reciprocal radiator. That is, as long as the=20 transmitted power doesn't get the ground-plane too hot where its = conductivity=20 decreases or it melts!
 
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