Return-Path: Received: from mtain-dc05.r1000.mx.aol.com (mtain-dc05.r1000.mx.aol.com [172.29.64.133]) by air-me08.mail.aol.com (v129.4) with ESMTP id MAILINME082-8bbd4d77bd97216; Wed, 09 Mar 2011 12:49:11 -0500 Received: from post.thorcom.com (post.thorcom.com [195.171.43.25]) by mtain-dc05.r1000.mx.aol.com (Internet Inbound) with ESMTP id D2BBB38000138; Wed, 9 Mar 2011 12:49:09 -0500 (EST) Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1PxNUp-0001NU-I8 for rs_out_1@blacksheep.org; Wed, 09 Mar 2011 17:48:27 +0000 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1PxNUo-0001NG-RT for rsgb_lf_group@blacksheep.org; Wed, 09 Mar 2011 17:48:26 +0000 Received: from smtpa1.mediabeam.com ([194.25.41.13]) by relay1.thorcom.net with esmtp (Exim 4.63) (envelope-from ) id 1PxNUl-0006lA-Ua for rsgb_lf_group@blacksheep.org; Wed, 09 Mar 2011 17:48:26 +0000 Received: from UMS02-SMTP3 (balancer04.mediabeam.com [10.100.1.40]) by smtpa1.mediabeam.com (8.13.8/8.13.1) with ESMTP id p29HmHf5026425 for ; Wed, 9 Mar 2011 18:48:17 +0100 From: uwe-jannsen@kabelmail.de To: rsgb_lf_group@blacksheep.org Message-ID: Date: Wed, 09 Mar 2011 18:48:15 +0100 X-Priority: 3 (Normal) MIME-Version: 1.0 X-Spam-Score: 0.6 (/) X-Spam-Report: autolearn=disabled,HTML_MESSAGE=0.001,NO_REAL_NAME=0.55 Subject: Re: LF: Re: VLF dj8wx on 8970.022Hz Content-Type: multipart/alternative; boundary="----6D3231C4BE9A4AFF82D92EE8CF48E6C2" X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: * X-Spam-Status: No, hits=1.1 required=5.0 tests=HTML_MESSAGE,NO_REAL_NAME, PRIORITY_NO_NAME autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false x-aol-global-disposition: G x-aol-sid: 3039ac1d40854d77bd94794e X-AOL-IP: 195.171.43.25 X-AOL-SPF: domain : blacksheep.org SPF : none X-Mailer: Unknown (No Version) ------6D3231C4BE9A4AFF82D92EE8CF48E6C2 Content-Type: text/plain; charset=Windows-1252 Content-Transfer-Encoding: quoted-printable Dear Jim,=20 thanks for estimating the ERP.=20 no buildings in vicinity, only some walnut trees, willow trees and oaks.= the topload of the aerial is some metres beyond those trees (still).=20 the diameter of the naked wire: 2,5mm . the isolation: 025mm of unknown ma= terial (no PVC).=20 the spacing of the triple wire: 30cm.=20 I cant let it go with the 3A radial current. the fence and even wett pols= of wood get under tension. =20 1A radial current is the max Ill work with.=20 best regards=20 Uwe/dj8wx=20 Von: rsgb_lf_group@blacksheep.org Gesendet: 08.03.2011 21:00 An: rsgb_lf_group@blacksheep.org Betreff: LF: Re: VLF dj8wx on 8970.022Hz Dear Uwe, One can make a very approximate, order-of-magnitude estimate of ERP and= =20 voltage from the info you have:- The effective height of a heavily top-loaded vertical such as your antenn= a=20 will be a bit less than the average physical height of the conductors -= =20 assume about 12m. The radiation resistance Rrad =3D 160 * pi^2 *=20 (Heff/lambda)^2 at 9kHz is therefore 200 micro-ohm - it will be reduced= if=20 there are trees, buildings etc. close to the antenna. The antenna current= =20 will be slightly less than the current in the radials, due to the=20 distributed capacitance of the loading coil, etc., at max power let us sa= y=20 2.7A. ERP will then be 1.8 * (2.7)^2 * 200u =3D 2.7mW. The exact capacitance will depend on the spacing and diameter of the ante= nna=20 wires, etc., but I estimate roughly 1200pF based on your drawing. The=20 reactance at 9k is therefore 15kohm. With 2.7A antenna current, the anten= na=20 voltage would be 40kV RMS. There are major possible sources of error when trying to measure the ante= nna=20 voltage in the way you describe - even at 9kHz, the effect of stray=20 capacitance will be very serious - a 10M resistor has the same magnitude= of=20 impedance as 1.7pF of stray capacitance, so only a short length of expose= d=20 wire will cause a large change in voltage at the scope input. Also, as yo= u=20 have seen, the strong E field can couple significantly to the internal=20 wiring of the measuring equipment unless it is very well screened. Any=20 potential divider must also be well screened, and take into account the= time=20 constants due to resistance and stray capacitance. Cheers, Jim Moritz 73 de M0BMU ------6D3231C4BE9A4AFF82D92EE8CF48E6C2 Content-Type: text/html; charset=Windows-1252 Content-Transfer-Encoding: quoted-printable


Dear Jim,


thanks for estimating the ERP.

 

no buildings in vicinity, only some walnut trees, willow trees an= d oaks. the topload of the aerial is some metres beyond those trees (still= ).

 

the diameter of the naked wire: 2,5mm . the isolation: 025mm of= unknown material (no PVC).

the spacing of the triple wire: 30cm.

 

I cant let it go with the 3A radial current. the fence and even= wett pols of wood get under tension.

 

1A radial current is the max Ill work with.

 

best regards

Uwe/dj8wx

 


Von: rsgb_lf_group@blacksheep.org
Gesendet: 08.03.2011 21:00
An: rsgb_lf_group@blacksheep.org
Betreff: LF: Re: VLF dj8wx on 8970.022Hz

Dear Uwe,

One can make a very approximate, order-of-magnitude estimate of ERP and voltage from the info you have:-

The effective height of a heavily top-loaded vertical such as your antenna=
will be a bit less than the average physical height of the conductors - assume about 12m. The radiation resistance Rrad =3D 160 * pi^2 *
(Heff/lambda)^2 at 9kHz is therefore 200 micro-ohm - it will be reduced= if
there are trees, buildings etc. close to the antenna. The antenna current=
will be slightly less than the current in the radials, due to the
distributed capacitance of the loading coil, etc., at max power let us say=
2.7A. ERP will then be 1.8 * (2.7)^2 * 200u =3D 2.7mW.

The exact capacitance will depend on the spacing and diameter of the anten= na
wires, etc., but I estimate roughly 1200pF based on your drawing. The reactance at 9k is therefore 15kohm. With 2.7A antenna current, the antenn= a
voltage would be 40kV RMS.

There are major possible sources of error when trying to measure the anten= na
voltage in the way you describe - even at 9kHz, the effect of stray
capacitance will be very serious - a 10M resistor has the same magnitude= of
impedance as 1.7pF of stray capacitance, so only a short length of exposed=
wire will cause a large change in voltage at the scope input. Also, as you=
have seen, the strong E field can couple significantly to the internal wiring of the measuring equipment unless it is very well screened. Any potential divider must also be well screened, and take into account the ti= me
constants due to resistance and stray capacitance.

Cheers, Jim Moritz
73 de M0BMU

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