Return-Path: Received: (qmail 13941 invoked from network); 22 Feb 2001 15:46:36 -0000 Received: from unknown (HELO murphys-inbound.servers.plus.net) (212.159.14.225) by excalibur.plus.net with SMTP; 22 Feb 2001 15:46:36 -0000 Received: (qmail 3861 invoked from network); 22 Feb 2001 15:46:31 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by murphys with SMTP; 22 Feb 2001 15:46:31 -0000 Content-Transfer-Encoding: 8bit Received: from majordom by post.thorcom.com with local (Exim 3.16 #2) id 14Vxpp-0005KO-00 for rsgb_lf_group-outgoing@blacksheep.org; Thu, 22 Feb 2001 15:39:13 +0000 Received: from bob.dera.gov.uk ([192.5.29.90]) by post.thorcom.com with esmtp (Exim 3.16 #2) id 14Vxpn-0005KJ-00 for rsgb_lf_group@blacksheep.org; Thu, 22 Feb 2001 15:39:11 +0000 X-Priority: 3 X-MSMail-Priority: Normal Received: by bob.dera.gov.uk; (8.8.8/1.3/10May95) id PAA20715; Thu, 22 Feb 2001 15:41:04 GMT Received: (qmail 30887 invoked from network); 22 Feb 2001 16:31:18 -0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from gauntlet.mail.dera.gov.uk (172.16.9.10) by baton.dera.gov.uk with SMTP; 22 Feb 2001 16:31:17 -0000 Received: by gauntlet.mail.dera.gov.uk; id QAA22596; Thu, 22 Feb 2001 16:20:13 GMT Received: from unknown(10.71.64.31) by gauntlet.mail.dera.gov.uk via smap (3.2) id xma021618; Thu, 22 Feb 01 16:17:44 GMT Received: from FRN-MAIL-3.dera.gov.uk (unverified) by mailguard.dera.gov.uk (Content Technologies SMTPRS 4.1.5) with ESMTP id for ; Thu, 22 Feb 2001 13:52:22 +0000 Received: by frn-mail-3.dera.gov.uk with Internet Mail Service (5.5.2650.21) id ; Thu, 22 Feb 2001 13:47:37 -0000 Message-ID: <65AECDF1F89AD411900400508BFC869F0D75E2@pdw-mail-1.dera.gov.uk> From: "Talbot Andrew" To: rsgb_lf_group@blacksheep.org Subject: LF: RE: ANTS: Higher L - higher ERP Date: Thu, 22 Feb 2001 13:47:54 -0000 MIME-Version: 1.0 X-Mailer: Internet Mail Service (5.5.2650.21) Content-Type: text/plain; charset=iso-8859-1; format=flowed Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: >From G3XDV > I contend that, although I may have my theory incorrect, using > elevated inductance can be worthwhile (even with the increased > resistive losses caused by the total inductance having to increase) > in circumstances where the top section is relatively small and > therefore the effective height is low, or where the lower part of the > vertical section is obstructed. The theory I defend is that the > effective height of the antenna is being increased by this method. > I look forward to further discussion on exactly what theory fits this > practical result. > Quite correct and the theory is very simple, the height is being increased quite a lot :-) No top loading at all results in a tapered current distribution from max at the bottom to zero at the top of the antenna. For a short antenna, << 0.1 lambda, the taper is linear. Averaging (integrating) over the total length gives an effective height exactly equal to half the physical height. Adding an INFINITELY big top loading capacitance to a perfectly conducting ground (infinite radial system) means the effective height is equal to the actual height of the antenna. Everything else is in between and not easily modelable (is that a valid word ?) If a sufficiently large L is raised to the top of the vertical to self resonate a practical top load, this behaves as an infinite load. Think about a series tuned circuit, at just below resonance the impedance drops quickly and this looks like a rapidly increasing capacitor, (until actual resonance which then appears as a short circuit or small resistance). Hence a fully resonant top load equals an infinite top load ! And current in the vertical section stays constant. Since the system is now resonant no loading coil is needed at the bottom. And the effective height has increased and ERP gone up by the square. In practice the loss resistance of the top loading coil - which now has to be bigger as the capacitance of the top section, which which it resonates, is less than top plus vertical, so gives an increased series resistance over that of a bottom coil alone. Also some bottom L is still needed to allow tuning over frequency I must admit, Mike, to being concerned as to why you got a serious reduction in current, but now you've explained about the change to a lossier plain wire coil from a Litz wound one that explains everything. When first on 73kHz I tried putting about 1mH at the top - just a loose coil of enamelled wire - not really expecting much improvement with a 22mH loading coil. It gave me about 5% reduction in antenna system resistance over no top loading so had to be more efficient, but no field strength measurements were ever made. The coil was too big to hang permanently from the centre of the Tee so got put away and forgotton until now. In fact the wire formed part of the 137 loading coil in use now. May be time to revisit the idea. And I bet my back garden at 10m x 4.8m is smaller than yours Mike :-( Andy 'JNT > -- The Information contained in this E-Mail and any subsequent correspondence is private and is intended solely for the intended recipient(s). For those other than the recipient any disclosure, copying, distribution, or any action taken or omitted to be taken in reliance on such information is prohibited and may be unlawful.