Return-Path: Received: (qmail 15005 invoked from network); 9 Apr 2001 18:32:15 -0000 Received: from unknown (HELO warrior-inbound.servers.plus.net) (212.159.14.227) by 10.226.25.101 with SMTP; 9 Apr 2001 18:32:15 -0000 Received: (qmail 16322 invoked from network); 9 Apr 2001 18:32:02 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by warrior with SMTP; 9 Apr 2001 18:32:02 -0000 X-Priority: 3 X-MSMail-Priority: Normal Received: from majordom by post.thorcom.com with local (Exim 3.16 #2) id 14mgLU-00029n-00 for rsgb_lf_group-outgoing@blacksheep.org; Mon, 09 Apr 2001 19:25:00 +0100 Received: from hestia.herts.ac.uk ([147.197.200.9]) by post.thorcom.com with esmtp (Exim 3.16 #2) id 14mgLJ-00029i-00 for rsgb_lf_group@blacksheep.org; Mon, 09 Apr 2001 19:24:50 +0100 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from [147.197.200.44] (helo=gemini) by hestia.herts.ac.uk with esmtp (Exim 3.16 #4) id 14mgKq-00054N-00 for rsgb_lf_group@blacksheep.org; Mon, 09 Apr 2001 19:24:20 +0100 Message-ID: <13641.200104091824@gemini> From: "James Moritz" Organization: University of Hertfordshire To: rsgb_lf_group@blacksheep.org Date: Mon, 9 Apr 2001 19:24:16 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 8bit Subject: LF: Coil Losses X-Mailer: Pegasus Mail for Win32 (v3.11) Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: Dear Mike, LF Group, Placing a ferrite core in a coil will always increase the inductance, and will also result in losses due to hysteresis, eddy currents, and dielectric loss in the ferrite, all of which result in heating of the core. Whether the Q increases or decreases depends on many things, such as the type of core material, the geometry of the core and the number of turns and disposition of the windings. At LF, ferrite losses are usually quite low. For a given overall size of coil, the reduced number of turns required to obtain a given inductance, and the thicker wire which can be used as a result, reduces resistive losses in the wire, and usually more than compensates for the additional losses due to the ferrite. So the ferrite cored coil can have a higher Q. However, you can usually make an enormous air-cored coil if you want, wheras very big ferrite cores are hard to come by, and are very heavy. But if small size is important, ferrite cores could be worth a go. One problem with ferrite cores is saturation - if the magnetic flux in the core exceeds a certain point, the core effectively loses it's magnetism over the peaks in the current waveform in the coil, which can give rise to harmonic generation as well as excessive heating of the ferrite. If you put a ferrite core into a coil carrying a particular current, the magnetic flux density there is in the core depends on the shape of the coil and core - a long, thin rod along the axis of the core will be subject to much greater flux density than a short, fat core. So poking ferrite rods into loading coils gives them a really hard time, and a given type of rod should show less heating if several short ones in a bundle are used instead of one long one . The flux density also depends upon the permeability of the ferrite - lower permeability means lower flux density, which may be why some ferrite rods work better than others. However, lower permeability or increased cross-section means that more ferrite is required to reach the neccessary inductance. The inevitable result is that a large mass of ferrite will be required for high power use. Another factor that is important when deciding the type of coil to make is the voltage it has to withstand. A 7mH coil with, say, 3A @ 136kHz going through it has about 18kV across its terminals, So insulating an inductor wound on a pot-core would be quite a challenge at this level, especially for outdoors use. Although it may give lower inductance for it's size, a solenoid has the advantage that the high voltage points are as far apart as possible. Cheers, Jim Moritz 73 de M0BMU