Return-Path: Received: (qmail 17084 invoked from network); 23 Feb 2003 21:51:48 -0000 Received: from warrior.services.quay.plus.net (212.159.14.227) by mailstore with SMTP; 23 Feb 2003 21:51:48 -0000 Received: (qmail 28766 invoked from network); 23 Feb 2003 21:51:49 -0000 Received: from post.thorcom.com (193.82.116.70) by warrior.services.quay.plus.net with SMTP; 23 Feb 2003 21:51:49 -0000 X-SQ: A Received: from majordom by post.thorcom.com with local (Exim 4.12) id 18n3yE-0005YA-00 for rsgb_lf_group-outgoing@blacksheep.org; Sun, 23 Feb 2003 21:47:38 +0000 Received: from [194.73.73.176] (helo=protactinium.btinternet.com) by post.thorcom.com with esmtp (Exim 4.12) id 18n3yD-0005Y1-00 for rsgb_lf_group@blacksheep.org; Sun, 23 Feb 2003 21:47:37 +0000 Received: from host213-122-210-198.in-addr.btopenworld.com ([213.122.210.198] helo=Main) by protactinium.btinternet.com with smtp (Exim 3.22 #23) id 18n3yC-0005vB-00 for rsgb_lf_group@blacksheep.org; Sun, 23 Feb 2003 21:47:36 +0000 Message-ID: <003401c2db85$335161e0$6507a8c0@Main> From: "Alan Melia" To: "VLF Group" Date: Sun, 23 Feb 2003 21:25:53 -0000 MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.50.4522.1200 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Subject: LF: IMd products Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Status: No, hits=1.0 required=6.0tests=SPAM_PHRASE_00_01,USER_AGENT_OEversion=2.43 X-Spam-Level: * X-SA-Exim-Scanned: Yes Sender: Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rsgb_lf_group-outgoing@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false Hi all, I think there could be problems implementing my tests but the usual check for IMDs in a receiver is to put an passive attenuator in front of the receiver. Say we use a 6dB attenuator the "real" signals will be attenuated by 6dB and the 3rd order products by 18dB. This is the easiest way to identify an IMD problem in the reciever. It is possible for there to be products from signals way above where you may be listening. The standard AM station spacing in Europe is 9kHz so there could be products at around 9kHz. Another example on the 136khz amateur band is intermod usually now in the front end of the RX between BBC Radio4 from Droitwich on 198kHz and MSF (Rugby) Standard Frequency transmissions on 60kHz appear on 138kHz. Since reducing either of these signals with a filter reduces or kills the IMD in most cases, it is within the Rx or pre-amp. It is quite possible for a strong signal at night to intermod with with a signal within your groundwave range. The difficulty is trying to disentangle the modulation to identify it. If the problem is ICM (Ionospheric Cross Modulation) The two stations are probably in a line from you. both modulatons will be heard on the product but the frequency response will be quite restricted. Say one is a BC station ...there will be little modulation above about 1kHz to be heard. ICM usually require a station with a power of around 200kW, and you may get ICM from stations further away. It is quite difficult (even for the professionals) to identify the stations involved. I am not sure whether this can lead to VLF signals but I cannot see why not as it is a mixing process. The usual effect is for the distant station to be heard as low level modulation in the background of the high power station (regardless of the frequency of the distant station) There is little you can do about this form of interference unless you can drop it into the null of a loop. I am afraid that rather amessy explanation as I know but I hope it gives you a few ideas. Cheers de Alan G3NYK