Return-Path: Received: (qmail 2163 invoked from network); 15 Apr 1999 22:50:22 +0100 Received: from magnus.plus.net.uk (HELO magnus.force9.net) (195.166.128.27) by guiness.force9.net with SMTP; 15 Apr 1999 22:50:22 +0100 Received: (qmail 29260 invoked from network); 15 Apr 1999 21:50:23 -0000 Received: from post.thorcom.com (194.75.130.70) by magnus.plus.net.uk with SMTP; 15 Apr 1999 21:50:23 -0000 X-Priority: 3 Received: from troy.blacksheep.org ([194.75.183.50] ident=root) by post.thorcom.com with esmtp (Exim 2.04 #3) id 10Xu03-00060a-01; Thu, 15 Apr 1999 22:48:44 +0100 X-MSMail-Priority: Normal Received: (from root@localhost) by troy.blacksheep.org (8.6.12/8.6.12) id VAA24409 for rsgb_lf_group-outgoing; Thu, 15 Apr 1999 21:49:09 GMT Received: from post.thorcom.com (root@post.unica.co.uk [194.75.183.70]) by troy.blacksheep.org (8.6.12/8.6.12) with ESMTP id VAA24405 for ; Thu, 15 Apr 1999 21:49:07 GMT X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from irwell.zetnet.co.uk ([194.247.47.48] ident=root) by post.thorcom.com with esmtp (Exim 2.04 #3) id 10Xtzn-00060X-00 for rsgb_lf_group@blacksheep.org; Thu, 15 Apr 1999 22:48:27 +0100 Received: from central.zetnet.co.uk (central.zetnet.co.uk [194.247.47.20]) by irwell.zetnet.co.uk (8.9.3/8.9.3/Debian/GNU) with SMTP id WAA29707 for ; Thu, 15 Apr 1999 22:48:24 +0100 X-ZSender: g3ldo@zetnet.co.uk Message-ID: <1999041521543568199@zetnet.co.uk> Date: Thu, 15 Apr 1999 21:54:35 +0100 To: rsgb_lf_group@blacksheep.org From: "Peter Dodd" X-Mailer: ZIMACS Version 1.20c 10000836 Subject: Re: LF: Receiving Sender: Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org Content-type: text/plain; charset=windows-1252; format=flowed MIME-Version: 1.0 Content-transfer-encoding: 8bit In the early days of 73kHz experimenting the only receiver equipment I had was a Datong converter and my mobile transceiver or the Drake 4c. The Daytong converter was designed for SWL use, had excellent sensitivity with a short length of wire but was easily overloaded when connected to an LF transmitting antenna. Yet most of my 73kHz DX contacts and some of the early 136kHz contacts were used with this receiver system. The secret was in the appropriate use of filters and a variable attenuator. The first stage in the filtering in my LF receiver is the transmitter antenna resonating and matching circuits. This was followed by two parallel tuned circuits, top-coupled; followed by an attenuator (comprising a potentiometer an a couple of resistors. With this circuit weak amateur signals could be extracted without any intermodulation from the high powered RTTY and broadcast stations However, it is well know that two parallel tuned circuits using capacitive top-coupling degenerates to a high-pass filter in the stop-band, i.e. gives good attenuation on the lower side of resonance but the attenuation is only around 40dB on the high side of resonance. If problems occur from signals higher than the filter resonance then it is better to use inductive bottom coupling on the filter. Since the advent of high performance broadband front-ends and VHF first IFs in modern receivers we seem to have lost the art of good receiver front end filter design - however there is a lot of design information around. I recommend Solid State Design for the Radio Amateur, by Wes Hayward W7ZOI. On page 117 (Advanced Receiver Concepts) he describes a tunable Cohn filter ( which is a four resonator inductive bottom coupled filter for 1.8MHz that can be scaled for other bands). Additionally, Reference [1], shows the use of filters in domestic radios. Fig 7, for example, uses switched filters to cover the long, medium and short wavebands. On the longwave filter a parallel bandstop filter is used to reduce interference from a known high-power transmitter. Reference [2] gives filter design data in a concise form, with appropriate formula and characteristics. There are many computer software filter design packages. Reference [3] is of one such package, analyzing the performance, for example, of a double-tuned parallel filter. The use of filters such as these are essential, particularly when using large antennas. The out of amateur band signals burned out the diodes of my MFJ-249 when I was trying to resonate my LF antenna for top-band. Having said all this, filters cannot help in my case of the in-band trash that comes from the Loran transmitter at Lessay on 100kHz. REFERENCES [1] Classified Radio Receiver Diagrams, E. M. Squire, Sir Isaac Pitman & Sons, 1943 [2 Radio Engineering, Volume Two, E.K. Sandeman, Chapman and Hall Ltd, 1949 [3] The ARRL Radio Designer. (computer software), The American Radio Relay League, Newington, CT06111, USA -- Regards, Peter, G3LDO