Return-Path: Received: (qmail 24851 invoked from network); 26 May 2000 09:03:48 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by grants.core.plus.net.uk with SMTP; 26 May 2000 09:03:48 -0000 Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: 8bit Received: from majordom by post.thorcom.com with local (Exim 3.02 #1) id 12vFqG-0002hH-00 for rsgb_lf_group-outgoing@blacksheep.org; Fri, 26 May 2000 09:51:40 +0100 X-Priority: 3 X-MSMail-Priority: Normal Received: from tele-post-20.mail.demon.net ([194.217.242.20]) by post.thorcom.com with esmtp (Exim 3.02 #1) id 12vFqE-0002hC-00 for rsgb_lf_group@blacksheep.org; Fri, 26 May 2000 09:51:39 +0100 Received: from telemetry.demon.co.uk ([194.222.19.114]) by tele-post-20.mail.demon.net with smtp (Exim 2.12 #2) id 12vFqB-000Jxn-0K for rsgb_lf_group@blacksheep.org; Fri, 26 May 2000 08:51:36 +0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Message-ID: Date: Fri, 26 May 2000 09:42:34 +0100 To: "RSGB LF Group" From: "Tom Boucher" Subject: LF: Diversity reception MIME-Version: 1.0 X-Mailer: Turnpike Version 3.03a Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: Mike, Alan, Larry and all, Surely the AR88s would be used with an external combiner? I would guess that the D suffix probably indicates the receivers had the necessary interface to an external combiner. Todays aerospace telemetry systems employ space, frequency and polarisation diversity, sometimes simultaneously which involves banks of receivers and combiners. Space diversity, using two physically seperate receive antennas, is used to overcome signal fades due to multi-path phase cancellation. Frequency diversity uses two seperate RF links on different frequencies to overcome radiation pattern nulls on larger vehicles. Polarisation diversity is used to overcome polarisation twisting of the received signal due to vehicle rolling or other factors. A fourth system is time diversity where the data is delayed and transmitted again to overcome short term signal attenuation such as on re-entry vehicles. Multiple receivers are necessary with a combiner which takes each receiver output and sums it in an optimal ratio depending on the S/N from each. A better system, known as Pre-Detection Combining, phase locks the two IF signals together and combines at IF frequency. Maximum theoretical improvement with two similar signals is 3dB but of course if one signal has a good S/N and the other is lousy, then the improvement can be much greater. Could be worth exploring some of these techniques for scraping LF signals out of the noise. Larry did you say you junked an AR88? What a terrible admission! I'm sure someone could have put it to good use, if only at the end of a long chain by a yachtsman. 73, Tom G3OLB.