From owner-qrp-l@Lehigh.EDU Tue Jan 14 12:41:32 1997 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id MAA26960 for ; Tue, 14 Jan 1997 12:41:28 -0500 (EST) X-Received-x: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id MAA26960 for ; Tue, 14 Jan 1997 12:41:28 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35003-42762>; Tue, 14 Jan 1997 12:41:13 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <34833-11531>; Tue, 14 Jan 1997 12:40:20 -0500 Received: from scubed.com (server1.scubed.com [192.31.70.21]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with ESMTP id MAA56126 for ; Tue, 14 Jan 1997 12:40:10 -0500 Received: from inferno.scubed.com by scubed.com (S3.4/solaris-main) id JAA05062; Tue, 14 Jan 1997 09:39:57 -0800 Received: from [192.31.66.229] by inferno.scubed.com (S3.4/s3-sgi-5) id RAA24042; Tue, 14 Jan 1997 17:36:28 GMT Message-Id: Date: Tue, 14 Jan 1997 10:45:59 -0700 Reply-To: ji3m@maxwell.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: ji3m@maxwell.com (James R. Duffey) To: "Low Power Amateur Radio Discussion" Subject: Copying signals with poor sig/noise levels Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" X-Sender: ji3m@192.31.66.42 X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Status: RO I am just now going through my e-mail which has accumulated during my travels and forced excile at home due to 2 large snow storms; one of 2 feet and one of a foot. (Only got 2 inches last night) I noticed a thread about copying weak signals in Friday's digest. I hope that these comments are timely and not duplicates. Here is what I do; 1. Use headphones, preferably those that cover the ear and eliminate any contribution from room noise. Headphones are worth 3 to 6 dB improvement in signal to noise ratio over a speaker. No kidding. 2. Turn off the AGC. Turn off the AGC. Turn off the AGC. At low signal levels, if it is working at all, it is just raising the noise level. This is why your AM radio gets noisy when you drive under a bridge. The signal fades due to shileding by the bridge, so the AGC tries to recover the lsot signal by increasing the gain, and eventually it is working on nothing but noise. As a result you have overpowering noise. 3. Use the narrowest filter you have available. Filters in the IF are best, audio filters also help. A filter between the IF and the product detector will helps reduce noise introduced by IF amplifiers. There will be less noise in a narrower bandwidth. The noise power goes as the square root of the bandwidth. A 250 Hz CW filter will have 1/3.2 times the noise of a 2.5 kHz SSB filter. You can hear the difference. The brain does some noise filtering, however, and the gain may be less for trained operators. 3. Reduce the RF gain until the signal is just perceptible. Crank up the AF gain to improve volume level. This too decreases the noise level somewhat relative to the signal level. Do this with the AGC off. 4. Practice copying weak signal levels. It is a learned skill and takes practice. Most people can get good copy on CW at a signal to noise ratio of 0dB. Those with good skills can copy CW at signal to noise ratios of -10dB. By contrast, SSB needs 10 dB or so signal to noise to get good copy. Did I mention turning off the AGC? - Duffey KK6MC/5 James R Duffey KK6MC/5 DM65 30 Casa Loma Road Cedar Crest, NM 87008