Return-Path: Received: from post.thorcom.com (post.thorcom.com [195.171.43.25]) by klubnl.pl (8.14.4/8.14.4/Debian-8+deb8u2) with ESMTP id x0DGftD8016512 for ; Sun, 13 Jan 2019 17:42:01 +0100 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1giikQ-000168-1O for rs_out_1@blacksheep.org; Sun, 13 Jan 2019 16:36:30 +0000 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1giikP-00015z-8J for rsgb_lf_group@blacksheep.org; Sun, 13 Jan 2019 16:36:29 +0000 Received: from mail-ed1-x536.google.com ([2a00:1450:4864:20::536]) by relay1.thorcom.net with esmtps (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.89) (envelope-from ) id 1giiji-00046M-QU for rsgb_lf_group@blacksheep.org; Sun, 13 Jan 2019 16:36:28 +0000 Received: by mail-ed1-x536.google.com with SMTP id d39so16767015edb.12 for ; Sun, 13 Jan 2019 08:35:36 -0800 (PST) X-DKIM-Result: Domain=gmail.com Result=Good and Known Domain DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:from:date:message-id:subject:to; bh=2Vh9ozYZsop+nes7eBEToQYHL6KjI6f7Y0MS+sXdREg=; b=IV+JDFnPJEtdvyCH9EGKOwMOi39YVRiOzeOMMkEZC5nwOvn8/zAmT/uFUDC+lwjv6e aQpzGvzLOcuf8GMSbAbyym+vu0gjVE7pHtLBQljnHZ0oPqxjl+ERuIgRbzzAztOlqvqo zCQ2Vig3E60WIYodUwQggydhcQ4c7bk4N/Lvu8t9HuKCo7QAW8+18opseVB4orjXmjgQ ppvpocbkJpsEztwcs82qMZnCvmAhlScmoUeZpsnPVeVy1pCsAvSfvDJkQ8ZitCcq6yDo Q1Sarm9Ua6+ySXLlBKxeRmihATewZVbf0hEvcvI8XWusg+20Nnl4CJZv+ZPU1DRMKKt2 TnvA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:from:date:message-id:subject:to; bh=2Vh9ozYZsop+nes7eBEToQYHL6KjI6f7Y0MS+sXdREg=; b=s5Xu4p0ypQb8jcuxx020uGgo0WGKG+nd9r9kCffKnVcM2HgZ3ZdjSN+Yjq/x2Eg5+Q b6J8nY0rmeqo4XqLF1hA81NCwReYgqQIlToYs/E8BDv2QGc4Y+SJxKLptMtTrOqBTRRd lpN7mPoa0I4rL4omB3+YgF5K8KEARZRwM/KKlY2il7KPaPaPH3NpOTIXZ4QvOw2JXWXB KySdWYMsvsAEJfToNyuN52a7buC6W5WUJNNyFIrQRP7AWhi9EvFA87Wg8i24mKh2r4il AfG54GG+KbTyZHh3cVUTPjDCAFV5wzuGxH1yXDHK793MG1FZQ36n/9I0vFFK8Wwl12qU ZmJg== X-Gm-Message-State: AJcUukeggnAwP/+wYwYRbOdXiyCeQ3FqNvAtAQrncEZYnmXbhKoRsaR8 fLXlEysX1Jn54CKuZtGnLi/iOsD5vYrRsn3QriQ1pYEyHmM= X-Google-Smtp-Source: ALg8bN7FHKg8sU8DGlhtzX+jlztpAy7PqxEdsNHBRCHKz+ZiNWZt2tQDCzKrsdVHvoN5N/nwMWk6wPGV0r9dNU2XKmA= X-Received: by 2002:a17:906:f6cb:: with SMTP id jo11-v6mr17539381ejb.80.1547397335137; Sun, 13 Jan 2019 08:35:35 -0800 (PST) MIME-Version: 1.0 From: Andy Talbot Date: Sun, 13 Jan 2019 16:35:19 +0000 Message-ID: To: rsgb-lf-group@groups.io, "rsgb_lf_group@blacksheep.org" X-Spam-Score: 0.0 (/) X-Spam-Report: Spam detection software, running on the system "relay1.thorcom.net", has NOT identified this incoming email as spam. The original message has been attached to this so you can view it or label similar future email. If you have any questions, see @@CONTACT_ADDRESS@@ for details. Content preview: It's only taken me about 20 years on LF to actually think about, and calculate, this... We all know we're supposed to attenuate harmonics and unwanted signals, so most LF amplifiers, especially switching types, incorporate a low pass filter - usually the traditional T or Pi design. Content analysis details: (0.0 points, 5.0 required) pts rule name description ---- ---------------------- -------------------------------------------------- -0.0 RCVD_IN_DNSWL_NONE RBL: Sender listed at http://www.dnswl.org/, no trust [2a00:1450:4864:20:0:0:0:536 listed in] [list.dnswl.org] -0.0 SPF_PASS SPF: sender matches SPF record 0.0 FREEMAIL_FROM Sender email is commonly abused enduser mail provider (andy.g4jnt[at]gmail.com) 0.0 HTML_MESSAGE BODY: HTML included in message 0.0 T_DKIM_INVALID DKIM-Signature header exists but is not valid 0.0 T_REMOTE_IMAGE Message contains an external image X-Scan-Signature: c40596c1cc3306f1877076595a921c57 Subject: LF: Low Pass Filtering Content-Type: multipart/alternative; boundary="0000000000007cc476057f59847e" X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.5 required=5.0 tests=HTML_40_50,HTML_MESSAGE autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false --0000000000007cc476057f59847e Content-Type: text/plain; charset="UTF-8" It's only taken me about 20 years on LF to actually think about, and calculate, this... We all know we're supposed to attenuate harmonics and unwanted signals, so most LF amplifiers, especially switching types, incorporate a low pass filter - usually the traditional T or Pi design. But I always felt the hi-Q antenna assembly would do a pretty good job of filtering in its own right, and didn't use any LPF on either the 700 Watt 137kHz Tx which has a tuned tank output (Ql = 6) or the 400W class E 475kHz unit. However, thought it was time to try to calculate the actual attenuation of harmonics. So, using the inductance of the loading coil which goes up proportional to harmonic number, N, Rrad of the antenna that goes up with N squared and a crude approximation of Rloss changing as 1/N, plugged all those into a spreadsheet and found the following on my 7m high Tee antenna At 137kHz the relative radiated level of harmonics is between 50dB down at 2nd and 3rd, dropping to 45db at 8-9. Considering the switcher PA plus tank is about -30dB to start with that looks well within the recommended -70dB for high power amps At 475kHz low order are only 33dB down and higher up -30dB. I don't have a figure for the class-E starting harmonic level, but IIRC it was of the same order as the 137 Tx, so again that looks near acceptable. Of course, your antenna / loading coil assembly will differ, and larger antennas / smaller coils will not provide as much attenuation but it is still enough to suggest the 5th order filters in use on some designs are a bit OTT. One thing I haven't considered is how distributed capacitance in the loading coil might compromise rejection of higher order harmonics, but that can be mitigated by the Tx producing less of them in the first place. Did the original Decca transmitters have any extra low -pass filtering ? Tee antenna design spreadsheet at http://www.g4jnt.com/DropF/teeantcalcs.xls Andy www.g4jnt.com Virus-free. www.avg.com <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> --0000000000007cc476057f59847e Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
It's only taken me about 20= years on LF to actually think about, and calculate, this...

We all know we're supposed to attenuate harmonics = and=C2=A0 unwanted signals, so most LF amplifiers, especially switching typ= es,=C2=A0 incorporate a low pass filter - usually the traditional T or Pi d= esign.

But I always felt the hi-Q antenn= a assembly would do a pretty good job of filtering in its own right, and di= dn't use any LPF on either the 700 Watt 137kHz Tx which has a tuned tan= k output (Ql =3D 6) or the 400W class E 475kHz unit.=C2=A0=C2=A0

However, thought it was time to try to calculate t= he actual attenuation of harmonics.=C2=A0 =C2=A0So, using the inductance of= the loading coil which goes up proportional to harmonic number, N,=C2=A0 = =C2=A0Rrad of the antenna that goes up with N squared and a crude approxima= tion of Rloss changing as 1/N, plugged all those into a spreadsheet and fou= nd the following on my 7m high Tee antenna

At 137kHz the relative radiated level of harmonics is between 50dB down = at=C2=A0 2nd and 3rd, dropping to 45db at 8-9.=C2=A0 =C2=A0 Considering the= switcher PA plus tank is about -30dB to start with that looks well within = the recommended -70dB for high power amps

At 475kHz low order are only 33dB down and higher up -30dB.=C2=A0 I don&#= 39;t have a figure for the class-E starting harmonic level, but IIRC it was= of the same order as the 137 Tx, so again that looks near acceptable.

Of course, your antenna / loading coil assem= bly will differ, and larger antennas / smaller coils will not provide as mu= ch attenuation but it is still enough to suggest the 5th order filters in u= se on some designs are a bit OTT.

One = thing I haven't considered is how distributed capacitance in the loadin= g coil might compromise rejection of higher order harmonics, but that can b= e mitigated by the Tx producing less of them in the first place.=C2=A0

Did the original Decca transmitters have any= extra low -pass filtering ?=C2=A0

Tee = antenna design spreadsheet at=C2=A0http://www.g4jnt.com/DropF/teeantcalcs.xls


= Virus-free. www.avg.com
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