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 x0DMAOtU018192 for ; Sun, 13 Jan 2019 23:10:25 +0100 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1ginrJ-000432-NC for rs_out_1@blacksheep.org; Sun, 13 Jan 2019 22:03:57 +0000 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1ginou-00042p-O5 for rsgb_lf_group@blacksheep.org; Sun, 13 Jan 2019 22:01:28 +0000 Received: from icp-osb-irony-out5.external.iinet.net.au ([203.59.1.221]) by relay1.thorcom.net with esmtp (Exim 4.89) (envelope-from ) id 1ginol-0004hP-JT for rsgb_lf_group@blacksheep.org; Sun, 13 Jan 2019 22:01:22 +0000 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: =?us-ascii?q?A2D6AAA2tDtc/wGhAg4NGjkEHAEBAQQ?= =?us-ascii?q?BAQcEAQGBZYFWgRSBKYQBlFEBAQEBAQEGgRAliS+OYYFnNIIcgikCTAEIgXY?= =?us-ascii?q?4EgEDAQEBAQEBAoEJgjSDIwEBAQMjJkALDQsqAgJXBwwIAQGDHgGBdBytMXG?= =?us-ascii?q?BLxoFhSOEXIJ+ilCBB4ERJwyCX4MeBBUDgRQBEgEDEoMTglcCiWOYIQmKbIc?= =?us-ascii?q?WHoouA4dOiXWJLUCIZ4EGcTMaH4NACYIeF4NLimdeAYgggj4BAQ?= X-IPAS-Result: =?us-ascii?q?A2D6AAA2tDtc/wGhAg4NGjkEHAEBAQQBAQcEAQGBZYFWg?= =?us-ascii?q?RSBKYQBlFEBAQEBAQEGgRAliS+OYYFnNIIcgikCTAEIgXY4EgEDAQEBAQEBA?= =?us-ascii?q?oEJgjSDIwEBAQMjJkALDQsqAgJXBwwIAQGDHgGBdBytMXGBLxoFhSOEXIJ+i?= =?us-ascii?q?lCBB4ERJwyCX4MeBBUDgRQBEgEDEoMTglcCiWOYIQmKbIcWHoouA4dOiXWJL?= =?us-ascii?q?UCIZ4EGcTMaH4NACYIeF4NLimdeAYgggj4BAQ?= X-IronPort-AV: E=Sophos;i="5.56,475,1539619200"; d="scan'208,217";a="194888288" Received: from unknown (HELO [10.1.1.88]) ([14.2.161.1]) by icp-osb-irony-out5.iinet.net.au with ESMTP; 14 Jan 2019 06:01:07 +0800 To: rsgb_lf_group@blacksheep.org, rsgb-lf-group@groups.io References: From: Richard Rogers Message-ID: <099b087b-8369-e59e-816e-fc726e807f25@netspace.net.au> Date: Mon, 14 Jan 2019 09:01:05 +1100 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: Content-Language: en-US X-Antivirus: AVG (VPS 190113-4, 14/01/2019), Outbound message X-Antivirus-Status: Clean X-Spam-Score: -0.7 (/) 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: G'day Andy, When Robert AX2TAR was experimenting on 180KHz, he tried a transmitter of my design without the low pass filter. I could hear his 3rd harmonic and the 17th, 19th, 21st and 23rd harmonics.   The 21st harmonic was approaching S9.   I think the HF coupling was via the stray capacitance between the turns of his [...] Content analysis details: (-0.7 points, 5.0 required) pts rule name description ---- ---------------------- -------------------------------------------------- -0.7 RCVD_IN_DNSWL_LOW RBL: Sender listed at http://www.dnswl.org/, low trust [203.59.1.221 listed in list.dnswl.org] -0.0 SPF_PASS SPF: sender matches SPF record 0.0 HTML_MESSAGE BODY: HTML included in message 0.0 T_REMOTE_IMAGE Message contains an external image X-Scan-Signature: f9bb43a66658129469273581af35b296 Subject: Re: LF: Low Pass Filtering Content-Type: multipart/alternative; boundary="------------82B146A189784C3734DD3B48" X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.6 required=5.0 tests=HTML_20_30,HTML_MESSAGE, HTML_TAG_EXISTS_TBODY 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 This is a multi-part message in MIME format. --------------82B146A189784C3734DD3B48 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit G'day Andy, When Robert AX2TAR was experimenting on 180KHz, he tried a transmitter of my design without the low pass filter. I could hear his 3rd harmonic and the 17th, 19th, 21st and 23rd harmonics.   The 21st harmonic was approaching S9.   I think the HF coupling was via the stray capacitance between the turns of his loading coil and the maximum radiation was when his aerial wire was about a half wave long. On 14/01/2019 3:35 am, Andy Talbot wrote: > 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 > > > -- 73, Ric, VK7RO --- This email has been checked for viruses by AVG. https://www.avg.com --------------82B146A189784C3734DD3B48 Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: 8bit

G'day Andy,

When Robert AX2TAR was experimenting on 180KHz, he tried a transmitter of my design without the low pass filter.

I could hear his 3rd harmonic and the 17th, 19th, 21st and 23rd harmonics.   The 21st harmonic was approaching S9.   I think the HF coupling was via the stray capacitance between the turns of his loading coil and the maximum radiation was when his aerial wire was about a half wave long.

On 14/01/2019 3:35 am, Andy Talbot wrote:
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


Virus-free. www.avg.com 
-- 
73, Ric, VK7RO
--------------82B146A189784C3734DD3B48--