Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on lipkowski.org X-Spam-Level: X-Spam-Status: No, score=0.0 required=5.0 tests=FREEMAIL_FORGED_FROMDOMAIN, FREEMAIL_FROM,HEADER_FROM_DIFFERENT_DOMAINS,HTML_MESSAGE,SPF_PASS, T_DKIM_INVALID,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 X-Spam-DCC: wuwien: mailn 1290; Body=2 Fuz1=2 Fuz2=2 Received: from post.thorcom.com (post.thorcom.com [195.171.43.25]) by lipkowski.org (8.14.4/8.14.4/Debian-8+deb8u2) with ESMTP id v8BB0YIe022028 for ; Mon, 11 Sep 2017 13:00:35 +0200 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1drMIS-00063c-1q for rs_out_1@blacksheep.org; Mon, 11 Sep 2017 11:50:32 +0100 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1drMIR-00063T-1n for rsgb_lf_group@blacksheep.org; Mon, 11 Sep 2017 11:50:31 +0100 Received: from omr-a013e.mx.aol.com ([204.29.186.60]) by relay1.thorcom.net with esmtps (TLSv1:DHE-RSA-AES256-SHA:256) (Exim 4.89) (envelope-from ) id 1drMIN-0008CM-0J for rsgb_lf_group@blacksheep.org; Mon, 11 Sep 2017 11:50:29 +0100 Received: from mtaomg-mbc01.mx.aol.com (mtaomg-mbc01.mx.aol.com [172.26.221.143]) by omr-a013e.mx.aol.com (Outbound Mail Relay) with ESMTP id 9BFB938000A4 for ; Mon, 11 Sep 2017 06:50:24 -0400 (EDT) Received: from core-acx15d.mail.aol.com (core-acx15.mail.aol.com [10.74.43.2]) by mtaomg-mbc01.mx.aol.com (OMAG/Core Interface) with ESMTP id 4B3E738000081 for ; Mon, 11 Sep 2017 06:50:24 -0400 (EDT) Received: from 80.146.228.119 by webjas-vaa155.srv.aolmail.net (10.96.33.158) with HTTP (WebMailUI); Mon, 11 Sep 2017 06:50:23 -0400 Date: Mon, 11 Sep 2017 06:50:24 -0400 From: Markus Vester To: rsgb_lf_group@blacksheep.org Message-Id: <15e708fc4d1-c05-5b015@webjas-vaa155.srv.aolmail.net> In-Reply-To: <6a4f2233-82b4-1264-6cad-0739ad154ed1@abelian.org> MIME-Version: 1.0 X-MB-Message-Source: WebUI X-MB-Message-Type: User X-Mailer: JAS STD X-Originating-IP: [80.146.228.119] x-aol-global-disposition: G DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mx.aol.com; s=20150623; t=1505127024; bh=5Cq9TkUfyJ9NKtlN+KUg2lNoEIF/avtbLg/ueGNP8YM=; h=From:To:Subject:Message-Id:Date:MIME-Version:Content-Type; b=zFFfAiI0HJN4BpfXNs7LdMQfhO0OwNvzfrZq6qhmXjPORvnd4RGO5JtxObv5KUqgH GL7zt2rjMki/xrCkxtVg/nrcGqVaaO21CH9SKigz4fFXqMatcYThYgTaaZN0VpgB9c glz8riEzKm6B6GQ7aTxXpIdDAG/oQ53yVqzlyZlk= x-aol-sid: 3039ac1add8f59b66a701132 X-Scan-Signature: fb42d311bf65eb51bbb8124890b4f977 Subject: Re: VLF: E-field receivers in Todmorden Content-Type: multipart/alternative; boundary="----=_Part_442247_231370214.1505127023824" 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 X-Scanned-By: MIMEDefang 2.75 Status: RO X-Status: X-Keywords: X-UID: 12565 ------=_Part_442247_231370214.1505127023824 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable There may be another pitfall with the Alpha's: Because three transmitters a= re sending dashes on the same frequency, the coherent sum of any single spe= ctral line depends on the location of the receiver (that's the essence of a= navigation system). At my location, the central line on 11904.7619 Hz happ= ens to have a minimum due to near-cancellation between Krasnodar and Novosi= birsk, which also depends on time of day. The superposition may create a no= ticeable signal difference if the two RX sites are spaced more than a few h= undred meters apart. In principle, calculating MSK station levels by summing power across say 30= 0 Hz should provide a good and accurate reference with low standard deviati= on. The difficulty may rather be that they are all transmitting above 16 kH= z, which might not be perfectly representative of preamp noise at lower VLF= . =20 Not sure if paralleling FETs is a useful idea to improve preamp performance= in an active probe. Paralleling does reduce noise voltage by sqrt(N), but = increases gate noise current by the same factor, leading to the same noise = figure at N-times reduced source impedance. Especially with a small capacit= ance probe with many megohms at ELF, paralleling may well worsen rather tha= n improve the noise mismatch. It will be difficult or impossible to see ZEV= S and Schumann resonances on a miniwhip, but on it is usually easy to simpl= y connect a larger wire antenna.=20 Best 73, Markus=20 -----Urspr=C3=BCngliche Mitteilung-----=20 Von: Paul Nicholson An: rsgb_lf_group Verschickt: So, 10. Sept 2017 18:17 Betreff: Re: VLF: E-field receivers in Todmorden The old rx is down at the moment, being measured on the work bench. The system noise floor is about 140nV/m in 1Hz. That's very high but quite low enough for natural radio (for which it is mainly used). The natural floor is about 10uV/m. But after sferic blanking (necessary for weak amateur signals) the natural floor may be down to 1uV/m and at that level the system noise from the old rx becomes noticeable. One reason for the new rx, which has about 5 to 10nV system noise depending on the front-end bias resistance I eventually decide to use. Unfortunately only the Alpha signals from the North East provide a suitable reference. Other signals eg MSK are too wide and amateur signals are too weak. Bernd, DF9RB wrote: > I plan to build this amplifier: >=20 http://dg4rbf.lima-city.de/Ultra%20Low%20Noise%20AMP%20V1.7f%20_WebSeite.pd= f > Please your opinion about this concept? Merging the currents from multiple front-end devices is a conventional way to achieve ultra low noise and this looks like a good implementation. I tried this technique with two devices but the doubling of the input capacitance reduced the signal by as much as the noise was lowered, no overall benefit (in fact slightly worse). This kind of thing shows up when you refer the system noise to field strength V/m instead of just input voltage. The system noise voltage is far lower than necessary and the gain is very high. Perhaps it is intended to work with a very small probe antenna. In that case the input capacitance is likely to cause a significant division of the signal voltage. I wonder how it performs in other respects. Dynamic range and intermod. Ultimately, a VLF pre-amp must be designed for a particular antenna. Measure the noise when the input is loaded by the equivalent antenna capacitance, and measure gain with a low impedance test signal injected through that capacitance. PS, this afternoon the old rx is back into operation. -- Paul Nicholson -- ------=_Part_442247_231370214.1505127023824 Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: quoted-printable
There may be another pitfall with the Alpha's: Because t= hree transmitters are sending dashes on the same frequency, the coherent su= m of any single spectral line depends on the location of the receiver (that= 's the essence of a navigation system). At my location, the central line on= 11904.7619 Hz happens to have a minimum due to near-cancellation between K= rasnodar and Novosibirsk, which also depends on time of day. The super= position may create a noticeable signal difference if t= he two RX sites are spaced more than a few hundred meters apart.
In principle, calculating MSK station levels by summing po= wer across say 300 Hz should provide a good and accurate referenc= e with low standard deviation. The difficulty may rather be that = they are all transmitting above 16 kHz, which might not be perfectly repres= entative of preamp noise at lower VLF.  

Not sure if paral= leling FETs is a useful idea to improve preamp performance in an = active probe. Paralleling does reduce noise voltage by sqrt(N), b= ut increases gate noise current by the same factor, leading to the same noi= se figure at N-times reduced source impedance. Especially with a small= capacitance probe with many megohms at ELF, paralleling may= well worsen rather than improve the noise mismatch. It will be d= ifficult or impossible to see ZEVS and Schumann resonances on a miniwhip,&n= bsp;but on it is usually easy to simply connect a larger wire antenna.=  

Best 73,
Markus 



-----Urspr=C3=BCnglic= he Mitteilung-----
Von: Paul Nicholson <vlf0403@abelian.org>
A= n: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: So, 10= . Sept 2017 18:17
Betreff: Re: VLF: E-field receivers in Todmorden

The old rx is down at the moment, being measured on the
work= bench. The system noise floor is about 140nV/m in 1Hz.
That's very hig= h but quite low enough for natural radio (for
which it is mainly used). = The natural floor is about 10uV/m.

But after sferic blanking (neces= sary for weak amateur signals)
the natural floor may be down to 1uV/m an= d at that level the
system noise from the old rx becomes noticeable. On= e reason
for the new rx, which has about 5 to 10nV system noise dependin= g
on the front-end bias resistance I eventually decide to use.

Un= fortunately only the Alpha signals from the North East
provide a suitabl= e reference. Other signals eg MSK are too
wide and amateur signals are = too weak.

Bernd, DF9RB wrote:

> I plan to build this ampl= ifier:
>
http://dg4rbf.lima-city= .de/Ultra%20Low%20Noise%20AMP%20V1.7f%20_WebSeite.pdf

> Plea= se your opinion about this concept?

Merging the currents from multip= le front-end devices is a
conventional way to achieve ultra low noise an= d this looks
like a good implementation.

I tried this technique w= ith two devices but the doubling of the
input capacitance reduced the si= gnal by as much as the noise
was lowered, no overall benefit (in fact sl= ightly worse).
This kind of thing shows up when you refer the system noi= se
to field strength V/m instead of just input voltage.

The syste= m noise voltage is far lower than necessary and the
gain is very high. = Perhaps it is intended to work with a very
small probe antenna. In that= case the input capacitance is
likely to cause a significant division of= the signal voltage.

I wonder how it performs in other respects. Dy= namic range and
intermod.

Ultimately, a VLF pre-amp must be desig= ned for a particular
antenna. Measure the noise when the input is loade= d by
the equivalent antenna capacitance, and measure gain with
a low = impedance test signal injected through that capacitance.

PS, this af= ternoon the old rx is back into operation.
--
Paul Nicholson
--
------=_Part_442247_231370214.1505127023824--