DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
  s=s1024; d=btinternet.com;
  h=DKIM-Signature:Received:X-Yahoo-SMTP:X-YMail-OSG:X-Yahoo-Newman-Property:Message-ID:From:To:References:Subject:Date:MIME-Version:Content-Type:Content-Transfer-Encoding:X-Priority:X-MSMail-Priority:X-Mailer:X-MimeOLE;
  b=5yWdW7oDiF0zFVtp90jzWrusv94xJG2Jk8ybTgvqRUPkWkNhXIn7R+gkxhEcwPTVtNo56UA91cpmHmOuTpM5eZ2XAdS9Bi388bqTg2RldvlCnameJkZMXYMiuYLfCMQ3GjNxqPPyghKrcCL+gAPHB4YQvHAtjrQPee0s9xvf2Ck=  ;
Message-ID: <BB503995CA3F4D5EA05CC7D701CC1751@lindavideo>
From: "Ken" <ken.h.wright@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <FDCC39D43D974DED978401025DB874DF@lindavideo> <005201cb9d61$8bdf3200$0201a8c0@Clemens04> <7958EBE930FB4CF695E06F93E5FEEDA1@JimPC>
Date: Thu, 16 Dec 2010 22:53:08 -0000
MIME-Version: 1.0
DomainKey-Status: good (testing) 
Subject: LF: Re: Re: Re: Sound Card mic Impedance
Content-Type: text/plain;
	format=flowed;
	charset="iso-8859-1";
	reply-type=original
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


Hello Jim.
I sorry if have caused every one a few problems tonight, but it certainly 
shows how complex our hobby is I must admit I was only concentrating on the 
output side of the filter and not the input where the greater problem is.

73.
Ken
M0KHW


----- Original Message ----- 
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Thursday, December 16, 2010 10:35 PM
Subject: LF: Re: Re: Sound Card mic Impedance


> Dear Ken, Clemens,
>
> As I recall, Ken is using a HF wire as a VLF RX antenna. The LF/VLF source
> impedance of such an antenna is basically the capacitance of the wire in
> series with some loss resistance - at 9kHz, Xc will probably be several 
> 10s
> of kilohms, Rloss several hundred ohms. The source impedance is therefore
> always much higher than the load impedance of Ken's sound card and/or 
> filter
> of 4 kilohms or less - so the RX sees a signal source that behaves
> approximately as a constant current source, and the voltage at the 
> soundcard
> input will be (signal current) * (soundcard Zin). So the signal voltage 
> will
> be nearly proportional to the input impedance. Reducing Zin from 4k to 50R
> will therefore reduce the signal voltage by a factor nearly 50/4000 -
> about -38dB. Actually, if a symmetrically terminated filter with Zin/Zout 
> =
> 50R is used, to achieve the designed frequency response it will be 
> necessary
> to  have shunt 50R resistors at both input and output, making the overall
> input impedance at the antenna terminal something nearer 25ohms, knocking
> off a further 6dB of signal. Loss of 44dB in signal level may well be too
> much. Also unfortunately, the antenna impedance will be much lower at
> broadcast frequencies - so the current due to broadcast QRM may be
> increased, depending on the circuit design, reducing the effectiveness of
> the filter.
>
> A better idea would be to design a singly-terminated filter. Connect a
> terminating resistor of as high a value as possible (say 1 or 2 kohm) 
> across
> the soundcard input to terminate the filter. Then design a low-pass filter
> with "infinity" source impedance, and load impedance equal to the new
> soundcard input resistance. The attachment shows my first effort with the
> AADE software, with 30k cutoff frequency and 1k load impedance. The 10k
> source impedance isn't infinity, but is close enough for practical 
> purposes.
> I fiddled with the values a bit to get preffered values. Note that the 
> input
> capacitor C2 can "absorb" the capacitance of the antenna and feeder. E.g.,
> if you connect the antenna via 10m of coax (100pF/m), and the antenna has
> 300pF capacitance, C2 could be reduced to 8.7nF to maintain the same
> response. The same also goes for the output side - but if the input
> capacitance of the soundcard  is really several nF as some have found, the
> filter would need to be re-designed.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>