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Date: Sun, 6 Mar 2011 13:13:47 +0000
Message-ID: <AANLkTikWf-OC5nhh0j1bupVD0PQL4tF6ns6px_JdNM1R@mail.gmail.com>
From: Gary - G4WGT <g4wgt@tiscali.co.uk>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Sub 9kHz antenna tests.
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Hi Jim,

Thank you for the very informative description, I now more fully understand
your loop system design criteria. It seems that I worsened the receive
capabilities in one way another by my twiddling with the loop etc. At least
now that I understand the system I can return the loop & pre-amp to it's
original state & note the difference.

What did take me by surprise that my two loops produced a much higher level
of noise than the Mini-Whip which I modified for VLF but since reading your
e-mail I can see the probable reason for that.

I will return my loop system to original & report my findings to the LF
group.

Regards, Gary - G4WGT.


On 5 March 2011 22:47, James Moritz <james.moritz@btopenworld.com> wrote:

> Dear Eddie, Gary, LF Group,
>
> The loop is tuned, but is very low Q, and has more of a low-pass response
> really. It was done this way partly in order to obtain a wide bandwidth
> around 9kHz, which allows effective use of noise blanking or clipping
> (implemented using the DSP facilities of Spec Lab) to reduce the impact of
> QRN impulses. It also avoids the need for re-tuning, which is very
> convenient. At the same time, it provides a decent amount of attenuation of
> signals above the VLF range, which would otherwise tend to overload either
> the preamp or the sound card input. The frequency response is defined by the
> loop inductance, the shunt capacitor at the input, and the input resistance
> of the preamp stage (about 300ohms, due to the series/shunt negative
> feedback). The low Q means the loop voltage is small, so a fairly low-noise
> preamp is needed. The band noise has always been >10dB above the internal
> preamp noise in practice, so this is satisfactory. The overall gain of the
> preamp is about 60dB at 9kHz, and this was chosen as a compromise - it is
> large enough so that the band noise swamps the internal noise and QRM in the
> sound card input, but small enough so that the utilities and QRN do not
> overload the sound card.
>
> You can get much higher signal voltage out of the same loop antenna by
> changing it to a high Q resonant design. I guess with the relatively high
> impedance load of the sound card input (several kohm), you might get a Q
> between 10 and 100. This would give a voltage gain of 20 - 40dB, more if you
> made a really low-loss loop and optimised a tapping point for the sound card
> input connection. But the bandwidth would also be greatly reduced, probably
> to only a few hundred Hz or less, and this is a bad thing. The narrow
> bandwidth "stretches" the QRN impulses making clipping / blanking less
> effective. Also, accurate tuning is then required.
>
> I suppose one motive for having such a high Q loop would be to eliminate
> the preamp. This might work, depending on how noisy the sound card codec
> chip and the PC it is attached to are. My experience has been that this
> varies a lot between different PCs and frequencies, and which audio input
> you use. E.g. the laptop I usually use has a "Mic" input with considerably
> higher gain than the "line" input. But for some reason, the SNR is worse
> even though the signal level is higher when using this mic input. Also, the
> input noise level may vary depending on the source impedance of the antenna
> connected to the input. Since the preamp components only cost pennies, and
> allow you to get ample SNR as well as several dB improvement through the use
> of noise clipping and "no-tune" operation, this seems a better and simpler
> solution.
>
> When comparing noise levels, it is important to check what the noise
> actually is, especially since PCs and associated stuff are potentially
> powerful noise sources. Listening to the output around 9kHz using the
> SpecLab VLF SDR function is useful - you are hoping to hear the band QRN and
> probably assorted mains-borne noises, rather than a lot of digital hash from
> the PC.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>

--001636c923ee3df003049dd026f5
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

Hi Jim,<div><br></div><div>Thank you for the very informative description,=
 I now more fully understand your loop system design criteria. It seems th=
at I worsened the receive capabilities in one way another by my twiddling=
 with the loop etc. At least now that I understand the system I can return=
 the loop &amp; pre-amp to it&#39;s original state &amp; note the differen=
ce.</div>
<div><br></div><div>What did take me by surprise that my two loops produce=
d a much higher level of noise than the Mini-Whip which I modified for VLF=
 but since reading your e-mail I can see the probable reason for that.</di=
v>
<div><br></div><div>I will return my loop system to original &amp; report=
 my findings to the LF group.</div><div><br></div><div>Regards, Gary - G4W=
GT.</div><div><br><br><div class=3D"gmail_quote">On 5 March 2011 22:47, Ja=
mes Moritz <span dir=3D"ltr">&lt;<a href=3D"mailto:james.moritz@btopenworl=
d.com">james.moritz@btopenworld.com</a>&gt;</span> wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1=
px #ccc solid;padding-left:1ex;">Dear Eddie, Gary, LF Group,<br>
<br>
The loop is tuned, but is very low Q, and has more of a low-pass response=
 really. It was done this way partly in order to obtain a wide bandwidth=
 around 9kHz, which allows effective use of noise blanking or clipping (im=
plemented using the DSP facilities of Spec Lab) to reduce the impact of QR=
N impulses. It also avoids the need for re-tuning, which is very convenien=
t. At the same time, it provides a decent amount of attenuation of signals=
 above the VLF range, which would otherwise tend to overload either the pr=
eamp or the sound card input. The frequency response is defined by the loo=
p inductance, the shunt capacitor at the input, and the input resistance=
 of the preamp stage (about 300ohms, due to the series/shunt negative feed=
back). The low Q means the loop voltage is small, so a fairly low-noise pr=
eamp is needed. The band noise has always been &gt;10dB above the internal=
 preamp noise in practice, so this is satisfactory. The overall gain of th=
e preamp is about 60dB at 9kHz, and this was chosen as a compromise - it=
 is large enough so that the band noise swamps the internal noise and QRM=
 in the sound card input, but small enough so that the utilities and QRN=
 do not overload the sound card.<br>

<br>
You can get much higher signal voltage out of the same loop antenna by cha=
nging it to a high Q resonant design. I guess with the relatively high imp=
edance load of the sound card input (several kohm), you might get a Q betw=
een 10 and 100. This would give a voltage gain of 20 - 40dB, more if you=
 made a really low-loss loop and optimised a tapping point for the sound=
 card input connection. But the bandwidth would also be greatly reduced,=
 probably to only a few hundred Hz or less, and this is a bad thing. The=
 narrow bandwidth &quot;stretches&quot; the QRN impulses making clipping=
 / blanking less effective. Also, accurate tuning is then required.<br>

<br>
I suppose one motive for having such a high Q loop would be to eliminate=
 the preamp. This might work, depending on how noisy the sound card codec=
 chip and the PC it is attached to are. My experience has been that this=
 varies a lot between different PCs and frequencies, and which audio input=
 you use. E.g. the laptop I usually use has a &quot;Mic&quot; input with=
 considerably higher gain than the &quot;line&quot; input. But for some re=
ason, the SNR is worse even though the signal level is higher when using=
 this mic input. Also, the input noise level may vary depending on the sou=
rce impedance of the antenna connected to the input. Since the preamp comp=
onents only cost pennies, and allow you to get ample SNR as well as severa=
l dB improvement through the use of noise clipping and &quot;no-tune&quot;=
 operation, this seems a better and simpler solution.<br>

<br>
When comparing noise levels, it is important to check what the noise actua=
lly is, especially since PCs and associated stuff are potentially powerful=
 noise sources. Listening to the output around 9kHz using the SpecLab VLF=
 SDR function is useful - you are hoping to hear the band QRN and probably=
 assorted mains-borne noises, rather than a lot of digital hash from the=
 PC.<br>

<br>
Cheers, Jim Moritz<br>
73 de M0BMU <br>
<br>
</blockquote></div><br></div>

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