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Date: Fri, 19 Mar 2010 12:40:11 +0100
Message-ID: <38A51B74B884D74083D7950AD0DD85E82A1B7B@File-Server-HST.hst.e-technik.tu-darmstadt.de>
Thread-Topic: RX antennas
Thread-Index: AcrHVWIe+z8rh46zTVaxcDxvkiiB9wAAYAhO
References: <Pine.LNX.3.95.1100316221541.251A-100000@noname.nodomain.nowhere> <4BA0A501.5050506@abelian.org> <38A51B74B884D74083D7950AD0DD85E82A1B66@File-Server-HST.hst.e-technik.tu-darmstadt.de> <4BA3187E.9010801@abelian.org> <005c01cac74b$ef5d3c60$8d01a8c0@JAYDELL> <4BA35BA3.9090105@abelian.org>
From: =?iso-8859-1?Q?Stefan_Sch=E4fer?= <schaefer@hst.tu-darmstadt.de>
To: <rsgb_lf_group@blacksheep.org>
Content-Type: multipart/mixed;
	boundary="----_=_NextPart_001_01CAC758.EF4488ED"
Subject: VLF: RX antennas
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


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Paul,=20
=20
Before the VLF theme came up, my special interest was the transmission of=
 the RX signal by an fiber optic cable. This greatly reduced the local noi=
se coupled into an active E-field antenna in my 137kHz experiments. I even=
 tested that method with a RX loop antenna (also shown on my site).
Do you think this could be another (just additional) method to even improv=
e a RX system? I mean, an isolating transformer is a good choice at all bu=
t there remains some stray capacitance between the coils and core you know=
. This capacitive coupling is rahter an issue on 137 than on 9 kHz and a=
 loop antenna is not so sensitive as a small E-field probe (that has just=
 some less pF coupling to the far field) but i want to give you this sugge=
stion to think about. Your opinion about this would be interesting.
Note that with such a fiber optic cable you can REALLY decouple the antenn=
a to all conducting wires in and out of your house by some 10m, if wanted!=
 And the preamp does not even has to be realised symmetric when using a sm=
all battery. This preamp consists just out of a FET (BF862), a C(abt 1uF)=
 and a TX LED(e.g. SFH756V) and the battery(9V block battery, maybe 2 accu=
s in change)!
That circuit worked fine on 137 and on 9 it would be even more unproblemat=
ic to realise...
=20
73, Stefan

________________________________

Von: owner-rsgb_lf_group@blacksheep.org im Auftrag von Paul Nicholson
Gesendet: Fr 19.03.2010 12:10
An: rsgb_lf_group@blacksheep.org
Betreff: Re: VLF: TA, 300m and new PA?




 > What do you consider suitable?

In order to squeeze out the maximum S/N, it looks like a
loop antenna is desirable in order to null as much
background noise as possible.

It must 'see' down to the natural VLF background, so a
basic sensitivity of say better than 5fT in 1Hz is
required.    Then the floor should be dominated by the
natural background.   This doesn't seem too demanding at
9kHz and it looks like people are having some success with
resonant antennas connected straight into the soundcard!

A pre-amp would enable a smaller loop to be used which
increases options for locating the antenna in a quiet spot.
Transformer isolation may be necessary to remove ground-
looped interference - even if the loop is balanced.

Sampling needs to be stable to better than 0.01Hz, which
may be a problem for low grade soundcards, especially if
timing is based on ceramic resonators rather than xtal.

Spectrum Lab contains tools for doing the initial conversion
rate calibration and also for making continuous correction
during reception by using a reference signal.  It is
important that the signal stays more or less within one bin
during the test.

It is worth also discarding the 5% or so of signal that
contains the strongest sferics.    May be worth recording
the test raw and post-processing to try various settings
for blanking and frequency binning.

If the signal crosses the Atlantic, it may be right on the
edge of detectability.   We used a pessimistic model for
the previous test and the signal was larger than expected.
But now we are probably using an optimistic model. Therefore
luck cannot be invoked and it is down to skill, ingenuity,
and probably patience too - a long integration will be
needed!

--
Paul Nicholson
--





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