Message-ID: <4BC774F3.9010709@telia.com>
Date: Thu, 15 Apr 2010 22:20:03 +0200
From: "Johan H. Bodin" <jh.bodin@telia.com>
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To: rsgb_lf_group@blacksheep.org
References: <4D95FB39404AA04CA998E7F63B1A2E5F0432C756@esealmw110.eemea.ericsson.se> <4BC7200F.3070208@telia.com> <4D95FB39404AA04CA998E7F63B1A2E5F0432C8A0@esealmw110.eemea.ericsson.se>
In-Reply-To: <4D95FB39404AA04CA998E7F63B1A2E5F0432C8A0@esealmw110.eemea.ericsson.se>
Subject: Re: LF: Amplifier required for LF DX/weak signals?
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Hi Daniele,

yes, absolutely. A simple resistor is very quiet compared to the noise
figure of most LF receivers. An ideal resistor has a noise voltage of
about 0.128 * R^0.5 [nV/(Hz^0.5)] at room temperature. Metal film
resistors come close but carbon resistors are somewhat noisier,
especially when they are exposed to some DC bias. Anyway, if you don't
see any noticeable difference in noise level between open input and
resistor terminated input, just plug the antenna in/out and listen. If
the noise from the antenna is considerably stronger then you don't need
any preamp.

More info here:
  http://en.wikipedia.org/wiki/Thermal_noise

73 de Johan SM6LKM

Daniele Tincani wrote:
> Hi Johan,
> Would a normal carbon resistor be quiet enough as a dummy antenna impeda=
nce for this test?
> Cheers
> D.
>=20
> -----Original Message-----
> From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@bla=
cksheep.org] On Behalf Of Johan H. Bodin
> Sent: gioved=EC 15 aprile 2010 16.18
> To: rsgb_lf_group@blacksheep.org
> Subject: Re: LF: Amplifier required for LF DX/weak signals?
>=20
> Daniele,
>=20
> the ususal rule of thumb is that you don't need any preamplifier if you
> get a big increase in the noise level when the antenna is connected.
> It is best to make the comparison with a dummy load as "quiet reference"
> since some receivers may not show their true noise floor when the input
> is unterminated.
>=20
> The RX noise floor become unimportant if it is 10..20dB or so below the
> "natural" antenna noise.
>=20
> 73
> Johan SM6LKM
>=20
> ----
>=20
> Daniele Tincani wrote:
>> Hi LF,
>>
>> =20
>>
>> Quite a general question: is a pre-amplifier needed in your opinion for
>> listening to LF weak signals (e.g. NDB DX, etc.)?
>>
>> I read different opinions in very famous articles on the Web: one
>> opinion is that a bigger non-amplified loop is better than a smaller
>> amplified loop and an amplifier should be used only if really necessary
>> (and only with the strictly required gain). On the contrary, another
>> famous article says that an antenna pre-amplifier is important for NDB
>> DX, despite it mentions quite a big loop as the antenna. I'm just a
>> beginner in LF and NDB listening with quite a simple system. I tried an
>> homebrew amplifier together with my small loop and it seemed to me that
>> it did not add anything but loudness to both the signal and the noise.
>> Basically, no improvements in signal readability. What's your opinion,
>> in particular when cheap portables (e.g. DE1103) are involved?
>>
>> =20
>>
>> Best regards
>>
>> Daniele
>=20
>=20
>=20