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Date: Fri, 16 Apr 2010 09:02:06 +0200
Message-ID: <4D95FB39404AA04CA998E7F63B1A2E5F0432C93F@esealmw110.eemea.ericsson.se>
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Thread-Topic: Re: Amplifier required for LF DX/weak signals?
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References: <4D95FB39404AA04CA998E7F63B1A2E5F0432C756@esealmw110.eemea.ericsson.se> <E551A7A523FC4A8193BC31AA7210C056@JimPC>
From: "Daniele Tincani" <daniele.tincani@ericsson.com>
To: <rsgb_lf_group@blacksheep.org>
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Subject: LF: RE: Re: Amplifier required for LF DX/weak signals?
Sender: owner-rsgb_lf_group@blacksheep.org
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Reply-To: rsgb_lf_group@blacksheep.org



Thank you very much, Jim.
Very kind of you to provide this comprehensive answer to my inquiry.
Best regards
Daniele

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@black=
sheep.org] On Behalf Of James Moritz
Sent: gioved=EC 15 aprile 2010 22.36
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: Amplifier required for LF DX/weak signals?

Dear Daniele, LF Group,

...> 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....>

If the correct combination of antenna/preamp/receiver is used, sensitivity=
=20
will be limited by external noise, without compromising other performance.=
=20
A general principle is that increasing the signal level to the receiver,=
=20
either when using a preamp or by increasing the size of the antenna, shoul=
d=20
be kept to the minimum neccessary to ensure the receiver noise does not ad=
d=20
significantly to the band noise. This will minimise the amount of=20
overload/IMD in the receiver. Also, unwanted strong signals can be reduced=
=20
by filtering, either by increasing the loop Q or with additional filter=20
elements. It is fairly easy to make preamps that generate lower levels of=
=20
distortion products than even good receivers, as well as lower noise, so=
=20
this is not really an argument against preamps. If there are no strong=20
ambient signals present, higher gain/bigger antennas can be used without=
=20
harmful effects, but this just "adds loudness to the noise" as you nicely=
=20
put it...

The signal output from a loop antenna depends basically on the loop area=
 and=20
bandwidth. If the loop is tuned with a high Q, even a small loop can produ=
ce=20
quite a large signal for the receiver. But many receive loop designs trade=
=20
off a reduced signal level for increased bandwidth and the convenience of=
=20
not needing to re-tune the loop every time the RX frequency is changed. Th=
e=20
design might restore adequate signal level by increasing the loop area, or=
=20
by adding a low-noise preamplifier.

Most communications receivers have emphasis on strong-signal performance,=
=20
and do not have very high sensitivity, especially for LF/MF - they are=20
primarily designed for use with relatively large antennas that are=20
reasonably matched to 50ohms, or active antennas or distribution amplifier=
s=20
with 50ohm preamp output impedance. Also, many amateur type transceivers=
=20
that have coverage of LF/MF on receive have reduced sensitivity below abou=
t=20
1MHz - the front-end circuit is really designed for HF, and performance=20
falls off at lower frequencies. So for these receivers, a large loop can=
=20
often be used to give good sensitivity, although a well-designed small loo=
p=20
and preamp will achieve the same result. Also, small loop antennas have th=
e=20
advantage that they are easy to move around to make use of the directional=
=20
null, or to find a position where the local QRM is a minimum.

On the other hand, small portables are often designed with high sensitivit=
y=20
for use with small internal whip and ferrite rod antennas. However, they=
 are=20
also usually much more susceptible to overload/IMD due to strong signals,=
=20
and have minimal signal frequency front-end filtering of unwanted signals.=
=20
For these receivers, a small, relatively high-Q loop will provide adequate=
=20
signal level, and also filter the input signal to reduce the level of many=
=20
of the strong off-frequency signals that could cause overload.

So a cheap portable RX with a fairly small tuned loop is probably a good=
=20
combination. But other good antennas are also possible, e.g. a larger=20
antenna with an attenuator and a preselector to provide filtering and=20
adjustment of the signal level. Active whip antennas are normally broadban=
d,=20
and so overload/IM problems in a portable receiver are likely, although it=
=20
would be perfectly possible to add a preselector between the whip preamp=
=20
output and the RX input.

BTW, if you follow SM6LKM's suggestion with the dummy load, do make sure=
=20
that the dummy load has an impedance reasonably close to the actual source=
=20
impedance of the antenna you are using - this may give a noise level quite=
=20
different to, say, a 50ohm resistor, depending on the antenna and the RX=
=20
input circuit. Another possibility with a loop antenna is simply to turn=
 it=20
so the plane of the loop is horizontal - you should find this nulls both=
 the=20
signals and the band noise (but perhaps not local QRM)

Cheers, Jim Moritz
73 de M0BMU