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
will be limited by external noise, without compromising other performance.
A general principle is that increasing the signal level to the receiver,
either when using a preamp or by increasing the size of the antenna, should
be kept to the minimum neccessary to ensure the receiver noise does not add
significantly to the band noise. This will minimise the amount of
overload/IMD in the receiver. Also, unwanted strong signals can be reduced
by filtering, either by increasing the loop Q or with additional filter
elements. It is fairly easy to make preamps that generate lower levels of
distortion products than even good receivers, as well as lower noise, so
this is not really an argument against preamps. If there are no strong
ambient signals present, higher gain/bigger antennas can be used without
harmful effects, but this just "adds loudness to the noise" as you nicely
put it...
The signal output from a loop antenna depends basically on the loop area and
bandwidth. If the loop is tuned with a high Q, even a small loop can produce
quite a large signal for the receiver. But many receive loop designs trade
off a reduced signal level for increased bandwidth and the convenience of
not needing to re-tune the loop every time the RX frequency is changed. The
design might restore adequate signal level by increasing the loop area, or
by adding a low-noise preamplifier.
Most communications receivers have emphasis on strong-signal performance,
and do not have very high sensitivity, especially for LF/MF - they are
primarily designed for use with relatively large antennas that are
reasonably matched to 50ohms, or active antennas or distribution amplifiers
with 50ohm preamp output impedance. Also, many amateur type transceivers
that have coverage of LF/MF on receive have reduced sensitivity below about
1MHz - the front-end circuit is really designed for HF, and performance
falls off at lower frequencies. So for these receivers, a large loop can
often be used to give good sensitivity, although a well-designed small loop
and preamp will achieve the same result. Also, small loop antennas have the
advantage that they are easy to move around to make use of the directional
null, or to find a position where the local QRM is a minimum.
On the other hand, small portables are often designed with high sensitivity
for use with small internal whip and ferrite rod antennas. However, they are
also usually much more susceptible to overload/IMD due to strong signals,
and have minimal signal frequency front-end filtering of unwanted signals.
For these receivers, a small, relatively high-Q loop will provide adequate
signal level, and also filter the input signal to reduce the level of many
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
combination. But other good antennas are also possible, e.g. a larger
antenna with an attenuator and a preselector to provide filtering and
adjustment of the signal level. Active whip antennas are normally broadband,
and so overload/IM problems in a portable receiver are likely, although it
would be perfectly possible to add a preselector between the whip preamp
output and the RX input.
BTW, if you follow SM6LKM's suggestion with the dummy load, do make sure
that the dummy load has an impedance reasonably close to the actual source
impedance of the antenna you are using - this may give a noise level quite
different to, say, a 50ohm resistor, depending on the antenna and the RX
input circuit. Another possibility with a loop antenna is simply to turn it
so the plane of the loop is horizontal - you should find this nulls both the
signals and the band noise (but perhaps not local QRM)
Cheers, Jim Moritz
73 de M0BMU
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