The great advantage of this sytem is that
the critical tuning components are nice and dry in the shack, no remote
control, no waterproofing, just twiddle and go!
The preamp has 50ohms Zin, and with 50ohms load gives
22dB gain. The noise figure is roughly 3dB, which is low enough for most
things, and has quite good strong signal performance - it can cope with 100mV
RMS input without clipping, so the RX will almost always be overloaded before
the preamp. The output impedance is more or less that of the 22ohm resistor,
which ensures stability with reactive loads, so it is not fussy about what load
it is connected to. The bandwidth with the transistors shown is about 5MHz, but
it is really meant for the LF/VLF range. TR1 can also be a ZTX650 with similar
results, also a 2N4401 or 2N2222 are OK but with 1 or 2dB more noise. A BD135,
BFY52 or 2N3053 work well for TR2 - this runs quite warm so a clip-on heatsink
is desirable. Extra filtering of the 12V will be required if there is
significant noise on the supply.
I have used the preamp with several different types of
antenna, which I hope to write up when I get time. But a simple one which is
also quite useful is also shown in the attached file. This is a fairly large
single turn loop, which is series tuned by an inductor and variable capacitor.
The inductor should be reasonably high Q - I used 2 x 1mH ferrite cored chokes
(RS components 233-5291), with a Q of about 80. The transformer allows one side
of the tuning capacitor to be grounded - almost any high-permeability ferrite
core should work OK. The tuning part is located in the shack, and connected to
the loop with coax; this can be quite long, since it effectively just increases
the inductance of the loop, which in any case is much smaller than the 2mH
tuning inductance. This is not the most efficient way of matching a loop to a
pre-amp, but it is very convenient, and the losses in the tuning are easily
made up for by increasing the size of the loop. It also gives a useful amount
of front end selectivity.
The area of the loop required depends on how deaf the
receiver is - I tried it with an IC718, which with 250Hz CW bandwidth needs
about 0.9uV for 10dB SNR at 136k, about 20dB down on the sensitivity at HF. The
20m^2 loop with the preamp gave ample signal-to-noise with this rig. If
sensitivity is inadequate, a bigger loop can be put up. The area is the
important factor which decides the signal level, height and shape are not
critical. This means it only takes a few minutes to put up, and can use any
available supports - the loop was just slung between a fence post and the
branches of a bush, with no additional insulation, a wire thrown over a small
tree also worked fine - hence the "Lazy" title. It could also be
useful as a /P receiving antenna. Of course, the loop should be mounted as far
from noise sources such as mains wiring as possible. If one end is made
moveable, the loop can be turned to null out QRM just as with a
"proper" loop. The tuning inductance can be changed to tune different
frequency ranges - increased to 200mH, it was possible to get good results
right down to the Alpha beacons on about 12kHz.
© Jim M0BMU.
Notes by G3YXM.
I have used the preamp with a smaller, multi-turn loop
which I have mounted on a rotator. This was originally intended to null-out
interference from local sources. The interesting thing is that the loop behaves
differently from the main aerial especially at night. Sometimes signals can be
much better on the loop, although orientation is critical. I recently worked
OH1TN with 100% copy on the loop, whereas if I listened on the main wire he was
a very difficult copy... and it wasn't local noise that made the difference.
The loop does seem to be able to pick out the best mode of propagation
The loop is supported on a "crucifix" made from
two 2m aluminium poles, one vertical and one horizontal bolted together at the
centre. The wire is wound round four self-tapping screws sticking out near the
ends of the poles. I ended up using 5 turns which seems to give the best
sensitivity on 136kHz. I can also tune down to 60k at reduced sensitivity,
useful for a quick listen on 73k without retuning the main aerial.
I use two inductors in the preamp box which can be
selected by a switch. One for 60-100kHz and one for 100-150kHz. This gave
better sensitivity on 73k than just padding the C down.
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