I had a few LM394CN left and used one for the pre-amp.
I would try to use another low-noise transistor. Furthermore, you do not
necessarily need a matched pair. The transistor on the right in the
schematic is merely used for biasing the amplifying transistor on the
left. The prices for matched pairs can get ridiculously high, so I too
would try to look for alternatives. If you still want a matched pair,
there's the SSM2212 from Analog Devices, which is priced lower than the
MAT12. You may also be able to find the earlier SSM2210, which now is
obsolete. Another alternative is the THAT300 from THAT Corporation,
which has 4 low-noise NPNs in a 14-pin DIP (great for experiments...)
Useful single NPN transistors I can think of would likely be the BC337
or perhaps the 2SC3324 which both have low Rbb, and they are quite cheap.
The "Ca. 3 Ohm" was indeed the estimated input resistance, including the
series resistance of the input capacitor as I recall.
Best regards
Ole
Den 17-07-2015 kl. 22:49 skrev Dr. David Kirkby (Kirkby Microwave Ltd):
On 16 July 2015 at 08:05, Ole Stender Nielsen <[email protected]> wrote:
I use a home-made untuned loop antenna with 4 windings of 2.5 mm2
insulated wire on a 80 x 80 cm wooden frame, and with a grounded base
pre-amplifier mounted on the antenna frame. A schematic is enclosed for you
to copy.
The pre-amplifier is powered through the cable, and loads the FS700 input
as required.
I live about 290 km from the island of Sylt, and get nice noise margin
figures from the FS700, normally about 40 dB, often up to 46 dB.
A couple of questions
1) Do you have any suggestions for a replacement for the LM394CN, which is
obsolete and unobtainable from any reputable source? There are plenty on
eBay for a few $'s from China, but the probability of them being fakes is
greater than 0.99. The MAT12 seems to be one possible candidate for a
replacement and while not cheap, is available from reputable sources like
Farnell.
2) What is the "Ca. 3 Ohm" to the left of your circuit? Is that what you
estimate the input impedance is? I've got 95 m of 2.5 mm^2 wire. The
resistance of that is about 7.41 mOhm/m so my 95 m would have a DC
resistance of around 7 Ohms if I used it all.
I have built the loop 1.0 x 1.2 m. Hopefully that will be ok to receive at
least one or both of
* Lessay, France, power = 250 kW, distance = 321 km, bearing = 211 degrees.
* Anthorn, England, power =- 250 kW, distance = 419 km, bearing 331 degrees.
I now need to work out how many turns to put on it.
Dave
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