160 and 80m high performance preamplifier
Here are some hints and tips for building a high performance antenna
preamplifier for the 160 and 80m bands, which can be used to boost signals of a
K9AY loop, beverage, etc.
The basis for the preamp was a schematic with a 2N5109 transistor, which is a
CATV device offering high signal capability at frequencies till about 1 GHz.(Download
the specs sheet here).In order to handle high signal levels, the
amplifier is operated in class-A configuration with a quite high current drain
(about 70 mA) and some feedback for a perfect linearity. Out of band signals are
filtered in order not to overload the receiver.
See the original
schematic diagram here. This design is with an collector/emitter current of
about 40 mA about. In order to increase the performance of high signal handling
(IP3 claimed as +39 dBm), W7IUV has changed the transistor working parameters to
increase the current to 70 mA about. See the
new parameters here. I suggest you copy these to the original design.
Some hints and tips :
- With the current of 70mA, the transistor needs to be cooled (a small
'star' shaped clip-on heatsink is suitable)
- It is best to place the preamp immediately at the antenna feed: at 1.6
or 3.8 MHz a lenght of coax will indeed not attenuate signals very much
under ideal conditions, with 50 Ohm in/out (thus low SWR). However, if SWR
is not optimal (K9AY loop? preamp input?) losses will deteriorate the
signals, so it is always better to connect the preamp input directly to the
antenna (off course through a suitable TX/RX relay for protection).
- The preamp needs to be powered by 12v which must be stabilized and
'RF-clean'. I initially used a µA7812 regulator with the classic
tantalum condensers on the in- and output, but my preamp was extremely noisy
on 160m! It was necessary to choose a µA7812 from another manufacturer and
insert a ferrite choke coil on the output and add ceramic capacitors
to have a perfectly clean power.... When the preamp is online, and no
antenna connected, you should heard the noise floor rise slightly in the
background, but the S-meter of your transceiver should not deflect at
all - otherwise there is really something wrong!
Some modifications ...
The preamp gain can be increased further by about 3 to 5 dB by decoupling the
emitter directly to ground (with the 100nF), but this is at risk of
oscillations!
I tested the performance of the preamp with a spectrum analyser.
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The original performances (with emitter directly
decoupled to ground). The horizontal line is the signal with the preamp
OFF., the curve with preamp ON.
Vertical scale = 10 dB / division, center frequency = 3 Mhz,
horizontal scale = 500 kHz / division.
Observe the smooth roll off past 4 MHz (at 4.5 MHz gain is unity) and
the sharp cutoff below 1.6 MHz.
It is a pity that there is a 'bump' around 1.8 Mhz.
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The pass band with slightly modified input filter (in
the BCB filter, the 1.500 pF capacitor in line to ground with the 8,2µH
was increased to 8.200 pF)
Now roll off on the high end past 3.8 MHz, on the low end sharp cut
off past 1.75 MHz, and no more 'bump'. Gain on 160m peaking to 20 dB, on
80m = 15 dB
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