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 :


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.

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.


The pass band with slightly modified input filter (in the BCB filter, the 1.500 pF capacitor in line to ground with the 8,2H 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