A modification / combination of the DJ9BV & I5TDJ
144 MHz preamp designs
The original I5TDJ design used a rather long coaxial input design. This was rather awkward to
put in my preamp box at the feed of my dish. I decided to use the DJ9BV helical input design instead.
Original designs were published in Dubus:
I5TDJ design: Dubus 3/94
DJ9BV design: Dubus Technik IV book
The preamp is shown here. Note that the number of windings of the input helical has been reduced with half a winding. Source inductance of about 2mm on each source leg of the MGF1801.
Another view of the preamp. Output circuit is the original I5TDJ design. Bias circuit is adapted to the parts in my junk box.
The result of the NF measurement. Something 0.25dB NF according to my home-brew DJ9BV noise figure meter.
Gain plot of the 144 MHz preamp. The simple input circuit has a gain drop of 10dB at 175 MHz and of 22 dB at 98 MHz.
This is better than many other designs. Thanks to Jack Smeets, PE1KXH for performing the measurements.
Plot of the preamps gain.
The I5TDJ schematic
The parts list for the I5TDJ preamp
Mechanical design for the DJ9BV design.