Topband: W7IUV preamp info

[Lee K7TJR]

      Here is an interesting tidbit of information about "feedback" types of RF
 amplifiers. Larry mentioned the importance of terminating filters connected
 to the input of his amp. All of the feedback type of RF amplifiers that I am
 aware of have a peculiar problem of changing their input impedance
 depending on what the load impedance is on the output terminal. This
 applies to W7IUV, DXE, probably others, and the Hi-Z Antennas preamp.
 If you are concerned with the amps input impedance it is critical that you
 load the amplifier with whatever impedance the amplifier was meant to
 drive. No, you cannot mix lengths of different impedance cable on the
 output, you cannot drive poorly designed filters, or even split off two
 signals without a proper splitter and expect the amps input impedance to be
 correct. Add a length of cable to this mismatch and the impedances are all
 over the map. Good RX antennas are a System design!
     One can have a little fun with this if you have the ARRL software
 package provided with ON4UNs book. In the LADPAC folder there is
 a program called FBA.exe . You can run this FeedBack Amplifier analysis
 program and put in different values to see what effects changing them has. 

As Larry says "Have Fun"

Lee  K7TJR OR