Preamplifier, assembled
I have built an RF preamplifier using the W7IUV design [504] This is one of the most popular and easy to build amplifiers, with a 20 dB gain, high dynamic range and incredible tolerance to overloading. The author claims that this preamp functions as a protective circuit itself and does not need a T/R switch or back to back diodes at the input.
The amp was built on a prototyping board and placed in an aluminum case.
As we will see later, a metal case is a must for a device as sensitive as this one.
The amplifier gain, measured with HamVNA
[502]
, was 21 dB at the low end of the HF band and dropped to 17.5 dB at
the high end, which is a little bit lower than specified by the author - probably
due to less then optimal board layout, since I used all components
recommended by the author.
The input impedance of the amplifier, measured with a 50-Ohm load connected to
its output, was close enough to 50 Ohms, as can be seen on the chart below.
The
transformer described in the previous
section, inserted in front of the amplifier, turned the input impedance into
550-600 Ohms across the whole 1-30 MHz frequency range.
