Without any doubts, the best preamplifier is a good antenna (antenna system) with low loss cable. This is the only way of obtaining sufficient RF amplification (by sufficient antenna gain) with the IP3 of the receiver kept constant. Nevertheless in some situations the antenna preamplifier is a good thing for VHF contests – particularly in situations when the receiving conditions are not worsened by a signal from close „top-gun“ station.
In my article, I will describe you my realization of a preamplifier designed by Dragoslav YU1AW.
My requirements on the LNA could be summarized into these points:
● dB(s21) @ 100MHz <= dB(s21) @ 144MHz -20dB
○ In other words the gain in the FM radio band is at least 20dB lower than in the 2m band.
● 25dB >= dB(s21) @ 144MHz >= 12dB
○ That means that the gain in the 2m band is within 12 and 25dB. The upper limit is set to
25dB to lower the risk of oscillations and not to overdrive the receiver input. The lower
limit is set to 12dB with regards to the cable loss.
● To put more emphasis on IP3 than on NF:
○ „higher“ IP3 (let's say IP3OUT >= 30dB)
○ NF <= 1,8dB
● Good availability and price(<100Kč) of the transistor
An optimal solution that suites these requirements uses BFG540 and was designed by Dragoslav
All the documentation is available at: http://yu1aw.ba-karlsruhe.de/low_noise.htm.