1: Use an RF transistor...2N5179 or equivalent (the 3904 has an ft of 300MHz, so a gain of 3 at 100MHz)
2: Use higher resistance divider for bias for Q3 (what you have is probably saturated) a couple of 100K would do better.
3: Use small coupling cap for antenna to minimize load pulling.
I built one as a baby monitor many years ago and used a logarithmic compressor on the audio so we could hear breathing but would not be blown out of the room when a plane flew overhead. It showed very little drift.
2: Use higher resistance divider for bias for Q3 (what you have is probably saturated) a couple of 100K would do better.
3: Use small coupling cap for antenna to minimize load pulling.
I built one as a baby monitor many years ago and used a logarithmic compressor on the audio so we could hear breathing but would not be blown out of the room when a plane flew overhead. It showed very little drift.
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Ragarding 2, I am aware of the close to saturation bias point, it is 8.63V at the base with a 9.09V supply. I was planning to change that too but it did not appear to be a major issue since it works but only with small audio inputs (that don't exceed around 0.3 Vrms).
I will try a coupling cap for the antenna too. As far the transistor is concerned, I will see if I can get it here. 🙂
The transistor is not the main problem, any standard si-type will do. In the old days I built an fm transmitter with 2xBC109 that covered a radius of 1~2km with a dipole antenna and 0.6W input power.