Rubbish. Varactors are not used for freqency mulitplication.
Google"varactor multipliers", still used. Popular technique in
the 60's for getting 70 cm output from 2m TX.
As osc control keep RF across diode low, high levels produce harmonics
and reduce tuning range. Most tuning varieties have max V = 30.
At low freqs power diodes work ok and have high V rev.