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One way to think of it is that the resonator has a narrow bandwidth, so for a given available Q (which is a given when you select your media, such as around 30 for microstrip, etc), the bandwidth that the resonator is trying to restrict the oscillator's operation to gets bigger. Q = BW/f0 So, when f0 goes up, your bandwidth goes up, so your oscillator noise bandwidth goes up too.
You can read some of the older papers by Leeson to understand where that phasenoise equation comes from.
If you go higher in frequency, to have the same phase noise you would need a somewhat better resonator Q, or a lot bigger oscillation power.
Do you mean the phase noise of VCO or LO divided by VCO?I think if VCO frequency is double,the divider will double,so the phase noise of LO will improve 6dBc/Hz from Bessel theory.
He may means the phase noise of VCO slopes with 20dBc/decade(6dbc/oct). But in fact, it slopes with 30dbc/decade for low frequency and 20dbc/decade for medium frequency. You can refer to Hajimiri‘s paper.
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