Is this interference avoidable if using the right gear correctly?

No, the analog signals would overlap.

In this case, KBOO on 90.7 is running HD format, which puts the lower digital shoulder at 90.5 MHz. So this station was operating on top of KBOO, compromising the KBOO signal. 90.5 - 90.6 MHz contained two signals, although the pirate operator didn't know that, as the digital hash sounds like static to the untrained ear.

200 kHz isn't that much. One can easily observe the effect by slightly tuning off radio and still hearing something.

isn’t this the usual spacing of these bands: 200 kHz apart? like 101.3, 101.5, 101.7 were all distinct stations growing up IIRC.

does KBOO have a right to 90.5 being unoccupied? e.g. did they purchase double the spectrum as those higher-frequency stations?

KBOO's signal, centered at 90.7, starts at 90.5 MHz. The edge of the digital HD signal touches 90.500 MHz. By a pirate operating on 90.5, half of the pirate analog signal is directly over the KBOO signal.

KBOO did not purchase spectrum as they are a noncommercial station, and those stations are not subject to application or regulatory fees. KBOO has the same rights to the 200 kHz for analog and the additional digital bandwidth on either side, as any licensed FM station in the US has (expect for perhaps some FM translators).

As for analog vs. analog on adjacent frequencies, they interfere with each other, because there's a little bit of overlap. Analog signals don't have that "cliff" that the band edge that digital signals have. Analog FM can fit right next to digital, because the digital signal stops right at the edge. But the little bit of overlap of a 101.3 signal onto 101.5 is doubled when you consider the little bit of overlap from 101.5 to 101.3. It's significant interference on a receiver.

If you look at the lineup of licensed stations in any one location, you'll notice that stations are spaced by at least one vacant channel. For example, in most cities you'll see an alignment like this: 101.3 (full power), 101.7 (low power or distant), 102.1 (full power), 102.5 (low power or distant).

A full power station can't be in the same vicinity as another full power station for 3 adjacent channels. That is probably overkill, as it's made with consideration for receivers made decades ago that are no longer in use.

> did they purchase double the spectrum as those higher-frequency stations?

Pretty much, yes. There are always guard bands allocated, plus careful calculations to avoid harmonics, intermodulation products, images, etc, which could interfere with other services.

KBOO didn't purchase any spectrum, let alone double spectrum. They're a noncommercial station, which means they're not subject to application or regulatory fees.

They have the rights to what every station on the FM dial has, except possibly FM translators/repeaters: 200 kHz analog signal centered on 90.7, and 100 kHz on each side of that for the HD signal, for a total of 400 kHz. So their signal starts at 90.50 MHz and ends at 90.90 MHz. The 90.5 MHz pirate was operating on top of the licensed 90.7 signal.

(I should note that you're correct about the guard bands with calculations to avoid harmonics, intermodulation products, images, etc)

Not that it's a critical decision point, but KBOO is rather old and well known in some circles. They aren't powerful but they are respected. When the favorite neighborhood watering hole is disrupted, people may speak up.

Yeah, good point. Those people may have complained directly to the FCC, or a donor may have mentioned to KBOO that they can no longer get the station. Action may have originated on the listener level with a station as loved as KBOO.