I'm going to use this spot to record the questions which come up for me regarding the FCC's LPFM Channel finder and whatever answers I learn.

What's that Direction?

I don't understand when the FCC's finder gives a bearing in degrees quite what it means. The FCC sometimes uses what they call "biased" latitude and longitude in their code and database to simplify certain calculations, and their (old anyway) source code to compute azimuth produces North as 0 degrees but positive degrees go counterclockwise (more like math, less like compasses). It has been easy for me to mess up bearings myself since I'm leveraging FCC code and database.

Anyway a good example is to search in Richmond, VA and look at the direction toward WVKL in Norfolk, which is close to Southwest of Richmond, which would give a bearing near 135 degrees or if it was reversed, 225 degrees.  FCC calls it 305 degrees.

This isn't a big deal for a channel finder's accuracy until one tries to orient the keyhole-shaped translator input protection regions.

Protect that Translator Construction Permit?

Well there are a ton of twisty things I've learned, and more yet to learn, about how to interpret the FCC's CDBS database.  Here's an issue which is either a problem, or my misunderstanding -- a missing translator.  Looking at the Richmond search again (see above), FCC suggests that 96.1MHz may be available, while RFree and myLPFM don't.  That's because, looking at the stations the FCC considered, their channel finder protected the licensed W295BF translator, but not the CP application for that translator.  Maybe that's because the CP is being treated as a major modification and thus subject to the upcoming translator dismissal process or something similarly subtle.  I don't know if this is a systemic behavior or just this one case.

Translator Input Protection?

I was looking for a place to test how the FCC's channel finder was dealing with translator input protection, which is something relatively new and which has frankly been a pain to implement, and I'm not finished with it yet.  One good test location I found is near Grants Pass, OR (42° 29' 20", 123 18' 21" (NAD-27)) which is much closer than 2km to translator K221CP, which is third adjacent to channel 242 or 94.3MHz.  I expected the FCC to exclude 94.3 from their list of possible channels since the translator is so close.  There are some additional ways, besides the keyhole-shaped protection zone, to prove non-interference, so technically I think their channel finder is correct, however I suspect they may just not be doing translator input protection yet, and will try to find an acid test another time, but it's a difficult task in part due to what appears to be significant ambiguous and missing data in the FCC database.

Kudos to myLPFM which at the same location gives a solid warning for 94.3!  RFree currently calls this a "yellow" channel, and it may turn "red" depending on further clarifications from the FCC.



The times they are a-changin'

Since I wrote the post above, stations and the FCC have been hard at work and the state of the world has changed and some of what I wrote is now not so well-proven.

The What's that Direction? piece is still true -- FCC's channel finder gives the direction to WVKL as 305 (and 313) degrees.

As for Protect that Translator CP, that CP no longer exists, and so I'd have to find another example to figure out what's happening.

Translator Input Protection is, I am fairly certain, still not being done by the FCC's channel finder.  I just re-wrote that stuff in RFree and doing it anywhere close to right is quite difficult.  The Grants Pass example is not quite a fun as it used to be.  Formerly the FCC channel finder offered 94.3 as a "green" no-waiver-required channel, but now it is offered with a waiver possibly needed -- same as RFree.  I think the change may have to do with a shift in KRRM in the interim, which now requires an LPFM waiver on 94.3 and didn't before.

This example is even more interesting... It turns out that running 100 watts at 30m HAAT at that location within translator K221CP's input-protection region (the LPFM is far less than 1km from the translator), not only violates the input-protection region, it also produces a U/D at the translator input which is not permitted either (see 73.827).  Aha I thought, the FCC is definitely not checking translator inputs!

But then I noticed something else.  30m HAAT at that location requires an underground antenna because of the nearby terrain.  When I put the antenna at a sane 20m AGL, the HAAT became 406m, and the transmitter power reduced to around 1w... and at 1w the U/D at the translator input is just fine and a waiver would presumably be granted.

So maybe the FCC channel finder had this subtle angle right all along...