Scott Fybush said:
Yes, it's a pre-1964 allocation, and yes, so is 92.7 in Arlington Heights. So if the issue were just those two stations, they could move as close to each other as they'd like - even co-located - without breaking the FCC's rules, because those rules say there is no second-adjacent protection between two pre-1964 grandfathered signals.
I did the math on this.. Assuming a transmitter on the John Hancock... and assuming use of contour protection under 73.215...
If 92.7/92.3 were *not* pre-1964 grandfathered signals.. 92.7 would be a problem for this upgrade UNLESS 92.3 downgraded to Class A in the process. As a 2nd-adjacent Class A, 29km minimum separation would be required -- the John Hancock is 40km from the existing 92.7 transmitter site. If 92.3 remained Class B, 67km of separation would be required; if it downgraded to Class B1, 46km would be necessary.
But those two stations operate on a dial that's crowded by plenty of other newer signals. Start with 92.5 in DeKalb: even if it's also pre-1964, which I believe it is, the rules are different for first-adjacent stations. Two grandfathered first-adjacent stations can move, but the net amount of interference between them can't increase. So moving 92.3 into Chicago would have to be balanced out by moving 92.5 somewhere further west to avoid creating any additional interference - and making that move means you start dealing with spacings to plenty of other stations out there beyond Chicago.
And this is the problem; downgrading 92.3 would not loosen the restrictions enough to make it fit. It would, however, come awfully close... The 92.5 transmitter is 94km from the John Hancock. First-adjacent separation requirements, if 92.3 remains Class B, is 145km. If 92.3 downgrades to Class B1, 114km is required. If it downgrades to A, 96km is enough. 92.5 would have to move just over a mile to the west.
**I think** (didn't spend quite as much time on this) that there's nothing -- in the way of other stations -- to prevent 92.5 from making that move. I don't know the lay of the land in DeKalb -- whether there is a plausible transmitter site a mile or so west of the existing site.
====================================
Next, we hit the point where I get over my head.....
To be fully spaced, 92.3 (as a Class A) and 92.5 must be 113km apart. Using 73.215 contour protection, the closest they can be is 96km.
By my reading of 73.215, the 64dBu F(50,50) contour of 92.5 **assuming it's running class-maximum 50kw/150m** may not be intersected by the 58dBu F(50,10) contour of 92.3.
The 64dBu F(50,50) contour of a 50kw/150m station extends 44km.
So the 58dBu F(50,10) contour of 92.3A could not exceed 52km. (or it would intersect 92.5's 64dBu) As a maximum-facility Class A on the John Hancock, 370w/399m, 92.3's 58dBu F(50,10) would extend 34.6km. Without doing anything to reduce its signal in the direction of 92.5, 92.3's signal would come nowhere close to prohibited overlap with 92.5's.
I think I'm missing something in this point.
====================================
Now, the next step... is that the city-of-license would have to change. If 92.3 is to downgrade to Class A, it will no longer be able to deliver a city-grade signal to Hammond. By my calculations, if 92.3 were to share the WXRT antenna (I forget what other stations are also on that antenna right now...) at 399m HAAT, it would be allowed 370 watts ERP. The 70dBu city-grade contour would extend 16.05km. The reference coordinates of Griffith, Indiana (next town southeast of Hammond) are 44.1km from the John Hancock. 92.3 Class A on the Hancock would come nowhere near providing 70dBu across all of Hammond. (I'm not so sure it would provide 70dBu across *any* of Hammond!)
Since there is an AM station (WJOB-1230) licensed to Hammond, the FCC would allow you to change 92.3's city of license. You'd have to find something such that the entire community lies within 16km of the John Hancock. Cicero, Oak Park, Forest Park, Berwyn, Stickney.
Chicago might be the obvious choice, but a Class A station cannot provide a city-grade signal across all of Chicago. The city is too (geographically) large.
(Scott, I forget: would they have to choose a city that doesn't already have at least one station? Of the five I mention above, three have AM stations and one has an FM..)
====================================
If I *am* missing something, about needing to limit the signal to something less than Class A maximum in the direction of 92.5, then things are going to get pretty tight in the city-of-license area. Since all the alternative cities-of-license lie due west of the transmitter, and even with full Class A facilities, they just barely fall within the city-grade coverage. If the signal due west of the transmitter has to be pulled back from full Class A, then it's possible no community exists that falls entirely within the city-grade coverage of the station.
====================================
So...
*MAYBE* they could do it. They would have to:
- Downgrade to Class A.
- Arrange to move 92.5 about 1.2 miles due west.
- Change the city of license to something just slightly west of the Chicago city limits.
- Hope that any power reduction necessary to comply with contour protection doesn't pull in the city-grade contours too tightly to cover even the closest suburbs, leaving them with no available alternate city of license.
Without taking the time to do a full study on WPWX, I'm pretty sure the biggest factors keeping it from moving in would be 92.5 in De Kalb and 91.9 in Joliet. I don't think the Wisconsin stations on 92.1 or 92.5 are insurmountable obstacles, but protecting De Kalb and Joliet probably makes a move into Chicago impossible.
I don't think WJCH is a problem, at least not if 73.215 is used. It's 76km from the Loop and by my figures only 71km is required even if 92.3 remains a Class B. The Wisconsin station on 92.5 is nearly 180km away (it's roughly 25 miles northwest of Milwaukee) with 145km required. 92.1 is closer to being a problem but isn't a problem -- it's almost 100km from the Loop, with 96km required. (the 92.1 transmitter is somewhat to the north of Racine)