Joe Nicholson wrote in a message to Phil Marlowe:

 -=> Quoting Phil Marlowe to Leonard Erickson <=-

 > How far away are these stations? Any idea where their *transmitters* 
 > are?

 PM> I'd guess less than 100 miles? Probably around 50-60.

 JN>  When the FCC was passing out station licenses in the late 40's 
 JN> through the 50's, the theoretical maximum distance for channels 
 JN> 2-6 was 100 miles, and for channels 7-13, only 50 miles.  That's 
 JN> the basis for not assigning same channels and adjacent channels 
 JN> within so many miles of each other.

That,  and the fact that early tv sets weren't as selective in their front
ends as later stuff was.  Cable kinda proved what would or wouldn't work in
that regard,  as they used all sorts of stuff adjacent to each other.

 JN> (note: channels 4 and 5 are not adjacent, thus 4 and 5 both are 
 JN> licensed in Los Angeles.)

And in New York.  I don't know much about what's happening on UHF there, 
as that was a fairly recent thing around the time I moved away,  but active
VHF channels were 2, 4, 5, 7, 9, 11, and 13.  I know there's 3, 6, and 10
in Philly,  probably the closest major metro area,  dunno about in other
directions.  Around here it's mostly UHF for some reason,  with them using
8, 15, 21, 27, 33, and 43.  There are occasional very faint images picked
up elsewhere,  like 49 and somewhere up in the 60s but I have no idea where
those are. 

 JN>  Well, lo and behold, people in San Diego were receiving L.A.'s 
 JN> channels, and on good days even channel 3 which is 215 miles away 
 JN> in Santa Barbara.

Sometimes people around here with a good high roof antenna can pick up
Baltmore stations.  I think there's too much terrain between here and
Philly to get much,  though I've very occasionally seen a faint image on
one of the VHF channels.

 JN>  Someone had not taken into consideration inversion layers which 
 JN> exist over some portions of the U.S.  They act as ducts, funneling 
 JN> signals hundreds of miles away.

Are these only in specific locations,  or what?

 JN>  (I have watched a channel 2 out of Texas after channel 2 in L.A. 
 JN> went off the air at night.)

 JN>  Most public service (police/sheriff/fire) was dispatched on 2490kc
 JN> and the mobile radios were transmitting somewhere in the 4Mhz gap 
 JN> between channels 4 and 5 -- and causing quite a bit of interference
 JN> on TV.  But the FCC stuck by its guns and said San Diego was beyond
 JN> the range of L.A.'s stations and they were not guaranteed to be
 JN> free  of interference.  (SDPD was the last agency to leave 2490, in
 JN> summer  1962.  Everyone by now was on either lo-band or hi-band
 JN> VHF.)

I can still remember some black-and-whites out there in the late 1960s that
had those big whip antennas bolted to the rear bumper.  Don't remember who
was using them,  though.

 JN>  Armchair cops could receive dispatch on their AM radio by tuning
 JN> to the image frequency, and hear the units in the field by tuning
 JN> a TV to either channel 4 or 5.  (TV and mobile units both FM)

 JN>  Sidebar: In summer 1973, I spent 90 days active duty as a 
 JN> reservist at NavyMARS in San Diego.  During that time, we received 
 JN> thousands of LAPD mobile radios which the RMC "thought" could be 
 JN> used by MARS.  He hadn't realized they were old, old, old 2490kc AM
 JN> receivers and 72-75Mhz FM transmitters and were worthless to hams
 JN> (or anyone else). 

Hm.  No way to easily modify that stuff,  eh?

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