Interference Between Distant TV Frequencies?

[Mark_Ericson]

I have a question.

I talked to an engineer at a TV station near me who informed me that the reason their station could not move any closer is due to channels 24 and 38 not being to co-exist. In addition, reading a thread on the Phoenix TV board, K53GF did a study and decided against channel 19 because of too much interference from KTVW-33.

Does anyone have a list of all the frequencies that cannot peacefully co-exist? I'm certain there's a list at the FCC site somewhere (why wouldn't there be?) but I can't find it.

- Trip<P ID="signature">______________
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[KeithE]

> I have a question.
>
> I talked to an engineer at a TV station near me who informed
> me that the reason their station could not move any closer
> is due to channels 24 and 38 not being to co-exist. In
> addition, reading a thread on the Phoenix TV board, K53GF
> did a study and decided against channel 19 because of too
> much interference from KTVW-33.

> Does anyone have a list of all the frequencies that cannot
> peacefully co-exist? I'm certain there's a list at the FCC
> site somewhere (why wouldn't there be?) but I can't find it.
>
>
> - Trip
>

<a target="_blank" href=http://a257.g.akamaitech.net/7/257/2422/12feb20041500/edocket.access.gpo.gov/cfr_2004/octqtr/47cfr73.698.htm>Here's the table of no-no's from CFR 47.698.</a> It lists channels where interference can happen. Channel 24 can put a sound image on Channel 38 and vice-versa. Same goes for Chs. 19 and 33. It's a plain-text file.

<a target="_blank" href=http://a257.g.akamaitech.net/7/257/2422/12feb20041500/edocket.access.gpo.gov/cfr_2004/octqtr/pdf/47cfr73.698.pdf>Here is a pdf file of the same data.</a>

 

[Garrett]

> I have a question.
>
> I talked to an engineer at a TV station near me who informed
> me that the reason their station could not move any closer
> is due to channels 24 and 38 not being to co-exist. In
> addition, reading a thread on the Phoenix TV board, K53GF
> did a study and decided against channel 19 because of too
> much interference from KTVW-33.
>
> Does anyone have a list of all the frequencies that cannot
> peacefully co-exist? I'm certain there's a list at the FCC
> site somewhere (why wouldn't there be?) but I can't find it.
>
>
> - Trip
>
Well, I've not seen a market where there was a 3 and a 4, and I have seen a market with 6 and a 7, or a 5 and a 6. But this could just be luck of the draw...

 

[KMRichards]

> > I have a question.
> >
> > I talked to an engineer at a TV station near me who
> informed
> > me that the reason their station could not move any closer
>
> > is due to channels 24 and 38 not being to co-exist. In
> > addition, reading a thread on the Phoenix TV board, K53GF
> > did a study and decided against channel 19 because of too
> > much interference from KTVW-33.
>
> > Does anyone have a list of all the frequencies that cannot
>
> > peacefully co-exist? I'm certain there's a list at the
> FCC
> > site somewhere (why wouldn't there be?) but I can't find
> it.
> >
> >
> > - Trip
> >
>
> Here's the table of no-no's from CFR 47.698. It lists
> channels where interference can happen. Channel 24 can put
> a sound image on Channel 38 and vice-versa. Same goes for
> Chs. 19 and 33. It's a plain-text file.

There's a different way of calculating some of that under DTV, as I recall; for example, adjacent channels can be used if they are at the same tower location.
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[KMRichards]

> Well, I've not seen a market where there was a 3 and a 4,
> and I have seen a market with 6 and a 7, or a 5 and a 6. But
> this could just be luck of the draw...

He means UHF separations other than adjacent, Garrett.

It is well known that on the VHF band, the only adjacent channels that can be used in the same area are 4/5 and 6/7.
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[KeithE]

> Well, I've not seen a market where there was a 3 and a 4,
> and I have seen a market with 6 and a 7, or a 5 and a 6. But
> this could just be luck of the draw...

Channels 4 & 5 are not adjacent. Neither are 6 & 7 or 13 & 14. They don't pose a problem being located in the same city.

Some cities now have LPTVs or Class-A's on adjacent channels to a full-powered station (both analog). I don't know how they make that work. Here in Phoenix, we have an LPTV on Channel 44, adjacent to full-powered KUTP/45. Being so close to the big South Mountain towers (I'm about 5 miles away), 44 is unviewable in the part of the city where I live.

 

[Mark_Ericson]

> There's a different way of calculating some of that under
> DTV, as I recall; for example, adjacent channels can be used
> if they are at the same tower location.

I was asking about analog, but an FYI, for digital, adjacent channels can be used as long as they are within 10-15 miles or so of each other. But no adjacents without protection for 80 miles if not within the 10-15.

Don't quote me on that, but that's what I recall hearing.

- Trip<P ID="signature">______________
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[Markxxx]

> > Well, I've not seen a market where there was a 3 and a 4,
> > and I have seen a market with 6 and a 7, or a 5 and a 6.
> But
> > this could just be luck of the draw...
>
> He means UHF separations other than adjacent, Garrett.
>
> It is well known that on the VHF band, the only adjacent
> channels that can be used in the same area are 4/5 and 6/7.
>
I often wonder how it works in the Duluth area.

I used to visit Hibbing and clearly got Channels 10, and 13. WIRT, Hibbing is a translator of WDIO In Duluth.

Recently Channel 6 opened up a full power translator on Channel 11. (Channel 11 wa originally assinged to Int'l Falls but moved to Chisholm, a few miles from Hibbing.)

I wonder if Channel 10 and 11 interfere with each other in Hibbing? I realize the transmitter must be located north of Chisholm, but considering how clear Channel 10 comes into Hibbing, I wonder
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[clichemoth]

How does Birmingham/Anniston have so many stations close together?

We have a tiny LP in Moody on 38, WJSU running 316Kw from Bald Rock Mountain in St. Clair Co. on 40, WIAT running 5000Kw from Red Mountain on 42, WPXH running 5000Kw from a tower near Highland Lake in Blount Co. on 44, and a CA in Sylacauga on 47.

40 and 44, in particular, have to be either too close together for the intermodulation rules or crammed in as close as physically possible. I've driven past them both on many occasions and they simply don't seem to be 19 miles apart.

Not to mention, there's a Gadsden translator for WBRC-6 on 29, which would be too close to 44 even now. And it existed prior to 44's move to Highland Lake, and they would have been even closer then.

 

[KMRichards]

Re: How does Birmingham/Anniston have so many stations close together?

> We have a tiny LP in Moody on 38, WJSU running 316Kw from
> Bald Rock Mountain in St. Clair Co. on 40, WIAT running
> 5000Kw from Red Mountain on 42, WPXH running 5000Kw from a
> tower near Highland Lake in Blount Co. on 44, and a CA in
> Sylacauga on 47.
>
> 40 and 44, in particular, have to be either too close
> together for the intermodulation rules or crammed in as
> close as physically possible. I've driven past them both on
> many occasions and they simply don't seem to be 19 miles
> apart.
>
> Not to mention, there's a Gadsden translator for WBRC-6 on
> 29, which would be too close to 44 even now. And it existed
> prior to 44's move to Highland Lake, and they would have
> been even closer then.

Simply put, the intermod rules are different for LPTVs and Class As than for full-power stations.

I'll give you an even stranger example: In Los Angeles, full-power KCET/28 at 2450kW co-exists with KNLA-LP/27 at 9kW, and they are both on the same mountain.
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[clichemoth]

Re: How does Birmingham/Anniston have so many stations close together?

> > We have a tiny LP in Moody on 38, WJSU running 316Kw from
> > Bald Rock Mountain in St. Clair Co. on 40, WIAT running
> > 5000Kw from Red Mountain on 42, WPXH running 5000Kw from a
>
> > tower near Highland Lake in Blount Co. on 44, and a CA in
> > Sylacauga on 47.
> >
> > 40 and 44, in particular, have to be either too close
> > together for the intermodulation rules or crammed in as
> > close as physically possible. I've driven past them both
> on
> > many occasions and they simply don't seem to be 19 miles
> > apart.
> >
> > Not to mention, there's a Gadsden translator for WBRC-6 on
>
> > 29, which would be too close to 44 even now. And it
> existed
> > prior to 44's move to Highland Lake, and they would have
> > been even closer then.
>
> Simply put, the intermod rules are different for LPTVs and
> Class As than for full-power stations.
>
> I'll give you an even stranger example: In Los Angeles,
> full-power KCET/28 at 2450kW co-exists with KNLA-LP/27 at
> 9kW, and they are both on the same mountain.
>

Both 40 and 44 are full power stations. But that LA example is odd, wouldn't KCET destroy KNLA-LP, at least if they were in one another's line of sight? Now, if they're separated by the peak of the mountain, I could see it possibly working.

 

[bpatrick]

Re: How does Birmingham/Anniston have so many stations close together?

> > We have a tiny LP in Moody on 38, WJSU running 316Kw from
> > Bald Rock Mountain in St. Clair Co. on 40, WIAT running
> > 5000Kw from Red Mountain on 42, WPXH running 5000Kw from a
>
> > tower near Highland Lake in Blount Co. on 44, and a CA in
> > Sylacauga on 47.
> >
> > 40 and 44, in particular, have to be either too close
> > together for the intermodulation rules or crammed in as
> > close as physically possible. I've driven past them both
> on
> > many occasions and they simply don't seem to be 19 miles
> > apart.
> >
> > Not to mention, there's a Gadsden translator for WBRC-6 on
>
> > 29, which would be too close to 44 even now. And it
> existed
> > prior to 44's move to Highland Lake, and they would have
> > been even closer then.
>
> Simply put, the intermod rules are different for LPTVs and
> Class As than for full-power stations.
>
> I'll give you an even stranger example: In Los Angeles,
> full-power KCET/28 at 2450kW co-exists with KNLA-LP/27 at
> 9kW, and they are both on the same mountain.
>
I know practically zilch about LPTVs and Class As but I do
know that Channel 42 in Birmingham and Channel 40 in Anniston
both came on the air in the '60s, and neither put an OTA signal
into the other city. In other words, I lived in Birmingham and
could never get 40; I had a girlfriend in Anniston who could never
get 42 (nor 13 from Birmingham nor 11 from Atlanta--she got NBC
on Atlanta's Channel 2, CBS on Atlanta's Channel 5 and Anniston's
Channel 40, and ABC on Birmingham's Channel 6). It's probably due
to any real overlap between 40 and 42 that both became CBS affiliates
in 1970. (40's going to ABC was probably more a marriage of
convenience between the network and Allbritton when 6 went to Fox.)

Channel 44 is in Gadsden, about 50 miles or so from Birmingham,
so that doesn't short-space 42; I think it's about the same distance
from Anniston, so 40 isn't short-spaced either.

 

[clichemoth]

Re: How does Birmingham/Anniston have so many stations close together?

> > > We have a tiny LP in Moody on 38, WJSU running 316Kw
> from
> > > Bald Rock Mountain in St. Clair Co. on 40, WIAT running
> > > 5000Kw from Red Mountain on 42, WPXH running 5000Kw from
> a
> >
> > > tower near Highland Lake in Blount Co. on 44, and a CA
> in
> > > Sylacauga on 47.
> > >
> > > 40 and 44, in particular, have to be either too close
> > > together for the intermodulation rules or crammed in as
> > > close as physically possible. I've driven past them
> both
> > on
> > > many occasions and they simply don't seem to be 19 miles
>
> > > apart.
> > >
> > > Not to mention, there's a Gadsden translator for WBRC-6
> on
> >
> > > 29, which would be too close to 44 even now. And it
> > existed
> > > prior to 44's move to Highland Lake, and they would have
>
> > > been even closer then.
> >
> > Simply put, the intermod rules are different for LPTVs and
>
> > Class As than for full-power stations.
> >
> > I'll give you an even stranger example: In Los Angeles,
> > full-power KCET/28 at 2450kW co-exists with KNLA-LP/27 at
> > 9kW, and they are both on the same mountain.
> >
> I know practically zilch about LPTVs and Class As but I do
> know that Channel 42 in Birmingham and Channel 40 in
> Anniston
> both came on the air in the '60s, and neither put an OTA
> signal
> into the other city. In other words, I lived in Birmingham
> and
> could never get 40; I had a girlfriend in Anniston who could
> never
> get 42 (nor 13 from Birmingham nor 11 from Atlanta--she got
> NBC
> on Atlanta's Channel 2, CBS on Atlanta's Channel 5 and
> Anniston's
> Channel 40, and ABC on Birmingham's Channel 6). It's
> probably due
> to any real overlap between 40 and 42 that both became CBS
> affiliates
> in 1970. (40's going to ABC was probably more a marriage of
> convenience between the network and Allbritton when 6 went
> to Fox.)
>
> Channel 44 is in Gadsden, about 50 miles or so from
> Birmingham,
> so that doesn't short-space 42; I think it's about the same
> distance
> from Anniston, so 40 isn't short-spaced either.
>

The COLs do not represent the actual tower locations in the cases of 40 and 44. Both of them have moved to new sites closer to Birmingham since 1996 (in the locations I mentioned) in order to serve it as well.

And, yeah, from where I'm at, I can't get 42 for beans. And I'm not close enough to Atlanta to get CBS 46 except during insanely awesome tropo. I can practically see 40 just by looking out the window (well, not really, but I don't need an antenna at all). However, before the tower move, both 40 and 42 came in equally watchable.

 

[bpatrick]

Re: How does Birmingham/Anniston have so many stations close together?

> > > > We have a tiny LP in Moody on 38, WJSU running 316Kw
> > from
> > > > Bald Rock Mountain in St. Clair Co. on 40, WIAT
> running
> > > > 5000Kw from Red Mountain on 42, WPXH running 5000Kw
> from
> > a
> > >
> > > > tower near Highland Lake in Blount Co. on 44, and a CA
>
> > in
> > > > Sylacauga on 47.
> > > >
> > > > 40 and 44, in particular, have to be either too close
> > > > together for the intermodulation rules or crammed in
> as
> > > > close as physically possible. I've driven past them
> > both
> > > on
> > > > many occasions and they simply don't seem to be 19
> miles
> >
> > > > apart.
> > > >
> > > > Not to mention, there's a Gadsden translator for
> WBRC-6
> > on
> > >
> > > > 29, which would be too close to 44 even now. And it
> > > existed
> > > > prior to 44's move to Highland Lake, and they would
> have
> >
> > > > been even closer then.
> > >
> > > Simply put, the intermod rules are different for LPTVs
> and
> >
> > > Class As than for full-power stations.
> > >
> > > I'll give you an even stranger example: In Los Angeles,
>
> > > full-power KCET/28 at 2450kW co-exists with KNLA-LP/27
> at
> > > 9kW, and they are both on the same mountain.
> > >
> > I know practically zilch about LPTVs and Class As but I do
>
> > know that Channel 42 in Birmingham and Channel 40 in
> > Anniston
> > both came on the air in the '60s, and neither put an OTA
> > signal
> > into the other city. In other words, I lived in
> Birmingham
> > and
> > could never get 40; I had a girlfriend in Anniston who
> could
> > never
> > get 42 (nor 13 from Birmingham nor 11 from Atlanta--she
> got
> > NBC
> > on Atlanta's Channel 2, CBS on Atlanta's Channel 5 and
> > Anniston's
> > Channel 40, and ABC on Birmingham's Channel 6). It's
> > probably due
> > to any real overlap between 40 and 42 that both became CBS
>
> > affiliates
> > in 1970. (40's going to ABC was probably more a marriage
> of
> > convenience between the network and Allbritton when 6 went
>
> > to Fox.)
> >
> > Channel 44 is in Gadsden, about 50 miles or so from
> > Birmingham,
> > so that doesn't short-space 42; I think it's about the
> same
> > distance
> > from Anniston, so 40 isn't short-spaced either.
> >
>
> The COLs do not represent the actual tower locations in the
> cases of 40 and 44. Both of them have moved to new sites
> closer to Birmingham since 1996 (in the locations I
> mentioned) in order to serve it as well.
>
> And, yeah, from where I'm at, I can't get 42 for beans. And
> I'm not close enough to Atlanta to get CBS 46 except during
> insanely awesome tropo. I can practically see 40 just by
> looking out the window (well, not really, but I don't need
> an antenna at all). However, before the tower move, both 40
> and 42 came in equally watchable.
>
You're far more up to date on this than I am; I haven't lived
in Alabama since the '70s and was describing the situation that
existed 1969-73, before either my friend or I had cable.

I'm not sure you can pick up CBS46 Atlanta until you get to
Heflin; their cable system, I know, carries it and 11 Alive.
I somehow doubt that 46 comes in OTA there, though.

 

[clichemoth]

Re: How does Birmingham/Anniston have so many stations close together?

>
> I'm not sure you can pick up CBS46 Atlanta until you get to
> Heflin; their cable system, I know, carries it and 11 Alive.
>
> I somehow doubt that 46 comes in OTA there, though.
>

They should carry all the Atlanta stations, not just those two. Clay and Cleburne counties are in the Atlanta market.

As for getting 46 OTA there, I don't remember. The only time I've ever operated a TV out that way was on the bus for a basketball game against Ranburne like 10 years ago. The Atlanta VHFs and 14 came in fine, but I don't know about 17/30/34/46/69.

 

[KMRichards]

Re: How does Birmingham/Anniston have so many stations close together?

> > I'll give you an even stranger example: In Los Angeles,
> > full-power KCET/28 at 2450kW co-exists with KNLA-LP/27 at
> > 9kW, and they are both on the same mountain.
> >
>
> Both 40 and 44 are full power stations. But that LA example
> is odd, wouldn't KCET destroy KNLA-LP, at least if they were
> in one another's line of sight? Now, if they're separated
> by the peak of the mountain, I could see it possibly
> working.

I'm not sure how it's done (perhaps by channel offset) but with an external antenna pointed at Mount Wilson, about 35 miles away, I can get both distinctly with no adjacent channel interference.
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[Mark_Ericson]

> I often wonder how it works in the Duluth area.
>
> I used to visit Hibbing and clearly got Channels 10, and 13.
> WIRT, Hibbing is a translator of WDIO In Duluth.
>
> Recently Channel 6 opened up a full power translator on
> Channel 11. (Channel 11 wa originally assinged to Int'l
> Falls but moved to Chisholm, a few miles from Hibbing.)
>
> I wonder if Channel 10 and 11 interfere with each other in
> Hibbing? I realize the transmitter must be located north of
> Chisholm, but considering how clear Channel 10 comes into
> Hibbing, I wonder

Realize that just because a station is clear doesn't mean it's strong. The opposite is true also. Before WFXR-DT signed on channel 17, I used to get WNCN-17 out of Raleigh all the time. Now I only get it during tropo. And WFXR-DT is only at about 3 kW (out of a possible 400 kW). Likewise, I can't get WPXR-38 analog clearly, but I can't make it go away either (except when WUVC-DT 38 is coming in via tropo).

VHF stations have to be 65 miles apart adjacent. KRII-11 and WDIO-10 meet that requirement (else they wouldn't be there). I assume the only place they have interference is about half-way between, and there it doesn't matter since WIRT-13 and KBJR-6 should be receivable should 10 and 11 harm each other. If KDLH wanted to put a channel 9 (I don't know if that would work, but it's an example) in Hibbing or Chisholm or Virginia or any of those towns, it too would be safe. If their channel 9 was harmed by WDSE-8 in Duluth, people could tune to KDLH-3. If WDSE-8 was not watchable due to their channel 9, watch WRPT-DT 31.

Now the market I've always wondered about was Monroe, LA/El Dorado, AR. They have KTVE-10 as their NBC and KAQY-11 as their ABC. IN THE SAME MARKET (they do make the 65-mile separation). Though to be fair, the southern half of the market can probably get NBC from KALB-5 in Alexandria.

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[fybush]

The magic answer

> > Both 40 and 44 are full power stations. But that LA
> example
> > is odd, wouldn't KCET destroy KNLA-LP, at least if they
> were
> > in one another's line of sight? Now, if they're separated
>
> > by the peak of the mountain, I could see it possibly
> > working.
>
> I'm not sure how it's done (perhaps by channel offset) but
> with an external antenna pointed at Mount Wilson, about 35
> miles away, I can get both distinctly with no adjacent
> channel interference.

Correct. And if you think about it, when your TV set is plugged into cable, it has no problem at all with multiple adjacent channels all stacked up next to each other.

Why is that? Because the factor that matters, in this case, isn't raw signal strength but the ratio of signal strength between the signal you're trying to receive and the one that might potentially interfere with it.

By co-locating adjacent-channel operations, you maintain a constant signal-strength ratio between the two signals, no matter whether you're right on Wilson or 35 miles away in the Valley.

This is the same engineering theory that's allowing FM translators on second-adjacent channels to colocate with full-power FM stations, and it goes a long way towards explaining situations like 27/28 (and, at one time, a 26 LPTV as well) on Wilson.

Some of the other restrictions (UHF channels +/- 15, for instance) are vestiges of antiquated TV receiver technology, and will cease to be issues when the DTV conversion is complete. (And indeed, the move to DTV has opened up many previously "taboo" channels for use in many markets.)<P ID="signature">______________
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[KMRichards]

Re: The magic answer

Scott Fybush reminds me why I never got into television engineering ...

> > I'm not sure how it's done (perhaps by channel offset) but
>
> > with an external antenna pointed at Mount Wilson, about 35
>
> > miles away, I can get both distinctly with no adjacent
> > channel interference.
>
> Correct. And if you think about it, when your TV set is
> plugged into cable, it has no problem at all with multiple
> adjacent channels all stacked up next to each other.

Well, my set is plugged into that external antenna primarily as a backup to my DirecTV receiver plugged into the A/V input of the set, but I get the point.

Technical explanation edited ... go back and read Scott's post if you missed it!

> Some of the other restrictions (UHF channels +/- 15, for
> instance) are vestiges of antiquated TV receiver technology,
> and will cease to be issues when the DTV conversion is
> complete. (And indeed, the move to DTV has opened up many
> previously "taboo" channels for use in many markets.)

Damned obsolete technology.

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[bpatrick]

Re: How does Birmingham/Anniston have so many stations close together?

> >
> > I'm not sure you can pick up CBS46 Atlanta until you get
> to
> > Heflin; their cable system, I know, carries it and 11
> Alive.
> >
> > I somehow doubt that 46 comes in OTA there, though.
> >
>
> They should carry all the Atlanta stations, not just those
> two. Clay and Cleburne counties are in the Atlanta market.
>
> As for getting 46 OTA there, I don't remember. The only
> time I've ever operated a TV out that way was on the bus for
> a basketball game against Ranburne like 10 years ago. The
> Atlanta VHFs and 14 came in fine, but I don't know about
> 17/30/34/46/69.
>
True that Cleburne County is in the Atlanta DMA. But if
you go to zap2it and look at the channel lineup for Charter
Communications in Heflin, you'll find that it carries only
WXIA, WTBS, and WGCL. By contrast, it carries WBRC, WVTM,
WTTO, and WJSU from Birmingham/Anniston. Is there a possibility
that WBRC and WJSU want their Atlanta Fox and ABC counterparts
kept off that particular cable system?