0.4 MHz spacing of co-located FM stations

[briankay]

0.4 MHz spacing of co-located FM stations in the US.

As a way to more efficiently use the limited FM spectrum, I wonder why this is not done (to the best of my knowledge) in the US rather than the current 0.8 MHz spacing for co-located transmitters. Now I understand combiner technology is not at the stage to combine 2 transmitters 0.4 MHz apart into 1 antenna, but what I am asking is why is it not possible with separate antennas on the same structure or tower when the ERP of both stations is equal. As far as I know, listeners should be able to receive both stations without adjacent channel interference from the other 2nd adjacent because there would be almost no location where one signal is sufficiently stronger than the other to cause interference (exception may be a few meters from the antenna).

This following paper appears to suggest that it is viable when both station are equivalent in ERP.
http://www.rsm.govt.nz/cms/pdf-library/policy-and-planning/broadcasting-1/vhf-fm-broadcasting-frequency-availability-and-allocation/narrow-channel-spacing/Criteria%20for%20400%20kHz%20Channelling.pdf

Has the FCC addressed this or studied this, or could anyone tell me why this is not feasible.

P.S. I know the reasons for having sufficient separation distances when 2 stations on 2nd adjacent frequencies to each other are in separate locations.

 

[rfry]

Now I understand combiner technology is not at the stage to combine 2 transmitters 0.4 MHz apart into 1 antenna, but what I am asking is why is it not possible with separate antennas on the same structure or tower when the ERP of both stations is equal.

Even though most FM receivers may not have a problem receiving these signals, such co-located systems using separate antennas on the same or nearby structures can produce r-f intermodulation products that can exceed the limits permitted by the FCC unless they are adequately de-coupled.

For detail, see paper 7 at http://rfry.org .

RF

 

[BobOnTheJob]

The spacing standards were designed in a day when FM radios had poor selectivity...long before ceramic filters. Speaking strictly from a receiver standpoint, equally strong signals 0.4mhz apart should be selectable on the vast majority of radios made today (but virtually none of the tube type radios made when the spacing standards were set). Many would even select signals at 0.2mhz. Consider cable TV systems where equal strength signals are on adjacent channels with zero interference.

I would far prefer having stations at 0.4mhz spacing rather than having IBOC interference. I have a receiver that will easily select DX signals adjacent to local signals...but IBOC interference on the adjacency brings my superb tuner to it's knees.

 

[LA_Guy]

It works well, and a couple of cans in the transmission line(s) takes quick care of the Intermod problem Richard speaks of-not to mention that one good thing to come out of IBOC is a huge improvement in close channel spacing combiner technology.

The second and third adjacent spacing rules are from a distant time when tube type tuners had about 25 db selectivity at +/- 400 kHz. Modern tuners typically have at least 70 db-and 80 db isn't uncommon.

BUT this makes WAY too much sense, so the FCC will never adopt it!

 

[briankay]

It works well, and a couple of cans in the transmission line(s) takes quick care of the Intermod problem Richard speaks of-not to mention that one good thing to come out of IBOC is a huge improvement in close channel spacing combiner technology.

The second and third adjacent spacing rules are from a distant time when tube type tuners had about 25 db selectivity at +/- 400 kHz. Modern tuners typically have at least 70 db-and 80 db isn't uncommon.

BUT this makes WAY too much sense, so the FCC will never adopt it!

It sounds like 0.2 MHz spacing on co-located transmitters would even have been feasible if IBOC on FM was not adopted. That said in practice 0.2 MHz spacing would probably be an allocation nightmare, but could have given some options for extra stations in crowded cities. I also read that combining analog only stations 0.4 apart into a master antenna is possible, but is not technologically feasible with IBOC FMs that are 0.4 apart as there would be only a 6 kHz guardband between their sidebands.

Getting back to 0.4 MHz spacing, I do notice that in Chicago, that a few low power translators are operatated (97.5 and 100.7) that is 0.4 away from the Class Bs from top of the Hancock and Sears Tower. Doesn't this break the FCC spacing rules?

That said those translators are received pretty decently up to 15 miles away even though they are 3 and 2 watts respectively. The interference experienced is from the not so distant co-channel stations. I admire the individuals that got these class D licenses, that 2 watts from the Sears Tower gives you a lot of bang for the watt. But it would have been far better use of spectrum to put a single full power class B station in those spaces rather than these class Ds and the 2 to 3 class A 100.7 stations at the fringes of the Chicago metro, that in total serve far far fewer people along with all the co-channel interfence issues.

 

[OKCRadioGuy]

"BUT this makes WAY too much sense, so the FCC will never adopt it"

Amen to that! In the ham world we did the equivelent of putting to UHF ham repeaters on the same site adjacent-channel. It worked great except occationally there were a few mobiles that were off frequency and slapped the adjacent's input a bit. It wasn't a big issue though. Looking back we probably should have established the newer repeater as a different PL tone to prevent that problem. (but we co-existed as sister orginizations for years happily next door to each other both in co-location and frequency). As long as it was the same owner forever in the broadcast world that agreed to never not co-locate I'd think it would be absolutely in the public's best interest, much more so than adding IBUZ service. If I were an owner I'd gladly trade in my IBUZ for another FM stick!

 

[OKCRadioGuy]

Another words the two licenses couldn't be seperated to two different licensees...

 

[LA_Guy]

The fact is this: In New York City, Los Angeles and Chicago it's been going on for over 40 years without any problems-and they're NOT co-located!

Let's take LA for example: First off there are two class Bs NOT co located that are 600 kHz apart 89.3 and 89.9. No one has any problems receiving them!

Next. starting at 89.9, and going all the way to 107.5, there are LA metro class B stations every 800 kHz (89.9, 90.7, 91.5, 92.3, 93.1, 93.9, 94.7, 95.5, 96.3, 97.1, 97.9, 98.7, 99.5, 100.3, 101.1, 101.9, 102.7, 103.5, 104.3, 105.1, 105.9, 106.7 and 107.5). Most if not all of these are GRANDFATHERED class Bs, which means they are running FAR more power then a normal class B station would. There also are surburban class A stations in between these (92.7, 93.5, 94.3, 95.9, 96.7, 98.3, 102.3, 103.1, 103.9, 105.5, 106.3 and 107.1).

Everything works FINE-nobody has any problems listening to anyone in LA!

Once again the FCC is using what I call FCC PHYSICS-which has little or nothing to do with the classical physical sceinces we learned about in school!

 

[OKCRadioGuy]

Yup. It all has to do with the greenbacks and those that control them.

 

[joebtsflk1]

Getting back to 0.4 MHz spacing, I do notice that in Chicago, that a few low power translators are operatated (97.5 and 100.7) that is 0.4 away from the Class Bs from top of the Hancock and Sears Tower. Doesn't this break the FCC spacing rules?

Normally w/ full-power spacing, station A's 60 dB contour can't have any overlap with the 100 dB contour of station B that's 2 or 3 clicks away. In other words, A's protected contour can't have any overlap with a 2nd or 3rd adj. that's 40 dB higher.

With translators against full power stations, the translator can't create interference to the other 2nd/3rd adjacent. That means the translator can be no more than 40 dB higher than the 2nd or 3rd adjacent's signal. Now, when the translator is inside the 2nd/3rd adjacent's protected contour (60/57/54 dB depending on where you are and/or class) the translator's contour...Well, here'e an example: Translator's 140 dB contour will by definition interfere w/ 2nd/3rd adj. 100 dB contour. But at low power levels say under 100 watts, the 140 dB contour may only go a hundred yards or so and with a four or six bay antenna, 140 dB might not even reach the ground.

So, just like the tree that falls in the forest doesn't make any noise if there's no one to hear it, there's no interference if no one on the ground can hear it, or so the logic goes.

 

[Ai4i]

The short answer:
The NAB lobbies to keep competition to a minimum.
The political structure in New Zealand is likely quite different.

 

[Schroedingers Cat]

LAGuy, I would agree that with a good receiver, the 0.4 MHz spacing in LA is not a problem. But I stayed at a hotel near LAX years ago and the Class As on 103.1 and 103.9 made it impossible to tune in KOST on 103.5 in the hotel room on the cheap alarm clock radio.

 

[radiofish]

I've wondered about the same thing. Yes, translators are 0.4 MHz spaced and can demonstrate no interference to population and permitted.

Why not write up your idea as a proposal and send it to the Secretary at the FCC as a "Petition for Rulemaking?" It wouldn't cost anything to do that. You could Cc: your two Senators on the idea and argue the merits. Allowing such spacing would also GREATLY reduce the need for a proliferation of new communications towers which some find to be a negative thing. It might even reduce the need for some towers.

Write it up and send it to the Commission as a "Petition for Rulemaking" so that it can be considered and debated.

Send it to:

Marlene H. Dortch, Secretary
Federal Communications Commission
Office of the Secretary
445 12th Street, SW
Washington, DC 20554

Cc: your two US Senators. How hard can it be?

0.4 MHz spacing of co-located FM stations in the US.

As a way to more efficiently use the limited FM spectrum, I wonder why this is not done (to the best of my knowledge) in the US rather than the current 0.8 MHz spacing for co-located transmitters. Now I understand combiner technology is not at the stage to combine 2 transmitters 0.4 MHz apart into 1 antenna, but what I am asking is why is it not possible with separate antennas on the same structure or tower when the ERP of both stations is equal. As far as I know, listeners should be able to receive both stations without adjacent channel interference from the other 2nd adjacent because there would be almost no location where one signal is sufficiently stronger than the other to cause interference (exception may be a few meters from the antenna).

This following paper appears to suggest that it is viable when both station are equivalent in ERP.
http://www.rsm.govt.nz/cms/pdf-library/policy-and-planning/broadcasting-1/vhf-fm-broadcasting-frequency-availability-and-allocation/narrow-channel-spacing/Criteria%20for%20400%20kHz%20Channelling.pdf

Has the FCC addressed this or studied this, or could anyone tell me why this is not feasible.

P.S. I know the reasons for having sufficient separation distances when 2 stations on 2nd adjacent frequencies to each other are in separate locations.

 

[Ai4i]

Why not write up your idea...it wouldn't cost anything to do that.

Aren't you forgetting the hundreds of millions, make that billions of dollars he would need to enclose within the envelope to counter the lobbying efforts of the NAB and other such groups intent on suppressing any and all competition to their member stations?

 

[bossbill]

Count me in. I happen to own 4 FMs, all of which are second-adjacent to Seattle stations. Get the Rulemaking done and I'm in line for a major move-in. Retirement here I come.