FM Translators in Albuquerque

[wastedtalent23]

I've recently noticed that there are differences in coverage areas of some translators on Sandia Crest(according to radio-locator).
There is K225CH (92.9), K240BL (95.9), and K245CD(96.9) that cover more area than say K233CG (94.5), K265CA (100.9), and K279BP (103.7).
All of the above have nearly the same technical details. They are all 250 watts and have a HAAT of about 1250 meters. According to the maps, 92.9, 95.9 and 96.9 blanket the metro well and can be heard clear up to Santa Fe, while 94.5, 100.9 and 103.7 only cover the ABQ city limits and the southern half of Rio Rancho. Why is this? I'm in programming, not an engineer. I just find it interesting.

 

[Kent]

The reason is that those translators are directional. Some are more directional than others. I'm not sure why, but 92.9, 95.9, and 96.9 have a sliver cut out of their signals to the southeast while 94.5, 100.9, and 103.7 have most everything not covering Albuquerque cut out. I do know that directional FM's work considerably differently from directional AM's. A directional FM only subtracts from a relatively circular coverage area while a directional AM works more like a balloon as you can increase coverage in one direction while reducing it in others.

Fybush could probably tell us more if he sees this thread, but I suspect those six translators are on two antennas. I don’t know how diplexing off of directional antennas works, but I would guess that diplexing another signal off of a directional FM would mean the second signal would inherit the limitations of the first if it uses the same set of bays. If my guess is correct, one of those signals may be directional to prevent interference while the others also had to be directional when they hopped onto the same tower.

Finally, I would be surprised if those translators at 92.9, 95.9, and 96.9 do as well in the real world as they look like they do on paper. I was last in Albuquerque in 2005, and I don’t remember any of those being on-air at the time. However, all the stations from Albuquerque or nearby except the Class C's on Sandia seemed to fade quickly after getting north of Rio Rancho. I had an easier time getting the Santa Fe rimshots north of Rio Rancho along I-25 than I did any of the lesser Albuquerque signals, despite being closer to them.

 

[joebtsflk1]

...directional FM's work considerably differently from directional AM's. A directional FM only subtracts from a relatively circular coverage area while a directional AM works more like a balloon as you can increase coverage in one direction while reducing it in others.

That quickly explains the difference very well.

To expand on that point, the FCC doesn't allow directional FMs to have a signal strength in any direction that would exceed the signal strength for a non-directional for its class. So, for a class D translator, that limits the power in the main lobe of a directional FM to 250 watts, where in the nulls there may only be 5 or 10 watts, for example. A directional full-power FM is regulated the same. A directional class A can only push its signal out in any given direction to 6 kW, a class C3 25 kW, etc.

Basically the thinking of the FCC when allowing directional FMs was to limit somewhat the chaos that happened on AM where the licensed power of stations like 1130 in Minneapolis at night is 25 kW, where the entire 25 kW transmitter output is dumped into a directional pattern that pushes the signal to the north like one could get with (I'm guessing here) 100 kW or more, non-directional.

 

[Greg Strickland]

Wastedtalent23-

Radio Locator map does not tell the whole story about coverage. A radio station should hire an engineer to know as much as possible about coverage. Consulting engineer can run down all the aspects, do the research, study with software and provide more insight to you than typical maps found on the Internet. For example, if Radio Locator software cannot locate the directional pattern for a station, it may plot a non-directional pattern. You could be coming to conclusions that are not correct.

A complete comparative study of these translators could take several full days of work. Would take more time to consider modification of them, and simulcast considerations. Just as in programming, engineering requires careful consideration of many things.

I'm not pitching for consulting work here. Just saying this is serious business, and someone may not be inclined to spend a weekend doing multi-station coverage analysis project to post the answer here.

 

[fybush]

I felt a disturbance in the force...

I agree with Greg - there are a lot of variables at play, even beyond the ones he mentioned.

Sandia is one of those sites where a DA isn't necessarily a bad thing. All the population is in one direction, which, conveniently enough, is the direction where all that population is a few thousand feet down below the site. If you're 92.9, for instance, and you have to have a deep null to the northeast to protect the co-channel station in Espanola, so what? That null doesn't fall over much population.

A very quick glance at some of the Sandia translators shows a variety of DAs in use, not to mention different combinations of polarity - some are V-only, some H-only, some H&V. There are different protections for each channel - because Sandia is up so very high, 250 watts throws the 40 dBu co-channel interfering contour out a long way, and so you have to consider, for instance, protection to 103.7 in Gallup from the 103.7 translator on Sandia.

And here's a very general statement that's not meant to imply anything about any specific translator: the rules are a little loose about certification of construction for a translator. A directional full-power FM station needs a surveyor's report when it files for its license to cover, to certify that it's using the correct antenna mounted at the correct orientation. A directional translator self-certifies. Is every translator in America built 100% the way the CP says it should? Ummm.....

(And even at that, you can be legal and yet not perform exactly the way the CP says - you don't have to 100% fill the envelope of the DA pattern you specify, so the maps might show coverage in some areas where it doesn't exist.)

It's complicated.

 

[DavidEduardo]

A complete comparative study of these translators could take several full days of work. Would take more time to consider modification of them, and simulcast considerations. Just as in programming, engineering requires careful consideration of many things.

It might even take more time, such as several of Dave Stewart's multi-station reallocation projects in Texas 15 to 20 years ago.

 

[DavidEduardo]

A very quick glance at some of the Sandia translators shows a variety of DAs in use, not to mention different combinations of polarity - some are V-only, some H-only, some H&V. There are different protections for each channel - because Sandia is up so very high, 250 watts throws the 40 dBu co-channel interfering contour out a long way, and so you have to consider, for instance, protection to 103.7 in Gallup from the 103.7 translator on Sandia.

You raise what is an additional interesting point.

When I moved HCTM1, 95.1, to a mountainside over Quito at about 13,500 feet AMSL and about 3500 to 4000 feet above the population, we experimented with polarization. We built our own antenna out of aluminum tubing, so we could easily rotate the elements 90° for testing.

We found that pure vertical was much better for both coverage and multipath in what has to be about the most rugged, irregular terrain you can imagine (from the site, though, on a good morning I could see at least 6 or 7 snowcapped mountains just a few miles south of the Equator).

 

[DavidEduardo]

I'm not pitching for consulting work here.

However, let me be the first to recommend it!