WAJD-1390

[w9wi]

WAJD may even be limited into what it can send West towards the Gulf of Mexico in order to avoid any overlap in the two signals over the Gulf.

I'm not absolutely sure this applies to AM, but on FM the FCC doesn't count predicted interference that happens over large bodies of water. Upgrades have been authorized in Milwaukee that have predicted contour overlap with stations in western Michigan, because the interference area lies entirely over Lake Michigan.

 

[jmtillery]

WAJD may even be limited into what it can send West towards the Gulf of Mexico in order to avoid any overlap in the two signals over the Gulf.

I'm not absolutely sure this applies to AM, but on FM the FCC doesn't count predicted interference that happens over large bodies of water. Upgrades have been authorized in Milwaukee that have predicted contour overlap with stations in western Michigan, because the interference area lies entirely over Lake Michigan.

If that is true, then WAJD and WOCA should have much more latitude in configuring a night-time signal for maximum coverage since the effected co-channel stations for both WAJD and WOCA are located across the Gulf of Mexico.

 

[Kmagrill]

If that is true, then WAJD and WOCA should have much more latitude in configuring a night-time signal for maximum coverage since the effected co-channel stations for both WAJD and WOCA are located across the Gulf of Mexico.

I know, for sure, that FM and TV do not have to protect contours if the overlap is entirely over water. With AM, it's probably the same, but I'm not up on the AM rules enough to say for sure. I do know that even small amounts of rf radiated towards another AM station can cause significant interference at night. It turns out that AM modulation is far more susceptible to interference than FM. This interference is usually manifest as a hetrodyne "Whistle" that increases in loudness as the interfering signal becomes stronger. It could be that WAJD has to control radiation towards WROA not due to overlap in the Gulf, but because of possible interference to one of WROA's service contours over land in their local vicinity.

 

[w9wi]

If that is true, then WAJD and WOCA should have much more latitude in configuring a night-time signal for maximum coverage since the effected co-channel stations for both WAJD and WOCA are located across the Gulf of Mexico.

They still can't throw much power at each other, because it won't take much to get all the way across the Gulf and start causing interference on land on the other side. I think you can reasonably assume WAJD's 51 watts of authorized non-directional night power reflects all they can send in the direction of WROA. (and probably WXTC/Charleston SC, possibly among other stations)

As Kmagrill says, a weak interfering AM signal causes a lot more interference than a proportionally similar interfering FM signal. AM interference is additive -- if there's another signal on the same frequency 30dB down, you'll hear its audio 30dB below the desired station's audio. FM has a "capture effect". If there's another signal on the same frequency 30dB down, you probably won't even know it's there.

Though the interference only shows up as a "whistle" if one or both stations are off-frequency. (the pitch of the whistle is equal to the difference in the two stations' frequencies. So one will have to be a LONG WAY off-frequency to hear a whistle! This *does* happen if the interference is coming from Europe, where 9KHz channels -- and enormous amounts of power -- are used.)

Usually interference between two Western Hemisphere stations results in a mix of the two stations' audio, sometimes accompanied by a low rumble if one or both stations are *slightly* off-frequency.

 

[Nostalgia]

Usually you can look at a stations PSRA and PSSA to get an idea of who they are mainly protecting. Strange in the case of WAJD their PSRA allows them 500 watts at 6am every month with NO limiting stations. However, the PSSA really hammers them down at sunset without mentioning any particular station. I guess, as far as the ionosphere is concerned they are protecting every 1390 station in the world!

And yes the interferance contour overlaps over water (or forrests, ect where nobody lives) is an FM and TV thing because they are line of sight signals. AM daytime ground wave and nighttime skip is a totally different animal to deal with interferance-wise. An AM station along the coast could make a case that the overwater interferance during the daytime is no big deal and probably would be OK with that like the examples given, but not at night, and especially since WAJD is still OMNI at night throwing that big 51 watts right up into the ionosphere in all directions.

 

[redneckriviera]

There was a time when people were hanging onto AM licenses in the hope that the commission was going to convert all AM and FM signals into a single digital band--making the AMs worth something, again. Didn't happen. So now a guy like Gillen looks at the prospect of spending hundreds of thousands of dollars to upgrade 1390 and recognizes that he'd never see that money again. Might as well light a match to it.

Firing it back up in order to scramble to sell $5 spots and working one's ass off to bill $15K in a good month--and $5K in the bad months--makes no sense if he (and crew) can invest the same amount of time, effort and money into selling FM spots for 6 or 7 times the rate and realizing significant profit.

The best thing he could do with 1390 (it's been done many times before in other places) would be to donate the SOB to UF as another "experiential learning" lab as a tagalong for RUF-A/F at a fictional price-tag acceptable to the feds for tax purposes. Despite having multiple properties, UF doesn't have a typical student-run radio station like most colleges have. Out in Boulder, UC has such an AM animal, IIRC.

There are a lot of us who cut our radio teeth on that kind of college radio, and the experience didn't hurt us a bit. Otherwise, he just pulls the plug--gets nothing for it--and another AM disappears...

 

[Greg Strickland]

In some cases, FCC staff may choose to not consider AM station contour overlap created by a salt water as overlap on a particular application. This would be groundwave signal, not skywave.

I think most nighttime power values reflect skywave interference created, especially the "dial a power" night values.

A station should seek the services of a consulting engineer to explore options.

 

[Kmagrill]

Well, for what it's worth, I was curious so I asked a consulting engineer friend of mine if "the FCC considers skywave interference from an AM station if the overlap occurs entirely over water, such as the Gulf of Mexico?" His reply was: "I don't believe so. I recently had a conversation with FCC Staff about protecting a Carribean island class A from skywave interference from a station in Texas. He told me that, even though the protected groundwave contour of the 'A' was huge, I only had to protect it from skywave interference to the borders of the island."

 

[MN Maniac]

Doesn't 820/Largo have a deep daytime null towards Fort Worth? If so, this would certainly be due to overlap over water. Fort Worth is too far away for any possible daytime overlap over land.

 

[jmtillery]

I'm wondering, other then the current AM pattern remedies, can anything else be done to reduce or eliminate skywave propogation, or does the natural laws of physics dictate this is simply an annoying natural phenomena, and fact, we have to live with, and there is nothing else that can be done based on our existing level of knowledge? It would be great if AM could be converted to an "FM like" medium, with all AM stations operating unlimited hours with a non-directional antenna pattern without the concern for skywave interference.

 

[Kmagrill]

I'm wondering, other then the current AM pattern remedies, can anything else be done to reduce or eliminate skywave propogation, or does the natural laws of physics dictate this is simply an annoying natural phenomena, and fact, we have to live with, and there is nothing else that can be done based on our existing level of knowledge? It would be great if AM could be converted to an "FM like" medium, with all AM stations operating unlimited hours with a non-directional antenna pattern without the concern for skywave interference.

I don't think anything can be done about skywaves, Mark. Skip is the result of the upper atmosphere acting like a giant mirror to certain frequencies. For AM, this occurs only after dark when the sun's rays are no longer disturbing the layer that the radio waves use to bounce back to earth. The frequency determines how high up the signal goes before it is reflected.


AM signals go to fairly low altitudes and are thus restricted to about 1,000 miles, or less, per hop (generally speaking). The angle of incidence of the radiation leaving the antenna has a decided effect on the range. The higher up the angle of radiation is, the shorter is the skip distance (within limits, of course), but the more powerful the signal is within the skip area. Even if all of the energy went straight up, some skip would still occur, not to mention the fact that nobody in the local service area would hear the station at all due to the energy all going skyward and great interference would occur to the frequency somewhere within the 1,000 mile skip zone. Generally, the FCC tries to limit the amount of skyward radiation by insuring that antennas are designed to radiate outward, not upward. That's about all that can be done, short of disrupting the atmosphere itself.

Shortwave penetrates higher and therefore travels further on each hop. Of course, there are lots of examples where AM has circled the globe, but that is the result of the signal bouncing off of the atmosphere, then a large water body and back up to the atmosphere over and over. Each bounce causes significant losses, so the mere fact that a signal from a 5kW AM station can be heard at all in places halfway around the world, after 5 or 10 hops, is indicative of how significant the problem can be.

 

[w9wi]

I'm wondering, other then the current AM pattern remedies, can anything else be done to reduce or eliminate skywave propogation, or does the natural laws of physics dictate this is simply an annoying natural phenomena, and fact, we have to live with, and there is nothing else that can be done based on our existing level of knowledge? It would be great if AM could be converted to an "FM like" medium, with all AM stations operating unlimited hours with a non-directional antenna pattern without the concern for skywave interference.

I'm afraid it's physics. You can limit it somewhat by concentrating as much power as possible along the horizon (limiting high-angle radiation) which is largely why the FCC has minimum tower height and ground system regulations. But that won't eliminate skywave altogether.

I suppose if we could find a way to "seed" the D-layer of the ionosphere, to keep it active after solar energy stops repleting it after sunset, we could eliminate skywave. Obviously not practical... (and shortwave hams would be chasing you with pitchforks!)

_________________________________________________

He told me that, even though the protected groundwave contour of the 'A' was huge, I only had to protect it from skywave interference to the borders of the island."

Is this island under U.S. jurisdiction?

Again I can only speak to FM, but on that band, interference from a U.S. station to a foreign station only "counts" if the interference happens in the foreign country. There's a station on 104.3 in Sault Ste. Marie, Ontario; the U.S. could authorize a station on the same frequency just across the river in Michigan if it could limit the interference to the U.S. side of the border.

It is possible this provision only applies against Canadian and Mexican stations -- as of course a U.S.-based FM station isn't capable of causing interference in any other country. (barring sporadic-E!)

 

[Greg Strickland]

The 5 microvolt groundwave contour of the Largo station cannot overlap the 100 microvolt groundwave contour of the Fort Worth station. 5 microvolts isn't much and a directional antenna may be necessary to protect Class A WBAP coverage on Texas soil.