FM Tropo Ducting - Is There a Solution to This Problem?

[everydayguy]

We have a lot of ducting issues this time of year thanks to inversion layers and being near an ocean. We can't be heard in some of our primary coverage area some mornings because of it.

Which, if any, of the following would help this situation:

1. Running in mono rather than stereo.

2. Reducing power when this happens (we're 25kw ERP).

3. Having the antenna higher (or lower) on the tower (we're at 500 feet).

4. Using a better, "louder" audio processor (we're using a modest processor now).

5. Are there any solutions at all or do we just have to "live with it"?

Thanks in advance --

 

[Sgeirk]

Live with it or switch to an aux antenna at a much lower height during the blessed tropo event.

 

[fybush]

Yup. If your signal's being sucked into a duct and carried somewhere else, it doesn't matter whether it's in mono or stereo, or how it's processed, and reducing power at the same antenna height doesn't help either (except perhaps to help out the co-channel station at the other end of the ducting path that's getting interference from you at the same time!)

But a lower aux antenna may very well make a difference, moving your signal under the duct so it can still get out to your local audience.

 

[Play Freebird]

One of the engineers at an FM station in the Tampa - St. Pete market presented a paper on this subject years ago. I have a copy somewhere and will try to find the reference.

Like multipath, tropo problems can be frustrating. An old cartoon from Stereo Review (reprinted in a Bruce Elving FM Atlas) comes to mind: It shows a customer in a hi-fi shop looking at a product with a big sign GUARANTEED TO SOLVE ANY MULTIPATH PROBLEM On the shelf, next to the sign, are several sticks of dynamite and a detonator box.

 

[BobOnTheJob]

Tropo Ducting is a 2 way street...if your signal is getting sucked into a duct and being spit out where you don't want it, operating with a lower aux antenna 'might' help--depends on how wide (vertically) the entry to the duct is. But much of the problem comes from a station at the other end entering the duct & being spit out right where your listeners are (or were). Say you normally have 63dbu at your target listening location. If the tropo duct is piping in a 70dbu signal from another station, you're toast. If there was a way to get your 63dbu signal up to 70dbu, you'd already be doing that. A lot of it depends on how much signal you have. If you have a 70dbu or better signal, you're going to win most of the time. If you're a rimshotter with a 54dbu signal, you're going to lose the battle a lot more often.

 

[nd2023]

The only solution is an aux antenna at a lower height and higher power, which will hopefully keep your signal out of the duct. It'll also reduce the interference your station causes in the other station's protected contour. Maybe you can try to get the engineer of the other station to also have a lower aux antenna for use during tropo, which would help you out.

E-skip season is about to peak in June, and it can happen anytime during the day. The interference can't be avoided and the stations are usually 700-1300 miles away. The signals are usually stronger than tropo signals. It affects stations with lower dial positions more often. In the analog TV days, it was almost impossible to watch channel 2 or 3 in June because of the daily e-skip. So it would suck for tropo to kill your signal in AM drive and e-skip to kill your signal in PM drive.

The New York City stations on top of the Empire State Building always suffer from tropo because of their high height and low power. Many times I can get a co-channel 105.9 from Hartford, 100 miles further from New York City and in the same direction as New York City, interfering (and sometimes completely wiping out) WQXR, because the 105.9 in Hartford has more power than WQXR. Tropo is usually strongest in the overnight hours, but it usually spills over into morning drive.

There isn't much that can be done for tropo and e-skip interference. Maybe tell the listeners to listen via their smartphones if there's DX interference.

 

[TomT]

Cure for FM Ducting? AM?? :

 

[Kmagrill]

If there was a sure-fire cure, we'd all be doing it. I've seen full class C FMs wiped out 15 miles from the transmitter and Class As unlistenable 3 miles out. On the other hand, we'd often get calls from the other end of the duct, 300 miles away where we were wiping out another class C.

This problem affects STLs and RPUs, too.

 

[BobOnTheJob]

The New York City stations on top of the Empire State Building always suffer from tropo because of their high height and low power.

Back in my high school days in the early 70's in Cincinnati (when I had time to DX), there was one opening into New York City via tropo. After logging a college station in NYC, the dial was searched on every Empire State Building frequency (568 miles)....none of them were there. WFME 94.7 in New Jersey made the trip, but they aren't on Empire. The same duct was accepting a signal from WKSS Meriden, CT (650 miles) as well, so the duct didn't stop at NYC. This has always been my proof that height does indeed have an impact on what gets into a duct and what doesn't. In this case, 250' got in, 679' got in but 1362' or so did not.

 

[secondchoice]

In the early or mid 1970’s I saw an Andrews(?) catalog with FM antennas. One of the pages had an antenna that looked like a arrow. It was suppose to allow you to “penetrate” an area. Would one of these pointed 10 or so degrees below the horizon be at an "angle" that would keep the signal out of the duct?

 

[chriscollins]

If ducting is horrible on one of my FM's (50 KW/500 Feet/15.5 Miles from town), we sometimes switch it to our backup (6.9 KW/3?? Feet/6 Miles from town). It has solved the problem locally MOST times... Then we hop back on the main after the event (usually 10:30 or 11:00 CST).

It's hit or miss, though.

 

[OKCRadioGuy]

The trick there probably was the closer proximity to the intended target more so than the reduced height and increased power. To make a bit of argument for more height and less power, we went from a 3-bay running just over 4k ERP to a new site where we are running 1.6kw at slightly over double the height. We are in OKC, in town. One would think that we are very flat around here... and we are, but there were 'holes' in our coverage from the old site that really showed up when we had tropo issues. I've had considerably less complaints since our recent change-over. What we are runnning is a 4-round ERI baby panel at one level (equiv. of a 1-bay). Since coverage is much more consistant, we have less opportunity for the sigals from outside the market to overcome us.

 

[Kmagrill]

In the early or mid 1970’s I saw an Andrews(?) catalog with FM antennas. One of the pages had an antenna that looked like a arrow. It was suppose to allow you to “penetrate” an area. Would one of these pointed 10 or so degrees below the horizon be at an "angle" that would keep the signal out of the duct?

That's the Jampro "Penetrator" line of antennas. They are a double "V" design that's naturally broadbanded, but they are not designed specifically to radiate at an angle. What you are referring to there is beam tilt which is used to put the center of the main lobe below the horizon, though not generally to reduce ducting. Most antennas can have electrical beam tilt by shortening an inner bay line to change the phase angle slightly.

 

[Smiths and Smittys]

Posted this in the Tucson section then saw this thread.
Had one of those DX experiences Friday morning. 98.5 what sounded like Jazz or Classical.
Turned the corner and 98.5 The Fox KRDX, Vail, AZ faded back in.
Looked on Radio-Locator.com and was unable to find a Jazz format or a 98.5 anywhere close by.
If anyone has thoughts and want's to chime in that would be cool.
Otherwise it was one of those weird DX radio geek moments!

 

[secondchoice]

I wonder if a non com or other station was ducted in with a IF harmonic that would work on 98.5?

 

[OKCRadioGuy]

So my question for the gang here is, could a guy get an over-height antenna with tons of downtilt and over-ride the ducting problem if your little FM system was an in-town? I believe the commission actually figures power based on what is sent into the horizion. Is there something here to play with to get more power to the listeners to help the ducting?

 

[BobOnTheJob]

So my question for the gang here is, could a guy get an over-height antenna with tons of downtilt and over-ride the ducting problem if your little FM system was an in-town? I believe the commission actually figures power based on what is sent into the horizion. Is there something here to play with to get more power to the listeners to help the ducting?

Just an opinion...I know of large number of stations (in addition to the ones who occasionally get eaten up by tropo) who'd be very interested in beefing up their signal inside the 60dbu contour without extending the FCC defined circle. I'd think this would have been universally done by now if it were allowable. The catch is at least partially in the ERP limit...100KW ERP is 100KW ERP whether it's aimed at the horizon or it hits the ground at 3 miles. Imagine the possibilities though if 100KW at the horizon was allowable, but with no ERP limit inside the horizon. Except for creating more receiver overload/intermod, there could be a real benefit here. Which leads me to a possible solution...an on frequency booster in the middle of the population center.

 

[kenglish]

Isn't it Houston that has all those huge towers right next to each other....the ones on that "tower climbing" video from a while back?
That could be an interesting experiment, assuming they get lots of tropo-ducting. Put an experimental FM on one tower, with a fixed antenna at the top, and another on a tram, that could go up and down the tower while operating. Have station engineers, hams, and students be "on call" to get a message when conditions are right for tropo (also, monitoring Bill Hepburn's site), and check out the signals, both at a distance, and close-in. A good time-reference and a log book could give a lot of useful info.
Looking at tropo from several different FM's that are on closely spaced towers, but at different heights, plus the variable "filler" antenna, might provide some interesting data.

As for "who pays", isn't that what the NAB is for?

 

[chriscollins]

So my question for the gang here is, could a guy get an over-height antenna with tons of downtilt and over-ride the ducting problem if your little FM system was an in-town? I believe the commission actually figures power based on what is sent into the horizion. Is there something here to play with to get more power to the listeners to help the ducting?

Just an opinion...I know of large number of stations (in addition to the ones who occasionally get eaten up by tropo) who'd be very interested in beefing up their signal inside the 60dbu contour without extending the FCC defined circle. I'd think this would have been universally done by now if it were allowable. The catch is at least partially in the ERP limit...100KW ERP is 100KW ERP whether it's aimed at the horizon or it hits the ground at 3 miles. Imagine the possibilities though if 100KW at the horizon was allowable, but with no ERP limit inside the horizon. Except for creating more receiver overload/intermod, there could be a real benefit here. Which leads me to a possible solution...an on frequency booster in the middle of the population center.

Remember that I am IT, when I ask this crazy question... I read a lot about boosters. How can you successfully do that? For instance, if my backup and main are on at the same time, the interference is horrible. Wouldn't that be the case with a booster? Would some kind of GPS time sync help that? Would antenna design be done to throw the interference into a non populated area? How does the on channel booster do its job without doing damage?

 

[LibertyNT]

Remember that I am IT, when I ask this crazy question... I read a lot about boosters. How can you successfully do that? For instance, if my backup and main are on at the same time, the interference is horrible. Wouldn't that be the case with a booster? Would some kind of GPS time sync help that? Would antenna design be done to throw the interference into a non populated area? How does the on channel booster do its job without doing damage?

Simple, It Doesn't. Where there's overlap the capture effect will swing back and forth between the booster and the main, pending which is stronger.
If the audio is synced up perfectly it would sound like a staticy mess.
Boosters are used (or at least I thought) in hills and valleys where the main has protected coverage, but cannot hit the area because of obstructions.

Boosters do work great on AM, space them right and sync the carriers it works well. FM (because of the nature of the modulation, cant have carrier sync *I think*)