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Help! Tropo Is Killing Us!

D

dx7

Our C3 FM antenna and tower are about 40 miles from the Gulf of Mexico -- during this time of year, tropo ducting causes us nightmares -- so bad that even in our 70 dbu signal area, we can't be heard on some mornings.

I'm not a broadcast engineer but learning, so my question is this:

Can anything be done to help minimize the impact of tropo ducting on our signal when weather conditions cause this to happen? Fewer antenna bays and a higher TPO? New antenna? Anything?

A few specifics -- the station is at 96.1-- the antenna is side mounted at 450 feet HAAT and is a very old six bay Jampro that we inherited from the previous owner...we had an engineer run a test on it and it appears to be an antenna that was tuned for 95.7 mhz. Would that make a difference with our ability to handle bad tropo? BTW reflected power averages 20 watts.

TPO is 5kw -- ERP is 13kw. Terrain is flat.

When conditions are right, we sound good and our signal booms out across a surprisingly large area -- but on some mornings, we can barely be heard a few miles past the tower.

Thanks in advance
dx7
 
I've seen some stations in tropo-ridden areas have success with an auxiliary antenna installed much lower on the tower (with correspondingly higher ERP.) When the trop starts rolling in, switching to the lower antenna can sometimes keep your signal down below the duct instead of getting sucked up into it. The downside, of course, is the expense of a second antenna and potentially a bigger transmitter to make the larger ERP.
 
That duct is going to act just like a big transmission line from the other station's locale, to the receiver near you.
A solution would only work at the receiving end...something like a more directional receive antenna, or some widely-separated space-diversity antenna array.
The only thing you could do is greatly increase the signal strength of your own signal, enough to overpower the ducted signal.

Maybe you could make some sort of "game" of it. Promo that "other stations are jealous of our audience, and are trying to take over", still explaining that it's (really) just a law of nature. Then, give away some prizes, or offer to play "anything you want, since no one can hear us right now...call in your request". Set up an "incoming signal watch" with listeners. Get some fun, free publicity out of it all.
Heck...call the DJ at the other station, and chat with him on the air.
 
kenglish said:
That duct is going to act just like a big transmission line from the other station's locale, to the receiver near you.
A solution would only work at the receiving end...something like a more directional receive antenna, or some widely-separated space-diversity antenna array.
The only thing you could do is greatly increase the signal strength of your own signal, enough to overpower the ducted signal.
Click to expand...

Which is why some stations drop down to a lower aux antenna at higher power. You're not getting rid of the incoming interference, but you at least have a shot at overpowering it. And by dropping down - say, to 400 feet instead of 1000 - you stand a chance of getting your outgoing signal down low enough that it's not going into the duct but rather out to your local audience.
 
Scott Fybush said:
And by dropping down - say, to 400 feet instead of 1000 - you stand a chance of getting your outgoing signal down low enough that it's not going into the duct but rather out to your local audience.
Click to expand...

Back when TV network signals arrived via terrestrial microwave, AT&T did this to protect against tropo.

When the inversion levels got low enough, the microwave signal from the transmitting dish would be refracted up, *above* the receiving dish. Hopefully the lower dishes would be below the inversion, allowing the beam to continue to be aligned.
 
Another possible solution: Beam Tilt.

Set the 3dB point of the upper side of the main lobe so that it is below the horizon.
 
The Houston area is famous for Summer ducting. I was told that one of the stations there had a full-wave spaced antenna below 500' on the tower that they could switch to at any time to get below the 1100' foot level where their regular antenna stood.

Once upon a time, living in Austin, it was not uncommon at all for stations from Houston to come blasting through like locals!
 
Scott Fybush said:
kenglish said:
That duct is going to act just like a big transmission line from the other station's locale, to the receiver near you.
A solution would only work at the receiving end...something like a more directional receive antenna, or some widely-separated space-diversity antenna array.
The only thing you could do is greatly increase the signal strength of your own signal, enough to overpower the ducted signal.
Click to expand...

Which is why some stations drop down to a lower aux antenna at higher power. You're not getting rid of the incoming interference, but you at least have a shot at overpowering it. And by dropping down - say, to 400 feet instead of 1000 - you stand a chance of getting your outgoing signal down low enough that it's not going into the duct but rather out to your local audience.

Probably need somebody (physicist?) with more knowledge of the troposphere than any of us, to answer all this ;D .

Like you say, if your power is going in to the duct (instead of going out to the local area), moving substantially higher or lower on the tower can cause the entry angle of the signal in to the duct to change. That would keep the duct from sucking up all the signal. It's similar to energizing a waveguide in microwave or UHF.

As for the local reception, maybe getting down lower AND at a higher power, overrides the incoming signal up close. That's a good way to try and override it, but I wonder what happens to your outlying and fringe audience. Might be an interesting study, if somebody wanted to take some FS readings all around the area of some ducting interference.

When KSL was on analog, on channel 5, we had all kinds of problems with Tropo affecting translators. We just had to "ride it out", since everything "lower on our tower" was made of dirt and rock (4500' mountain, above a 4500' AMSL city).

Anybody know how high these ducts usually are?
Click to expand...
 
Well, I got to thinking...Bill Hepburn would likely have an answer about the height of the ducts. Here it is, from his site:

"As far as classifying tropo..there is not a sharp line between enhancement and ducting. Rather than classify the type using the actual physics involved, the heights of the inversions can be used as a general rule of thumb. So typically "enhancement" is caused by inversions below 450 m (1500 ft) above ground, and "ducting" is caused by inversions above 450 m. (The layer of the troposphere below 450 m is called the "boundary layer" in meteorology)."

I would have thought the ducts would be located much, much higher in the troposphere. Maybe that's our answer on how different transmit antenna heights change it. Are we seeing "enhancement", rather than actual "ducting"?

http://www.dxinfocentre.com/propagation/tr-modes.htm
 
Question for the O.P.:
Is your signal just becoming weaker than normal, or is it being overpowered by another station coming in through the duct?
That would indicate whether it's power is being sucked in to a duct, or is being overridden. And, it might indicate if the signal (from a high point on your tower) is being blocked from the ground-based listeners, possibly by that "enhancement layer".
 
Unfortunately there's nothing you can do about tropo, just have to live with it. Hundreds of miles away, another station's engineer is cursing the fact that your station is overpowering them. Stations located closer to the coast are usually enhanced by tropo, inland stations are usually hurt by tropo. The best you could do is make a deal with your co-channel interfering stations to mutually reduce power during tropo openings. I usually notice some of my semi-local stations are weaker during tropo (since their signal is going into the duct) and distant stations override them.

Just last week during a strong tropo opening, I saw a Facebook status from someone "101.9 RXP became a rap station, as if we need any more of that crap". 101.9 is a rock station, and that morning they were overpowered by WIKS 101.9 Kiss FM from 350 miles away. I've also heard anecdotal evidence that police radar guns are inaccurate during tropo because they pick up speeds of vehicles far away.

Now e-skip on the other hand is unpredictable and changing your height won't mitigate it.
 
Sgeirk said:
The Houston area is famous for Summer ducting. I was told that one of the stations there had a full-wave spaced antenna below 500' on the tower that they could switch to at any time to get below the 1100' foot level where their regular antenna stood.

Once upon a time, living in Austin, it was not uncommon at all for stations from Houston to come blasting through like locals!
Click to expand...

You are right about the Houston area. A couple of weekends ago I was scanning the dial here in Charlotte, NC and came across a new “rap station” here in town. It was a very strong signal and was scannable for what seemed like hours. Found out that the station was a Houston 100kw FM. That’s a reception distance of over 900 miles.
 
Question for the O.P.:
Is your signal just becoming weaker than normal, or is it being overpowered by another station coming in through the duct?
That would indicate whether it's power is being sucked in to a duct, or is being overridden. And, it might indicate if the signal (from a high point on your tower) is being blocked from the ground-based listeners, possibly by that "enhancement layer".
Click to expand...

kenglish, we have experienced both -- the most common one is when there is "white noise" background static and a very limited signal range (regular listeners call us to say they can't hear us)...other times, a 100 kw co-channel station 250 miles to our south and another c1 about 200 miles to our west take turns booming into our coverage area depending on conditions.

As I said earlier, our antenna is at 450 feet haat, 5kw TPO, 13kw ERP, so we get slaughtered at times by the co-channel 100kw stations that are just a few hundred miles away.

The idea of having an aux antenna lower on the tower sounds like a good possibility, but being in a fairly small market, not sure we could afford to do that anytime soon. Maybe I should try to find an AM translator ;)
 
Friday through Sunday we heard E of Indianapolis WKOA on 105.3 cover up 2 translators like they didn't exist in areas within their 60dbu.

I listened to this station 70 miles distant on a portable radio, antenna down, hand over the antenna with no noise.

Several others like locals. WITZ Jasper 104.7. Not sure of the mileage- 70? Class A from Bedford Indiana 105.5. Local signal.

Then the reports of interference to 93.1 our local NT station. At 25 miles in Lebanon the Class B was unlistenable. MANY others.

New cume builder????

It literally SUCKED the signale of local stations and ran them elsewhere.
 
I'm a meteorologist and former radio engineer, on a class A in Central TX in the 80s. We got destroyed by the 100kw stations in Houston in weather like this.

You can actually see the inversion on the weather balloons launched every morning around sunrise. There isn't any near Houston, but here is a link to the SkewT from the weather balloon in Fort Worth http://www.rap.ucar.edu/weather/upper/fwd.gif and Lake Charles http://www.rap.ucar.edu/weather/upper/lch.gif. The bottom is the surface and the red line is temperature, green is dew point. If the temperature gets warmer with height, just above the surface, that is an inversion. It will go away during the day as the temperature (red line) gets warmer (moves to the right). (The inversion will probably show up only on the 12z sounding -AM)

You can also see the inversion on area weather radars as the beam is tilted back toward Earth and ground clutter can fill up the screen over 100 miles away. We call this AP (Anomalous propagation). http://radar.weather.gov/radar.php?rid=hgx&product=N0R&overlay=11101111&loop=no
 
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