• Get involved.
    We want your input!
    Apply for Membership and join the conversations about everything related to broadcasting.

    After we receive your registration, a moderator will review it. After your registration is approved, you will be permitted to post.
    If you use a disposable or false email address, your registration will be rejected.

    After your membership is approved, please take a minute to tell us a little bit about yourself.
    https://www.radiodiscussions.com/forums/introduce-yourself.1088/

    Thanks in advance and have fun!
    RadioDiscussions Administrators

How to Decrease FM Tropo Interference Without Raising Power

O

olmjax

I want to know if it is possible to decrease tropo interference on FM stations without increasing the power, especially for directional transmitters. Will FM processors help alleviate some of the inference problems?
 
It's a fact of life in the summer. Short of asking the stations interfering with you to lower their power, there's nothing you can do. Your signal gets "sucked into" the duct in your local coverage area, and escapes the duct over 100 miles away. Same thing with other stations, except their signal comes down in your coverage area.

Last week I heard a caller from Belmar, NJ (a town right on the beach) complain about not hearing 92.3 Now that night. There was tropo bringing in 92.3 PRO-FM from Providence, RI, and it must have been blasting in right along the beach.
 
I know of stations in areas such as the Jersey Shore and Gulf Coast where tropo is a daily fact of life that will switch to licensed auxiliary antennas when the ducting starts to happen. It's sometimes possible to get under the duct, so that instead of the desired signal being sucked into the duct and overwhelmed locally by something else coming out of the duct from somewhere else, the local signal stays local.
 
We have our antenna connected to a big winch. When we start to get tropo interference, we just lower the antenna down until it improves.

That works better than the old way. We used to have the antenna mounted on a pivot system. When we would get tropo we would pivot the antenna to a 45 degree angle. Problem was, it would only improve on one side of the city and get worse on the other side. We had to pivot it from side to side every few minutes to keep listeners on both sides happy.


Ok, just kidding, don't do either of those... :D
 
Thing is, lowering the antenna would require the power to be raised. It's hard to lower an antenna on a whim. For example, in NYC, it's impossible to lower the antenna so when tropo hits, the stations are sucked into the duct and come down far away. And sometimes the duct extends to the ground, so lowering the antenna and raising the power will only cause more interference elsewhere without helping the local coverage area.

Which stations on the Jersey shore change their antenna height if there's tropo? I remember being right on the beach a few miles from 94.3 The Point, and that station was weak, and overpowered by 94X from Long Island. Even a 10 watt translator from Long Island was coming in like a local. It was like the signal hit a wall a certain distance from the ocean.
 
Some stations have a licensed auxiliary at a lower height on the same tower. Sometimes it has a higher ERP. The higher ERP may overpower and capture better during tropo interference. Also, if the auxiliary antenna is much closer to the population to be served, as it is with some "rimshot" stations, the higher signal strength may again overpower and capture during tropo. The licensed auxiliary may not be fully spaced, so it couldn't be licensed as main transmitter site. Maybe that's what they meant.
 
Do I understand that the goal is to reduce the amount of tropo ducting to reduce interference to distant stations? Short of having the distant station alert you that the duct is in place, how would you even know that you were creating interference? And short of being a good neighbor, what would possess you to reduce your own bread & butter coverage to possibly minimize your impact on a market hundreds of miles away? And how would you know when the duct shut down? Do you really think that the lack of listener complaints for X number of minutes will cause the interfered with station to alert you that the coast is clear & to go back to normal facilities?

Before you assume that lowering the height reduces the ducting, consider this : In 1971, I lived in Cincinnati and experienced a good duct to the northeast. NYC was into the duct, but only 1 station...a relatively low height educational band station. At 2AM back in those days, there were a lot of open frequencies that had NYC stations on them...not 1 station from the Empire State Building made it into the duct.
 
My recent experience is with WKOA Lafayette. 105.3. They are a class B at reduced height. I also hear another station from Linden Indiana. Any time I hear one I hear the other. My location is East of Indianapolis. I can never hear WKOA normally. When this ductwork opens I hear them on a tabletop radio with no external antenna.

105.3 drowns out our translator that is 10 miles distant with better line of sight.
 
BobOnTheJob said:
Do I understand that the goal is to reduce the amount of tropo ducting to reduce interference to distant stations? Short of having the distant station alert you that the duct is in place, how would you even know that you were creating interference? And short of being a good neighbor, what would possess you to reduce your own bread & butter coverage to possibly minimize your impact on a market hundreds of miles away? And how would you know when the duct shut down? Do you really think that the lack of listener complaints for X number of minutes will cause the interfered with station to alert you that the coast is clear & to go back to normal facilities?
Click to expand...

I don't think that's the goal.

I think what they're getting at is... that if *your* antenna is in or above the duct, *your* signal will be conducted through the duct. Some of your power will be wasted, being delivered to another market & causing interference there, instead of serving listeners within your own market. That's a bit simplistic but is the general idea.

In other words, it's not about preventing someone else from propagating into your market, but about keeping your signal stronger so the interference from someone else doesn't cause as much trouble.

I'm in TV -- back when we used to receive the network signals via terrestrial microwave, there were two sets of dishes at different elevations. During severe tropo events, the microwave signal was deflected into the other dish..
 
w9wi said:
BobOnTheJob said:
Do I understand that the goal is to reduce the amount of tropo ducting to reduce interference to distant stations? Short of having the distant station alert you that the duct is in place, how would you even know that you were creating interference? And short of being a good neighbor, what would possess you to reduce your own bread & butter coverage to possibly minimize your impact on a market hundreds of miles away? And how would you know when the duct shut down? Do you really think that the lack of listener complaints for X number of minutes will cause the interfered with station to alert you that the coast is clear & to go back to normal facilities?
Click to expand...

I don't think that's the goal.

I think what they're getting at is... that if *your* antenna is in or above the duct, *your* signal will be conducted through the duct. Some of your power will be wasted, being delivered to another market & causing interference there, instead of serving listeners within your own market. That's a bit simplistic but is the general idea.

In other words, it's not about preventing someone else from propagating into your market, but about keeping your signal stronger so the interference from someone else doesn't cause as much trouble.

I'm in TV -- back when we used to receive the network signals via terrestrial microwave, there were two sets of dishes at different elevations. During severe tropo events, the microwave signal was deflected into the other dish..
Click to expand...
Interesting...I've never thought about the impact on the local signal...does the duct "suck away" a portion of the local signal?
 
Interesting...I've never thought about the impact on the local signal...does the duct "suck away" a portion of the local signal?
[/quote]

Somebunny said sumptin about energy: it cain't be created or destroyed. :-\
 
The duct would refract the signal from the antenna away from the ground in the local coverage area, making it seem weaker. Due to the curvature of the earth the radio waves would eventually come down to the ground far away. The strength of the duct (difference of the temperatures at the bottom and top of the inversion layer) determines the amount of the signal refracted, and the length of the duct determines where the signal will be heard. So not only is the local station weaker, a distant co-channel station's radio waves are coming down.

I recently experienced a tropo opening to Boston last week. NYC and Boston are the same direction from me. The NYC stations were struggling to come in, and some of the Boston stations were coming in strong enough to be heard in HD. The stations from the Prudential in Boston were stronger than the stations from Route 128, possibly because the Pru is higher and located closer to the ocean. The next 3 days after that tropo opening, it was over 100 degrees.

Along the ocean, almost every time the sea breeze starts, a tropo duct forms. Those ducts can get pretty strong, but the tropo fades at the same point the sea breeze stops.
 
When I built 101.5 in RI, I put up two DA antennas-one at 950 feet AAT and one at 640 feet AAT. They can switch back and forth with no local coverage loss (OK in the fringes the tall antenna does better). In this way they can get under the duct most of the time.
 
w9wi said:
I'm in TV -- back when we used to receive the network signals via terrestrial microwave, there were two sets of dishes at different elevations. During severe tropo events, the microwave signal was deflected into the other dish..
Click to expand...

My old station had dual dishes at a receive site, but more to deal with the issues related to temperature inversion more than tropo ducting.

Unless ducting is caused by temperature inversion, then my previous line is moot.

And Dana, 101.5 still has both antennas, and both are connected to transmitters.
The main is tied to the BE-FM30T/FMi-703 HD and the aux is tied to the BE-FM30. The aux TX can be fired up fairly quickly and feed the aux antenna in a hurry "as needed". Those are on a tower I oversee now.
 
Nice packet radio paths. These are packet stations that regularly communicate. How is this evidence of tropo? Am I missing something?

I looked at the Indiana paths and see these paths as stations that I know the locations of that communicate regularly. The paths don't outline the duct from Lafayette to Indy for example. The map seems to show existing links.

How did the dual antennas and transmitters work out?

I recall WIBC having 2 transmitters in Indy at 93.1. Sometimes two transmitters at once just created a mess that sounded like self hetrodyne.

Did someone just switch transmitters as needed?
 
Status
This thread has been closed due to inactivity. You can create a new thread to discuss this topic.


Back
Top Bottom