• 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

Interference vs Contours?

W

wonkastophat

Just trying to learn more about all this.. I've always been confused to the relationship between contours and interference.

For example, we have WCWP, WDNJ, WARY, and WYGG - which all share FM 88.1 and are spaced somewhat close together. There is some overlap between the "fringe" reception in some cases.. how does this overlap translate into interference?

Is there a distance rule of thumb for stations on the same channel?
 
For stations in the reserved band (88.1-91.9), the relevant protections are found in section 73.509 of the FCC's rules:


For commercial-band FMs (92.1-107.9), it's generally governed by 73.207, but there are also rules in 73.213 and 73.215 that apply in many cases.


These are just the basic framework of what can be a complicated regulatory system, especially when Canadian and Mexican borders and other such issues come into play. I've spent over 20 years learning all the intricacies and am still finding nuances.
 
If I'm reading 73.509 correctly, co-channel overlap is permitted, but not if the 40 dBu contour overlaps the 60 dBu contour of the other station (and vice-versa?)
 
wonkastophat said:
If I'm reading 73.509 correctly, co-channel overlap is permitted, but not if the 40 dBu contour overlaps the 60 dBu contour of the other station (and vice-versa?)
Click to expand...
That's correct. There's always some degree of "overlap" between signals, but the rules were built to provide a reasonable amount of protection from interference. The FCC's assumption is that a "desired" signal should be at least 20 dB above an "undesired" interfering signal, hence the 40-to-60 protection... but 60 dBu is somewhat arbitrary as a contour protected from interference. Class B commercial FM stations are protected to 54 dBu, while class B1 is protected to 57.
 
What does it sound like when you're in the middle of that overlap? Let's say the two co-channel stations are legal on those requirements, I'd imagine if there were two different audio signals it would try and switch between them and be unpleasant.

Maybe if they were both the same audio stream it wouldn't sound as bad?
 
wonkastophat said:
What does it sound like when you're in the middle of that overlap? Let's say the two co-channel stations are legal on those requirements, I'd imagine if there were two different audio signals it would try and switch between them and be unpleasant.

Maybe if they were both the same audio stream it wouldn't sound as bad?
Click to expand...
It sounds like interference. If you're in a car, you might go back and forth rapidly between locking on one signal or the other. Or it might just be a noisy mess.

Synchronizing both to the same audio source can be done and helps some, but it's still not going to be a really stable usable signal in most of the interference area.
 
The F(50,50) and F(50,10) contours are based on fairly flat terrain, like plains within a few miles of some rivers and large bodies of water, left by ancient receding waters. If there are substantial hills between the two signals, the hills block the signals from the other side or in terrain holes. In these cases, you may receive the interfering signal within the predicted protected contour. These situations are predicted more accurately using Longley Rice modeling. The proliferation of translators on tall buildings and TV towers in large cities, on second adjacent channels to locals, have resulted in many real interference situations. Those interfere in terrain holes of the protected contours of stations in nearby markets, which tend to only have station HAATs in the 100-200 meter range, leaving the holes within the predicted service area, or near the limits of earth curvature, just beyond the horizon, with interference. This was apparent with a translator on 104.7 beyond the 54 dBu contour of WIOT Toledo, OH, resulting in the translator being ordered to reduce their signal or go off the air. The rules for such complaints are now written to be so complicated so as to discourage complaints, in my opinion.
 
Last edited:
When I visit in Vallejo, I have 2 stations going back & forth on 101.7

KKIQ Livermore & KHTH Santa Rosa on my Sangean DT-200X turn right I get KKIQ turn left I get KHTH
 
wonkastophat said:
Just trying to learn more about all this.. I've always been confused to the relationship between contours and interference.

For example, we have WCWP, WDNJ, WARY, and WYGG - which all share FM 88.1 and are spaced somewhat close together. There is some overlap between the "fringe" reception in some cases.. how does this overlap translate into interference?

Is there a distance rule of thumb for stations on the same channel?
Click to expand...

I worked for an FM station that had a on channel fill in booster... not a translator, but a transmitter on the same frequency as the main transmitter that was placed inside the local 60dbu contour intended to fill in a hole in reception caused by terrain.

When you came out of the bowl where the booster was, the main and booster would clash and it would just be kinda staticy, like a signal fading in and out.
 
SomeRadioGuy said:
I worked for an FM station that had a on channel fill in booster... not a translator, but a transmitter on the same frequency as the main transmitter that was placed inside the local 60dbu contour intended to fill in a hole in reception caused by terrain.

When you came out of the bowl where the booster was, the main and booster would clash and it would just be kinda staticy, like a signal fading in and out.
Click to expand...
Interference fringes where the main signal and the booster. Radio waves are coherent. Light is incoherent. You cannot change the Laws of Physics. There may be things you can do to minimize how it sounds in the fringes. You can play with the ERP of the booster, so that the area of interference is minimized. Sometimes a Directional Antenna can help. Usually, unless the area to be filled in is behind a mountain, less ERP is usually better than high ERP. If you have a small shadow area, sometimes a few watts will fill it. You can sometimes see the shadow area on a Longley Rice Map by zooming way in, and try to put the booster close to the middle of the shadow. "Driving the signal" is often the best way to optimize it.
 
Schroedingers Cat said:
Interference fringes where the main signal and the booster. Radio waves are coherent. Light is incoherent. You cannot change the Laws of Physics. There may be things you can do to minimize how it sounds in the fringes. You can play with the ERP of the booster, so that the area of interference is minimized. Sometimes a Directional Antenna can help. Usually, unless the area to be filled in is behind a mountain, less ERP is usually better than high ERP. If you have a small shadow area, sometimes a few watts will fill it. You can sometimes see the shadow area on a Longley Rice Map by zooming way in, and try to put the booster close to the middle of the shadow. "Driving the signal" is often the best way to optimize it.
Click to expand...

The booster in this question was in a bowl.

they ended up turning the booster off and handing in the license. Whenever it went off, we only got one complaint. .every single time, from the same person... the goal was to give advertisers no reason to say.. we cant hear the station and try and sell in that market more. never worked.
 
SomeRadioGuy said:
The booster in this question was in a bowl.

they ended up turning the booster off and handing in the license. Whenever it went off, we only got one complaint. .every single time, from the same person... the goal was to give advertisers no reason to say.. we cant hear the station and try and sell in that market more. never worked.
Click to expand...
Why not just put a 1 watt booster right next to the person's house? With the same polarization as the person's receiver antennas, H, V, or 45 degree mount? One watt gets out amazingly well. During maintenance at one radio station, they put the exciter into the main antenna. From the digital dB signal readout on the Technics, the ERP was about 30 watts ERP, and it was getting out 15 miles without issues. All you had to do is adjust the telescoping antenna on a portable to get a very listenable signal.
 
Schroedingers Cat said:
Why not just put a 1 watt booster right next to the person's house? With the same polarization as the person's receiver antennas, H, V, or 45 degree mount? One watt gets out amazingly well. During maintenance at one radio station, they put the exciter into the main antenna. From the digital dB signal readout on the Technics, the ERP was about 30 watts ERP, and it was getting out 15 miles without issues. All you had to do is adjust the telescoping antenna on a portable to get a very listenable signal.
Click to expand...

Wasnt worth the expense any longer in any form or fashion.

One day when the man dropped off after a quick maintenance job on the exciter and wouldnt come back on, the booster was heard at my house 30 miles away.. but i was up on a hill.

(They had a spare exciter and quickly put it in)
 
Back
Top Bottom