Picket Fencing in Stereo Broadcasts

[shreveville]

I work at a 100,000 watt FM. There is a lot of picket fencing in the stereo broadcasts in the mountains. What do you think is the best antenna or transmitter power combo that would have the least amount of picket fencing in the mountains on a class c station? Smaller powered stations don't have as much interference.

 

[rcs]

Some more information would be helpful. Do the lower powered stations that appear to have less multipath have their antennas at the same location as the 100,000 watt station? If not, are they anywhere nearby and how does the height of their antennas compare to the 100k stations antenna? Are all of the stations using circular polarization?

Can you give us an idea of what part of the country the station is in.

Has the engineer checked the transmission system's AM noise? Excessive AM noise can exaggerate multipath problems.

The more information you provide the better we will be able to help but you will really need to have a professional engineer with RF experience look at the site and the general area and do some path studies on the various stations.

 

[fm-engineer]

I work at a 100,000 watt FM. There is a lot of picket fencing in the stereo broadcasts in the mountains. What do you think is the best antenna or transmitter power combo that would have the least amount of picket fencing in the mountains on a class c station? Smaller powered stations don't have as much interference.

As RCS mentioned, check the AM synchronous noise in the system. In the old days, -40 in the field was good for tube systems. That is around 1%. Today, any modern system should see -50 dB if the transmitter and antennas are tuned correctly. It is not uncommon to see -60 dB with solid state. We really need more info: type of transmitter, exciter, processing, antenna, etc. Excessive processing (clipping, dense compression) can make things far worse. Transmitters and antennas do not generate multi-path or picket fencing. That phenomenon is directly related to the terrain. The previously mentioned factors can make the multipath seem much worse than it is. Good luck.

 

[audiophile.]

I work at a 100,000 watt FM. There is a lot of picket fencing in the stereo broadcasts in the mountains. What do you think is the best antenna or transmitter power combo that would have the least amount of picket fencing in the mountains on a class c station? Smaller powered stations don't have as much interference.

As RCS mentioned, check the AM synchronous noise in the system. In the old days, -40 in the field was good for tube systems. That is around 1%. Today, any modern system should see -50 dB if the transmitter and antennas are tuned correctly. It is not uncommon to see -60 dB with solid state. We really need more info: type of transmitter, exciter, processing, antenna, etc. Excessive processing (clipping, dense compression) can make things far worse. Transmitters and antennas do not generate multi-path or picket fencing. That phenomenon is directly related to the terrain. The previously mentioned factors can make the multipath seem much worse than it is. Good luck.

I would never say never. The vertical pattern of the antenna could play a role (especially in mountains), a burnout or damaged interbay section (causing a phase shift), an inverted bay, a transmitter "christmas treeing", etc...

 

[radiorob2.0]

I work at a 100,000 watt FM. There is a lot of picket fencing in the stereo broadcasts in the mountains. What do you think is the best antenna or transmitter power combo that would have the least amount of picket fencing in the mountains on a class c station? Smaller powered stations don't have as much interference.

Depending on the type of mountains it may be an issue you can't resolve. Eastern Kentucky, for example, is multipath hell because of the thick mountains. There is one class C in Hazard that overcame the multipath problem by staying in mono. Several times through its history the station attempted stereophonic broadcasting only only to shut it off because stereo destroyed their coverage.

 

[fm-engineer]

I work at a 100,000 watt FM. There is a lot of picket fencing in the stereo broadcasts in the mountains. What do you think is the best antenna or transmitter power combo that would have the least amount of picket fencing in the mountains on a class c station? Smaller powered stations don't have as much interference.

As RCS mentioned, check the AM synchronous noise in the system. In the old days, -40 in the field was good for tube systems. That is around 1%. Today, any modern system should see -50 dB if the transmitter and antennas are tuned correctly. It is not uncommon to see -60 dB with solid state. We really need more info: type of transmitter, exciter, processing, antenna, etc. Excessive processing (clipping, dense compression) can make things far worse. Transmitters and antennas do not generate multi-path or picket fencing. That phenomenon is directly related to the terrain. The previously mentioned factors can make the multipath seem much worse than it is. Good luck.

 

[fm-engineer]

Agreed. Terrain is the key element. Where there are mountains, there’s multipath, period! Everything else; antenna patterns, phase, bandwidth, synchronous noise, pilot and subcarrier injection levels, audio clipping, etc. must all be crafted to minimize the effect of multipath. His problem may be something simple as too much stereo enhancement. I work in the Western PA, Maryland, and WV. You would be amazed at how the stereo coverage improves by just turning off the stereo enhancers in the current generation of digital audio processors. I would start with the transmitter. Make sure it’s tuned correctly. Check the AM noise. Look at the composite spectrum for any garbage around the pilot and 38 kHz. Check subcarrier levels. High RDS levels can degrade stereo coverage. Check the antenna tuning with a spectrum analyzer / tracking generator.

I work at a 100,000 watt FM. There is a lot of picket fencing in the stereo broadcasts in the mountains. What do you think is the best antenna or transmitter power combo that would have the least amount of picket fencing in the mountains on a class c station? Smaller powered stations don't have as much interference.

As RCS mentioned, check the AM synchronous noise in the system. In the old days, -40 in the field was good for tube systems. That is around 1%. Today, any modern system should see -50 dB if the transmitter and antennas are tuned correctly. It is not uncommon to see -60 dB with solid state. We really need more info: type of transmitter, exciter, processing, antenna, etc. Excessive processing (clipping, dense compression) can make things far worse. Transmitters and antennas do not generate multi-path or picket fencing. That phenomenon is directly related to the terrain. The previously mentioned factors can make the multipath seem much worse than it is. Good luck.

I would never say never. The vertical pattern of the antenna could play a role (especially in mountains), a burnout or damaged interbay section (causing a phase shift), an inverted bay, a transmitter "christmas treeing", etc...

 

[rrsounds]

Agreed. Terrain is the key element. Where there are mountains, there’s multipath, period! Everything else; antenna patterns, phase, bandwidth, synchronous noise, pilot and subcarrier injection levels, audio clipping, etc. must all be crafted to minimize the effect of multipath. His problem may be something simple as too much stereo enhancement. I work in the Western PA, Maryland, and WV. You would be amazed at how the stereo coverage improves by just turning off the stereo enhancers in the current generation of digital audio processors. I would start with the transmitter. Make sure it’s tuned correctly. Check the AM noise. Look at the composite spectrum for any garbage around the pilot and 38 kHz. Check subcarrier levels. High RDS levels can degrade stereo coverage. Check the antenna tuning with a spectrum analyzer / tracking generator.

shreveville,

The first thing I would do would be to carefully document the status quo, by taking the most comprehensive set of readings possible from the transmission and monitoring systems (including AM noise, and spectral data if possible), and noting the settings and levels of all stages of processing.

I would then find a few of the more notable reception trouble spots and make some audio recordings for reference. Since a few feet can make a great difference, move around a bit to find the best spot within the worst spots. You may even want to record as you move around, to get a somewhat arbitrary 'feel' for the depth of interference. (When I was CE at Z100 I was 'fortunate' that a reliably bad multipath spot was under a bridge just down the street from the studio. If an adjustment made our signal clearer there, I knew I was on the right path).

Once you are convinced you have your present situation documented, then try some of fm-engineer's steps [above] one at a time, and note after each step whether there seems to be any improvement at the trouble locations.

Yes, it's a lot of work, and don't be surprised if you find no improvement no matter what you do. It may indeed be the transmitter location, or an antenna issue. But having documented your before and after conditions, you may find a lead somewhere that helps you head the right direction.

BTW, my experience is that, as a general rule, fewer bays with higher transmitter power will help reduce multipath interference. If your present system has more than five bays, that may be a contributing factor. But it is a major undertaking to change!


Good luck, and keep us informed!

Kind Regards,
David

 

[shreveville]

Thanks so much for your help!