• 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 Question

C

C414

Guest
I work for an audio-video contractor that installs professional sound systems. We've got a church we did an install at that is within sight of an AM stick/1kW XMTR combo. We installed balanced audio cable throughout (except for the speaker wire, of course) and the electrical system AFAIK is grounded properly. All the wireless mic systems in the church are UHF and true-diversity.

Here's the problem....we're getting complaints from the church that the AM station is bleeding through on their sound system. It's not LOUD loud, but it's loud enough to be heard when there's no audio passing through the system (mixing console is in full mute). Here's the other problem...there are other churches within the vicinity that are complaining of the same situation.

Whose responsibility is this to correct? I'm thinking it may be the station, but am not for sure. Is there a device that would plug inline that filters this type RFI and if so, what is it???

BTW & FWIW, the freq is 1450 kHz. Won't disclose more info than that for now.<P ID="signature">______________
Failure to plan on your part doesn't constitute an emergency on my part.</P>
 
Don't blame the radio station - it just means they're gettin' out good. What's probably happening is the speaker wires are acting like an antenna and dumping considerable RF back into the amplifier where it is being demodulated. I would try some 0.001uF ceramic disc capacitors from each speaker terminal at the amplifier to the chassis. Designers of most PA systems don't even consider RF ingress as a possible problem (the same goes for most telephone systems). Low level inputs such as microphones can be suseptable also. Try unplugging each input one at a time to see if you can isolate the culprit. I have 5kW AM transmitters running in the next room without a problem so I know it can be solved. Good luck!
 
>
> Whose responsibility is this to correct? I'm thinking it may
> be the station, but am not for sure.

Have you informed the offending station? It is indeed their responsibility
to respond in kind and make every effort to help solve the interference complaint, especially since it is occurring in several establishments in the area. Make sure they know it is more than one location by banding together and writing the station a letter, but don't mail it until after you have contacted them in person. If they blow you off in person, you can send a copy of the letter you are sending them to the FCC, but don't go that route unless absolutely required.

> Is there a device that
> would plug inline that filters this type RFI and if so, what
> is it???

Every interference situation is not always easily solved. I would start by going to the power amp(s), removing all input lines (one at a time) (yes, I know you said they were balanced, but do it anyway .. all it takes is one bad XLR) and evaluate. If the speaker lines are found to be inducing the RF back into the final outputs, see the other post for a possible solution ... make sure the power amp's ground has not been compromised ... there is no magic bullet device to solve AM interference. AM is easily demodulated by a single diode junction, whether that be part of a transistor in the audio stages or in the power supply. In high fields, it can sometimes be a real dog job to totally eliminate it out of a P.A. system with thousands of feet of input cable and output speaker lines. <P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>
 
> >
> > Whose responsibility is this to correct? I'm thinking it
> may
> > be the station, but am not for sure.
>
> Have you informed the offending station? It is indeed their
> responsibility
> to respond in kind and make every effort to help solve the
> interference complaint, especially since it is occurring in
> several establishments in the area. Make sure they know it
> is more than one location by banding together and writing
> the station a letter, but don't mail it until after you have
> contacted them in person. If they blow you off in person,
> you can send a copy of the letter you are sending them to
> the FCC, but don't go that route unless absolutely required.
>

That's not entirely true. Since the PA is a part 15 device, it is required to "accept" any interference. The radio station is not required to eliminate the problem. Just like with telephones.

Now, having said that, most radio stations are good neighbors. I have been known to grab a bunch of toroids and help clean up RF problems. The key is to be polite when calling and don't threaten to call the FCC, etc.

The only time the FCC will get involved is if there is interference with a licensed user. The most you'll get from the FCC is a few tips on reducing RF interference.<P ID="signature">______________
</P>
 
> >
> > Whose responsibility is this to correct? I'm thinking it
> may
> > be the station, but am not for sure.
>
> Have you informed the offending station? It is indeed their
> responsibility
> to respond in kind and make every effort to help solve the
> interference complaint, especially since it is occurring in
> several establishments in the area. Make sure they know it
> is more than one location by banding together and writing
> the station a letter, but don't mail it until after you have
> contacted them in person. If they blow you off in person,
> you can send a copy of the letter you are sending them to
> the FCC, but don't go that route unless absolutely required.

Excuse me??? YOU HAVE GOT TO BE KIDDING!! If the AM is only a KW and the church is picking up the station on their wiring, there is NOTHING requiring the station to fix the problem UNLESS they are operating illegally (I assume this is a station that has existed for a while at this site)..but to make such a blanket statement as you did above is totally ludicrous.
Legally, they are required by Part 73 to correct any illegal problem...but other than that, ANY RF overload, etc situation is NOT their problem to fix.
 
> > >
> > > Whose responsibility is this to correct? I'm thinking it
>
> > may
> > > be the station, but am not for sure.
> >
> > Have you informed the offending station? It is indeed
> their
> > responsibility
> > to respond in kind and make every effort to help solve the
>
> > interference complaint, especially since it is occurring
> in
> > several establishments in the area. Make sure they know it
>
> > is more than one location by banding together and writing
> > the station a letter, but don't mail it until after you
> have
> > contacted them in person. If they blow you off in person,
> > you can send a copy of the letter you are sending them to
> > the FCC, but don't go that route unless absolutely
> required.
>
> Excuse me??? YOU HAVE GOT TO BE KIDDING!! If the AM is only
> a KW and the church is picking up the station on their
> wiring, there is NOTHING requiring the station to fix the
> problem UNLESS they are operating illegally (I assume this
> is a station that has existed for a while at this site)..but
> to make such a blanket statement as you did above is totally
> ludicrous.
> Legally, they are required by Part 73 to correct any illegal
> problem...but other than that, ANY RF overload, etc
> situation is NOT their problem to fix.
>


Ditto. I can't beleive he said that.

Yes, the station is responsible...to assure they are operating within licensed parameters. That's it. And I would bet the station predates the new PA system by a few decades...

<P ID="signature">______________
Never hold a cat and a dustbuster at the same time.</P>
 
Um, Check out 73.88...

> Excuse me??? YOU HAVE GOT TO BE KIDDING!! If the AM is only
> a KW and the church is picking up the station on their
> wiring, there is NOTHING requiring the station to fix the
> problem UNLESS they are operating illegally (I assume this
> is a station that has existed for a while at this site)..but
> to make such a blanket statement as you did above is totally
> ludicrous.
> Legally, they are required by Part 73 to correct any illegal
> problem...but other than that, ANY RF overload, etc
> situation is NOT their problem to fix.

Sec. 73.88 Blanketing interference.

The licensee of each broadcast station is required to satisfy all
reasonable complaints of blanketing interference within the 1 V/m
contour.
 
I had a similar problem several years ago at my church. Everyone said the station was running higher power than they should be. I knew better, I was the contract engineer for the station.
Replaceing the speaker wires with shielded cable fixed the problem. Grounded the shield to a good ground worked better than to the amp frame ground.
 
Re: Um, Check out 73.88...

> Sec. 73.88 Blanketing interference.
>
> The licensee of each broadcast station is required to
> satisfy all
> reasonable complaints of blanketing interference within the
> 1 V/m
> contour.
>

Keep digging! 73.88 also says "For more detailed instructions concerning operational responsibilities of licensees and permittees under this section, see Sec. 73.318 (b), (c) and (d)."

"Sec. 73.318 (b)"
<snip>
"These requirements specifically do not include interference
complaints resulting from malfunctioning or mistuned receivers,
improperly installed antenna systems, or the use of high gain antennas
or antenna booster amplifiers. Mobile receivers and non-RF devices such
as tape recorders or hi-fi amplifiers (phonographs) are also excluded."
<snip>
 
> Don't blame the radio station - it just means they're
> gettin' out good. What's probably happening is the speaker
> wires are acting like an antenna and dumping considerable RF
> back into the amplifier where it is being demodulated. I
> would try some 0.001uF ceramic disc capacitors from each
> speaker terminal at the amplifier to the chassis. Designers
> of most PA systems don't even consider RF ingress as a
> possible problem (the same goes for most telephone systems).
> Low level inputs such as microphones can be suseptable
> also. Try unplugging each input one at a time to see if you
> can isolate the culprit. I have 5kW AM transmitters running
> in the next room without a problem so I know it can be
> solved. Good luck!
>

Sometimes at AM frequencies, a .01uF will work better, but COULD possibly roll off some high-end. Try it in combination from the leads to ground and across the leads on the speakers and the inputs. Also, don't forget the AC power. Radio Shaft has several power strips that have broadband noise filters that provide over 40db filtering that will likely help the situation. Also, grounding the chassis MAY help. Try it and see.. Sometimes adding a ground CAN cause MORE problems, but in most cases it'll help reduce the problem. As the other guy mentioned, try removing all the inputs and see if it still sings. Adding stuff back one by one with the amp on, you'll see what is causing issues if it's an input problem. Incedently, you might have some better luck with XLR connectors if you ground the outside case tab to pin 1. Power filters and .01s will likely you're best friend on this project. If you still have issues, give me a shout back here and I'll try to help further. Needless to say, I've been down the RF-hell road more times than I can count, and most of the time I win. ;)


<P ID="signature">______________
--- THE Insultant ---</P>
 
The station is responsible within a certain distance. Thank God for the internet: http://www.scott-inc.com/html/73318.htm

As said in lower threads, isolate the problem and go from there. Once you find the source, either replace the cable or use an isolation transformer. Also, as mentioned below, be kind to the station as honey goes a lot further than vinegar.

Of course there is door number three, build a Faraday cage over the church.

http://en.wikipedia.org/wiki/Faraday_cage

If door number one or two doesn't work, come back and we'll try again.
<P ID="signature">______________
The radio business is a cruel and shallow money trench, a long plastic hallway where thieves and pimps run free, and good men die like dogs. There's also a negative side.</P>
 
> > >
> > > Whose responsibility is this to correct? I'm thinking it
>
> > may
> > > be the station, but am not for sure.
> >
> > Have you informed the offending station? It is indeed
> their
> > responsibility
> > to respond in kind and make every effort to help solve the
>
> > interference complaint, especially since it is occurring
> in
> > several establishments in the area. Make sure they know it
>
> > is more than one location by banding together and writing
> > the station a letter, but don't mail it until after you
> have
> > contacted them in person. If they blow you off in person,
> > you can send a copy of the letter you are sending them to
> > the FCC, but don't go that route unless absolutely
> required.
>
> Excuse me??? YOU HAVE GOT TO BE KIDDING!!

It's in the rules, if you'd bother to read them.

I wrote "It is indeed their responsibility
to respond in kind and make every effort to help solve the interference complaint"

The rules state, "(d) Following the one year period of full financial obligation to satisfy blanketing complaints, licensees shall provide technical information or assistance to complainants on remedies for blanketing interference."

This means ALWAYS. Whereas the station may not be financially responsible, they are still mandated TO RESPOND to every interference complaint, no matter the time frame of complaint and program test authority (age of license).

>UNLESS they are operating illegally ... a blanket statement as you did above is totally ludicrous.

What's ludicrous is your familiarity and practical interpretation of the rules.

It's still the station's best defense to prove interference is being caused by
the complainant's equipment, which may be exempted from licensee financial committment to repair or modify, but nowhere does the notion of operating outside parameters come into play. This can only be achieved by cooperating with the complainant, not blowing them off as you apparently would do.

<P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>
 
> Ditto. I can't beleive he said that.
>
> Yes, the station is responsible...to assure they are
> operating within licensed parameters. That's it.

You are also misinformed, as is CW.

See my response to him for chapter and verse.

It may be true that licensee's are becoming more difficult to challenge at renewal time, but the rules of interference and the obligation of licensees to investigate EVERY interference complaint no matter what the circumstance have remained intact.<P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>
 
Re: Um, Check out 73.88...

> > Sec. 73.88 Blanketing interference.
> >
> > The licensee of each broadcast station is required to
> > satisfy all
> > reasonable complaints of blanketing interference within
> the
> > 1 V/m
> > contour.
> >
>
> Keep digging! 73.88 also says "For more detailed
> instructions concerning operational responsibilities of
> licensees and permittees under this section, see Sec. 73.318
> (b), (c) and (d)."
>
> "Sec. 73.318 (b)"
>
> "These requirements specifically do not include interference
>
> complaints resulting from malfunctioning or mistuned
> receivers,
> improperly installed antenna systems, or the use of high
> gain antennas
> or antenna booster amplifiers. Mobile receivers and non-RF
> devices such
> as tape recorders or hi-fi amplifiers (phonographs) are also
> excluded."
>

While your at it, read (d) or see my response to CW.
Stations must still respond and investigate and help the complainant, how else can one know whether the equipment is at fault. AM stations of age develop ground radial discrepancies that cause some weird interference problems. I know of a 45 year old AM site that began demodding on a water pipe in several homes ... and further investigation found they had broken ground radials reradiating into the water system in the area. I suppose you would have told all the people in the neighborhood that "we're operating legally and water pipes are not on the list of stuff we have to fix." Great p.r.
<P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>
 
Re: Um, Check out 73.88...

>> The licensee of each broadcast station is required to
>> satisfy all reasonable complaints of blanketing interference
>> within the 1 V/m contour.
________

According to the FCC's propagation charts, the 1 V/m contour radius for a non-directional, 1 kW AM on 1450 using a 1/2-wave tower and a ground conductivity of 15 mS/m is about 0.24 miles. This station's parameters most likely are not this good, and the radius would be less.

If the station was directional and the church was located in a direction where the station's ERP was 5 kW, the 1 V/m radius would be 0.51 miles (other conditions the same).

So if the church is located beyond the appropriate distance above from this AM station, the station would have no legal obligation to take action, but might choose to do so as a "good neighbor."

Even if the church was within the 1 V/m contour, the interference is being caused to a non-RF device, which is not covered by the station's legal obligation to correct blanketing interference, in any case.
 
RFI response and (non-invasive) cures

> The rules state, "(d) Following the one year period of full financial
> obligation to satisfy blanketing complaints, licensees shall provide
> technical information or assistance to complainants on remedies for
> blanketing interference."

> This means ALWAYS. Whereas the station may not be financially responsible,
> they are still mandated TO RESPOND to every interference complaint, no matter
> the time frame of complaint and program test authority (age of license).

That's right! Although there are some misconceptions in other posts about the FCC requirements. Also some good troubleshooting ideas in some of the posts.


I've done RFI resolution for many years. What does it take for a station to "RESPOND"? Depending on the circumstances and the client station a "response" could be anything from a RFI housecall to my form letter and mailing (approved by the client's legal department).

I've made hundreds of RFI house calls a year on behalf of AM and FM stations and the vast majority of cases are NOT covered by the FCC and are not the fault of the radio station - it's hard to convince the neighbors that the problem is theirs (but that's the PR part). Some stations do want to be good neighbors and go the extra distance while others just don't care. PA systems, telephones, cassette recorders and many other items are NOT covered by the FCC with regard to RFI resolution.

A quick sweep with an AM Field Intensity meter (FIM) will show if there is a re-radiation problem. An isolated RFI case is one thing, a sudden surge of RFI complaints just might indicate a station problem.

Although many RFI cures can be done inside equipment, I do not want the liability. Every measure I take on behalf of a client station is done externally and can be removed in minutes to restore the situation to the way it was prior to any RFI abatement. In rare cases I have purchased a similar device (usually telephone), filtered it internally and then GIVEN (as in gift) the item to the neighbor. No liability if the device breaks. I don't even like to open wall plates or phone Network Interfaces (but I do, and sometimes add filters at those points).

For some information about RFI with sources of filters (and soon, a do-it-yourself RFI troubleshooting guide) can be found here:

Broadcast RFI

Good luck. <P ID="edit"><FONT class="small">Edited by Uncle AL on 04/15/06 09:51 PM.</FONT></P>
 
Re: Um, Check out 73.88...

>
> While your at it, read (d) or see my response to CW.
> Stations must still respond and investigate and help the
> complainant, how else can one know whether the equipment is
> at fault. AM stations of age develop ground radial
> discrepancies that cause some weird interference problems. I
> know of a 45 year old AM site that began demodding on a
> water pipe in several homes ... and further investigation
> found they had broken ground radials reradiating into the
> water system in the area. I suppose you would have told all
> the people in the neighborhood that "we're operating legally
> and water pipes are not on the list of stuff we have to
> fix." Great p.r.
>

I don't think anyone is saying that. What most here seem to be saying is simply that there is no legal obligation to do anything (which is correct).

I can't speak for others, but I know that 99% of the time I'll help in the spirit of being a good neighbor.

Now, I tend to be a little less helpful if the first sentences contain the phrases "you must" and "I'll call the FCC".

My unwritten policy is that I will provide one phone filter/AC filter/torroid along with instructions on how to use them free of charge. There is a list of places to buy more included in the instructions. I WILL NOT install them, due to liability issues. Again, with this policy I have never had a return call to deal with interference issues.

Bottom line, I can't see any of the Engineers here just telling someone to go pound sand. On the other hand, someone with an interference issue asked about the responsbility of the offending station, and we correctly replied that there was none. I think the reason we went out of the way to point that out is that there is no quicker way to lose the cooperation of the station's management then to get on the phone and start making demands with the threat of calling the FCC.<P ID="signature">______________
</P>
 
> > > >
> > > > Whose responsibility is this to correct? I'm thinking
> it
> >
> > > may
> > > > be the station, but am not for sure.
> > >
> > > Have you informed the offending station? It is indeed
> > their
> > > responsibility
> > > to respond in kind and make every effort to help solve
> the
> >
> > > interference complaint, especially since it is occurring
>
> > in
> > > several establishments in the area. Make sure they know
> it
> >
> > > is more than one location by banding together and
> writing
> > > the station a letter, but don't mail it until after you
> > have
> > > contacted them in person. If they blow you off in
> person,
> > > you can send a copy of the letter you are sending them
> to
> > > the FCC, but don't go that route unless absolutely
> > required.
> >
> > Excuse me??? YOU HAVE GOT TO BE KIDDING!!
>
> It's in the rules, if you'd bother to read them.
>
> I wrote "It is indeed their responsibility
> to respond in kind and make every effort to help solve the
> interference complaint"
>
> The rules state, "(d) Following the one year period of full
> financial obligation to satisfy blanketing complaints,
> licensees shall provide technical information or assistance
> to complainants on remedies for blanketing interference."
>
> This means ALWAYS. Whereas the station may not be
> financially responsible, they are still mandated TO RESPOND
> to every interference complaint, no matter the time frame of
> complaint and program test authority (age of license).
>
> >UNLESS they are operating illegally ... a blanket statement
> as you did above is totally ludicrous.
>
> What's ludicrous is your familiarity and practical
> interpretation of the rules.
>
> It's still the station's best defense to prove interference
> is being caused by
> the complainant's equipment, which may be exempted from
> licensee financial committment to repair or modify, but
> nowhere does the notion of operating outside parameters come
> into play. This can only be achieved by cooperating with the
> complainant, not blowing them off as you apparently would
> do.
>

You are incorrect. The rules apply to interference to licensed services. (IE: Blanketing interferece to TV or other Radio stations). It DOES NOT apply to PA systems or phones, or anything of the sort.<P ID="signature">______________
</P>
 
> The station is responsible within a certain distance. Thank
> God for the internet:
> http://www.scott-inc.com/html/73318.htm
>
> As said in lower threads, isolate the problem and go from
> there. Once you find the source, either replace the cable
> or use an isolation transformer. Also, as mentioned below,
> be kind to the station as honey goes a lot further than
> vinegar.
>
> Of course there is door number three, build a Faraday cage
> over the church.
>
> http://en.wikipedia.org/wiki/Faraday_cage
>
> If door number one or two doesn't work, come back and we'll
> try again.
>

Yes, thank God for the internet indeed:

These requirements specifically do not include interference
complaints resulting from malfunctioning or mistuned receivers,
improperly installed antenna systems, or the use of high gain antennas
or antenna booster amplifiers. Mobile receivers and non-RF devices such
as tape recorders or hi-fi amplifiers (phonographs) are also excluded.<P ID="signature">______________
</P>
 
> > Ditto. I can't beleive he said that.
> >
> > Yes, the station is responsible...to assure they are
> > operating within licensed parameters. That's it.
>
> You are also misinformed, as is CW.
>
> See my response to him for chapter and verse.
>
> It may be true that licensee's are becoming more difficult
> to challenge at renewal time, but the rules of interference
> and the obligation of licensees to investigate EVERY
> interference complaint no matter what the circumstance have
> remained intact.
>


Again, simply not true. I'll bold the important part for you:

[C F R]
[Title 47, Volume 4, Parts 70 to 79]
[October 1, 1996]

FM blanketing interference.

Areas adjacent to the transmitting antenna that receive a signal
with a strength of 115 dBu (562 mV/m) or greater will be assumed to be blanketed. In determining the blanketed
area, the 115 dBu contour is determined by calculating the inverse
distance field using the effective radiated power of the maximum
radiated lobe of the antenna without considering its vertical radiation
pattern or height. For directional antennas, the effective radiated
power in the pertinent bearing shall be used.
(a) The distance to the 115 dBu contour is determined using the
following equation:

D (in kilometers)= [ .394 times the square root of P ]
D (in miles)= [ .245 times the square root of P ]

Where P is the maximum effective radiated power (ERP), measured in
kilowatts, of the maximum radiated lobe.
(b) After January 1, 1985, permittees or licensees who either (1)
commence program tests, or (2) replace their antennas, or (3) request
facilities modifications and are issued a new construction permit must
satisfy all complaints of blanketing interference which are received by
the station during a one year period. The period begins with the
commencement of program tests, or commencement of programming utilizing
the new antenna. Resolution of complaints shall be at no cost to the
complainant. These requirements specifically do not include interference
complaints resulting from malfunctioning or mistuned receivers,
improperly installed antenna systems, or the use of high gain antennas
or antenna booster amplifiers. Mobile receivers and non-RF devices such
as tape recorders or hi-fi amplifiers (phonographs) are also excluded.
(c) A permittee collocating with one or more existing stations and
beginning program tests on or after January 1, 1985, must assume full
financial responsibility for remedying new complaints of blanketing
interference for a period of one year. Two or more permittees that
concurrently collocate on or after January 1, 1985, shall assume shared
responsibility for remedying blanketing complaints within the blanketing
area unless an offending station can be readily determined and then that
station shall assume full financial responsibility.
(d) Following the one year period of full financial obligation to
satisfy blanketing complaints, licensees shall provide technical
information or assistance to complainants on remedies for blanketing
interference.<P ID="signature">______________
</P>
 
Status
This thread has been closed due to inactivity. You can create a new thread to discuss this topic.


Back
Top Bottom