Tech Forum Archive

Assisted Listening device interferring on Amateur Radio band

24 postsstarted by studerje 13,612 viewsoriginal topic

studerje
New Member
#1
Not sure where to start, as the building equipment is clearly interfering with FCC frequencies licensed for amateur radio use when the chapel microphone is switched on.

A non-member friend of mine, and fellow ham, lives right down the street from a local chapel and reports the following:
The harmful interference is taking place on the 2m Amateur Radio Band, unfortunately in the low end or weak-signal portion of the band. That is the portion of the band reserved under the ARRL Band Plan for listening and working weak stations, meaning either far off (DX) stations or closer stations running low power. Worse, the transmitted interference is FM mode, but is received in the Single Side Band portion of the 2m meter band, meaning that there are heterodynes and the interference extends over a relatively wide range of frequencies (relative to narrow-band SSB operation) in the vicinity of 144.200 to 144.214 Mhz. Finally, since the transmission is FM mode the carrier is continuous, meaning that the interference is constant even if people aren’t speaking or music isn’t playing.

At the amateur radio end of things, the interference makes hearing and working amateur stations impossible in the vicinity of the interference. The timing is very unfortunate since most interference has been observed on Sundays, and it happens that Sunday mornings are when hams attempt to make long distance contact in this portion of the band. And also because the interference falls next to the National Calling frequency of 144.200 that is used to make these weak signal contacts. In simplest terms, the interference destroys the ability to participate.

At the transmitting end, this situation would be thought to be equally unwelcome since live voice transmissions can be plainly heard ‘outside the walls’ by third parties, and the content could be considered by those speaking to be of a private or sensitive nature. Note that reception is possible by anyone with a publicly available radio scanner, and not just by Amateur Radio Operators.

My hunch is that there is a wireless mic that uses a frequency somewhere around 72.103 MHz and that harmonics are not properly being suppressed, resulting in the out-of-band interference. Although this is a violation of FCC regulations, my primary interest is not so much to point fingers as it is to arrive at an effective and speedy resolution.
Where should I start?
russellhltn
Community Administrator
#2

Re: Assisted Listening device interferring on Amateur Radio

I'd suggest the FM group to contact the equipment manufacturer. The equipment ultimately falls under the FM Groups responsibility. The box itself is just a commercial unit. If nothing else, perhaps the channel could be changed to throw the harmonic into a different part of the band where it's less of a problem.

Note that it's the nature of the beast that harmonics can't be completely suppressed. They can only be filtered to the point where they are within legal guidelines. So it's possible the unit isn't defective, it's just an unfortunate combination of neighbors. Then again, a defective connection in the antenna could cause problems.
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Mikerowaved
Community Moderators
#3

Re: Assisted Listening device interferring on Amateur Radio

My hunch is that there is a wireless mic that uses a frequency somewhere around 72.103 MHz...
Yes, the church's Assisted Listening System operates in the 70 MHz band, in fact the default Channel "A" is indeed 72.100 MHz. Here's a breakdown of the channels available:

Code: 

CHAN  FREQ
----  ----
A #1  72.1 MHz
B #2  72.3 MHz
C #3  72.5 MHz
D #4  72.7 MHz
E #5  72.9 MHz
F #6  75.5 MHz
G #7  75.7 MHz
H #8  75.9 MHz
I #9  74.7 MHz
J #10 75.3 MHz
My suggestion would be to have the FM group change the frequency to "F" or above. This would throw any 2nd harmonic harmlessly up into the 150 MHz business band.
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russellhltn
Community Administrator
#4

Re: Assisted Listening device interferring on Amateur Radio

Mikerowaved wrote:My suggestion would be to have the FM group change the frequency to "F" or above. This would throw any 2nd harmonic harmlessly up into the 150 MHz business band.
The question is what can the receivers do? Can all of them tune 75MHz? (Not just the new ones, but the existing ones in use.)

"E" would throw the harmonic to 145.8 MHz. Still in the ham band, but outside of the weak signal area. (Borderline between "Miscellaneous and experimental modes" and "OSCAR subband")
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russellhltn
Community Administrator
#5

Re: Assisted Listening device interferring on Amateur Radio

As near as I can tell, all receivers can tune all of the channels - but I can't find the spec sheets for the older receivers.

I did find the specs for the current transmitter: It's licensed under Part 90, Output power is 120 mW, harmonic suppression is "Better than 50 dB below carrier".
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StevePoulsen
Church Employee
#6

Re: Assisted Listening device interferring on Amateur Radio

We received this same communication from the FM, I have made recommendations to him.
hopefully the situation is resolved shortly.
Steve Poulsen - Meetinghouse Facilities Technology Engineer
russellhltn
Community Administrator
#7

Re: Assisted Listening device interferring on Amateur Radio

russellhltn wrote:The question is what can the receivers do? Can all of them tune 75MHz? (Not just the new ones, but the existing ones in use.)
I uncovered a cache of Comtek PR-72b receivers. They're single-channel and do not appear to be tunable.
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StevePoulsen
Church Employee
#8

Re: Assisted Listening device interferring on Amateur Radio

russellhltn wrote: I uncovered a cache of Comtek PR-72b receivers. They're single-channel and do not appear to be tunable.
I trust you on this, I know the receivers have evolved over time, and that they were more limited than they are now, I always thought they had some range of tune-able frequencies/channels, but perhaps these are older than the ones i remember, that or my memory is failing me.
Steve Poulsen - Meetinghouse Facilities Technology Engineer
russellhltn
Community Administrator
#9

Re: Assisted Listening device interferring on Amateur Radio

StevePoulsen wrote:I always thought they had some range of tune-able frequencies/channels, but perhaps these are older than the ones i remember, that or my memory is failing me.
The ones I found are crystal controlled. There are some internal adjustments, but I don't know how much range you have to "fine tune" them. There are no user controls for tuning. Only a volume knob and a LED for power. These are easily identified by the color - brown center stripe with off-white front/back panels. The pictures you find on the Internet show a channel indicator, but the ones we have don't have it.
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StevePoulsen
Church Employee
#10

Re: Assisted Listening device interferring on Amateur Radio

russellhltn wrote:
StevePoulsen wrote:I always thought they had some range of tune-able frequencies/channels, but perhaps these are older than the ones i remember, that or my memory is failing me.
The ones I found are crystal controlled. There are some internal adjustments, but I don't know how much range you have to "fine tune" them. There are no user controls for tuning. Only a volume knob and a LED for power. These are easily identified by the color - brown center stripe with off-white front/back panels. The pictures you find on the Internet show a channel indicator, but the ones we have don't have it.
These are indeed old. If they remain functional, I see no reason to replace them, but they will die and need to be replaced eventually.
Steve Poulsen - Meetinghouse Facilities Technology Engineer
russellhltn
Community Administrator
#11

Re: Assisted Listening device interferring on Amateur Radio

StevePoulsen wrote:These are indeed old. If they remain functional, I see no reason to replace them, but they will die and need to be replaced eventually.
Agreed. They were found in a library clean-out after the retirement of the long-time librarian. But since they work on our system, I'll let the new librarians know how they work. As it happens, there's one librarian that complains of difficulty in hearing during sacrament meetings due to the echos in the chapel.
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aclawson
Senior Member
#12

Re: Assisted Listening device interferring on Amateur Radio

Until the problem is resolved it is critical that you stop using the devices immediately: it is a significant violation of federal regulations and I believe the fines start at $29,000 for each violation. Since you have been notified of the problem it is your (the church's) responsibility to stop the violations ASAP. The amateur radio operators have first right to those frequencies and have every right to file a formal complaint with the FCC which will take action.
russellhltn
Community Administrator
#13

Re: Assisted Listening device interferring on Amateur Radio

aclawson wrote:Until the problem is resolved it is critical that you stop using the devices immediately:
Not that simple. The transmitter is wired into the chapel sound system and comes on automatically. The cabnet is typically locked and local leaders do not have a key.

aclawson wrote:it is a significant violation of federal regulations and I believe the fines start at $29,000 for each violation.
I'm not entirely sure that's true. Keep in mind that unlike many sources of interference that are unintentional transmitters, this is an intentional transmitter. So far we have no evidence that it's not meeting it's specifications.

That's not to say there is no responsibility to resolve it, but it strikes me as unlikely that the FCC will issue any fines unless they feel that the church not addressing this with appropriate speed.
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aclawson
Senior Member
#14

Re: Assisted Listening device interferring on Amateur Radio

That's taking a gamble - there is no "we'll get around to it" exemption. Those frequencies belong to the amateur radio operators and are commonly used during emergency situations.

The situation is address by §15.5 of Title 47. Once somebody reports the problem to the FCC then you must stop using the device immediately and may not use it again until the problem is resolved.

§15.5 General conditions of operation.
(a) Persons operating intentional or unintentional radiators shall not be deemed to have any vested or recognizable right to continued use of any given frequency by virtue of prior registration or certification of equipment, or, for power line carrier systems, on the basis of prior notification of use pursuant to §90.35(g) of this chapter.

(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, or by an incidental radiator.

(c) The operator of a radio frequency device shall be required to cease operating the device upon notification by a Commission representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful interference has been corrected.
russellhltn
Community Administrator
#15

Re: Assisted Listening device interferring on Amateur Radio

COMTEK runs under Part 90, not Part 15.

Emergency communications do not normally use that frequqncy. It's usually in the voice section.
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Mikerowaved
Community Moderators
#16

Re: Assisted Listening device interferring on Amateur Radio

If changing the channel is not a viable solution, then it would be a simple matter (for someone with access) to install a low-pass filter on the transmitter. Something like the $35 Mini-Circuits BLP-90+ would reduce 144MHz emissions by ~40dB, while only adding a negligible 0.5dB insertion loss at 72MHz.
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aclawson
Senior Member
#17

Re: Assisted Listening device interferring on Amateur Radio

russellhltn wrote:COMTEK runs under Part 90, not Part 15.

Emergency communications do not normally use that frequqncy. It's usually in the voice section.
Emergency communications use whatever happens to work at the time.

Knowingly causing interference to the amateur bands is highly illegal and can bring a lot of trouble upon the church - I checked with an RF engineer I know who spends every day dealing with FCC violations and he relates that the FCC has a well established history of walking into church during services and shutting things down over this kind of thing. Further, Part 90 subpart N explicitly states that the operator must take reasonable steps to eliminate harmful interference.

Beyond that, blocking amateur radio frequencies is being a bad neighbor. Skip the very real risk of fines and bad PR, eliminating the harmful interference is simply the right thing to do.

Don't have the key to the cabinet? Get FM out there immediately - if they are too busy then this is definitely an instance where the SP should get involved. This is serious business and really can't wait. The FCC will not accept "well, the guy with the key to the cabinet was too busy to get around to this" and if you have a crotchety amateur willing to send a constant stream of complaints to the FCC then bad times are in store.

OP is almost two months old and even though there does not seem to be any sense of urgency in the thread it really is a big deal.
aebrown
Community Administrator
#18

Re: Assisted Listening device interferring on Amateur Radio

aclawson wrote:OP is almost two months old and even though there does not seem to be any sense of urgency in the thread it really is a big deal.
There's been some really helpful information posted in this discussion. It's clear that certain kinds of interference can create significant legal liability for the Church.

But I would note that just about everything posted after the original post about what may or may not have been done is speculation. The only real piece of information about the actual situation mentioned in the OP is this post from a Church employee. There's no reason to assume that the FM group he contacted did not take prompt action and deal with it. The original poster has never responded to let us know about the resolution (or non-resolution) of the situation.
Questions that can benefit the larger community should be asked in a public forum, not a private message.
russellhltn
Community Administrator
#19

Re: Assisted Listening device interferring on Amateur Radio

aclawson wrote:I checked with an RF engineer I know who spends every day dealing with FCC violations and he relates that the FCC has a well established history of walking into church during services and shutting things down over this kind of thing.
I'd be interested in reports of such actions. I'm guessing it's done for far more egregious violations such as unlicensed equipment.

aclawson wrote:Further, Part 90 subpart N explicitly states that the operator must take reasonable steps to eliminate harmful interference.
Subpart N doesn't look like a good fit for this service. It appears to be more for two-way communication.

Subpart B looks to be the correct section. The frequency in question places a burden to assure no interference to TV channels 4 or 5.

aclawson wrote:Beyond that, blocking amateur radio frequencies is being a bad neighbor. Skip the very real risk of fines and bad PR, eliminating the harmful interference is simply the right thing to do.
I've not suggested otherwise. But I think your statements are a little over the top and do not reflect the most likely sequence of events. It's not something to be ignored, but I wouldn't go into a massive panic over it.

aclawson wrote:OP is almost two months old and even though there does not seem to be any sense of urgency in the thread it really is a big deal.
We never heard back from the OP what the resolution was, but if you look at Post #6, you'll see that people where involved. It probably got resolved shortly after.
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aclawson
Senior Member
#20

Re: Assisted Listening device interferring on Amateur Radio

No, bringing awareness of the potential of breaking federal law and international treaty is not "over the top". The prevailing attitude is "sure, dude, whatever, we'll do whatever we want and nobody will care".
it strikes me as unlikely that the FCC will issue any fines unless they feel that the church not addressing this with appropriate speed.
is just such an example.

I am a licensed amateur radio operator. Operating rules and regulations matter to me a great deal, especially in a world where people have no respect or consideration for such "unimportant" laws. It was in clerk discussions years ago about running networks over power lines within a meetinghouse where it was pointed out that (especially at the time with such new solutions) such things could completely block amateur communications. Nobody cared.

I hear of various wards/stakes investing in two way radios for use during various events. Point out that it is illegal to use the GMRS frequencies without a license and you are either ignored or get a comment along the lines of "if they were illegal you couldn't buy them" or "nobody cares about that law". Doubly so if you are in one of the areas subject to the international exclusion of GMRS use on 462.650, 467.6500, 462.7000 or 467.7000 MHz. Again, try to point this out to almost anybody and the response is "eh, nobody cares. Don't be over the top."

The point is that this kind of thing isn't something that should be messed with or taken lightly. That guy in Florida received notice of $16,000 fines *per incident* because he didn't think that it was important to put a filter on his well pump. I'm not in the loop on the fines and enforcement actions, the RF engineer is. That's his job. He says he just heard of somebody fined $7,500 for refusing to replace a faulty dimmer switch.

Do not ignore these things. Read the manuals. Keep up with regulation changes. There is no call to dismiss cautionary notices out of hand as is being done here. That is all.
russellhltn
Community Administrator
#21

Re: Assisted Listening device interferring on Amateur Radio

aclawson wrote:Do not ignore these things. Read the manuals. Keep up with regulation changes. There is no call to dismiss cautionary notices out of hand...
We're in agreement.

aclawson wrote:... as is being done here.
And on this, I disagree. We may not be handling it with the same sense of urgency you desire, but no one is advocating to ignore it.
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lajackson
Community Moderators
#22

Re: Assisted Listening device interferring on Amateur Radio

aclawson wrote:There is no call to dismiss cautionary notices out of hand as is being done here. That is all.
I do not believe anything is being dismissed out of hand here. There is nothing to dismiss. No one here has any idea of the real situation because the original poster has not returned. And even if they did, no one here has any authority or path toward a resolution.

If the original poster returned and there was still a problem, they would probably be invited to contact their FM Group with the encouragement that they continue to escalate the matter until it is completely resolved, because it is a serious matter and not something the FM Group can just ignore. Other than that, there is not much more anyone here can do.

You have made your point very well.
russellhltn
Community Administrator
#23

Re: Assisted Listening device interferring on Amateur Radio

lajackson wrote:If the original poster returned and there was still a problem, they would probably be invited to contact their FM Group with the encouragement that they continue to escalate the matter until it is completely resolved,
It should be noted from Post #6 that the FM group did contact a church employee over meetinghouse technology about the issue. So I think it's safe to say that FM group has been involved for some time now.

It seem to me there are a limited number of possible solutions:
  • Verify the antenna is installed correctly. (Maybe I'm remembering the wrong restriction, but I do remember a limitation to avoid horizontal polarization.)
  • Replace the transmitter and antenna on the chance that it's defective
  • Add a low pass filter
  • Change channels
None of these should take much time to do if no one drops the ball.
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StevePoulsen
Church Employee
#24

Re: Assisted Listening device interferring on Amateur Radio

Good Mormning gentlemen.

I am chiming in as reading the forum communication so far it appears to me that tow separate conversations have merged into 1, and it is causing confusion.
The original post by studerje "was in reference to a problem where the Assisted Listening System" ALS appears to be causing interference problems at approximately 144.3 Mhz range.

I have personally reached out to the FM group responsible for the building and equipment in question and made recommendations for repair, and believe that the situation has been resolved. If anyone knows differently, let me know and we will resolve it.

Before I had reached out to the FM group, it was suggested that the problem could be resolved by changing the transmission frequency, so that the harmonics were thrown up into another spectrum where they would be less noticeable. This spawned the question of weather or not the old receivers could tune to those higher frequencies

Shortly after I originally reached out to the FM group,
russellhltn wrote:The question is what can the receivers do? Can all of them tune 75MHz? (Not just the new ones, but the existing ones in use.)
As it turns out they can not. russellhltn posted such after he went and looked at the receivers in his own bldg. please note this sin not the same building as the original poster had concerns about.

The equipment in the original bldg was replaced with new equipment that resolved the issue the same week as my original post that we were working with he FM group to resolve the issue.
I can assure everyone that the church as a whole takes the issue of complying with all laws. And upon being notified of the issue, moved as fast as we passably could to ensure compliance.

Thank you all for your concerns.
Steve Poulsen - Meetinghouse Facilities Technology Engineer