Assisted Listening device interferring on Amateur Radio band

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studerje
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Assisted Listening device interferring on Amateur Radio band

Postby studerje » Mon Mar 10, 2014 3:38 pm

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
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Re: Assisted Listening device interferring on Amateur Radio

Postby russellhltn » Mon Mar 10, 2014 4:08 pm

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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Re: Assisted Listening device interferring on Amateur Radio

Postby Mikerowaved » Mon Mar 10, 2014 4:23 pm

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: Select all

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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Re: Assisted Listening device interferring on Amateur Radio

Postby russellhltn » Mon Mar 10, 2014 4:42 pm

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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Re: Assisted Listening device interferring on Amateur Radio

Postby russellhltn » Mon Mar 10, 2014 4:59 pm

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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Re: Assisted Listening device interferring on Amateur Radio

Postby StevePoulsen » Wed Mar 26, 2014 8:12 am

We received this same communication from the FM, I have made recommendations to him.
hopefully the situation is resolved shortly.

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Re: Assisted Listening device interferring on Amateur Radio

Postby russellhltn » Sun Apr 06, 2014 12:07 pm

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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Re: Assisted Listening device interferring on Amateur Radio

Postby StevePoulsen » Wed Apr 09, 2014 8:24 am

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.

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Re: Assisted Listening device interferring on Amateur Radio

Postby russellhltn » Wed Apr 09, 2014 10:21 am

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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Re: Assisted Listening device interferring on Amateur Radio

Postby StevePoulsen » Wed Apr 09, 2014 2:36 pm

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.


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