FCC Actions Against FM

Seeing the numbers you have taken the time to present goes a long way to allay the uncertainty recently stirred.

Of course within the reality of it is the fact that FM broadcast using a certified transmitter according to proper instructions for Part 15 is of little use for reaching an audience in the surrounding neighborhood.

But, those of us who have "very near field" uses for FM transmitters can perhaps stop talking about abondoning FM.

Thank you Neil.

Very interesting data!

I would never abandon Part 15

FM - I love to play with FM transmitters.

In my case, I just do the house and 

yard, though.  That's all I'm comfortable with.

This is great data - very interesting.

Bruce, The Dog Radio Group

The link below shows the power needed for a 1/2-wave, center-fed dipole to produce the fields at the distances shown, and as reported in the NOUOs mentioned by Neil.

It may be a surprise that the majority of them needed less than 10 mW to result in an NOUO.

Little toy FM transmitters like the C.Crane only have an 11" telescoping antenna so are probably safe, although we don't actually know what the output power is.

The Wholehouse 2.0 came with an instruction sheet for cutting the supplied wire to 1/2 wavelength, but that's only the top half of a dipole and I notice that adding the antenna doesn't add much influence to the transmitter's obervable output on a spectrum analyzer, but we are still groping around without knowing the actual output power.

A lady friend moved away with my Ramsey 25B, so that saves me any complications that might come from operating that transmitter. Maybe SHE will get an NOUO!

But I'd say we are perpetually on square one with Part 15 FM, as demonstrated by Rich: we are swimming underwater in a gulf polluted by the oils of the unknown.

rich can you plot 50dbmv (output of my lpb fms2000 cable modulator) apllied to the terminals of a 1/2 wave and a 1/4 wave?

just use 3 meters as the distance

50dbmv is (IIRC) 1mW into 50 hms and 2mW to 75 ohms (or maybe the other way around)

i also sent you an email rich abut a part 15 am transmitter design.

It's funny how the brain works. I was just listening to a radio program in which a scientist said, "There is no way of accurately measuring low dose exposure to radiation. We can only estimate." This statement made me remember something about the field strength readings reported by the FCC on NOUOs.

I remember asking, "Why do we trust the FCC's reported readings so readily?" There are many reasons why the claimed readings might be "estimates" and not accurate.

No one yet has talked about getting a second opinion. But if a second opinion was sought, where would it come from? Who is available to provide a professional and accurate reading of small field strengths?

The comment about having no way to accurately measure low level RF exposure is based on more than just the energy density of the signal. One factor which is difficult to assess is the energy which is absorbed by living tissue and the heating effect thereof. The penetration of the energy into the tissue is also a factor and these factors are not well known.

However, measurement of the field strength incident on a test receiving antenna is not that difficult since the physical parameters are well defined and it becomes a matter of proper calibration. It is therefore not likely that a FS measurement made by the FCC would not be accurate enough that a reading of over 100 times the allowable FS would result from a compliant transmitter.

We, as hobbyists, are left with some options. One is to use a certified transmitter operated according to the instructions which presumably has been lab tested to be compliant and another is to use the FCC's guidelines for this which estimate the range to be 200 feet with a compliant transmitter. This second option is risky since the range depends on many factors beyond the field strength at 3 meters.

As Rich's calculations of the antenna power needed to produce the FS for these NOUOs illustrate it is very easy to produce field strengths which are too high, especially with kits, since kits generally operate with milliwatts of power output capability. It remains, then, to adjust the antenna length to achieve the recommended range and hope for the best.

I fear that many people buy a non-certified unit or build a kit transmitter with the idea the "if it wasn't OK they wouldn't sell these" without actually understanding what actually is involved which leads to problems for them.

We do need to be careful but compliant FM operation is possible.

Neil

I have never done the 200-foot test because my radios perform so differently from one another and I don't know which one to trust.

The auto radio is probably a good way to go, but two other examples are these:

My Grundig FR-200, which is great with Part 15 AM and excellent with high power FM, is so poor with Part 15 FM signals I cannot be heard in another room of the house nor right out in front of the house.

But, by dramatic contrast, the TECSUN PL-310 picks up the Part 15 FM certified signals EVERYWHERE in the entire house and yard without so much as a slight fade-out.

Given that variablity, if I used the weak Grundig radio to pump up the FM range to 200-feet then the other radios would pick it up for a mile!

"Given that variablity, if I used the weak Grundig radio to pump up the FM range to 200-feet then the other radios would pick it up for a mile!"

This clearly illustrates the major problem with the 200 foot suggestion. The conditions of measurement were not specified. It is best to take this 200 foot range as an indication of what to expect rather than a specification to set the transmitter.

The FM situation here is similar to your observation. On three portables the signal is only usable inside the house and perhaps 20 feet or so outside, but car radio provides a usable signal past the curb and beyond. I have adjusted the transmit antenna to give the minimum strength which works with the portables inside the house and this is not a very assuring way to proceed but it is the best I can do without getting a certified unit. It is a dilemma in that I don't want to invest in a certified unit only to find that it will not work with these particular portables. On the other hand, risking non-compliant operation is also a dilemma.

This whole problem would go away for me if our local broadcasters provided something worth listening to and I could abandon FM transmitting.

At least with AM under 15.219 there is some assurance that compliant operation is doable and perhaps my solution will be to switch to AM since the portables don't reproduce stereo anyway. But around here the noise level on AM is quite high due to CFLs and dimmers. More study by me is needed on this.

Neil

The transmitter I used today is a C. Crane Version 1 (they now have a Version 2, but I have no idea what is different).

The transmitter sits atop the computer in a corner of the middle-most room of a brick house, the telescopic antenna was raised to its maximum of 11"

On the car radio right out at a front corner of the house I tuned the silent carrier, and it was completely quiet.

Backing down the driveway 100-feet to the street some noise began to creep into the carrier, but not much.

Driving east I went another 100-feet and by then noise had closed in, but there were spots where quieting returned to the carrier.

Suppose the FCC picked one of those "special spots" where the signal improved? It would otherwise be surrounded by FM noise.

Alright. I would say this test "passed" the 200-foot test, but...

What if I put the transmitter up higher, perhaps in the attic?

What if I enclosed the transmitter in a weatherbox and raised it on a chimney pole?

Same transmitter, same certification, better conditions.

Would I be a "pirate?"

Same transmitter, same certification, better conditions. Would I be a "pirate?" 

In the U.S., not as long as the true field intensity at some arbitrary, far-field distance from the transmit antenna was relevant to the permissible field 3 meters away from it, as defined by FCC §15.239.

My FM transmitters all sit on the first floor of

the house - in a room facing the back yard.

I try to rig them so they are compliant with 15.239 -

my rule of thumb is that if it's in the house

and yard and not too far beyond that with 

an average portable it's OK.  (Carl, my FR-200

is very insensitive on FM also - the range with 

that radio is a lot less.)  

The car radio can hear

my FM transmitters 500 or 600 feet

away - or maybe a little more -

but again - the car radio also 

hears stations 50 to 150 miles

away - so it's a whole different

deal.  If I thought I was into "pirate

radio" territory, I would make adjustments

accordingly.  

By the way - GNAT 90.9 and SLUG 88.3

follow those rules - they are really just

humorous names to identify some of these

transmitters.  I'm sure nobody is listening.

Oh yeah, nobody is listening to FISH 107.5

either.   These signals are just for me.

By the way - when I listen to the Low

Power Hour - or any other Part 15 station's

streaming signal or downloaded files - they

always go out though SLUG 88.3, because 

that FM transmitter is closest to the computer.

The AM transmitters are another matter.  When

they were on - I was looking for as much range

as possible.  

Bruce, The Dog Radio Group (And other 

forms of life, too, I guess - SLUG, GNAT,

and FISH are usually small creatures, 

so I thought those names would be 

appropriate for teeny little radio stations.)