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Part 15 air chain idea

W

W1DAN

Hi:

A friend is building a part 15 FM station for a college. He is looking for a manual (if only to copy) for a Wheatstone A500A console.

He is thinking of feeding the output of an automation PC running SAM Broadcaster to the console, then back into the PC (or into another one) for processing via Breakaway Broadcast. The composite out from an inexpensive 192Khz soundcard (probably not a Marian) would feed a low power transmitter.

We are thinking of using a Radio Free Berkeley 1 watt PLL transmitter with a fair amount of coax on the output feeding a ground plane or J-pole antenna. He will use a Bird wattmeter to adjust the output of the transmitter down to the FCC allowed power level at the antenna. The transmitter is composite/mono in (no SG).

Has anyone done this signal flow?

Thoughts on the soundcard to transmitter interface?

The transmitter with LPF is about $150.00. Do you have a similar one that you like better?

Thanks,
Dan
 
W1DAN said:
...Radio Free Berkeley 1 watt PLL transmitter with a fair amount of coax on the output feeding a ground plane or J-pole antenna. He will use a Bird wattmeter to adjust the output of the transmitter down to the FCC allowed power level at the antenna.
Click to expand...

Just to note that the FCC doesn't define/limit the power level of an unlicensed FM system at the antenna, per se. The FCC limit for unlicensed FM systems given in Part 15.239 is a 250 microvolt per meter field intensity measured 3 meters in any direction from the transmit antenna.

Physics shows that a 1/2-wave dipole will radiate that peak legal field when a matched power of only 11.43 nanowatts (0.000 000 01143 watts) is applied to its feedpoint. A ground plane or j-pole antenna wouldn't need a largely different input power than the dipole.

So likely it will be difficult to comply with Part 15.239 using the equipment and techniques described.
 
Nick said:
For a college campus, the limit is 250 mv/m measured 3 meters from the property line.
Click to expand...

As I read it, the special "college dispensation" applies only to AM stations under §15.221; depending upon which frequency it operates, a college station can have a field strength not exceeding roughly 15 µV/M, measured at 3M beyond the boundaries of the campus. On FM, it looks like §15.239 applies: 250 µV/M @ 3M, but there's no mention of campus perimeters--3 meters is 3 meters.
 
The link below leads to plots of the free space field intensity vs distance from the transmit antenna possible from an unlicensed FM setup that is Part 15 compliant (the red trace), with three that are not.

The legal field is down to 10 µV/m at about 61 meters (200 feet), which needs a rather good FM receiver and receive antenna to produce good quieting, especially in stereo.

http://i62.photobucket.com/albums/h85/rfry-100/6417f684.gif
 
AM will really serve a meaningful area more easily.
But everything else is good in the plan unless you're talking about a very small housing area.
If FM is important, consider a few "nodes" linked by a stream routed locally on campus to a few transmitters.
Placement of the antennas to incidentally couple into power lines will help reception on those alarm clock radios.
 
It is possible to serve a campus with part15 FM. You have to put in leaky coax all over campus to do it, though. It's a pretty big job to do it on a large campus. Running a 1W transmitter into any sort of radiator will get you into trouble. Perhaps a better solution is to wait for the next LPFM filing window and apply for a 100W license.
 
Folks:

Good thoughts here.

Looks like my friend will need to use an FSM at 3 meters for power calibration on FM. We know broadcast engineers in the area who can assist us. If anyone finds the college campus signal strength rule, please link to it.

For AM he will use a part-15 transmitter into the standard limited sized antenna with the coil at the base. We may be looking for an old CRL style airchain there.

My main question is the Breakaway soundcard mpx'ed output to transmitter input interface. Has anyone here done this?

Thanks much for your ideas!

Dan
 
Dan,

I have a feeling that your friend is already over at my forums judging by the comment on the needed manual.

An antenna-based Part 15 FM is pretty impractical for many standpoints from building penetration to just trying to limit the signal radiating off campus. I myself used an engineered solution for the campus-limited FM I did for a high school using radiating coaxial cable suspended in the ceiling. It provides a solid signal within the building without opening up the issues of having an externally located antenna. We also have a free-radiating AM which provides a nice signal for the parking and athletic fields and I'm hoping that the kids will consider doing play-by-play of their home games.

I believed one of my forum participants already covered the use of the Breakaway processing as well so that topic should be there for his/your reference.
 
Bill DeFelice said:
An antenna-based Part 15 FM is pretty impractical for many standpoints from building penetration to just trying to limit the signal radiating off campus.
Click to expand...

The above quote is surprising, considering its author.

As has been pointed out earlier in this thread, Part 15 has no provision for unlicensed "campus broadcasting" radiating in the FM broadcast band equivalent to that permitted for unlicensed systems radiating in the AM broadcast band.

Legal, unlicensed FM transmit systems are defined only by Part 15.239 whether they are used on a campus, or anywhere else -- which limits the peak radiated field to 250 µV/m 3 meters from the transmit antenna itself, not the field existing 3 meters past the campus perimeter.
 
R. Fry said:
As has been pointed out earlier in this thread, Part 15 has no provision for unlicensed "campus broadcasting" radiating in the FM broadcast band equivalent to that permitted for unlicensed systems radiating in the AM broadcast band.

Legal, unlicensed FM transmit systems are defined only by Part 15.239 whether they are used on a campus, or anywhere else -- which limits the peak radiated field to 250 µV/m 3 meters from the transmit antenna itself, not the field existing 3 meters past the campus perimeter.
Click to expand...

As mentioned many times by myself and others, campus FM broadcasting utilizing radiating coaxial cable was first realized by LPB, Incorporated. The accepted measurement point was 3 meters from the exterior wall of the campus building, not the campus perimeter as you state. You might wish to review my previous discussions regarding radiating coax FM usage in campus applications for clarification.

I know of one very such system that had been inspected at the same time a school's amateur radio station received an FCC inspection due to a neighborhood interference complaint. Neither the amateur station or the campus limited FM were cited for being out of compliance with FCC regulations.

While I'll agree with Mr. Fry that there may not be a particular Part 15 regulation specifically addressing radiating coax FM installations in campus applications, installations that adhere to the field strength stated in 15.239 while using a measurement distance of 3 meters from the building's exterior has a history of acceptable operation.
 
Bill DeFelice said:
You might wish to review my previous discussions regarding radiating coax FM usage in campus applications for clarification.
Click to expand...

Are you suggesting that your previous discussions contain legally defensible information superseding Part 15.239?
 
R. Fry said:
Are you suggesting that your previous discussions contain legally defensible information superseding Part 15.239?
Click to expand...

My past discussions regarding radiating coaxial cable FM for campus applications regarding inspection and acceptance by field representatives of the Federal Communication Commission, the authority ranking higher than any of your charts and graphs may suggest. A duly authorized agent of this government agency, with all the knowledge and measurement equipment at their disposal, citing an engineered campus FM system is in compliance means much more than your armchair observation.
 
Bill DeFelice said:
... A duly authorized agent of this government agency (FCC), with all the knowledge and measurement equipment at their disposal, citing an engineered campus FM system is in compliance means much more than your armchair observation.
Click to expand...

The anecdotal and undocumented statement quoted above is not unlike those that have been posted on many boards for many years to the effect that Part 15 AM transmitters with long, radiating paths to a true r-f ground are legal because some such installations (reportedly) have survived an FCC inspection without receiving an NOUO.

OTOH, physics shows that such installations are not functionally legal with respect to 15.219(b). And in fact, the FCC has issued NOUOs to the operators of some of these systems.

The same could happen to operators of "campus FM systems," so anyone with a vested interest in the promotion, installation and operation of such a system might want to get an official statement from the FCC to learn if anything other than 15.239 applies to these systems, and if so, the exact details.
 
R. Fry said:
The same could happen to operators of "campus FM systems," so anyone with a vested interest in the promotion, installation and operation of such a system might want to get an official statement from the FCC to learn if anything other than 15.239 applies to these systems, and if so, the exact details.
Click to expand...

So, the written statement obtained by the educational institution from the district director of their local field office doesn't hold water, eh? I was unaware that your vast knowledge trumps statements made by government officials.
 
Bill DeFelice said:
So, the written statement obtained by the educational institution from the district director of their local field office doesn't hold water, eh?
Click to expand...

The above-quoted point of view is still anecdotal, and has not been provably and publicly documented.

Until the official, policy-making sources in the FCC Office of Engineering and Technology (OET) and/or the FCC Enforcement Bureau (EB)* publicly confirm in legal statements that a "campus FM system" needs only to meet a 250 µV/m limit 3 meters beyond the exterior walls of a "campus" building such as suggested by Mr DeFelice, rather than the 250 µV/m limit 3 meters from the transmit antenna as defined by 15.239, then all those accepting and following such advice as true and valid may be exposed to possible FCC action.

* not just a particular FCC field inspector from a local field office
of the FCC, as alleged (if true)
 
You could very well do what I did: I called LPB and they gave me several customers installations as a references. I picked the one utilizing their radiating coax for FM since that was my interest. Since I was planning the exact type of installation for my campus project I even took the time to take a road trip to visit the particular educational institution, speak with their own staff overseeing their station project as well as the outside engineering firm who both performed the installation of the campus FM station and also documented its as-built field intensity readings. This was all for the asking and it was no secret 7+years ago during the planning for my project. LPB was only too happy to supply me with that info.

Going by both the documentation supplied to me by LPB as well as seeing how other installations passed inspection I personally would not have any problem with this sort of installation as long as it was performed as outlined in their documentation.
 
I have such an installation that was looked at by a field office employee last year. He asked many questions, looked at the transmitter and the leaky cable. Our cable runs throughout plus up the tower at the facility.
 
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