Part 15 for a Class D Daytimer?

[edarmsttrong]

Not sure if this is the right forum for this....but I was curious what the thoughts are on an AM daytimer using a Part 15 compliant solution for when it is not running the 'big rig'.

At 250w when the sun is up, this station is not a flame thrower to begin with...but there is zero chance of nightime authorization ever, and FM translators are in short supply in the area this station is located.

Would the Part 73 authorization preclude the station from running any AM transmitter, during times the license stipulates it should be 'off the air'?

I am looking for thoughts, as a Part 15 solution could actually be practical and simple for this station to pull off.

I am curious if anyone thinks it's legal to do, however...

 

[Tom Wells]

Depends on co-channel "chatter" from others, efficiency of the radiating system, local noise levels,
and expected useful service area.

Part 15 AM operation is basically designed to be a "no harm to anyone else" scenario.
Comply with that and it should not matter whether you are a teenager in the basement or a "real" station.
It means there had better be listeners close by, because you're not likely to make much more than a mile if yoiu're truly compliant.

 

[druidhillsradio]

Though it would not be legal, it would be fun if you could switch the Part 15 transmitter in and out of the existing station antenna.

 

[edarmsttrong]

Thanks for the thoughts...

The location of the Part 15 setup would be on a 12 story glass and steel building across the street form the studios. It's the tallest building downtown, and centrally located. I know that height would not really matter much, but my thought was that the steel of the building would make a good counterpoise. (At one time, this particular station contemplated relocating its main TX facility here, due to lease issues at it's tower site.)

With a mile radius, this would put the strongest signal right into center city. This would enable the station to continue remotes at special events, many of which are held downtown, HS sports, and the like...after all, some signal would be better than no signal.

The Part 15 could be fed from the existing STL. That's how close the studio is to this location.

 

[radiotower40]

This is a very interesting subject. I think in very small rural areas this could work. I was looking into this at a 250 watt D that I was with for a short time, they sold the station and we were all shown the door for total syndication.

The first thing you have to do is drive around the area at night and see what you get. Important things would be if on a clear, how strong is that clear on most nights?
If you have a 50kW coming in strong at night you won't get much out of such low power.

Secondly what is on your adjacents? Do you get splatter from other stations? Will you get knocked out by HD sidebands?

Any of these would make it very hard to get that signal out. If you are lucky enough to be on a somewhat quiet channel at night, this could work for you.

You mentioned one transmitter, have you thought of doing 3 or 4 sycronized transmitter. If your conditions are right, you could possibly do better than a station which is only allowed 1 or 2 watts at night.

You should also look at the possibilty of getting 1 or 2 watts with your current setup. The FCC has authorized many stations with a watt or two where I thought it would be impossible.

If you could get that and get a couple of part 15 rigs, you could possily get a useable signal for the area you want to serve.

I really like this idea, let us know if you decide to go forv

 

[rfry]

I know that height would not really matter much, but my thought was that the steel of the building would make a good counterpoise.

Unfortunately the steel frame of the building will act as a very large diameter conductor which will transform the antenna for the Part 15 AM transmitter from a ~3-meter whip to a long, off-center fed dipole, making that installation non-compliant with §15.219(b).

Even though the building steel may be solidly connected to earth ground at its base, those r-f ground conditions are not present on the part of the steel frame that is not buried in the earth -- so it will radiate.

 

[Groove1670]

Too much noise and skywave. We have a AM that runs 43 watts here at night and has chatter 500 feet from the transmitter. Using Part 15 power makes it almost impossible.

 

[RadioMan203]

Too much noise and skywave. We have a AM that runs 43 watts here at night and has chatter 500 feet from the transmitter. Using Part 15 power makes it almost impossible.

That was a powerhouse compared to the AM I engineered that had a post-sunset power of 9 watts. I use to joke that the only ones listening were the gophers at the transmitter site.

 

[Tom Wells]

I know that height would not really matter much, but my thought was that the steel of the building would make a good counterpoise.

Unfortunately the steel frame of the building will act as a very large diameter conductor which will transform the antenna for the Part 15 AM transmitter from a ~3-meter whip to a long, off-center fed dipole, making that installation non-compliant with §15.219(b).

Even though the building steel may be solidly connected to earth ground at its base, those r-f ground conditions are not present on the part of the steel frame that is not buried in the earth -- so it will radiate.

Here is one place where I see the issue differently.
Lacking any prohibitions specifically regarding height of installation, or the type of building it may be mounted on,
this situation seems to be one where the setup may work far better than the 3 meter total radiating system as defined.

Such "accidental", yet ideal places for such installations can hardly be considered the fault of users.
No one can be expected to replace their steel and glass building with a brick building, and then use battery power only
to eliminate currents to ground.

IF the law were written for AM pt 15 with only field strength as impoartant, then it would be easy to define how such
an installation would be "adjusted" to comply.

IF a separate long lead down to ground were provided, the installation is clearly not compliant.
If the transmitter is bolted directly to the side or top of the "grounded" structure, the resultant (better) radiation is "nobody's fault".

Even if there is no connection to ground for rf currents, at the top of 12 story steel structrure,
the rf current WILL couple into the building, as this is still the easiest path to ground.

This does sound like an ideal place to try the idea.

 

[rfry]

Lacking any prohibitions specifically regarding height of installation, or the type of building it may be mounted on, this situation seems to be one where the setup may work far better than the 3 meter total radiating system as defined. ... Such "accidental", yet ideal places for such installations can hardly be considered the fault of users.

The definition given in FCC §15.219(b) doesn't define the height of the radiating lengths authorized by that subpart, but it does define the length of the radiating conductors that legally may be used by unlicensed operators, which is 3 meters.

It doesn't matter to the FCC whether or not the radiating length of such conductors used by unlicensed AM operators is "accidental," or if that condition is the "fault" of that operator.

The subject of interest to the FCC in these matters is whether or not such unlicensed Part 15 systems actually comply with Part 15.

Such a system installed on top of a building, and using its steel frame as a "ground" connection will not meet the FCC requirements for legal, unlicensed AM transmissions, as a matter of physics.

 

[Tom Wells]

"Accidental" and "fault" were the best words I could find to describe such conditions.

As such a transmitter should be grounded for safety, we can't really ask someone to run in an unsafe manner.

WHEN a person has >installed< a conductor with a purpose, that conductor may be rightly considered a part of the
radiating system. When such conductive conditions permit safe grounding of a transmitter, the
building can not suddenly be re-defined AS a conductor. It is a building, in fact.
It may be conductive, and it certainly may radiate well, and it cetainly IS a conductor in a matter of physics and
safety, but if one needs a conductor, they go out and buy WIRE, not a building.

The phrase "intentional radiator" is quite loaded. We can "intend" for something to not radiate, but
if it still does, that's the inescapable physics. We may well intend for something to be an efficient radiator and yet
it is not. IF a building were erected specifically to mount a transmitter on with intent to use the structure as "ground", to take advantage of the situation, that building may called a conductor.
It would be difficult to argue that an already existing building was an elaborate ruse to create a hidden "intentional conductor" for use in some future radio project.

It may be that Pt 15 AM should have included maximum height from earth specs, but they did not.
This is one of the pitfalls of defing compliance in such terms ( as Pt 15 AM is) yet not acknowledging the
VERY wide range in resulting radiation (or grounded "ungrounded-ness") depending on whose "ground" is concerned.
Just as in racing, there are ways to fully comply with rules and still do far better than expected.

 

[edarmsttrong]

Oh well...it was just a thought. I was worried this would become murky.

I guess I will have to suggest to them they need get that smart phone app working,
for their Internet stream, instead.

 

[Shiny Knob]

Under the circumstances, I'd give it a try if I were in your boots.

You're doing a good faith effort to serve the community here, and this doesn't violate the letter of the law. And the SPIRIT of the law has long since been exorcised by the FCC anyway.

You could always plead forbearance if the FCC took exception. There's no good reason to believe the FCC would take exception.

And if you want to know for sure, hook it up and ask the inspector to drop by, making it clear that if there is a problem, you will promptly cease operation. Also let him know WHY you want to do it. I can tell you from personal experience that it's not all black and white, even at the FCC. They really DO want stations to have a fair shake, generally. Assuming you haven't ticked them off on a regular basis.

Too many people have the fear of God put in them by the straight necked interpretations of FCC rules. The FCC mandate is to provide for the public convenience and necessity. This idea seems to fit that mandate to a T.

 

[rickradio]

Would the Part 73 authorization preclude the station from running any AM transmitter, during times the license stipulates it should be 'off the air'?

Well, the reason daytimers are daytimers in the first place is to protect the nighttime contours of higher class co-channel stations from getting "holes" punched in them. For example, if a station in, say, Vallejo, CA wanted to run Part 15 power on AM 1190 at night, KEX's nighttime signal would have a hole in it (even if microscopic), which would make it illegal because KEX's nighttime contour extends down to Monterey (750 miles south of Portland). But then, KEX's signal would probably destroy the Part 15er, anyway. When looked at this way, seems kind of self-defeating and therefore, pointless.

Simply put, nighttime is no friend of the Part 15 signal as Part 15 power is no match for even a distant licensed signal, except for in the immediate vicinity of the Part 15 transmitter.

What I'm also wondering is whether the same antenna would be used, or a Part 15 antenna. I say that because if you hook a Part 15 transmitter up to a 200- or 300-foot tower, would any of that Part 15 power even get radiated, or would the tower just absorb it?

At any rate, I'm thinking the licenses of both stations would have to stipulate any of this before a station could actually do it. Which means petitioning the FCC, getting permission from the licensed station to punch a hole in their nighttime footprint, etc., etc., etc.

In the end, is it worth it all?

 

[pianoplayer88key]

I'm under the impression that under part 15, if you were going to use the tower as your radiating antenna, you'd be bound by the field strength limit in 15.209, as your antenna would be longer than 3 meters.
Was wondering ... with a 1/4-wave radiator over a standard ground radial system, what transmitter power output would be typically used to comply with 15.209?

 

[KeithE4]

I'm under the impression that under part 15, if you were going to use the tower as your radiating antenna, you'd be bound by the field strength limit in 15.209, as your antenna would be longer than 3 meters.
Was wondering ... with a 1/4-wave radiator over a standard ground radial system, what transmitter power output would be typically used to comply with 15.209?

Probably somewhere in the single-digit microwatts. At about 0.02 wavelength, a 3-meter antenna is extremely inefficient at 1.7 MHz, so higher power is allowed. Most of the transmitter's output is not radiated.

 

[rfry]

Was wondering... with a 1/4-wave radiator over a standard ground radial system, what transmitter power output would be typically used to comply with 15.209?

This is an interesting question.

A perfect 1/4-wave monopole driven against a perfect r-f ground plane produces a maximum (groundwave) field of ~314 mV/m at a distance of 1 km, when radiating 1 kW.(1)

For the AM broadcast band, FCC §15.209 requires the field strength in µV/m at a distance of 30 meters from the transmit antenna not to exceed 24,000 / (frequency in kHz). The maximum field for 540 kHz is ~44.4 µV/m, and for 1700 kHz is ~14.1 µV/m.

At 1700 kHz, the 1 kW field 30 meters from this antenna system would be 1,000/30 * 314 mV/m = 10,467 mV/m. To reduce that field to 0.0141 mV/m (= 14.1 µV/m), radiated power needs to be reduced by the square of the field reduction: (0.0141/10,467)2 * 1000W = 0.000 000 0018W, approx, or 1.8 nanowatts.

Calculating the power reduction for such a system on 540 kHz is not that straightforward, unfortunately, because a distance of 30 meters from that radiator is still in its near field -- where the change in E-field radiation is greater than the inverse distance value. The near field boundary is located about 0.16 wavelength from the radiator, which is 28.2 meters at 1700 kHz, and 88.8 meters at 540 kHz. So the reduction in radiated power for the system on 540 kHz would be greater than found by the approach above.

As a practical matter though, accurate measurements of fields this low would be very difficult to impossible to make outside of a screen room, due to the r-f noise present in most other receive environments.

(1) The radiation efficiency of a typical 1/4-wave monopole driven against 120 x 1/4-wave buried radials is about 95%.

 

[Schroedingers Cat]

I say that because if you hook a Part 15 transmitter up to a 200- or 300-foot tower, would any of that Part 15 power even get radiated, or would the tower just absorb it?

I have had this conversation every once and a while since the 1970s. My answer is that the losses are PROPORTIONAL to the input power. The percentage of power lost would be the same as the percentage lost at full power. If all the power was lost in the antenna system, why would the FCC care what antenna you used? If it were all lost, none would radiate.