Step-by-step for the clueless (Part 15)

[KDus]

Here's the reality of Part 15 AM. I don't want to burst bubbles, but we're talking about LEGAL levels here.  The 100mW limit is only part of the legal barrier. You can use up to 100mW to achieve a field strength of about 20microV per meter at 30 meters (98 feet) from the antenna.  (24000[/f(kHz)] uV/m @ 30 meters) The antenna can't be longer than 3 meters.

If you have a good signal past 200 feet, you probably aren't Part 15 compliant. The only way to be sure is to get your hands on a field strength meter that reads that low.

[sillyperson]

If you have a good signal past 200 feet, you probably aren't Part 15 compliant.

Holy. Crap.

So... it's pretty much impossible to remain legal, and broadcast to the High School just 1,500 feet from my rooftop?

[KDus]

Get a transmitter from a company that claims to be legal and then you can blame them.

[vonu]

If you want part 15 coverage, you'd get it cheaper by putting up a sign in your yard.

[KDus]

For AM, yes, get a magaphone.
For FM, height will get you close to the high school.

[vonu]

Don't even bother with Part 15 FM.

Agreed, just make sure not to interfere with anything within 2 channels on either side, and there will be no reason to track you down.

[marknh]

like someone else mentioned part15 fm is good for about 200 feet max.
and part 15am if i recall is about 200mv at 200 feet going on memory here.
and toss in the ant length and high above ground.

you might as well bootleg a few watts on fm.

[KDus]

Here's the reality of Part 15 AM. I don't want to burst bubbles, but we're talking about LEGAL levels here.  The 100mW limit is only part of the legal barrier. You can use up to 100mW to achieve a field strength of about 20microV per meter at 30 meters (98 feet) from the antenna.  (24000[/f(kHz)] uV/m @ 30 meters) The antenna can't be longer than 3 meters.

If you have a good signal past 200 feet, you probably aren't Part 15 compliant. The only way to be sure is to get your hands on a field strength meter that reads that low.

I have been very remiss in making a point about part 15. This is because I support running LRN, which is a full time broadcast.
There is another option within Part 15, for periodic broadcasts. You can run 500microVolts at 3meters, which is twice the full time limit. So, you could transmit a scanner or porc411 when something happens. The  problem is that broadcast receivers don't mute when the signal isn't present.

[RFBurns]

Here's the reality of Part 15 AM. I don't want to burst bubbles, but we're talking about LEGAL levels here.  The 100mW limit is only part of the legal barrier. You can use up to 100mW to achieve a field strength of about 20microV per meter at 30 meters (98 feet) from the antenna.  (24000[/f(kHz)] uV/m @ 30 meters) The antenna can't be longer than 3 meters.

If you have a good signal past 200 feet, you probably aren't Part 15 compliant. The only way to be sure is to get your hands on a field strength meter that reads that low.

Indeed. Pretty simple math and techniques to check field strength.

And on another thread...someone mentions about an operator claiming 8 miles coverage. Ya...with multiple 100mw transmitters scattered everywhere.....but just one single unit....well let the math explain it.

RFB

[RFBurns]

If you have a good signal past 200 feet, you probably aren't Part 15 compliant.

For FM this is most of the time true, however if conditions are right, a Part 15 FCC certified FM transmitter can send its signal well beyond 200 feet.

For AM, well this is a hit and miss best guess because of the variables such as ground conductivity, the grounding system utilized and the coupling/tuning efficiency to the 3 meter antenna. Ive seen 70mW in a very well designed Part 15 2.19 system send a signal out to 3/4 mile no problem. Daytime of course. Night..well at that point 200 feet or so is about as much to expect for a clear signal, after that...it ends up buried in the skywave noise.

The only way to be sure is to get your hands on a field strength meter that reads that low.

According to the rules, a quasi-detector with no AGC is the industry standard for measuring AM transmission field strengths. These are easy to build and can be made just as sensitive and accurate as the infamous Potmac FS unit the FCC engineers use. I know...I built one and use it monthly to verify my CC station's emission limits. I calibrate it to a fellow engineer's Potmac FS and every time, it indicates the same readings as the expensive Potmac.

A good communications service monitor can also serve as a fairly accurate FS meter for close to nth decimal point measurements, at least within the ball park.

After playing around with the "carrot on a stick" method for a number of years and always being let down by the poor range even at 3/4 mile on a good day with moist ground, carrier current is the way to go with AM. Though many state the lower band is best for carrier current, I found it to be opposite and that the upper portion works best for my installation. My TCU-30 couples to the neutral and grid ground lead in's right off the drop. Fortunately that ground drop is isolated and with the addition of 4 ground rods at 15 foot intervals, 5 watts was all that was needed to get my signal down the line and into listener's homes and without all that nasty hum.

If your really wanting to reach more listeners and expand the range of your station LEGALLY, then carrier current is the way to go.

Although LPB is not manufacturing new CC units, Radio Systems does, however their unit is limited to 10 watts carrier power while one can find an LPB unit on ebay that can pump out 3 to 5 times that power level.

And its great to see a 1982 design work so well with a 2000 era C-QUAM exciter. Chris never expected to see one of his transmitters mounted into an LPB TX as an exciter. I bought 3 more of his units as spares plus one that was un-assembled to have individual parts on hand if needed.

And if you cannot find an LPB or cannot afford a Radio Systems unit, there are plenty of schematics and plans out here on the web for building a transmitter and coupling circuit.

2 watts coupled through a 10mH inductor and a .1mf 3kv ceramic cap onto the neutral line will work with fantastic results! However to maximize the RF induction, use an antenna tuner such as an MFJ 900 and add 6 more turns to the upper portion of the inductor coil and add two 500 pf caps across each of the variable caps and with the above 10mH inductor and .1mf cap, you got yourself a very effective coupler that works just as well as the infamous LPB TCU-30 and TC-8 couplers.

Happy building and broadcasting!  



RFB

[RFBurns]

like someone else mentioned part15 fm is good for about 200 feet max.

Yep...but even a fully certified FM unit can send a signal beyond 200 feet with good conditions such as hight of the unit so its antenna is above obstructions and in line of sight.

..and part 15am if i recall is about 200mv at 200 feet going on memory here.
and toss in the ant length and high above ground.

Actually its 15mV/30m (45.715/f for metric , 157000/f for feet). It amazes me how many mix up the differences between the FM field strength limits and the AM field strength limits. They are not the same 200 foot range limit syndrome.

you might as well bootleg a few watts on fm.

The biggest mistake most make by bootlegging a few watts on the FM band is that they do not take the time to ensure their signal is clean and not filling the spectrum with spurious. Also most forget that the IF frequency of FM receivers is 10Mhz. So if your pumping a signal on 90.1 Mhz, and someone down the street is trying to tune in a station on 100.1 Mhz, guess what happens....your signal will ride along with the other station's signal via the IF and the listener of the 100.1 Mhz station hears two stations! Next day a letter or email is sent off to the FCC in the form of a complaint. Shortly thereafter...the interfering bootlegger receives a NOUO..or worse a NAL.

I would be outright lying if I said I never threw up an FM with a few hundred watts. I have...all home built equipment pumping out a stereo signal on 107.7 at 400W into a 2 bay dipole antenna system constructed of 1/2 inch copper tubing and PVC fittings mounted at 40 feet on a tv receiving mast. Yep it was fun, for a 3 full years....till a "friend" who I thought was a good guy decided to contact the FCC and a few weeks later, both the FCC and the Sheriff's department showed up to partake in the party..cept their plans were not to enjoy a beer and pizza.

At least the FCC guy let me announce the station was going off the air. He even commented on how clean the signal was compared to licensed stations in the area. I told the field agent.."that is due to well designed home built equipment using good engineering practices and building the thing to conform to the spirit of the rules for spectral purity". The agent actually looked surprised after saying that to him. I think I even caught a grin from the agent's expressions.

The key words in the rules is "harmful interference". It is the harmful interference that lands an operation in trouble....though not the only thing that can land an operation into problems. Things like indecent programing and foul mouthed fools will no doubt bring unwanted visits. It is this kind of operations that even I would hunt down and turn in myself. These types of operations ruins it for the rest of us who take the time to properly set up a station in the spirit of the technical parameters that licensed stations adhere to.

Just slapping up a signal and ohh ahh'ing over "Im on the air!" is not good enough...be it Part 15 or Part Bootleg. If your going to risk throwing out some watts, then do it RIGHT and at least impress the field agents when they come to bust your butt. The least that will happen is they smile while they take your beautiful home made transmitter! (UGGGHHHH!!!) Just dont tell them what I said as they walked out the door with my home made transmitter and antenna system..."You can take that equipment but you cant take whats in my head"...meaning the schematics to the unit they were taking as well as many other ideas were not on paper, but in my mind! Ha Ha! 



RFB