Part 15 Primer

[jlw]

I'm someone who has worked in broadcasting since 1964. Before that I experimented with Knight Kits, Lafayette kits and whatever else was available.  The Part 15 broadcaster of today has some unique advantages I didn't have as a teenager...solid state electronics, manufacturers who make Part 15 transmitters that really work, coils, plans for antennas that are legal and go great distances on 100mw, and most of all the countless how-to's from the internet.  100mw is a lot of power if you use it right!  I'd also like to share a few thoughts from my days of professional broadcasting about the FCC. 

1.During the CB revolution of the 1970s it became very hard for the FCC to chase down everyone using what was termed "foot warmers", offensive  language and the interference of one operator over another.  This had a numbing effect on the FCC that they couldn't regulate everything and they kind of gave up trying ...who could blame them...people went crazy.  Instead, today's FCC serves the public by answering complaints about amateur radio interference to TV sets, unauthorized powerful transmitters that radiate harmonics up the audible spectrum, and increasing complaints about a growing number of illegal FM stations that serve a particular listening audience in large metro areas.

2.The FCC is more concerned about complaints regarding interference or a reported interference which they will investigate.  Notice I said a reported interference.  This applies to FM more than AM....a legal AM 100mw signal is easily lost in the clutter of noise from electrical towers.  10 watt FM exciters were easily ordered from places like Broadcast Supply West in the 70s & 80s with the capability to range from 1 to 12 watts power with only a 30" whip antenna attached directly to the back of the transmitter setting in a garage on a workbench giving coverage of up to 4 miles.  When you put it in the attic you could cover 15 miles by raising the height just 10 more feet.  I know this because I owned one of these before selling it to Harris Radio's used equipment store in Indiana.  Believe it or not, that FM exciter I bought 26 years ago wasn't all that much fun because there weren't any challenges about engineering it for more distance.  If you are using an AM SSTran or Hamilton and using it correctly you probably have nothing to worry about unless you have a neighbor who just wants to complain. It certainly won't have anything to do with interference.  Another mistake young people and beginners make while "endless testing" is playing the same record over and over because it sounds good on the radio.  This always gets people's immediate attention and being good citizens with a lot of time on their hands they'll call the local FCC office because nothing like that is normal on the AM band.  Be as professional as possible!  Again, if you program responsibly (no paid commercials) (that means a log and inspections from the FCC), abusive language, any radical\racist or off the wall political programming will almost guarantee you'll wind up on the FCC radar but not for interference but irresponsible programming.  The Fairness Doctrine died in 1988...but I can guarantee that's it's very much alive for the unlicensed Part 15 broadcaster.

3. An inexpensive RF meter is a great way to guage efficiency in signal strength.  The old story about the 1973 CB radio antenna mounted on top of a Volkwagen Beetle is true.  The individual did get tremendous range at night (state to state on 5 watts) due to atmospherics and the beautiful reflectivity\contour of the VW's perfectly formed roof acting as an almost perfect ground plane.  An RF meter will measure very little on a vertical pipe or wire cemented into the ground unless you connect it physically with an alligator clip because it's just a grounded vertical metal pole.  It will measure a dramatic jump when the meter is put close to the ground and moved over the array of radials of the ground system and the actual reflection can be seen on 10\10 foot radials.  The 10 foot antenna with the coil (a must) recommended by the manufacturers works in concert with the ground system as a reflector thus giving AM its local daytime coverage (nightime is different due to the lowering of the Ionosphere for AM's state to state coverage).  The vertical part of the iron, copper or aluminum mast (and it should be no longer than 3-4 feet) acts to support the antenna\transmitter and makes the connection to the ground array.  The amount of radiated energy is "extremely negligible" on the metal support pole until you get to the bottom and start seeing the meter jump dramatically as it comes within 12" of the ground system array.  It also should be noted that the highest reading comes when you hold the RF meter high in the air adjacent to the antenna (5-10 feet away).  You're actually seeing the highest RF from the antenna and you'll always see that whether you have a ground system or not but you won't get "real" coverage w/o the added ground system (and the coil of course).  If you want coverage with a Part 15 transmitter you must have both ground radials and the antenna working together with the ground reflection and the radiation from the antenna.   
  Hope this is helpful especially to those young people with the desire to become engineers, radio announcers or eventually leading to owning your own station someday.

[darklife]

Part 15 broadcasting can be very useful and fun. If using a certified part 15 transmitter on AM and legal length 10 foot radiator (includes coaxial length and ground rod/radials/cable length) there is a real possibility of reaching half a mile or more if the conditions are really good.
The only true useful time to broadcast on AM at 100mw levels is day since at night propagation is useless with the clutter and skywave isn't very useful at that power level if even at all.
The best way to get an audience and strong signal is by word of mouth and putting the transmitter up on a tower, use a long 10 foot pipe as the antenna insulated from the tower and running power and audio up to the box. Good high quality modulation at 100%+ helps a ton. Of course this is much easier sounding than it is in reality.
Using any kind of feed line or grounding system is not practical if keeping with the rules as this adds length to the permitted radiator even if it's not actually radiating anything.
But there is where the rules and regulations get goofy. Some officials will tell you antenna length and power to the final amplifier is all that matters to keep within regulations, others will tell you actual radiated power at a distance is the deciding factor. So it all boils down to how much your neighbors hate or love your radio show, if the FCC official had his morning coffee and so on.
The general rule is antenna length being 10 feet, doesn't matter about HAAT either, using no earth or radial system, and keeping your show clean and fun.
Even if someone gives you trouble the most that will happen is someone who works for the FCC will tell you to shut it down or check your setup for legality.
However this is really worrying over nothing. 100mw is flea power! You'd be lucky if a setup could get out 1/4 mile on average and even get listeners.
Stay off of 1710khz also as this is not covered in the part 15 rules. 530-1700khz is the OFFICIAL frequencies free to use. But then who cares lol, I use 1710khz sometimes at night since it's clear. The higher mediumwave frequencies are best to use with low power since antenna efficiency increases with frequency when using shortened radiators.

Now FM is another beast all together. The maximum radiation distance is like a few hundred feet. The rules can be bent though and no one will care. 10-25mw is thought to be about the legal power on the FM dial. Coax cable and grounding are fine and play no role in the rules for FM. HAAT also doesn't matter. Mount the antenna as high up as possible. Using antennas with gain over a standard dipole is only allowable if you already have a few db loss in the coax. J-pole/slimjim antennas will give the best results with up to 5db power increase and are very cheap to construct.
Once all is worked out there is the possibility of covering the same distance as on AM using part 15, but the rules aren't greatly defined and the FCC likes to bend them to get people off the air they or their community don't like.

Few good links on this topic...
All about part15 related stuff:
Part15.us
Forums for pirate radio and section on part15, info still applies to legal broadcasting also:
The free radio forums
The must have handbook for AMers:
LPAM Handbook (PDF)

It's a great hobby, that's for sure!

[Bill DeFelice]

Just a few additional comments:

The best way to get an audience and strong signal is by word of mouth and putting the transmitter up on a tower, use a long 10 foot pipe as the antenna insulated from the tower and running power and audio up to the box.

You will have to install ferrite cores on the power and audio lines to prevent them from radiating (for Part 15 compliance), which is usually addressed in the transmitter's manual if you purchase one of the high-end high performance units like the Rangemaster or Procaster.

The general rule is antenna length being 10 feet, doesn't matter about HAAT either, using no earth or radial system, and keeping your show clean and fun.

Any elevated transmitter should be appropriately grounded per the National Electrical Code. A lightning strike that takes the unit out could very well cause damage to the neighboring dwelling and insurance companies are not likely to pay out should they discover an improper installation.

Now FM is another beast all together. The maximum radiation distance is like a few hundred feet. The rules can be bent though and no one will care. 10-25mw is thought to be about the legal power on the FM dial. Coax cable and grounding are fine and play no role in the rules for FM. HAAT also doesn't matter. Mount the antenna as high up as possible. Using antennas with gain over a standard dipole is only allowable if you already have a few db loss in the coax. J-pole/slimjim antennas will give the best results with up to 5db power increase and are very cheap to construct.

Part 15 FM can be using in a variety of setups to legally gain more range than the typically handful of feet. Radiating coax cable can be used in specialized installations, such as educational institutions, to cover the entire campus while remaining totally legal to the Part 15 regulations.

You may also wish to avail yourself to the resources of HobbyBroadcaster.net as the site provides educational, technical and legal references, instructional how-to's, equipment and written and video product reviews as well as online forum community that discusses the many aspects of Part 15 broadcasting.

[R. Fry]

Now FM is another beast all together. The maximum radiation distance is like a few hundred feet. The rules can be bent though and no one will care. 10-25mw is thought to be about the legal power on the FM dial. Coax cable and grounding are fine and play no role in the rules for FM. HAAT also doesn't matter. Mount the antenna as high up as possible. Using antennas with gain over a standard dipole is only allowable if you already have a few db loss in the coax.

Everyone is free to operate as they wish, but the FCC is very clear about the limits allowed for unlicensed FM under Part 15.  Such systems using a single antenna such as a 30" whip, a 1/2-wave dipole or even a directional type such as a Yagi cannot legally radiate a field intensity greater than 250 microvolts per meter when measured 3 meters in any direction from the transmit antenna.  Antenna height and coax loss are unimportant -- only radiation from the antenna.

In reality it takes a very small amount of radiated power to generate the maximum legal field, using any of those antennas.  In the case of a 1/2-wave dipole it is only 0.000 000 0114 watts.  Using a 10-25 mW FM transmitter with a dipole will certainly exceed the Part 15 FM limit unless the system radiates almost none of that power.

Accurately measuring FM field intensity takes very good (and expensive) test equipment.  About the best approach for hobbyists trying to observe Part 15 FM limits is to install/adjust their system so that the maximum coverage radius to a good FM receiver in any direction over a free-space path is close to 200 feet.

As to bending the rules... that's at the option of the operator, but there are many cases in the public records of the FCC where people have been issued "NOUOs" when physics shows for the conditions given in the NOUO that the system could have been using a transmitter operating at 25 mW or so.

Apologies to those who already know this, and have read such posts before.

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