Reprinted by self permission from thealpb(dot)com originally posted Jan. 2016
Some are naturally disappointed by the 1/10th Watt of power allowed for legal Part 15 radio transmission, or, by the same token, equally downcast about the 3-meter antenna limit.
But my experience tells me the 100mW 3-meter figures were very intelligently calculated by the FCC as the right signal level to allow for compliance with 15.219.
Consider these observations and tell me if you don't agree...
About 6-feet from the metal window frame serving as the antenna for an AMT5000 transmitter is a Panasonic table radio tuned to that transmitter. The radio is very sensitive and extremely useful for DXing far away AM stations.
But with the AMT5000 set at exactly 100mW, the antenna length measured to a precise 3-meters, the radio is over-saturated and the audio sounds like it's coming from under water.
The FCC obviously calculated this delicate relationship, and with the further recognition that most Part 15 users are living near other households, any more power and Part 15 stations would be jamming their nearby neighbor's radios leading to a steep rise in complaints.
On the other side of the coin the Part 15 rule for FM, 15.239, is set ridiculously low without any obvious rationation, virtually guaranteeing violation of anyone trying to make use of the FM band for low power operation.
It is reasonable to assume that when the FCC adopted the 100 mW limit for 15.219 that this was done with consideration for minimizing interference but why did they not specify a lower power?
One possible reason can be speculated and it is based on vacuum tube technology which was predominant when the rules were configured. An early use of Part 15 transmitters was the so called "phono oscillator". These were built into phonographs to enable the use of a nearby radio to produce the higher powered audio output without requiring including an amplifier and speaker in the phonograph. This could have been done by simply wiring the phonograph low level signal into the radio electronics but by using RF instead the need to standardize the audio signal parameters was eliminated and any radio could then be used for this purpose.
The simplest and least expensive phono oscillator would consist of one vacuum tube, probably a pentode, with an included diode section to rectify the line voltage to provide the DC operating voltage. For simplicity and lowest cost no transformer would be used and the tube filament would be powered from the 120 Volt line with the motor winding in series used to drop the line voltage to a level required by a suitable tube. The B+ would be derived from the 120 Volt AC line and would be around 170 Volts DC.
A tuned coil would be in the plate circuit with a feedback winding connected to the grid to produce an oscillator generating the RF signal. The audio signal could then be applied to the screen grid to provide for modulation.
A tube oscillating in this manner would do so by swinging the plate voltage at the maximum allowed by the circuit which would be double the plate voltage. Attempting to limit this swing by reducing the feedback would cause the oscillation to cease so the designers were "stuck with it".
For reasonably linear performance vacuum tubes have a lower limit on operating current and voltage. For example, from the data sheet for the 50C5 pentode, the plate voltage should be at least 100 volts and the plate current should be about 5 to 10 mA. These numbers will establish a minimum required DC input power and using 170 VDC plate voltage derived from the line and assuming a typical minimum plate current of 5 mA the DC input power to the final would be 5 mA X 170 V = 85 mW.
To reduce this power while maintaining the plate current would require lowering the plate voltage but since this voltage is derived from the 120 VAC line lowering would require a heat dissipating resistor and another bypass capacitor. Even if the voltage was reduced to around 100 VDC, nearly the minimum required by the tube, the input power would be 50 mW. A design goal of 100 mW input power would mean that these additional components would not be required and sufficient tube voltage and current could be used. It could be that the FCC took into account the balance between circuit design and minimizing interference by choosing 100 mW as a reasonable balance between the two objectives. This is only speculation on my part since the deliberations by the FCC technical staff are not known, but it makes technical sense.
My presentation about the design of an early phono oscillator is based on my experience with one which was given to me years ago and was my first AM transmitter. I reverse engineered the circuit and it was as described. I remember playing a record on this and going next door to ask my neighbor if I could try to hear the signal on their radio. Success! The signal was strong and listenable.
Neil
Neil, your speculation fits right in with my belief that the FCC did some intelligent calculations of one kind or another to arrive at 100 mW, and your proposition certainly fits with mine, perhaps we can combine them?
On Saturday night your reasoned Power-Point-Like Exhibition will be entered as "Revisionist History Origin of 100 mW 15.219 Rule", upon endorsement to be submitted to the Federal Register as "true history" in the American Manner of compiling this Nation's history. All reference to Indians or Slave Trade have been redacted.
Turning to the problem of 15.239, the Part 15 rule for FM, we see a more recent style based on "guessing low" so as to avoid the time and trouble of devoting real engineering thought to the matter. Probably drafted by hippie attorneys much more occupied with big money rule making for the nascent wireless industry.
I'm sure I'm right.
I just think that they wanted it so low to avoid all interference concerns. Since the usual application for a Part 15 transmitter is to stream MP3's to home audio or car systems, that kind of field strength is more than enough. It's only when you get into broadcasting to others that it's woefully inadequate - but then, Part 15 was never intended for that purpose.
as tim in bovey and many others have proven over the years you can do a lot with a 3m antenna and 100mW input when you have a good installtion. TIB gets out a couple miles from his procaster, on the other hand my installation goes no further than my part 15 fm because of my landlord imposed restrictions on antenna placement.
My post was concerning Part 15 FM, in case there was any confusion.
yea, unless you live in an apartment building or just looking to cover your own yard, part 15 fm is useless.