A “eureka moment” was finally reached when today I finally grasped (I hope) how INPUT TO THE FINAL STAGE is calculated.
A “eureka moment” was finally reached when today I finally grasped (I hope) how INPUT TO THE FINAL STAGE is calculated.
As everyone knows, the limit for Part 15 AM broadcast is that “No more than 100mW may be input to the Final RF Amplifier Stage.”
Here is the simple (but wrong) way I saw it.
Suppose you had a loudspeaker and some rule stated, “The input to the speaker may not exceed 10-Watts.” Well, all you would do is get a 10-Watt audio amplifier and things would be fine.
Therefore, in radio, I figured you’d get a transmitter whose 2nd-to-the-last-stage, also called “the buffer,” would have an OUTPUT power of 100mW, which would then go to the Final Stage Input. It’s so simple.
But that’s not the way it is there in reality, where everyone else lives. The actual formula is clearly explained in a July 30, 2011 posting from Radio8Z titled “POWER INPUT,” over on the Big Talker thread.
Take care because an earlier posting has the same title.
Input power
There are two “input powers.” The one in the rules is usually calculated as the DC input power to the final RF amplifier stage with no modulation, although technically it could also include the power to the final stage from the plate/collector/drain modulator, and also the portion of the power from the previous RF stage that feeds through to the output of the final stage.
The second “input power” is the power applied to the antenna system from the final RF stage. The output power of the antenna is the power radiated into the ether by the antenna. For an antenna at ground level, the radiated power might be about 10-100 MICROwatts, or so, and an efficient elevated antenna might radiate a couple of milliwatts. Although two milliwatts doesn’t sound like much, it can give more than a mile of range, and might get the attention of the FCC.
Input Power
It’s interesting to see how ambiguous that Part 15 rule really is…
If the authorities do not want you on the air, for any reason, you will not be on the air.
Not Ambiguous At All
I do not agree that the part 15 rule regarding AM power is ambiguous. The techniques for measuring the power input to an AM transmitter final have been known since the beginning of radio broadcasting.
The rule states that the power input to the final amplifier, excluding filament power, is limited to 100 mW. For a tube amplifier this would include both the power input to the plate and to the screen grid. For solid state amplifiers there is no filament or screen grid so the only powers to consider are the DC power applied to the final which is the DC voltage across the final device times the DC current through it. Engineering practice is to measure this with no modulation. The other power to consider is the power into the base of the transistor (or into the gate of a field effect transistor). This power is small compared to the input to the collector (or drain) and is usually neglected, but according to the rule they technically need to be included. For a typical RF amplifier with a gain of 10 dB or greater this signal input power will be about 10% of the total so it could be accounted for if so desired but it is usually neglected for standard measurements of AM transmitter power.
The rule does not address the output power which was described earlier as the “input power to the antenna” so there is no need to worry about it from a compliance point of view.
Neil
Ambiguous Rules
§ 15.219 Operation in the band 510–1705 kHz.
(a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100 milliwatts.
Where does it say ‘amplifier’? I see ‘stage’…I guess we could argue semantics all day long. What is the true context of the word ‘stage’, in this particular Part 15 regulation?
Let’s face it, the authorities like to make some of their rules (not just this one) ambiguous for a reason: If they don’t want you on the air, you are not on the air…period!
Heck, I like tubes, but at the end of the day, however, by continuing to include the words ‘(exclusive of heater or filament power)’ demonstrates just how ‘out of date’ many of these Part 15 rules really are.
Stage
“…What is the true context of the word ‘stage’, in this particular Part 15 regulation?”
As it is customarily used in electronics, “stage” is a part of a circuit which provides gain or signal processing by means of an active device (tube, transistor, etc.). Hence, in the rules the “stage” referenced is the same as the “final amplifier”.
Neil
About the Filament and Heater Power
Do I recall correctly that someone suggested the reference to “filament & heater power,” as worded in 15.219, might be “out-dated?”
Here is why it is not out-dated.
First, many transmitters today still use tube technology. If you argue that those are only high power transmitters, I would say…
There is a great interest in Part 15 tube transmitters and there are some available from very modern sources. Also to mention legacy transmitters, like the old Knight kits.
Still, I have a point of confusion someone can help with….. to me a filament IS a heater, because it causes the tube to warm up. But I sometimes wonder if “heater” means more than that…???
Part 15 tube transmitters
From another blog, some fun toob xmtrs
Tube transmitters for the nostalgic
Low voltage vacuum tube transmitter
Some unusual transmitters available now