Rules for AM Power

"Power to the final RF stage" is what it says. Most people interpret this to mean "Power to the final RF amplifier stage", but I am a holdout, since I believe the antenna is the final RF stage, thus I think 100mW to the antenna is the rule.

Nobody agrees with me.

But I will never change my mind.

You are right.

"Output power to the antenna would be easier to understand".

Anyway, the RF final amplifier is inefficient and CANNOT produce more power than is applied to the input. In fact, it cannot pass 100% of the power.

Many circuits only pass 25% or some low factor, but there are high efficiency circuits, for example the AMT5000 from sstran.com which has a very high efficiency RF amplifier, somewhere near 90%.

100 mW "Input Power" does not refer to an RF signal applied to the input of the final stage.  Rather, it is the DC power applied to the final stage.

As Carl stated, amplifiers are not 100% efficient so you never get OUT what you put IN.  Except for some very efficient Class E output stages at nearly 80 to 90% efficient, you are lucky to get 60 to 70% efficiency.

In the FCC rule, power to the filament of a tube amplifier is excluded and I forget how the input drive signal figures in.  But, generally speaking the product of the DC voltage and current applied to the final stage (not the whole transmitter) is considered the input power.

If you obtain 60 to 80 mW output you're doing good.

The posting just above by MRAM is a clear explanation of the input power to the final RF stage, and I am thinking about this topic because I just did a power reading on the AMT5000.

I've recently been experimenting with lower than legal power, and am amazed that my desired coverage area is still well served during the daytime with less than 100mW. .

This morning's power reading showed 76mW to the final RF stage.

Then I got to thinking about the power monitoring required by the FCC in the case of licensed stations, and correct me if I'm wrong, but I think they are allowed to deviate up to 5% ABOVE licensed power!

In the case of Part15.219, 5% above legal power input to the final RF stage would be 120mW.

Seems reasonable, and it would compensate somewhat for the low efficiency transmitters.

5% above 100 mW will be 105 mW.

Some math can be used to predict the effect of this on the range. Range is proportional to the field strength which is proportional to the square root of power so the increase in range normalized will be the square root of 1.05 which is 1.025 or 2.5%.

Calculating the other way for Carl's 76 mW the range would be 0.87 or 87% of that expected with 100 mW input.

Operating near the 100 mW limit doesn't require exact precision adjustment in terms of range. It is just needed to stay compliant if running near or at 100 mW.

Neil

super modulation. 100mW RMS DC input to final 300% positive modulation peaks. cunninghams CM-3050 does this using cathode modulation, the solid state part 15.219 version of this would be panaxis am100 both acheive 300% positive modulation. the am100 is 100mw dc input while using modulator tricks to acheive a clean 300% positive modulation.

Much of the reason that AM transmitters with greater than 100% positive-going modulation peaks and 99.99...% negative-going modulation peaks sound louder at the output of a consumer-level AM broadcast receiver is the result of the audio distortion products present in that demodulated waveform.

The higher peak and average power in the AM sidebands of such waveforms relative to the carrier power cannot be demodulated accurately by such AM broadcast receivers.

There is a practical, positive modulation peak that produces ~tolerable performance in such receivers. The FCC limit for this for licensed AM broadcast band stations in the US is +125% modulation.

There is a Part 15 transmitter design which I have seen, but do not recall where, which claimed "higher than 100% modulation" to improve loudness and range. Rather than processing the audio to achieve this the device monitored the audio amplitude and through a long time constant circuit changed the DC voltage applied to the final amplifier raising the carrier component power with the audio. Such a system would produce 100 mW input power with no audio but this would increase with modulation which would keep the balance of carrier and sideband powers correct. It is the electronic equivalent of an operator adjusting the power to 100 mW at idle and then increasing the carrier power with audio applied.

I wonder if this sneaky method would pass an inspection.

Neil

What if a low power station ran very low modulation but increased the input to the final RF stage way up.

A listener would have a sense of "far away."

Of course on an FCC spectrum analyzer the full carrier would appear.

But if no complaint had been made, this could be a matter of years.

It's called "controlled carrier modulation" and was used in the popular Heathkit DX-40 ham transmitter and others of the same vintage. Also Harris Corp. sold at least one AM broadcast transmitter that increased carrier power as a function of higher modulation level.

The intent was to conserve power when modulation was low, not to raise the power to exceed legal limits.

It is certainly possible to implement controlled carrier modulation in a Part 15 AM transmitter and has been done in at least one transmitter claiming a 300% modulation level.

A good sounding implementation would require careful design to make sure the carrier power level change closely matches the audio waveform.

We might be tempted to confine our thinking to the fact that carrier power is normally measured as the input power to the final RF stage with 0% modulation. Peak power needs to be considered because our ever mindful FCC watchdogs use calibrated field strength meters that measure "average power", "peak power", and "quasi peak power". These sophistocated meters can detect when the ratio of your peak power to idle power is higher than would be expected from a typical 100 mW transmitter when modulated to maximum limits. They aren't just seeing your 0% modulation signal strength, they can see your avereage or your peak modulated signal strengths with the flip of a switch.

There really isn't any theoretical limit to the maximum peak power of a controlled carrier transmitter. It could be 1, 10, 100, or 1000 watts with an idle power of only 100 mW.

It would be fun to experiment, but there is no guarantee that you won't raise attention! It's kind of in the gray area like antenna top hats and really fat 3 meter antennas.

There really isn't any theoretical limit to the maximum peak power of a controlled carrier transmitter. It could be 1, 10, 100, or 1000 watts with an idle power of only 100 mW.

However such wide swings in the total r-f output power of an AM transmitter are likely to produce unwanted artifacts in AM receivers as their AGC (automatic gain control) circuits try to follow it. 

It also could be possible that AM receivers beyond a relatively short distance from the transmit site would produce ~100% noise output during pauses in program audio.  The field strength produced by the unmodulated carrier at those locations and times would not be high enough to "quiet" the receiver.

Wouldn't the modulator power also have to increase?

If a signal is at 100% modulation and you increase only the carrier power, wouldn't the percent modulation decrease?

Now I suppose if you were controlling a linear amplifier following a modulated exciter you'd get the effect but as Rich points out it would drive the AGC nuts.

Asymetrical modulation using high level (plate) modulation seems more logical as it would truly increase sideband power which is where your audio is.

I thought the Panaxis (long ago) used carrier control in the AM Transmitter.

Rob jogged my memory about where I had read about the carrier power modification technique. It was in the Panaxis AM100 manual. Here's a quote from the manual:

"Instantaneous peaks in audio will produce more than 100% modulation on occassion. The carrier will adjust to compensate for rapidly recurring peaks. It will readjust itself back down in power when modulation input drops. Control in this way allows up to 300% modulation on peaks with minimum distortion. The circuit tries to keep an overall 100% modulation at all times."

Neil