Discussion in 'Amplitude Modulation' started by WA1HLR, Mar 20, 2018.
Can you point to some ARS cases?
Amateur Radio Service? cases? Not off the top of my head.
That is only partially correct. FIM measurements can be disputed, because unlike broadcast, ham antennas are all over the place and are not design regulated. So yes, the FIM is used to take a reading, then FCC engineers pay a call on the ham, go in and have him transmit with their Bird or whatever in the line out of his rig, and call out to an agent outside manning the FIM to find out if the field intensity is the same as before and if it is they take the power reading or have the ham increase power until it is the same then measure power.
This is what happened when the hammer fell in the big bust around L.A. in the early '50s when FCC went in one night during the International DX contest and nailed around a dozen hams. Back then the rigs were all class C phone and CW and the ops all had variacs on the PA power supply primaries. Contesters with big mauls; Kalifornia Killowatts. FCC took the FIM readings, rang the doorbells, the suspected ham now scared s**tless, grabs the variac and dials it down, FCC walks in, says "transmit," only back then they read tx Ep and Ip on meters for input power (hams never thought to have trick shunts and multipliers) call out to the van and have the ham bring up the variac until the FIM reads the same as before and....BUSTED. They got everyone except W6AM, the biography of whom has this account in it, so that's how I know about it.
FCC probably has competent employees but the real problem is Congress defunding the hell out of them, closing field offices everywhere, super low morale because of this, and so on. Power in AM broadcast is measured by antenna current to known impedance. They don't worry about PEP, but limit modulation to 125% positive. To read current, they get about 5 seconds of dead air from the studio. If FCC can do all that, they can do something similar for ham AM if they have to. The PEP power rule was just a dumb down power rule to make it easy for hams, except as SM0 stated, PEP is used for other services such as transoceanic air--my hunch is such services never get measured by enforcement--PEP is just a self-regulation law. You can't have a measurement law where the measurement is so difficult for the regulated population that they can't perform it.
The power levels form part of the transmitter specifications which in turn are derived from systems planning criteria
(ICAO Annex 10 and ITU-RR Appendices 17,25, 26Aer and 27Aer).
For example, Air/Ground SSB planning assumes an effective radiated power of about 100 W for the airborne side,
and about 1000 W for ground systems.
These figures enter into coverage area and circuit reliability delibrations, but no one bothers how these watts are generated,
a service provider could be using a low-power transmitter with high-efficiency directional antenna systems,
or a higher-powered one with a small and more lossy antenna.
What sometimes is monitored is frequency stability, distorsion and adjacent-channel performance.
Engineering staff at both the ground stations and at the airlines are expected to check up their
transmitter performance regularly.
Even if open-wire feedlines are used, it is not impossible to quite accurately measure the output power
if you know what you are doing.
Early in my career, I had some involvement with the ancient Marconi SWB8, SWB11 and the Wilcox 96D
HF transmitters that formed part of the HF radiotelegraph sub-system in Swedish coast radio.
Those used open-wire lines feeding folded dipoles and in some cases rhombics.
According to 1930s practice, they used two RF ammeters in their feed-lines, and the
output power was calculated from RF current and voltage readings both into the
balanced dummy load and with the antenna connected.
The antenna impedances (magnitude and angle) were known from measurements taken at commissioning, and by measuring
the voltage at each feedline branch using a high-voltage probe and then multiplying with the current,
the apparent power was found. Finally, by multiplying with the cosine of the load phase angle the active power was determined.
Admittedly, this was very seldom done, the usual procedure was to tune the tank and coupling circuits for maximum current, while
staying within final tube (valve) and power supply ratings.
In order to apply this to a measurement of PEP output from a hypothetical AM transmitter,
the FCC engineer would have to use these steps:
First, tune up the transmitter into a balanced dummy load at rated power
(remember, you are supposed to have and use a dummy load)
Apply 100% sine-wave AM and check for gross envelope distortion either by an oscilloscope or some demodulating device
Record the RF ammeter readings with and without modulation
Determine the average active power from the readings.
The 100% modulated average power reading should be 50% higher than the unmodulated
Connect the antenna
Retune for maximum antenna current using same amount of DC input power that was previously used into the dummy load
Re-apply single-tone modulation and check that the feed-line currents actually become 1.22 times their unmodulated values.
Determine PEP by either measuring the p-p voltage at the crest of the envelope and divide it by the p-p
voltage generated by the carrier alone, square this ratio and multiply with the known power level (preferred),
or by knowing the modulating waveform and the time-averaged feed-line currents.
This also yields the PEP for simple waveforms with negligible distorsion.
However, the chances (or risks) of today encountering an FCC engineer with the necessary knowledge and time available to perform such evaluations can at best be described as remote...
IMHO, in the ARS, the licensee should have the option to choose to measure power either by PEP output (1500 watts max) or DC input (1000 watts max). IOW, the new way or the old way.
I think the REAL reason for the change to 1500 watts PEP output was the shenanigans of folks like W3PHL:
What "Fearless Freddy" did was to use a form of "supermodulation" in which the DC input to the final amplifier was 1000 watts DC but the audio input (modulator output) was many (up to 14!) kilowatts. The result was "double sideband reduced carrier".
He claimed legality under the existing rules despite the enormous PEP that resulted. After all, the DC input to the final amplifier was indeed 1000 watts!
The existing rules were rather vague about such schemes, so FCC changed them.
From what I have read and been told, W3PHL wasn't the only one doing this sort of thing.
Now, some may argue that Fred met the letter of the law as it was in those times. But he sure didn't meet the spirit of the law, and so FCC changed the law.
I mean.....14 kW of modulator power?
73 de Jim, N2EY
I think some are missing the delineation between Peak Envelope Power and carrier power in terms of Power/PEP measurements:
Peak envelope power (of a radio transmitter) [PEP]: The average power supplied to the antenna transmission line by a transmitter during one radio frequency cycle at the crest of the modulation envelope taken under normal operating conditions.
1) I.e, the measurement is taken somewhere at the transmission feed line going out to the antenna, so the feedline impedance has to be taken into account for any such measurement, but where on the feedline? At the output of the transmitter/feedline interface, somewhere along the feedline (where maxima current and voltage nodes may occur along a feedline), where?
2) one radio frequency cycle at the crest of the modulation envelope taken under normal operating conditions; what ARE the normal operating conditions for your transmitter/feedline/antenna/modulaton?
3) the average power supplied during the crest is a no-brainer with a storage type oscilloscope or good SA, but one has to know the feedline impedance before one can determine PEP = (Vrms)^2/RLoad, where the Vrms is measured as 0.707XVpeak at the CREST of the modulating envelope, and RLoad is the real part of the feedline impedance.
My point is, there are certain technical factors and considerations that have to be taken into account when and if a PEP measurement is taken, and it appears, at least to me, those measurements have to be taken in situ, i.e, at the ham shack.
Yes, one can monitor the carrier Power-at-a-Distance and using mathematics estimate the power of the power source, but even then, various factors have to be taken into account such as path distance and path attenuation, transmitting antenna gain, calibrated monitor receiving antenna capture area, calibrated receiver RF front end gain, coax loss to the RF front end from the calibrated monitor receiving antenna, etc.
The last paragraph describes essentially what a Field Intensity Meter (FIM) does.
Was that total modulated power (carrier plus modulation) or the power of the modulator alone?
It was the audio power of the modulator alone. The modulated stage ran at about 1000 watts DC input, so W3PHL claimed it was legal.
The regulations of that time stated that the power limit was 1000 watts DC input to the final stage. No mention was made of modulator power. A typical plate-modulated AM final consisting of a pair of triodes in push-pull running at 3000 volts 333 mA on the plates would be considered perfectly legal by FCC.
The modulator for that final stage could consist of another pair of triodes running at similar levels of power input to produce 500+ watts of audio, but the FCC didn't count that power at all. Only the DC power to the final amplifier mattered to them.
So W3PHL built what was essentially a high powered balanced modulator. With no audio, it ran at about 1000 watts DC input. With 14 kW of audio, what you got was both sidebands and several hundred watts of carrier. W3PHL claimed it was "AM", because both sidebands and the carrier were present. And since the plate voltmeter and ammeter for the final stage DC supply showed less than 1000 watts input, he contended it was all legal. I suspect FCC was stumped, because that's what the letter of the law said at the time. So....they changed the law.
IMHO, such shenanigans were a big factor in causing FCC to change the rules.
Read the article linked in my previous post. What should FCC have done?
73 de Jim, N2EY
The FCC alluded to this in the power limit proceeding.
They could have circumvented Fred's loophole by defining "AM power" in terms of the carrier power of full carrier double-sideband, while requiring the pee-e-pee standard for SSB and DSB reduced or suppressed carrier. The W3PHL scheme would thus be disqualified, since W3PHL-type "super modulation" becomes DSB reduced carrier, as evidenced by phase-reversed bumps of the modulated rf carrier at the troughs of negative modulation peaks, clearly visible on the envelope pattern of an oscilloscope.
I'm not sure that Fred even met the letter of the law under the old rules, since there used to also be a rule on the books stating that "in no case shall the emitted carrier be amplitude modulated in excess of 100 per cent". This limitation of 100% modulation did not apply specifically to either positive or negative peaks, but to the mean percentage of modulation on voice peaks per the definition of percentage modulation: ratio of half the difference between the maximum and minimum amplitudes of the amplitude modulated wave, to the average (unmodulated) amplitude of the carrier, expressed in percentage. Under this rule, it was illegal to exceed this limit even when modulating an RF final capable of ample headroom with asymmetrical audio without breaking the negative peak baseline. (With asymmetrical voice modulation, it was still legal to modulate over 100% in the positive direction as long as the difference in amplitude between the positive and negative peaks did not exceed the above definition, and the modulation capability of the final wasn't exceeded.)