Typical frequency used with SSTRAN 3000

From Reply 15:  "A transmitter w/ unmodulated carrier at 100 mW must necessarily reduce that carrier power when modulation is applied."  ...

There appears to be confusion/misapplication stated in the clip above between FCC Rules applying to legal, licensed amateur radio stations and legal, unlicensed setups permitted by FCC §15.219.

AFAIK, nothing in §15.219, or elsewhere in Part 15 supports this concept.

I may have missed it, so would be grateful to anyone who can quote such a requirement stated in the FCC Rules for Part 15.

Rich,

FCC part 15.219 states "The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100 milliwatts."

Note that's total input power, not output power.

Consider a perfectly efficient transmitter w/ 100 mW input power. Imagine that transmitter output as an unmodulated carrier at 100 mW.  Now apply modulation (audio power) to that carrier. That will necessarily increase the power input (and the power output) beyond 100 mW. Therefore, to keep within 15.219, the unmodulated carrier power must be reduced before applying the audio power to modulate the carrier.

Mike

The average value of the r-f carrier itself when amplitude-modulated by a sinewave remains constant during modulation.

The belief that the 100 mW limit of FCC §15.219(a) applies to the peak r-f power of the amplitude-modulated waveform is "unconventional."

Below is an inquiry I made to the FCC this morning, concerning this issue.  Will post their response here when it arrives.

Power is an instantaneous value. It is measured at a single point in time. It makes no difference what the "average value" of the carrier is either with or without modulation. If the total power drawn by the final amplifier stage at any moment in time exceeds 100 milliWatts, then it exceeds FCC limits.

650 and 700 khz are a no go for me, 700 WLW and 650 WSM are on either side of me.
600 and below might work. Central Kentucky.

That is not true.

From Reply 8:  ... studies as to whether the decreased antenna efficiency at those lower frequencies is less than the increase in propagation capability.

A NEC4.2 analysis shows this, for the system described:

• 2.8-meter, Base-fed, Base-loaded, Vertical Monopole
• Monopole Base Elevation = 0.2 meters above the Earth
• R-F Ground = 16 Buried Radials, 3 meters in length (each)
• Radial Common Point Concentric with Vertical Axis of Monopole
• Earth Conductivity = 5 mS/m, d.c. 13 (about average)
• Applied R-F Power at Loading Coil Input = 80 mW, unmodulated

At 1650 kHz...

• Loading Coil R-F Resistance at System Resonance = 20 Ω (assumed)
• Groundwave Field Intensity at 1 mile = 84 µV/m

At 570 kHz...

• Loading Coil R-F Resistance at System Resonance = 30 Ω (assumed)
• Groundwave Field Intensity at 1 mile = 34 µV/m

Rich,

If I follow that logic, then I can legally transmit a 100 watt burst and still be compliant with 15.219(a). 

If I transmit a 1 mSec 100 watt burst every one second, then I am averaging 100 mW each second and my average DC power input to the final stage is only 100 mW.

Mike

Mike -

The FCC stated that their §15.219(a) limit of 100 mW for the d-c input power to the final r-f stage applies _only_ to the input power needed to produce the unmodulated r-f carrier.

Amplitude-modulating that (continous) carrier wave to +/-100% by an audio sinewave increases the average r-f output power by 1.5X, and the peak-peak r-f output power by 4X, relative to the unmodulated value.

The average and peak d-c input power to the final input stage needed to generate that modulated waveform then might exceed 100 mW -- but that is NOT prohibited by FCC §15.219(a).

If your concept was valid, then (for example) the carrier power of licensed, 1 kW AM broadcast stations would need to be reduced to 250 W in order to be sinewave modulated to +/-100%.

That is absolutely NOT the case.

Rich,

If that were the case then how would one measure the input power to an unmodulated part 15 transmitter that used SSB instead of AM?

If that were the case then how would one measure the input power to an unmodulated part 15 transmitter that used SSB instead of AM?

Using a d-c voltmeter and d-c ammeter, whether or not modulation is present.

But the use of single-sideband (SSB) transmissions in the AM broadcast band would result in very few listeners, as very few members of the general public have receivers able to demodulate an SSB signal.

Mike,

Unless you are planning on selling this mod, either as a modified 3000, a kit of parts for this mod, or just a design package, I'd suggest you instead focus on where the most open frequencies are in YOUR area, and design accordingly.  You can start with  radio-locator.com, but then confirm by listening to the most likely frequencies morning, afternoon, evening, and overnight.  In my area (Philadelphia), I was told the only option was 1370khz, but since these transmitters are more effective at the high end of the band I first chose 1660 as the only other station was in Jersey City, NJ.  I should have audited that frequency but did not, and once I started broadcasting I got a ton of interference from that station. No doubt I wasn't interfering with them but they really killed my signal. So after doing a bunch of auditing at various times day and night I moved to 1610. 

Jim,

I'm not planning to sell this mod, however, if others want the details they are welcome to them.

I'm simply experimenting and having some fun with part 15 transmitters. I'm a ham (AB1AW) and also have a part 5 experimental license (WI2XRE) for 630 meter band. I enjoy the challenge of QRP so part 15 devices fit right into that.

I have no intention of actually broadcasting on AM, so operating frequency choices for me is strictly for my convenience only. I may select a frequency for a med-fer beacon transmitter at the upper end of the broadcast band but that would mostly likely not be on a standard AM frequency and would be a CW, PSK or MFSK signal.

Mike

Rich,

Your responses got me thinking. The possibility of being able to legally transmit a 100 mW unmodulated carrier, and thus a 200 mW PEP AM modulated signal, is enticing.

I did a little more research, and contacted the FCC.

My questions to the FCC and their responses are below. In their respose the FCC cited section 7.3 of IEEE ANSI C63-10-2013 as the actual procedure for measurement of the total input power. I've included the text of that section below.

Both the FCC part 15.219(a) and the IEEE procedure are ambiguous about whether or not modulation should be applied when performing the measurement. For FM/MFSK types of signals this is irrelelevent. However, for AM/SSB/PSK/QUAM and similar signals, the presence of a modulating signal in providing a portion of the power (in SSB all the power) seems to imply that modulation must be necessary to make the measurement. This seems (to me) especially true if one adheres strictly to the 3rd paragraph of section 7.3.

However, I'm inclined to also believe that the FCC intentionally left this all ambiguous in order to give them some latitude during the investigation of any particular part 15 transmitter operating in the AM band and using AM modulation [remember, there are med-fers that use CW, PSK, WSPR and other modulations for antenna and propagation studies).

I'm inclined to agree with you regarding the measurement based on this research. However, I'm also inclined to restrict my amplitude modulated signal to 100 mW PEP just to err on the side of caution.

Thanks for fueling this discussion. I would have never dug into the IEEE spec otherwise.

Section 7.3 : Input power to final RF stage for certain types of unlicensed wireless devices  

For unlicensed wireless devices subject to a limit on total input power to the final radio frequency stage (exclusive of filament power or heater power),62 the following procedure shall be used.

The total input power shall be determined by first identifying and reporting the circuit elements and details that constitute the final output stage. The current and voltage of the bus power to the output stage shall be measured and the net dc power shall be computed for the final stage.  

The final radio frequency stage shall be the dc bus power that is supplied to the final RF amplifier components, inclusive of all bias, transistor, module, and circuit portions that provide the final radio frequency excitation energy to the radiating element.

a) Identify the final radio frequency stage and all passive and active components associated therewith.

b) Measure the dc voltage applied to the final radio frequency stage.

c) Measure the dc current into the final radio frequency stage; if the net voltage and currents cannot be measured directly, then it is permissible to sum up the total power supplied by the battery or power supply.

d) Compute the total dc power by multiplying the dc voltage times the dc current according to Equation (9):

P = SUM(Vn * In)

where

Vn and In  are the individual components to the final stage.