SStran class E basis for SW xmitter

A few years ago, I purchased and built an AMT-3000... which is the predecessor to this model. It was fun to play with as an AM band transmitter. I also used it in some demonstrations in classes at NJIT, in conjunction with a simple SDR receiver and a laptop computer. I wanted to display the spectrum of an AM signal, and to show how the SDR's digital filters affected both the displayed spectrum and the output audio of the SDR. The NJIT buildings acted as Faraday cages to AM signals; and blocked even local AM broadcast signals... so I brought along my own AM transmitter; and used my iPod to provide recorded audio input to the transmitter.

One thing that I "discovered" (although I'm sure it is a well known phenomenon to those who have used these low power AM transmitters) is that there was huge difference in 60Hz hum on the audio output any AM receiver, tuned to this transmitter's frequency, when using an AC adaptor v. a pair of 9V batteries as the power source.

At first, I assumed that the problem was hum from the AC adapter getting into the transmitter's circuitry; but the real cause of the problem was something else.

Since the FCC rules for low power, unlicensed AM broadcast band transmitters put a tight limit on the length of the antenna and the length of the "ground" wire (which is actually the other half of the antenna)... this transmitter will conduct RF through the AC adapter... even with ferrite chokes on each separate wire between the adaptor and the transmitter. [I don't know if this is actually in compliance with FCC rules and regulations]

From the FCC rules and regulations:

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.

(b) The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.

(c) All emissions below 510 kHz or above 1705 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by measuring the radiated emissions.

The chokes did not provide a high enough RF impedance to prevent RF current from flowing from the transmitter into the house/building wiring. The house/building wiring can then become the dominant component of transmitter's RF-radiating (antenna) system. Various things connected to the AC wiring, and/or various imperfections in the wiring itself, can introduce non-linearities that result in mixing between the RF current in the house/building wiring and the 60Hz AC. The AC adapter itself acts as a mixer. It contains a diode rectifier. During a significant portion of each AC cycle (even for a bridge rectifier or a full wave rectifier) all of the diodes are reverse-biased. During those portions of each cycle, the common mode path between the transmitter's RF and the AC wiring has a much higher impedance than it does when some of the diodes are conducting. As a result, the power converter's rectifier acts like a multiplier between the RF and the 60Hz AC Therefore, the RF current flowing through the wiring of the house/building (and the RF radiated by those wires) has 60 Hz modulation on it... and that is what results in the hum at the output of a receiver tuned to this frequency.

Using a battery, instead of an AC adapter, made the hum completely disappear.

Stu