Understanding the Power Efficiency of SSB vs AM Modulation

Introduction

Radio communication involves various types of signals and modulations. Single Sideband (SSB) and Amplitude Modulation (AM) are two common methods, each with its own unique characteristics and power requirements. A radio ham reminded me that 400 watts SSB is equivalent to about 100 watts AM. This may seem counterintuitive, considering the traditional understanding that SSB should be more powerful than AM. Let's delve into the details to understand this concept better.

Key Differences Between SSB and AM

AM (Amplitude Modulation) and SSB (Single Sideband): AM is the earliest form of voice communication and involves a carrier signal with two sidebands, while SSB transmits only one of the sidebands and removes the carrier. The efficiency of these methods plays a crucial role in the overall power consumption during transmission.

AM Transmission Power

In AM, a carrier signal is modulated by the audio signal. The power of the carrier and the sidebands are combined to transmit the audio information. For a 100-watt modulated carrier, the peak envelope power (PEP) can be calculated as follows:

The peak power PEP in AM can be calculated as:

For a fully modulated carrier where the power in the sidebands and carrier are given by:

Maximum carrier voltage: ( V_{rms, text{carrier}} approx 71 ) volts at a 50 ohm load.

Maximum sideband voltage: ( V_{rms, text{sideband}} approx 35.5 ) volts.

Maximum sideband power: ( P_{text{sideband}} frac{V_{rms, text{sideband}}^2}{50 , Omega} 25 ) watts.

Total AM power: ( 100 , text{watts carrier} 25 , text{watts sideband (each sideband)} 150 ) watts.

The maximum PEP in AM occurs when the sidebands are in phase with the carrier, which results in a peak power of 400 watts. This is a crucial point to understand the power requirements of AM.

SSB Transmission Power

SSB, on other hand, is more power-efficient. By suppressing the carrier and one of the sidebands, SSB can transmit the same information using significantly less power:

The SSB power efficiency can be expressed as around 30-40% of the power required by AM, inverted it's 60-70% more efficient.

In SSB, only one of the sidebands is transmitted, and the carrier is removed. For a SSB signal, if 25 watts are required for each sideband, the total power for the same information would be 50 watts, compared to 400 watts for 100 watts AM (with both sidebands included).

Practical Implications

The key difference here is that while AM requires more power due to its full representation of the carrier and both sidebands, SSB uses a more efficient method by transmitting only the necessary parts of the signal. This has practical implications in terms of power consumption and transmission distance. An often-cited rule suggests that SSB can transmit an equally intelligible signal with significantly less power, which is why you might require 400 watts SSB to achieve the same results as 100 watts AM.

The math shows that an AM transmitter with a 100-watt carrier and 100% modulation would require a power amplifier capable of delivering 400 watts. In contrast, SSB transmitters can use lower power levels and still achieve the same coverage and intelligibility. Therefore, the final amplifier in an AM transmitter must handle 400 watts (PEP) for optimal performance, whereas an SSB transmitter with the same efficiency can run with only 100 watts.

Conclusion

Signed off, understanding the power efficiency between SSB and AM is crucial for those involved in radio ham operations and amateur radio communication. The rule of thumb that 400 watts SSB is equivalent to about 100 watts AM is rooted in the power dynamics of these different modulation techniques, emphasizing the power-saving and efficiency benefits of SSB over AM.