## 1 Introduction  
It may come as a surprise that in the era of wideband personal digital communications (Long-Term Evolution, 5G, etc. ), commercial aviation is still using analog radio systems, with no change in sight, at least in the next few years. This is due to the enormous inertia imposed by the need for international coordination agreements for the adoption of any kind of change, so that any new aeronautical communications system that succeeds should at least be compatible with the traditional system that has been used for the last decades. Recently, new projects have been undertaken that seek to achieve the global ranges provided by satellite communication systems, while maintaining compatibility with traditional communication systems at the very high frequency (VHF) band. These services are a new developing technology to improve air traffic management, especially in remote areas and oceanic areas where terrestrial infrastructures cannot provide adequate coverage. Since it is not possible to achieve high directivity gains in VHF antennas of sizes suitable for deployment on a satellite, most of the burden of achieving signal levels suitable for this type of service falls on the satellite transponder power amplifiers (PAs). These systems, based on the aggregation of several communication channels, result in wide fractional bandwidth (i.e., the signal bandwidth divided by the center operating frequency and expressed as a percentage) multicarrier signals with high peak-to-average power ratios (PAPRs). The required linearity figures for the transmitter cause the radiofrequency (RF) PA to operate in a linear mode away from the saturation region, where higher energy efficiency could be obtained. This results in low-energy-efficiency figures for the transmitter if conventional RF PA architectures are used [1]. The limited power resources of satellite platforms (due weight and size limitations for economic reasons) require energy-saving transmitters, while the distortion of the amplified signals must be kept to a minimum. There are some linearization techniques that allow the RF PA to operate near or under saturation conditions, while preserving its linearity [2]. As a result, these linearization methods help to increase the power efficiency of RF PAs. One solution that can potentially provide very good power efficiency results at the VHF band is polar-mode PAs [3; 4; 5; 6; 7], which have been studied over the course of many years. This technique (that sometimes can also be known as envelope elimination and restoration, or the Kahn technique) basically consists of a combination of a high-efficiency envelope amplifier (EA) that modulates the power supply port of a high-efficiency RF PA, where the amplitude modulation of the RF carrier takes place (high-level modulation principle). Figure 1 graphically compares a conventional transmitter PA (powered by a constant-level voltage supply) and a polar-mode PA architecture. The main advantage of using a polar-mode modulation technique is that, since the RF PA operates in saturation, the level of the amplified signal at the output of the PA (\(v_{O}\)) depends on its drain supply voltage (\(v_{E}\)) and not on the input signal (\(v_{DR}\)), meaning that a high-efficiency switchmode RF PA (class-D, class-E, etc.) can be used. When using polar-mode linearization systems at VHF, it must be noted that even moderate bandwidth communications signals exhibit a very wide fractional bandwidth. This poses an additional problem to those associated with EA and RF PAs for polar-mode operation with wideband signals: the RF PA power port must handle wideband envelope signals, while maintaining isolation of the RF output port. This is a major challenge that is still unresolved at these relatively low operating frequencies (VHFs), far removed from the microwave frequency bands, where most research effort and resources are currently focused on envelope tracking (ET) techniques [8; 9].