Methods of reducing antenna bandwidth requirements for envelope elimination and restoration transmitter
Authors: Nguyen D.C. 
Published: November 2023

The task of radio transmitting path efficiency increasing is relevant both for stationary high-power applications and for autonomous low-power devices. The highest efficiency can be achieved in switching operating modes (classes D, E, F). For the amplification of modern spectrally efficient broadcast and telecommunication signals with variable amplitude, for example, with OFDM, the use of switching operating modes is possible only with the use of "synthetic" amplification methods. The most promising for application can be considered Envelope Elimination and Restoration (EER) method, which is successfully used in the field of high-power broadcasting in the LF, MF and HF bands, and is the object of a large number of studies aimed at expanding its scope. Transmitting devices of radio communication and broadcasting systems in some cases are forced to work with an electrically short antenna. However, when operating with OFDM signals to a narrow-band load, high-efficiency EER transmitters require matching with an antenna circuit with an SWR of at least 1.05 in the signal frequency band. Thus, the task of antenna bandwidth requirements reducing for EER transmitter is relevant. To solve it, a simulation model was previously developed and verified to study the operation of EER switching RF power amplifiers for a narrow-band load. With its help, the dependence of output signal distortion level on the envelope path LPF parameters was studied. This article proposes and analyzes hardware circuit solutions aimed at reducing the transmitter sensitivity to antenna bandwidth. The analysis of hardware methods for reducing the EER transmitter sensitivity to the resonant antenna SWR (passband) showed the following: 1. The damping circuit, together with a double-sided loaded envelope path LPF, allows you to reduce antenna SWR requirements to a maximum value of 1.22, while the allowable antenna bandwidth can be reduced to five signal bands. 2. The high-pass filter diplexer proposed in this paper, together with a double-sided loaded envelope path LPF, makes it possible to reduce antenna SWR requirements to a maximum value of 1.47, and the allowable antenna bandwidth can be reduced to 2.5 signal bands. The disadvantages of this method include the need, depending on the antenna used, for individual design and manufacture of high-pass filter diplexers installed in the low-frequency path of each amplifying cell of the transmitter. To expand the range of antennas used, it is possible to switch capacitors of the high-pass filter diplexer from a set of 2 ... 3 pieces. Data on the production of transmitters with the proposed HPF diplexer has not been found to date. 3. Multi-phase PWM when using a 2nd order unilaterally loaded low-pass filter and installing 2nd harmonic rejectors of the operating frequency in the cells of the RF path allows you to work on antennas with a bandwidth of up to half the signal bandwidth. Restrictions on the antenna bandwidth and its SWR are imposed only by the uneven antenna frequency response in the transmitted signal band. The disadvantages of this method include the need to install narrow-band rejectors that block the power supply circuit of each RF cell in terms of the 2nd harmonic, which makes such a transmitter a custom product for a fixed operating frequency. 4. The most promising solution should be considered the use of a high-pass filter diplexer, which allows relatively simple means to expand the permissible SWR of a narrow-band antenna in the signal band from the initial value of 1.05 to 1.47.