The carrier pulses contain harmonics and the RC5 spec is rated at 25%~33%. Since the receiver has a BPF and AGC, attenuation of the fundamental will have a direct impact on path length. The question asks how to do this by analog methods, but in reality, any method that controls the fundamental Tx amplitude is identical including reducing the the PWM duty cycle(d.c.) of carrier pulses.
The Q of Rx BPF determines the bandwidth f/Q and approx. limitation for smallest d.c. for steady carrier.
Using fourier spectral analysis, I measured duty cycle ratio vs fundamental , f1 below, using a peak pulse at 0dB;
d.c. (~) f1 [dB]
1/2 . . . +2.1 ( Note: f1 pk is greater than squarewave pk)
1/3 . . . +0.9
1/4 . . . -0.9
1/8 . . . -6.4
1/12. . . -9.5
1/20. . .-13.7
1/32. . .-17.4
1/80. . .-26.0
These pulses will be filtered by the Rx BPF and added to the baseline noise to give a SNR and path loss for threshold of errors est. 15~20dB which in turn controls range.
Define range reduction, get Friss loss then choose duty cycle(d.c.) of carrier pulses for that loss on f1 from above table.
no need to add any circuit.