Using a Passive Front-End Filter
Though only on the periphery of my work experience, I suspect (in agreement with others) that lack of adequate front-end filtering could be the main issue with 5G interfering with radio altimeters. However, I have an additional thought on the differences between passive and active filters.
I wrote on this a couple of days ago, on Tim Worstall's website: https://www.timworstall.com/2022/01/is-us-5g-different-from-eu/ I don't know if anyone (more informed than me) has a view, but will repeat the gist of my view here.
Consider the wanted radio altimeter ground-reflected signal (so significantly attenuated from that transmitted from the aircraft) within an around 200MHz bandwidth (4200 MHz to 4400 MHz) is received together with a much more powerful direct path 5G signal (within the different bandwidth of 3700MHz to 3980MHz). Without adequate very-front-end filtering, there is risk of the unwanted 3700..3980MHz signal saturating (the amplitude of) the first front-end active circuit (that is supposed to filter out that unwanted signal). This means that the 3700..3980MHz unwanted signal would be clipped (even if the active filter would otherwise have provided adequate attenuation. This would cause the lower level wanted signal (ground reflection) to be overwhelmingly distorted by the (non-linear) clipping. Alternatively there might be no clipping but the wanted signal could be attenuated to a low level that becomes dominated by circuit noise. Alternatively a combination of these two highly undesirable effects could occur.
Note that the clipping explanation does not require the 5G antenna to be transmitting out of its allocated band. It only requires the received (RX) 5G signal level to be so much above expected out-of-band signal levels that clipping occurs in the first active circuits of the radio altimeter. Such active circuits are likely to be for a mix of amplification, automatic gain control and active (and so stronger) filtering of out-of-band signals.
If this is the cause, it is quite likely that the problem itself can be largely suppressed or totally eliminated by installing a passive bandpass filter into the RX antenna lead, that significantly attenuates much of the interfering 5G signal before the first RX active circuit in the radio altimeter electronics.
The key issue in this is that the very-front-end should include a passive filter rather than there first being an active filter. This is so the filter operates against the interfering 5G signal before there is clipping or loss of dynamic range. [Note: without such a passive filter, there would be some filtering from the RX antenna, but not enough for adequate suppression of other signals at adjacent frequencies.]
Upgrade with insertion of an in-line passive filter into each antenna cable strikes me as worth serious consideration as a practical and cost-effective solution for existing equipment. Obviously a designed-together pair of filters (passive first, then active) would be an even better solution, for new radio altimeter equipments. If there is already some front-end passive filtering, more passive filtering would provide a solution tolerant of higher 5G transmission levels.
Insertion of such a passive filter would improve air altimeter tolerance of 5G signals that are at a very high level, even though they have correct 5G design to avoid out-of-band transmissions. There would be problems with even higher level 5G signals, but perhaps not of those that would be desired in practice.