Military VHF Comms
The situation can vary widely from one country to another. Military comms on UHF and with cryptos are where the funding goes. Legacy systems on VHF and HF can be old equipment that is not serviced as effectively as it should be.
In some countries military VHF tx and rx equipment may be wide band valve-based kit. That means that the equipment uses channel bands that are twice or four times the width of current equipment and therefore incompatible.
A frequent problem can be antennae-based. Some military ATC has upgraded VHF to conform to exactly the same rules as civil systems including physical separation between tx and rx equipment and antennae systems. What can happen is that VHF antennae can end up with the worst position on the mast or tower and may be quarter wave aerials that are too close to the newer UHF aerial arrays.
Poor antennae location can lead to screening of signals in some directions and distortion of A planes and E planes. Thirty years back when computer modelling arrived for radio masts, most operators found that their masts would theoretically fall down in storm conditions and that changed the way that the antennae support structures were designed and monitored but did not change established practice for mounting antennae, resulting in heavy shielding by support structures that creates large blind spots.
Cabling may not have been replaced since Marconi was a boy and water ingression is a common problem with old cabling. When metered, water-logged cable shows a varying resistance as the transmitter goes to tx.
One further problem that can be suffered by some military radio systems is harmonic distortion that results from the wealth of equipment transmitting signals in close proximity. If the VHF systems are located at a military airfield, there will be many different ground comms systems on E band and A band VHF, T band and U band UHF, microwave systems in the 1-3 GHz range, together with all the HF, VHF and UHF land mobile and air mobile radio nets and radar systems on the ground and in the air. Older VHF radio and aerial systems are particularly vulnerable to interference form these other systems.