FCC rules won't allow encryption. I'm not sure why the rules were made originally.
If you combine these things you end up with encrypted transmissions raising the noise floor for everyone in a band with no way to know if that noise is a transmission or actual noise. A lot of ham bands are already very noisy from natural sources, a bunch of encrypted transmissions increasing the noise floor just serves to make those bands inaccessible.
The propagation makes this problem even worse because encrypted transmissions can raise the noise floor for hams hundreds or even thousands of miles away. Even if an encrypted transmission uses a very narrow bandwidth (no worse than other digital modes) you're still monopolizing part of a very small band with a private conversation preventing others from using the shared resource.
To make matters worse encryption necessitates some form of error correction. Whether this is forward error correction, retransmission, or whatever it's necessary for a receiver to receive a correct ciphertext in order to decrypt a message. Error correction necessitates a higher duty cycle in order to send the error correcting overhead. So encrypted transmissions last longer than the equivalent unencrypted digital mode tying up limited bandwidth for a longer duration.
The difference, from an RF perspective, of an encrypted signal and a non encrypted signal is non-existent as the encryption takes place before the modulator. Therefore encrypted signals won't impact the noise flow for everyone in the band. This would only occur if the operator were using frequency hopping.
>you're still monopolizing part of a very small band with a private conversation preventing others from using the shared resource
Depending on the technology used in the receiver, this also isn't true. There are numerous techniques to allow for multiple carriers on the same band. CDMA for example would allow for this. It would take some coordination to make sure each carrier is using a different code but should be feasible for amateur radio operators. It all depends on how sophisticated their receiver is.
>Error correction necessitates a higher duty cycle
This is true and is why encrypted signals have lower data rates. So instead of having a longer transmission you lower the data rate to get the same effect.
While the encrypted transmission will look like a regular digital mode the fact it can't decrypt the decoded symbols means it's effectively noise. Ham bands are for all hams. There are innumerable other ways to have a private conversation with targeted recipients.
> Depending on the technology used in the receiver, this also isn't true. There are numerous techniques to allow for multiple carriers on the same band. CDMA for example would allow for this. It would take some coordination to make sure each carrier is using a different code but should be feasible for amateur radio operators. It all depends on how sophisticated their receiver is.
Ham bands already have de facto CSMA, you wait for open air to transmit.
The "no commercial use" rule ensures companies aren't using the limited ham radio spectrum as a workaround to getting their own dedicated license and frequency allocation.
If encryption were allowed, there'd be no way for other users to verify whether traffic is commercial or not.
No business wants their comms in the clear with no means of verifying the sender of the message - so ham radio would be useless to them.
The exact wording is in 97.113: "No amateur station shall transmit... messages encoded for the purpose of obscuring their meaning".