I am curious about something. There are repressive governments that engage in censorship.....so do they put out jamming interference to prevent an exchange of radio traffic? I would expect this in parts of the Middle East, North Korea, and--perhaps--The People's Republic of China.
I apologize in advance for the long answer.
Jamming is certainly a possibility, but an expensive one. A better answer for most repressive governments would be to monitor and track signals. HF is more difficult to direction find (DF) than the VHF/UHF transmitters because of the size of the antenna and the propagation paths for HF. During WWII, spies had to deal with cumbersome radio sets, and still made good use of them. When a transceiver powerful enough to reach anywhere in the continental US can be built into an Altiod's tin and run on a 9Vdc transistor radio battery, it will be an interesting game to play.
The main cost of an amateur radio is the receiver. You're trying to pick up a very weak signal. The strength of the received signal will always be inversely proportional to the square of the distance, so it is really saying something that my 100 Watt transmitter carries my voice clearly to Europe without a problem. 100Watts is pretty standard for HF communications and is called "operating barefoot". The licenses allow up to 1,500 Watts output and some make a sport of covering the entire continental US with 5 Watts. Any receiver has two critical statistics. Sensitivity and selectivity. Sensitivity refers to the ability to amplify very weak signals. Selectivity (lacking in cheaper radios) refers to the ability to differentiate the signal you want from other, nearby signals, static, etc.
FM radios enjoy a phenomenon called "capture effect" blanks out all but the strongest signal. In my rural home, two FM stations miles away are on the same frequency and as I move from one room to another, one station drops and the other is instantly there.
AM and Single Side Band are different beasts. In these modes, your receiver will amplify and present you with any and all signals within the "pass band". In the ham world, an AM signal is around 6,000 Hz wide and single side band is around 2,300 Hz wide to carry the same information. Really good receivers will have adjustable pass band filters so you can clip out nearby signals and any noise that is outside your target frequencies. A strong signal close to the pass band will overwhelm the receiver (The stronger the interfering signal, the further it can be from your target frequency). This is called receiver desensitizing and is the reason that someone with a linear amplifier on their CB radio renders the nearby channels useless.
To jam a signal, you need a transmission source that will overwhelm the receivers. The more of the spectrum you are trying to jam, the more power you're going to have to pump out, so jamming the entire ham radio spectrum would be difficult. Because in the history of radio, hams have come up with many of the innovations, the ham radio spectrum is also broken up into small pieces all up and down the frequency scale to allow experimentation in different propagation ranges. For this reason, jamming us, would mean jamming commercial and government radio as well.
