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"So, there is no "signal" to find. The power used is trivial, and looks like noise. Even a Direction-finding antennae needs to be tuned to a band; it can't just listen to everything."

That's incorrect. A spectrum analyzer does indeed "listen to everything."

With a directional antenna and a spectrum analyzer, a DF crew can sweep a given geographic area for electromagnetic emissions (e.g. radio waves). If your antenna is emitting electromagnetic radiation on *any* frequency when a directional antenna (hooked to a spectrum analyzer) is pointed your way, then your distance and signal strength can be judged/calculated.

If two such directional antennas see you emit electromagnetic radiation, then your precise position can further be calculated.

It does not matter if you are emitting a weak radio signal from your antenna. It does not matter if the radio signal you emit is on a specific frequency or zipping through multiple frequencies. It does not matter if you encrypt the data on your radio waves. It does not matter if you send data digitally or via analog encoding.

What does matter is that energy left your antenna when a directional receiver was pointed at you.

This does not mean that an enemy can discern the *contents* of the data that you are transmitting from your antenna.

However, it does mean that for the past century an enemy has been able to locate your position based upon you sending radio waves from your antenna.

Random, super-high-speed frequency hopping is no defense against modern DF'ing, either. A spectrum analyzer sees all.

I see you still did not get it.

A properly designed secure radio would be indistinguishable from noise, both in frequency pattern and in amplitude.

There really is NOTHING to analyze. Spectrum analysis needs to run a FFT against the noise to find an active frequency; that's how the "transient" detection they talk about is done. It needs to be able to find an anomaly - power, or a period of time of a frequency which is not noise like. If you start with two noise regions and choose randomly which noise region to pick from, it is impossible to tell it is not noise unless you have the one-time use code book.

But if a soldier did something stupid, like bring a cell phone that was on to the battle, then you could DF that. Cell phones use a very specific range of frequencies, and give off easily detectable pulses every minute, so that they can receive calls. And even though I know you won't ask, yes, I worked on those systems.

Military secure communications are way ahead of commercial - because commercial needs to be affordable, military needs to be the best.