Plausable deniablity / steganography in radio transmission
Spread spectrum does this: -



The power spectral density of a transmission of direct sequence spread spectrum (DSSS) remains below a certain "noise level" (as shown in the diagram above). As will be shown later in this answer, due to DSSS's "processing gain", the actual received power level can be below the receiver's noise threshold across a large range of distances from the transmitter. This makes a transmission quite difficult to detect unless you have a matching receiver and you know the "code sequence" used in the transmission.

Because many individual low power "carriers" are simultaneously generated, there can be a negative SNR at any point in the spectrum but, the sheer multiplicity of simultaneous "carriers" means that a receiver can operate with a decent headroom of SNR. The receiver HAS to know or be able to learn the code sequence. The term "carriers" is not quite correct but for anyone not familiar with DSSS it's still a useful analogy.

This is a nice introduction: Spread Spectrum Steganography


  Encryption solves a part of the problem. Now the enemy cannot decipher
  your messages but can still know that you are transmitting.


If the "enemy" is close enough to detect the spectrum above the noise thresholds then they are likely to know where you are by visual means. See THIS for another good article that explains how many spreading frequencies are needed to get the transmission level progressively lower than the thermal noise of a typical receiver.



EDIT

Here's a basic and much simplified example. Let's say you have a payload data rate of 1 kbps and your code rate runs at 1024 x the payload rate. This will give you a processing gain of 30 dB when you receive the signal. 

FYI processing gain = 10log(PN codes per payload bit)

Let's say you transmit 1 watt (30dBm) at nominally 1 GHz. If this transmission is "spread", the nominal power in an equivalent bandwidth of the unspread data is 30 dB lower (1 milli watt).

Next, the free-field theoretical received signal power at various distances (based on the Friis link loss equation) is: -


1 km Rx power is -92.4 dBm
10 km Rx power is -112.4 dBm
20 km Rx power is -118.4 dBm
40 km Rx power is -124.4 dBm


The above numbers assume an isotropic antenna i.e. power is transmitted in all directions. This doesn't effect the overall conclusion if any other antenna type were used.

A plain ordinary radio receiver requires a signal power at its antenna that is determined by the bandwidth of the transmission. A commonly used formula is required received signal strength is -154 dBm + 10log(bit rate) 

The bit rate in the formula above is 1kbps x 1024 i.e. the "spread" bit rate. Therefore, a plain ordinary receiver needs -94 dBm to fully pick up the spread transmission. This means that a plain ordinary receiver is not going to detect any transmission further away from the source of transmission than about 1 km - it'll be in the noise at a greater distance.

However, a spread spectrum receiver can use its process gain (30 dB) to still receive the signal at distances up to nearly 40 km away. 

It's never as clear cut as this with radio but on a good day, you should be able to receive the signal quite easily at many times the distance of a "non-code-aligned" receiver.