NVIS propagation to circumvent jamming and direction finding

Before the bombs started dropping on Ukraine, I was trying to write something about the ban on ham radio there.  Now I’ll do the same thing, but not try to be clever or funny about it. I don’t know that this information will be particularly useful to the people of Ukraine *right now*, but it might come in handy for any of us later on.  
 
There are essentially two ways a ban on ham radio can be enforced.  
 

1. Jamming: basically, jamming is the deliberate interference of a radio signal by transmitting on the same frequency as the target signal, usually at a higher power. So, if you are transmitting at 5 watts, a 6 watt signal on the same frequency would drown you out. Or even a 1 watt transmitter if it is very close to the receiver. This is especially easy on FM due to a phenomenon called the “capture effect”.  

We saw the use of jamming a few months ago when the Cuban government used powerful transmitters to block signals on the 40 meter band which is used for long distance communication.  

 

2. RF direction finding: in movies and TV shows they often call this “triangulation” and it’s a real thing. If you’re on VHF/UHF your antenna most likely transmits in an omnidirectional radiation pattern, meaning your signal travels roughly the same distance in any direction. The signal is strongest at the source – near the antenna – and gets weaker as you get farther away. An attacker can find the source of your signal by the use of one or more *directional* antennas. These are antennas that have a more focused RF radiation pattern for both transmission and *reception*. Two common types of directional antennas are the Yagi-Uda antenna (featuring a large pole with a handful of smaller perpendicular bars) and the more well known satellite dish. The attacker would sweep an area, swinging the antenna in different directions to find out where the signal is strongest.   

An attacker can, of course, jam a signal using these directional antennas. However, they may be just as likely to send someone over to find the source of the transmission and shut it down themselves.  
 
Solution/circumvention  
VHF/UHF 
If an attacker is jamming these line-of-sight radio frequencies, you are probably a bit stuck.  
 
Assuming the jammer is only working on a narrow portion of the spectrum, you can try switching to a different frequency. Coordinate with your affinity group beforehand to select contingency frequencies.  
 
If the attacker is jamming on a huge chunk of frequency space, your only options are to switch to higher power, or switch to a different band.  
 
HF  
It might be possible to get a signal out using different modes of operation such as CW (Morse Code) or weak signal digital modes such as JS8Call. The narrow-banded nature and other properties of CW make it easy for human ears to pick out of the noise and decipher. Weak signal digital modes like JS8Call can be decoded deep into the noise floor. 
 
Neither of these options are *guaranteed* to be immune to jamming. It’ll depend on the situation. The only way to try is by experimentation.  
 
NVIS  
Near Vertical Incidence Skywave propagation is a technique for HF radio comms that involves *lowering* your antenna, generally to between 1/4 and 1/10 of a wavelength off the ground.  

 

Diagram comparing NVIS propagation to “DX” or normal skywave propagation.
To clarify: under normal circumstances, the ideal height of an HF antenna should be 1/2 wavelength above the ground. So if you’re on the 40 meter band (~7MHz), set it 20 meters up. 20 meter band (~14MHz), 10 meters up, etc. Antenna science is complicated, and there are many exceptions to this, but it’s a general rule of thumb. This allows your signal to radiate *outward* toward the horizon as much as possible, where it can reflect off the ionosphere once again beyond the horizon.  
 
If you’re doing NVIS on 40 meters, you would want a *horizontal* dipole between 4 meters and 10 meters off the ground. For 20 meter NVIS, you would want your antenna to be between 2 meters and 5 meters off the ground.  
 
By setting your antenna *lower* to the ground, you do the opposite. You are trying to make sure that as much energy as possible is directed skyward. Hence “Near Vertical Incidence Skywave”. This technique gets your signal
 out farther than line-of-sight (VHF/UHF), but for shorter distances than normal HF propagation. It can also be used to get a signal over mountainous, or possibly militarized, terrain.  
 
Direction finding is also more difficult with NVIS because in a way, you are using the earth as part of a directional antenna, which means an attacker’s *directional* antenna is going to have a tough time figuring out which direction a signal is coming from, as you no longer have an omnidirectional signal.
 
NVIS antenna videos
 
https://www.youtube.com/watch?v=fvZ7z-6wAy0
 
https://www.youtube.com/watch?v=yYXVKtu3nwk
 
https://www.youtube.com/watch?v=CO-0qSpFE20