Saturday, May 08, 2010

Near Vertical Incidence Skywave (NVIS)

Here I thought high frequency radio transmission (HF) was so yesterday that only aging amateur radio operators used it. Now I read in the May 2010 QST Magazine that

HF NVIS propagation has been battlefield proven in the Balkans, and more recently in Iraq and Afghanistan, as a method of communicating over mountains that you can't put VHF repeaters on. It has gained traction and is used for EmComm in the US, especially if conditions take out repeaters.

"VHF" means "very high frequency" and transmissions on VHF are good for line of sight communications only. "Repeaters" are what our cell phone systems depend upon.

The HF bands are between the broadcast bands at the lower end and the TV bands at the upper. (The TV bands are VHF and some U[ltra]HF.) HF bands are usually thought of as DX (long distance) frequencies, because the radio waves at those frequencies can "skip" several times back and forth off the ionosphere and the surface of the earth until they reach literally around the world. The radio wave that skips this way is called a "skywave." (I have discussed the miracle of propagation before.)

(Sometimes you can hear a transmission from someone that skips both ways - that is you hear the person from both directions - very nearly simulataneously. Very nearly is the key phrase, because one transmission is just a bit delayed: it would have traveled a little further than the other, unless the transmission originated exactly half way around the world from you. Thus, those sort of transmissions have a sort of echo.)

Usually if you are making an HF transmission, you want your antenna to be as high off the ground as possible, so that the radiation is a nearly as horizontal as it can be. This gives the signal a good bit of distance to travel before it bounces of the atmosphere the first time. Furthermore, the angle of the radiation when it hits the ionosphere will be acute.

But if your antenna is on or near the ground (that is, 1/4 wavelength or lower), the signal tends to go mainly up ("near vertical), hit the ionosphere, and then bounce back down (thus the "near vertical incidence skywave" or NVIS). If the incidence of the skywave were exactly vertical, then you couldn't reach anyone with your signal unless he was standing beside you. But the skywave is never completely vertical, so an HF NVIS can travel, according to the QST report, out to about 1000 miles.

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