NVIS

First published CARC Newsletter April 2000

For a little while I have been interested in a type of HF propagation mode known as NVIS systems. This short article summarises what I have found out about subject. 

WB5UDE (see below) gives a good introduction to the subject. “NVIS, or Near Vertical Incidence Skywave, refers to a radio propagation mode which involves the use of antennas with a very high radiation angle, approaching or reaching 90 degrees (straight up), along with selection of an appropriate frequency below the critical frequency, to establish reliable communications over a radius of 0-200 miles or so, give or take 100 miles. Many of us have used this mode when making nearby contacts on 160 meters or 80 meters at night, or 80 meters or 40 meters during the day. We may have thought of these nearby contacts as necessarily involving the use of ground-wave propagation, but many such contacts involve no groundwave signal at all, or, if the ground-wave signal is involved, it may hinder, instead of help.

”Deliberate exploitation of NVIS is best achieved using antenna installations which achieve

some balance between minimising groundwave (low takeoff angle) radiation, and maximising

near vertical incidence skywave (very high takeoff angle) radiation.

”As hams, we often faithfully follow the advice: get your antenna up as high as you can get it!

We do this, and other things (like choosing antennas that have a low angle of radiation) in order

to maximise the distance over which we can communicate. An antenna with a particularly high

angle of radiation is often somewhat disparagingly referred to as a “cloudwarmer”, the implica-

tion being that if the signal isn’t radiated at a low enough angle, it’s being wasted. For NVIS, we

ignore all this traditional advice, and select instead techniques which will maximise not our DX,

but our ability to reliably communicate with other stations within a radius of 0-300 miles.

”Not just any old frequency will work for NVIS. Successful NVIS work depends on being able to

select, or find (through trial and error), a frequency which will be reflected from the ionosphere

even when the angle of radiation is nearly vertical. These frequencies usually are in the range of

2-10 MHz, though sometimes the limit is higher. The trick is to select a frequency which is

below the current critical frequency (the highest frequency which the F layer will reflect at a

maximum—90 degree—angle of incidence) but not so far below the critical frequency that the D

and/or E layers mess things up too much.”

Some interesting experiments are reported by WA6UBE (see below) who was interested in using

the mode for emergency service operation. He suggests the following “Rules of Thumb” for

operation on the 40 metre band:

1. A Assume a half-wave dipole at 1/4 wavelength above ground as a reference for comparison
2. half-wave dipole at 6 to 7 feet off the ground will have an attenuation of approximately -4dB
3. half-wave dipole 10-1/2 inches off lossy ground will have a worst-case attenuation of approximately -20 dB
4. Assuming correct choice of frequency and a 10.7 cm solar flux value in the 200 range, a half-wave dipole at 1/4 wavelength above the ground would provide a 20 dB over S9signal reading at the distant station when the transmitter has a power output of 100 Watts.
5. If the transmitting station uses antenna “B” above, the resultant signal strength would be: 16 dB over S9
6. If the transmitting station uses antenna “C” above, the resultant signal strength would be: S9

This web page has some great photos of NVIS antennas, including a system that uses tent pegs to

hold the antenna at height.

The key parameter of an NVIS antenna is its height above ground, with a height of 0.1 lambda

often being quoted. I have been experimenting with a 100 foot long wire antenna 2m off the

ground (tacked along my neighbour’s wooden fence). Comparison with my G5RV at 10m (a

typical installation) shows that at some ranges the lower antenna produces a stronger receive

signal, and that at others it produces a weaker receive signal, but a much better signal to noise

ratio. This is a frequently reported result of these antenna types.

There are a number of contests that the NVIS antenna arrangement would be well suited to,

particularly the 80M AFS series and 160M Club Calls. It would be interesting to try an optimised

NVIS antenna called a Shirley at the club station for one of these contests. A Shirley antenna is

basically a pair of parallel dipoles 0.125 - 0.25 lambda off the ground separated by 0.65 lambda,

and fed in phase. At 80M this represents a pair of 80M dipoles (132' long) at about 33', separated

by 175'. This is an arrangement that we should not find too taxing to install.

Another interesting consequence of this work is to emphasis that for club net operation on the

low frequency bands, a low antenna put up for the purpose should work well enough. Thus

operators with restricted antenna facilities should not be put off participating in these events just

because they do not have the ability to put up a serious 160M DX chasing dipole - quite the

reverse.

If you are interested in reading more about this subject, here are some NVIS related links. For

those of you receiving the electronic version of this document should be able to just click on the

link and go there through the magic of hypertext.

http://www.qsl.net/wb5ude/nvis/index.html

http://www.ci.san-jose.ca.us/oes/races/hfradio.htm

http://www.gordon.army.mil/acd/tcs/hf/2418xtr2.htm

http://www.wr6wr.com/products/book_nvis.html

There is also a good article on the subject by VE3VIA in the ARRL Antenna Compendium series

that describes the mode and the design of the Shirley NVIS antenna.

Stewart G3YSX