80m Nvis Antenna
80m Nvis Antenna
80m Nvis Antenna
A Near Vertical Incident Skywave (NVIS) antenna is designed to radiate most of the signal up at a high angle thus making local/regional HF communications out to a few hundred miles much more reliable. One type of NVIS antenna is a center fed dipole that is mounted low to the ground, normally within 0.05 wavelength. Because of the low mounting height, an NVIS dipole is omnidirectional with most of the signal being radiated at a high angle between 75 and 90 degrees to the earth's surface.
Standard Dipole
Good for DX communicationsexhibits a high average gain of 4.9 dBd between 15 to 35 degrees to the earth's surface Maximum gain5.5 dBd at 30 degrees to the earth's surface Attenuates high angle signalsvirtually deaf from above due to an average loss of 6.9 dBd between 50 to 90 degrees to the earth's surface Bidirectional radiation pattern Optimum performance when mounted 1/2 wavelength above ground
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As you can see, a standard 1/2 wavelength dipole mounted horizontally at 1/2 wavelength above the ground is an excellent antenna for working DX. However, it is virtually useless for working stations within a few hundred mile radius (its null area). The opposite can be said of an NVIS dipole since it does an excellent job of hearing nearby signals. However, the NVIS dipole is virtually useless for working DX stations.
Excellent Option for Local and Regional Communications (Emergency and Disaster Relief)
The NVIS dipole is an excellent option for working emergency and disaster relief communications within your local area and regional area (out to a 150 mile radius, or more). This antenna provides omnidirectional communications with most of the signal being radiated at a high angle between 75 and 90 degrees to the earth's surface. This is important as it naturally helps to attenuate DX signals arriving at low angles, while amplifying nearby signals arriving at high angles. This results in more reliable local and regional communications with much less DX noise. However, all communicating stations must have antennas designed for high angle radiation. (More on this below.) For R.A.C.E.S. and ARES organizations, this means reliable HF communications that can cover a regional area of 70,000 square miles (150 mile radius) or more without repeaters. As a comparison, if your 2m or 70cm repeaters go down during a disaster, expect to cover a maximum of about 2,500 square miles (30 mile radius) using simplex, and that is a very optimistic figure.
What makes the NVIS dipole an excellent option for local/regional emergency and disaster relief communications?
Extremely portable as it weighs only a few pounds. Easy to install temporarily or permanently. Omnidirectional, so you are more likely to hear other stations from any direction. It is adapted to local and regional communications while keeping DX noise to a minimum.
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What kind of coverage can I expect from an NVIS dipole with reflector?
As they say, you mileage will vary. There are many factors to consider which we will not go into in depth in this document. Factors include band conditions, time of day, time of year, weather conditions, atmospheric conditions, frequency, et cetera. What I can tell you is how well mine performs. My shack is located in EM12ko in Arlington, Texas. I can usually work stations in about a 600 mile radius on 80 meters. That is over 1 million square miles. However a 600 mile radius is approaching the limitations of this antenna and communications are not always good, or even possible, at that range. As you can see from the coverage map, that includes the entire states of Texas, Oklahoma, Kansas, Arkansas, Lousiana, and Mississippi, most of Missouri and New Mexico, plus the Gulf Coast region from Southern Texas through Mississippihurricane alley. I can very reliably and consistently work stations within about 150 mile radius, illustrated by the small circle in the coverage map. That is 70,000 square miles and more than adequetely covers the greater Dallas/Fort Worth metroplex.
What if my lot is too small to stretch out a 130 foot antenna in a straight line?
The coverage from my NVIS dipole and reflector is pretty good, and mine isn't even mounted under ideal conditionsnot by a long shot. I strung the dipole along the top of my wood fence about 6 feet above the ground. That is a height of less than half the recommended height of 13 feet. I merely laid the reflector on the ground directly beneath the dipole rather than elevating it to the recommended height of 5 feet. Plus my lot is too small to conveniently stretch out the whole dipole/reflector in a straight line, so as you can see from the image I had to bend it around quite a bit. It might look like a strange configuration for a dipole, but it still works, and quite well I might add. Once again, do you what you can with what you have.
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On the other hand, an NVIS dipole works best when mounted 0.05 wavelengths above the ground: about 13 feet for 80m or 7 feet for 40m. To make an NVIS dipole work even better, you can mount a reflector wire (that is 5% longer than than the total dipole length) at 0.02 wavelengths above the ground: about 5 feet for 80m or 3 feet for 40m. This basically turns the antenna into an upward pointing yagi.
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The mounting height figures of 0.05 and 0.02 wavelength are for the optimum installation. However, as I showed with my antenna, you do not need to stick with those exact heights. A dipole laying on the ground will radiatenot well, but it will radiate. Try to get your dipole a few feet off the ground, as close as possible to the 13 feet recommendation. Also try to elevate your reflector, but it will definately work even if it is on the ground. Do use a reflector as it will add up to 3 dBd of gain over just a low mounted dipole by itself.
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I used a short 10 foot (3 meter) piece of RG-58 coax to make my RF choke balun. I connected the short end of the RF choke balun to my antenna, and the other end was connected to a 50 foot (15 meter) RG-8 coax cable that runs to the tuner in my shack. This method allows me to use the RF choke balun with any coax cable, which is handy for portable use.
73 de W5JCK
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