KB7INO's NVIS Page

NVIS is Near Vertical Incident Scattering

NVIS Explanation

Most of the time you want to work far away or DX, to accomplish this you try to have low angle antenna.
In doing this, you have "skip zones" areas where you do not have coverage because in that area, the radio signal is bouncing off the ionosphere
Many time you will hear a station 1000+ miles away at much stronger than a station 100 miles away

What the NVIS technique does is take your signal and shoot it straight up ( i.e. high angle or near vertical ).
In doing so, the signal bounces back down off of the ionosphere and you have great coverage in a ~500 mile radius... but usually not much further.

Why would you want this :

• local HF nets
• good coverage without the need of a repeater
• many times you do not have line of site with your friends due to mountains/hills or buildings in the way
• When I was in TX, a buddy of mine lived about 20 miles away, there was a hill in the way and NVIS was our only option
• Talking to Portland, OR from Seattle, WA, there are mountains in the way
• reduces incoming interference ( remember, most antennas are reciprocal )

NVIS works great  at about the 75/80 meter band.

· Above 10 MHz,  very iffy.

· Above 15 MHZ, not at all.

Most Amateur applications happen at 75/80 meters, then 40 meters.

· Time of day really doesn't matter !

---

If what I said is hard to comprehend, consider this example :
  You are in a large dark room filled with boxes, and you have a flashlight.

  You point your flashlight at the wall :

• You can see the wall
• Given the angle of the light leaving your flashlight, the area by your feet is poorly lit.
•  You are casting shadows behind the boxes, and there is no "light coverage" there

or

  You point your flashlight at the ceiling :

• You cannot see the wall all that well
• Given the reflection of the light off of the ceiling, the area by your feet is well lit.
• Given the reflection of the light off of the ceiling, there are no shadows behind the boxes within close proximity to you.

How to make a NVIS antenna

It's real easy:
  Basically cut yourself a dipole as you normally would.
  Technically, put it up between 0.1 and 0.15 wavelengths above the ground.
  Practically, put it up about 10-15 high.

If you are out camping or for day event, put it up about 6 or so feet.

• life just gets easier when antennas are low !
• you can put as low as 1 foot or on the ground, but by this point the losses are pretty high.
• this will work though
• 6+ feet is good
• easy to put up
• losses are not too bad
• people can walk under it without too much problems

Now I got a question, is "putting up a NVIS antenna" an oxymoron?!? ;)

---

My NVIS Story :

OK, I kept reading NVIS articles and how the NVIS antenna was the local area gap filler.
I read all the reasons that a station would use a NVIS antenna, so I had to find out for my self.

One weekend I ran a 100' wire from my shack to the back of my yard.
I loaded it up as an end fed, from my shack to the side of my house about 20 feet high.
Then from the side of my house to a tree in the yard about 15 feet high.

I just tuned it up and listened to the local "NW Country Cousins" Net on 75 meters.
I did some A/B comparisons between this temporary NVIS and my 80 meter vertical (i.e. tri-bander).

I was amazed, some stations were much stronger on the NVIS when compared to the vertical.

Portland, Oregon is about 150 miles south of me, with large mountains in the way.
I could hear stations on the NVIS that I was barely able ot hear on vertical.
Also, the NVIS noise floor was much lower, which made it easier on the ears.
I also listened to the Washington ARES Net, and from eastern Washington (again 150+ miles away) I got very similar results.

WOW! I am a believer in NVIS antennas!!!


NVIS Radio Links

• Yahoo! NVIS Group
• HF Pack  -  includes optimal NVIS  Frequencies
• For more NVIS Links, please go to my Links page.

Please click here to return to my main page.