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N.V.I.S. Portable Antenna

Stephen T. Reynolds (W4CNG) on November 18, 2002
View comments about this article!


N.V.I S. Portable Antenna

By Stephen T. Reynolds W4CNG

A.R.E.S. DEC Metro-Atlanta

There are several articles describing the N.V.I.S. antenna theory and a few ways to build them. Portable and Mobile operation using this style of antenna produces a good path of communications in the 100-250 mile range. Adapting a mobile station quickly and easily is the main feature of this article, plus low cost and ease of construction. Here is how to do two bands really easy for about $20. First is the hardware list.

Home Depot 100ft 12Ga Stranded RED wire. ($11.00)

2 #12 Wire Ring-Lugs with 3/8 inch holes. (.30)

2 3/8x24

4 3/8 inch flat washers (.20)

2 Quick Disconnect antenna bases ($6.00) HRO/other Mail order house.

2 Dog bone end insulators and 40 ft nylon rope. Junk box

Take the wire and cut at 33ft. This gives you 33ft for 40 meters, and 67 feet for 75 meters, the two most popular N.V.I.S. bands for A.R.E.S. use. Crimp and solder one of the ring-lugs to one end of each piece of wire. Using the 3/8x24 fine thread bolt and two flat washers, bolt the ring lug to the quick disconnect base (one flat on top and one on bottom of lug). You can have an option here. Use one quick disconnect for both sets of wire lugs (dual band antenna) or build 2 separate antennas and deploy the one you will be using. I built two separate antennas. The quick disconnect was chosen as lot's of us use singleband mobile HF antennas with quick disconnects. For portable operation, remove the mobile vertical antenna, connect the N.V.I.S. antenna and quick disconnect to your antenna base on your vehicle.

The picture below shows how the finished product looks connected ready to test. This is one of the one piece 3/8x24 single hole to SO-239 mounts (solid brass center pin, no pressed fitting).

0x01 graphic

Here is the testing of the antenna. The ground side of the antenna is your mobile. String out the antenna away from your mobile to a fixed structure further than the 33/66 ft of antenna you have. Red wire was chosen, as it is easy to see when strung at 3-5 ft above the ground. You do not want the antenna higher than 5 ft. The testing and tuning here is in the authors' deed restricted neighborhood front yard. The antenna wire is strung across the front lawn at about 3 ft above the ground and tied to the tree in the far left of the picture.

0x01 graphic

Tuning/trimming of the antenna is then done for resonance in the part of the band you will be operating. I allow an extra 12 inches of wire folded over at the end of the wire for future adjustments. There is no substitute for a good antenna bridge to get an antenna resonant. The antenna shows a near perfect match at 47ohms. No tuner will be needed here.

0x01 graphic

The #12 stranded wire gives +/- 80Khz on 40 meters and +/- 60Khz on 75 meters for less than 1.7:1 VSWR. Not bad for a simple wire antenna that took 30 minutes to build and 15 minutes to tune.

0x01 graphic

Both antennas coiled up and ready to put into the ready bag for deployment.

Good Luck

Member Comments:
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N.V.I.S. Portable Antenna  
by K5MAR on November 18, 2002 Mail this to a friend!
OK, I'll ask the stupid question: What does N.V.I.S. stand for? Just what are these antennas, and why are they good/bad compared to other antennas?

Mark
K5MAR
 
RE: N.V.I.S. Portable Antenna  
by M0DRH on November 18, 2002 Mail this to a friend!
Hello Mark,

Try this for a start:
http://www.qsl.net/k5eph/nvis.htm
Do a search of NVIS on Google, shed loads of info.

Rgds - David - M0DRH
 
RE: N.V.I.S. Portable Antenna  
by VK3YE on November 18, 2002 Mail this to a friend!
Near Vertical Incidence Skywave (?)

A low dipole radiates most signal at high angles (ie straight up or nearly so). Thus it's no good for DX, but it's good for short and medium distance communication on bands like 3.5 and 7 MHz.

A higher antenna (1/2 wavelength up or more) radiates more towards the horizon, and is thus better for DX.

There is nothing magic about NVIS antennas. Pretty much any bit of wire connected to an L-match ATU will do the job. Put it up higher if you want to work DX.

Peter
 
N.V.I.S. Portable Antenna  
by AE5X on November 18, 2002 Mail this to a friend!
Did an experiment with an NVIS antenna on 40m with excellent results. Details at:

http://www.ae5x.com/wichita.html

 
N.V.I.S. Portable Antenna  
by WB2WIK on November 18, 2002 Mail this to a friend!
Nicely done.

Although, for 100-250 mi coverage on 80/40 meters from the mobile station, I've done something for many years that's far simpler. The only requirement is a well-placed mobile "ball" mount, the mount, a stainless steel spring, an insulated windshield clip, cheapie mobile antenna tuner (MFJ-945E or equivalent) and 102" stainless steel "CB" whip. This antenna can be used while driving, or parked, equally well and is self-contained to the vehicle.

Install ball mount on rear fender (L or R) of the car. Install antenna spring into ball mount, and CB whip into spring. Install whip clip near front of windshield somewhere, in a suitable place that it will grab. (Radio Shack sells these for $3.95.) Pull tip of whip into the clip, so the whip is now shaped like an upside-down "U," and is more horizontal than vertical. Install the tuner in the trunk, within a few inches of the ball mount, and use a single insulated wire to feed the tuner (using "single wire" antenna connection), with coax going to the front of the vehicle for connection to the rig.

Transmit, and tune for a match. Close trunk, and mobile down the road. 100-250 mi contacts very workable all day long on either 40 or 80, parked or in motion, and much stronger than when using a conventional "vertical" whip mounted anywhere on the vehicle. MUCH stronger.

I use something like this, but with an SGC-230 Autotuner so I can easily change bands without manually adjusting anything, and it's like having a mobile NVIS antenna. If you unclip the whip from the retainer and let it stand back up straight again, then the same whip becomes a low-angle performer for the higher bands (20-17-15-12-10m) for working DX. Pull it back over and clip it to the windshield again for working locals on 40-80.

The wire antenna shown here probably works better, but my approach works while you're going 75 mph!

WB2WIK/6
 
N.V.I.S. Portable Antenna  
by WB6BYU on November 18, 2002 Mail this to a friend!
Low horizontal wires, either dipoles or quarter wave
wires such as this, provide high-angle radiation that
gives good coverage out to several hundred miles when
operating below the "critical frequency". This is
called "Near Vertical Incident Skywave" propagation
by the military, while hams have used "short skip" for
years.

However, remember that "low" is a relative term when
describing antenna height. A dipole 1/4 wave in the
air will give high-angle radiation and have a higher
efficiency than something just a few feet above the
ground. A good rule is that the MAXIMUM heigh for an
NVIS antenna in feet is the band designator in meters,
so the maximum practical height for a 40m NVIS antenna
is about 40'. (Above that the high angle radiation
is reduced.)

So performance should be at least as good - and quite
likely improved - if the wire is installed higher off
the ground when suitable supports are available. That
would also keep it out of the way of people. Getting
the end up 12 to 15' would be a good start.

The military often installs their NVIS antennas at
low heights for a reason: it minimizes the ground wave
signals radiated from it. This is important when they
want to keep their location secret because the ground
wave is easy for the enemy to DF. (Some antennas are
even used laying on the ground for this reason.)
But this is rarely a problem in amateur use, and the
antenna efficiency drops sharply at heights below about
0.08 wavelengths. This corresponds to about 20' on
75m and 10' on 40m. Although antennas will still work
at lower heights when supports are not available, a
more efficient antenna allows for the same coverage
with less power, thereby extending battery life.
 
N.V.I.S. Portable Antenna  
by JJ1BDX on November 21, 2002 Mail this to a friend!
A good NVIS antenna is surely a strong tool to gain popularity on a nationwide contest in Japan, since the distance between populated areas of ham radio stations are within 500 to 1000km of radius. One of the reasons why a balcony whip-type antenna with an automatic antenna tuner such as ICOM AH-4 is gaining popularity among hams in Japan for the 30m, 40m and 80m bands.
 
RE: N.V.I.S. Portable Antenna  
by K1MKF on November 24, 2002 Mail this to a friend!
Is it true that a good NVIS is 10-20 feet above ground on 40m and 20-40 feet on 80m? Anybody compare loops to dipoles?
 
RE: N.V.I.S. Portable Antenna  
by W4CNG on November 24, 2002 Mail this to a friend!
NVIS antennas are those that are a few inches above ground to 3-6 feet above ground. They are known as "Cloud Warmers" because their radiation pattern goes straight up and comes straight back down across a 100-200mile path. Low hanging loops do the same thing. I have a fullsized 80/75 meter full wave loop at my 5-Land home and it is a fantastic cloud warmer, up thru 40 meters, then it works out to 1500 miles on 20-10 meters when used with a tuner.

Good Luck Steve W4CNG
 
N.V.I.S. Portable Antenna  
by K1WCC on November 29, 2002 Mail this to a friend!
Try a counterpoise, just a wire laying on the ground under the antenna.
 
RE: N.V.I.S. Portable Antenna  
by KC8NNK on December 3, 2002 Mail this to a friend!
Is it possible to perform NVIS on 10 meters? You mentioned 20-10 at 1500miles. Is this a radius from the antenna or does the 1500 miles delineate the beginning of a coverage area?
 
N.V.I.S. Portable Antenna  
by KY6O on December 3, 2002 Mail this to a friend!
I built my first NVIS this past weekend, for 40m. Fun stuff! In order to get it to match without a tuner, I finally had to cut (Wrap, actually) 10' off *each end*. Thank goodness for a digital antenna analyzer!

But the performance (receive-wise) is amazing. You really need to try it to experience it. Most of the "hash" you usually hear on 40m sails right over the top of this thing. Just don't plan on cross-country QSO's with it, but for 300 to 500 miles, I'm sold.

My advice, however (Since this is the first one Ive ever built) is build a NVIS dipole, don't waste your money buying one, like I did.

For $34, here's what I got:
1. A 1:1 balun. These are < $11 at most radio stores.
2. 75 feet of copper wire, with instructions to "cut each leg to 34', and solder to the balun lugs"
3. Two ceramic insulators (For the ends of the wire)
4. A hunk of cotton rope.

Trust me, you probably have these components already, in your junk drawer!

Warning! NVIS voltages can get rather high. Please keep people/pets well away from the wire when it is "hot".

Rich de KY6O
 
RE: N.V.I.S. Portable Antenna  
by K1OU on December 5, 2002 Mail this to a friend!
There is some great data about this at www.cebik.com, and look for an article entitled "Some Notes on NVIS Cloud Burners".
 
N.V.I.S. Portable Antenna  
by N1RWC on December 21, 2002 Mail this to a friend!
I found that if you have a couple of wrenches in the car you can omit the Quick Disconnect. I live up here in New England where the primary winter driving conditions are white snow, brown snow, and white salt. The problem with the quick disconnects are that they corrode quickly, and when they do, they become useless. I have had the same 3/8-24 Mount for 4 years of this with no corrosion internally yet. I have found using Stainless Steel Bolts, or Grade 8 Bolts, to work well. I priced out my investment on this, and came up with $.97 for a 300 foot roll of string, $.69 for tape,$15.00 for a 500 foot spool of 14GA Wire,$.94 for Solder Lugs, and $1.10 for the 3/8-24x3/4 Bolt and Nut. Grand Total $18.70, Less than $20.00 for the project, and plenty of wire left over to experiment on other bands also. I could form my own Eye at one end and omit the solder lugs, and cut the cost down some more. I have tried it out and the receive signals were impressive with a wire at 5 feet off the ground. 73 Matt N1RWC
 
RE: N.V.I.S. Portable Antenna  
by KC7JDS on June 21, 2005 Mail this to a friend!
I remember setting up NVIS antennas in our Army Signal units. We called them "inverted V's". Just a cut-to-length dipole with the ends dropped close to the ground. We didn't tie the antenna ends to the ground, there was a length of rope in between the antenna end & ground.
 
N.V.I.S. Portable Antenna  
by N7GSU on June 21, 2005 Mail this to a friend!
Here's a military grade, man-portable combination U/VHF-HF NVIS antenna system that can be deployed by one or two ops in under five minutes WITHOUT tools, on ANY SURFACE and in the dark, if needs be:

www.emcomm-products.com/rads911a.html
 
RE: N.V.I.S. Portable Antenna  
by N5EAT on July 18, 2005 Mail this to a friend!
I have worked NVIS(Near Vertical Incident Scattering) antennas since I became a ham. Most have been 40 and 80 meter hand-made dipoles 10 feet or less off the ground, and in the case of 80 - on the ground. I found that if I got my 40 meter antenna 8 or 9 feet off the ground, I could work locals and folks up to 1000 miles distant. I generally made the antennas from insulated speaker wire so as to keep contact with the environment to a minimum.

These antennas are meant to work via the splatter principle. For me, the key has always been to cut the antenna so that it did not require a tuner. This entailed simply making the dipole long, and cutting it until it's swr was <2.0.

I did this a lot when I only had novice hf spectrum. I stopped doing it when people told me it didn't work. However, it does work, and depending upon your location, it can work really well.
 
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