Author Topic: HF NVIS for reliable local and distant (more secure )communications  (Read 3411 times)

Offline Carl

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  Ham radio HF communications is most often talked about because of it's long range ability.
VHF and UHF are great for line of sight (25 miles without repeater) local communications but
all radio suffers from terrain and most people live in the valley so reliable communications is just a dream.

But what good is long range when you need to talk 50 miles away and typical propagation carries
your signal 500 to 5000 miles away ? Well,you can get the distant station to relay your message back
to someone closer to where you need or want to talk, but this is sketchy ,at best.

 You can plan ahead for local (1 to 250 mile) communications that terrain will not absorb and is predictable and reliable.
First is to understand you HF signals pass through the "D" layer of the ionosphere and then,when conditions are right,
reflect off the "F" layer(about 75 miles up) and bounce back to earth...typical take-off angles of your antenna will make
the first bounce back to earth at some 400 miles away or so and whoever answers is not likely to bring you
a gallon of gas etc,so not much help.

  When help ,or conversation,news,ets is needed ,we need to talk closer in...enter NVIS or Near Vertical Incidence Skywave .
While NVIS sounds tough...all it is is to raise the angle of your antenna so the radio signal bounces back to earth
CLOSER TO YOU. The advantage is you cover more area closer to you and due to the higher angle ,you don't have as
much signal loss due to "D" layer absorption, so you can communicate closer in while still having (slightly diminished) distance capability.

  While NVIS sounds highly technical..it is basically just having your antenna closer to the ground...WHAT? That sounds EASY ,
and it is though how low and adding a wire ,on the ground,below your low dipole can help..all you need is a dipole at 10 or so feet above the ground (.15 wavelength is best) for a 40 ,60 or 80 meter dipole or inverted "V".

  NVIS while mobile(stationary) can be done by attaching a 1/4 wavelength of wire in place of your mobile HF (often a 3/8 X 24 antenna mount) ,so a bolt and wire can replace you marginally effective mobile whip with a effective wire. This assures communications within the 250 mile zone that VHF does not cover without need for crowded or out of service repeaters.
ALSO due to the "F" layer reflection mountains will not keep you from talking to the valleys around you,or the distant stations.

  Secure communications with HF...well here are my thoughts .

Direction finding is not so good when the signals come from a reflection overhead and is scattered due to reflection.

Frequency snooping....There are not channels and even finding you (besides your direction) talking would be difficult.

Jammers...are not effective against a signal from above as jamming depends more on line of sight or ground wave,

LOW POWER works well with NVIS and this decreases you 'distance' signal and leaves you harder to find still.

DIGITAL MODES,even with a PSK phone app using the MIC and speaker make fast efficient TEXT comms
                    requiring knowledge  to intercept.


I have a huge set of NVIS files that I can host on my Google drive for download for any interested and will,as best I can,answer any questions you may have on this  or most any other radio communications .

Link to my NVIS foler:

https://drive.google.com/folderview?id=0B5ZIZLZV4AwIfi11UXVCMG4xWnlOX0I0a2VVZEpxTTNDQWF5SThKcUduWldCcHFvUEVPR0k&usp=sharing

Link to 2259 (Military NVIS design) antenna files :

https://drive.google.com/folderview?id=0B5ZIZLZV4AwIY3NoMkthTEVQRE0&usp=sharing

And all of this data was found free on the internet
« Last Edit: May 09, 2015, 10:43:13 AM by Carl »

Offline Carl

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Here is a good description of NVIS ,with pictures for a better description of NVIS:

http://www.coares.org/publications/COARES/NVIS.pdf



And a link with graphs for those who like math:

http://www.radiohobbyist.org/blog/?p=245
« Last Edit: May 09, 2015, 11:38:18 AM by Carl »

Offline NR5P

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I did some reading from a military study some time back that showed from their info that below a quarter wave length above ground little was gained because of ground loss for nvis. Of course I guess that would depend on soil type etc. Just curious if you all have tried a quarter wave and compared to a very low wire. I doubt there is much advantage on 80 meters (we are all pretty much nvis compared to wavelength). But I would like to see now info on say 10ft compared to 50ft above ground.

Offline Carl

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I would like to see now info on say 10ft compared to 50ft above ground.

I have found that while transmitted signal strength is much the same below 1/4 wavelength ,the receive noise level is greatly reduced at lower antenna height and this is most helpful for reception of the scattered (weakened)  signal...also as many people already know ,I try to do things the easy and simplest way.(because it is faster,easier,and simpler)

My 40 meter BURIED LOOP   http://thesurvivalpodcast.com/forum/index.php?topic=51280.msg587036#msg587036    antenna is 2 to 6 inches below the sod and soil and yet is my best UNSEEN antenna because it hears very little noise and is effective with transmitting (even with ground loss) while it remains out of sight.
Maybe the best answer is "Because I said so"

Here is a quote :
The Near Vertical Incident Skywave (NVIS) antenna is a half-wave dipole antenna mounted not over 1/8th wave above ground (at the highest operating frequency). While 1/8th wave works reasonably well, better coverage is obtained if the antenna is mounted at about 1/20th wavelength above ground. A second advantage of lowering the antenna to near 1/20th wavelength is a lowering of the background noise level. At a recent S.E.T. communication on 75 Meters was started with a dipole at approximately 30 feet. We found communication with some of the other participants to be difficult. A second 1/2 wave dipole was built and mounted at 8 feet off of the ground. The background noise level went from S7 to S3 and communications with stations in the twenty-five and over mile range were greatly enhanced. Simply stated, you want as much of your signal going up as possible and ten to fifteen foot height has shown to function very well.

Any horizontally polarized antenna will have an NVIS component in its radiation. To maximize the NVIS component, you need to run the antenna at ten to fifteen feet above the ground. Will it work if lower? Yes it will, reference WA6UBE tests. Will it work if it is higher? Yes, but the NVIS efficiency goes down. Field tests have proven that the best NVIS efficiency is obtained at the ten to fifteen foot height for frequencies in the 40M to 75M range.

From this web page:

http://www.w0ipl.net/ECom/NVIS/nvis.htm

And this page:

http://www.tactical-link.com/field_deployed_nvis.htm





Offline armymars

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  High Carl,
       As you know I do a lot of NVIS work in Mars. Two things I have found out is how to cut ground loses. First is to design an antenna with a strong H wave. This is the magnetic portion of the wave and has much lower ground lose the an antenna with a high E Wave or voltage component.  The second antenna is a CCD antenna. This is made up of sections of wire and a capacitor with a bleed resistor to protect the cap. Each section of wire is a resonate circuit at the chosen frequency. These antennas work very well as low as 1 to 3  feet above the ground.
     The down side is on an antenna with a stronger H wave you have low radiations resistance so that means a high voltage capacitor in the matching circuit. Think small transmitting loop. In a CCD ( continuous current dipole )   it is a full wave length long. On frequencies that work well for NVIS that's a lot of real estate . Losses at 80 meters don't become excessive until your antenna is below 27 feet. My favorite band for NVIS is still 60 meters until noon or so then I jump up to 40 meters. 80 and 160 meters is reserved for late night.
     Did you hear that we might be getting some new frequencies around 160 KHz? Now how many bad guys can listen in on the 1024 meter band. Grin.....