Author Topic: NVIS  (Read 1136 times)

Offline BillFoster

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NVIS
« on: June 05, 2019, 07:20:08 AM »
It’s been a while since I have been active here (7 years). During that time, I studied and got my AE license, joined a local club, and found that because of terrain, I can’t use the club repeater that is only 14-miles away. For the same terrain reason, I can use a repeater that is 55-miles away.

During a SHTF event, things that happen within 100 miles from my location would be more important to me than what is happening in say, Europe or Japan. That is what got me interested in NVIS. I was wondering if anyone on the TSP Ham forum has used NVIS and what successes they have had.There are a couple of good articles on the subject if some (like me)  have not experimented.

http://www.gwinnettares.org/wp-content/uploads/2017/03/presentations/ARES%20NVIS%20multiband%20antenna.pdf

http://static.dxengineering.com/pdf/WP-NVIS-Rev2.pdf




Offline gnoticpasta

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Re: NVIS
« Reply #1 on: June 05, 2019, 07:28:16 AM »
I've been fascinated by NVIS for many years, and my current plan is to use a buddipole configured for NVIS. But, to answer your question.. no, haven't played around with it to much yet.. On an upcoming camping trip, me and a buddy of mine will be using NVIS antenna you have in your second link (just started getting the parts this past weekend)


GP

Offline armymars

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Re: NVIS
« Reply #2 on: June 06, 2019, 08:48:58 AM »
  I have been using NVIS for local MARS HF operation. Right now FOF2 is running about 4 MHz to 6MHz by day and 4MHz to 2MHz buy night. FOF2 is the highest frequency you can run where the signal will go straight up and come down. It changes with latitude, time of day and sunspot numbers. 200 KHz can make a big difference during day light hours. For myself I use the data from Alpena, MI.  Google real time FOF2 to find a list of sights near you for data.
  On 80 and 60 meters I find a dipole at 16 feet to work best. Good luck and good hunting.
 

Offline Smurf Hunter

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Re: NVIS
« Reply #3 on: June 06, 2019, 11:46:12 AM »
I'm actively working on building a full wave 80 meter loop for field day.  I've got an old Dentron Super Tuner (transmatch), and 50 feet of ladder line ready.  I'll share pics later.

Offline Mr. Bill

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Re: NVIS
« Reply #4 on: June 06, 2019, 12:31:36 PM »

Offline Smurf Hunter

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Re: NVIS
« Reply #5 on: June 06, 2019, 12:47:34 PM »
@bill

A little background...

VHF/UHF are line of sight.  This includes most any type of walkie talkie type bands you can think of.  If you put a really tall tower on a really high mountain peak, you might get 50+ miles of coverage, similar in concept to a cell phone tower.

HF is a very different beast.  It's long wavelengths (like am radio) do not escape the earth's ionosphere and will "bounce" back to earth.  That is how someone in Seattle can talk to someone in Beijing.  Assuming you believe the earth is not flat, this is achieved by HF propagation.  While it's not exact or precise, think of two parties who are 1000 miles apart, each aiming their signal a mid-distant point 500 miles from each up in the sky.



As cool as this can be, even if it's not always reliable, one other weakness is local communication.  Just as a mortar is not a weapon for attacking an enemy 10 yards away, HF typically has a "dead zone" of 300 miles or so.  NVIS is a trick to mitigate the skip zone by aiming vertically.

Again, imagine our mortar aimed almost 90 degrees straight up, with the hopes of hitting someone next to us.



Offline Smurf Hunter

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Re: NVIS
« Reply #6 on: June 06, 2019, 12:57:10 PM »
Here are some teaser photos of the horizontal loop.  It's actually a square, but it radiates upward, making it a form of NVIS.  In order to limit loss, rather than using coaxial cable, we use old style TV antenna ladder line or twin-lead.

Aside from just being a geeky project, I'm going to be using an old manual transmatch tuner that our late friend Carl gave to me when I came down to visit him last year.  While down there he helped me overhaul the thing, lubricating the rolling inductors, blowing out dust, and tightening up screws.  It's got to be over 60 years old, but I hope it will bring joy to get some use from it:



This is the mounting plate (can you tell I don't have a 3D printer?)

The loose copper wire ends will splice with the antenna wire.


Offline Mr. Bill

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Re: NVIS
« Reply #7 on: June 06, 2019, 01:01:25 PM »
Again, imagine our mortar aimed almost 90 degrees straight up, with the hopes of hitting someone next to us.

I don't have an amateur radio license, but I'll try it with the mortar.

Offline PhatForrest

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Re: NVIS
« Reply #8 on: June 06, 2019, 04:48:42 PM »
Using a 40 meter dipole 15 feet off the ground, I've worked most of New England SSB and digital.

3 bucks worth of Lowes speaker wire and a BNC cobra head adapter make a simple and effective NVIS antenna.

Offline Pathfinder

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Re: NVIS
« Reply #9 on: June 07, 2019, 08:17:09 AM »
When I was in KY, I lived in an HOA so a permanent antenna was a no-no.
I would set up a random wire end-fed antenna (Packtenna Mini to be precise) running from my deck to the back yard fence. We lived on a hill, so the deck end was about 6-8' off the ground. The fence end was 3.5' up. The ground sloped in 2 directions under the wire. I figured it would never work.
I hooked the antenna up to my IC-7200 via a tuner, and to my surprise I was able to hear lots of signals. From Lexington KY I actually (inadvertently) broke into a net with the net control in Tallahassee FL, some 600 miles or so.
I just received a Chameleon MPAS (somewhat similar to a Buddipole) and am looking to try it as NVIS as well.

As for the mortar example, it's a poor analogy. A mortar is a single round that goes up, then down and can be affected by wind.
The value of the NVIS is that it is a cone of radio signals that roughly describes a circle on the ground hundreds of miles across. NVIS is best used to stay in contact locally or when the geography (hills, mountains, etc.) get in the way of direct or low angle signals.
Our military uses NVIS - whenever you see a military vehicle with an antenna arched forward and secured, that's usually not a tied-down vertical whip, it's an NVIS and it is used in that tied-down position.

Offline armymars

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Re: NVIS
« Reply #10 on: June 07, 2019, 09:45:38 AM »
  Think of NVIS as a water hose pointed straight up and the water raining down.

Offline Smurf Hunter

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Re: NVIS
« Reply #11 on: June 07, 2019, 09:54:30 AM »
A picture is worth 1000 words, so here's a video I made for the local club last night.  Sorry I have poor video production skills.

https://www.instagram.com/p/ByZPc3fAe7y/?igshid=fe4ms81yipgz

Offline BillFoster

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Re: NVIS
« Reply #12 on: June 07, 2019, 07:43:47 PM »
 Very cool. Simple, inexpensive, and it works. Thanks for sharing.

Offline Canadian Prepper

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Re: NVIS
« Reply #13 on: June 09, 2019, 09:29:36 AM »
I have the Canadian Basic with Honours license (all band privileges up to about 560W PEP transmitting power) and have played more with HF than VHF/UHF.

While I haven't set up a strictly NVIS antenna array, in practicality many of the antennas I have put up (i.e. a 10-40m dipole at about 16 feet and long end fed wire that perhaps got just over 20ft elevation at its highest likely worked as NVIS antennas on 40 and 80m). We have some ongoing local 40 and 80m nets that consist mostly of people within a few hundred km from me, and I suspect that I would have been able to connect locally with other hams. Higher bands like 10 provide local distances comparable to many repeaters and it is not difficult to hear people locally on 40 and 80m. Even if someone was in somewhat of a dead zone on 40-80m there would probably be someone else close by that could relay. All of my contacts have been on 100W max, so I think it's reasonable to say that HF would work fine for you.

The one other consideration might be how many other HF operators transmit from within the area that you are interested in. That's where getting on the air, joining all of the local nets, making CQs and getting others into the hobby might pay off dividends. It would be good to develop the networks and trust now, rather than once you need those comms in more pressing circumstances.