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Author Topic: The Importance of NVIS on 40/80m and a great antenna book.  (Read 8504 times)

gil

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Hello,

I just found an absolutely awesome old Special Forces antenna book! Problem is, the cover is missing, and I'd like to identify it to buy another one in better shape, as mine is falling apart. Chapter 7 is "Indoor Antennas in Rural Areas." If you have it, please post the title, reference number and year of publication. Thank you!

I have been thinking of the different ranges of communications required for prepping. I basically came up with three:

  • Local: Up to about 20 miles.
  • Regional: 20 to 400 miles.
  • Global: Over 400 miles.

Each range requires different approaches in antenna design, modes and frequencies.

We all pretty much have the local range covered with 2m and 70cm handhelds. The only way to cover twenty miles of course is a good elevation. Longer contacts are possible with a little more power, a directional antenna and a tall mast. Even CB radios will provide a good local coverage. MURS, FRS and GMRS will work for very close contacts, a mile or so, sometimes two. The HF radios used for global communication will ork line of sight as well. Basically, any radio with an output of at least one Watt and a good antenna will do a fine job.

Most Hams like a good DX contact with stations all around the world. Antenna systems are usually set-up with a low take-off angle, ten to thirty degrees for long distance skip. We tend to shun stations from the same state. I have been guilty of it myself. A three-hundred mile contact doesn't seem like a challenge and is often an accident due to our antenna radiating in an unexpected direction. CB will skip sometimes when the conditions are good and provide contacts thousands of miles away, but don't expect any reliability.  We need an HF SSB radio for global range; usually a requirement fulfilled right after solving the local conundrum. We forget about the few hundred miles in that first skip zone...

In a SHTF situation, while I would like to know what goes on in my immediate surroundings, if I was part of a group, knowing what goes on beyond those twenty miles might be of vital importance. A few dozen to a few hundred miles can be covered by vehicle really quickly, and hell can be upon you before you can say "CQ DX." That means long range patrols or regional contacts within that range. If you are using a vertical antenna you might have noticed that regional contacts are pretty rare... If your dipole is pretty high or you cavort the higher bands you probably noticed the same. So, how do we cover those pesky few hundred miles close to home?

NVIS (Near Vertical Incidence Skywave) is the answer, on 40m during the day and 80m at night. Check the Wikipedia article, there is no reason for me to explain it all: https://en.wikipedia.org/wiki/Near_vertical_incidence_skywave.
Basically you use a low horizontal antenna to shoot your signal straight up to the ionosphere and have it rain down all around. It only works on lower bands.  Who has an antenna long enough for 80m? Not me... Well, I just ordered the 80/40m end-fed tuner from http://sotabeams.co.uk. All I need now is 135ft. of #534 wire from http://thewireman.com. Yeah, it's a bit long... I don't even have an 80m rig yet, but one thing at a time...

I do believe regional coverage is of the utmost importance. In fact, all three ranges are of the utmost importance and need to be addressed. So, don't forget about NVIS. Most often, that means an adequate antenna system, which can be a simple long enough dipole. The book I mentioned above has great examples. I really do hope someone identifies it.

As to long range patrols, make sure you mind the weight of the station and current draw... You know my opinions on the subject... Some radios will do it all, like the FT-817nd, but specific-purpose radios might do a better job in each case. Privacy is an issue as well for signals that will radiate far away. Think one-time-pads and low power. NVIS has some advantages in that regard as well.

What's your NVIS solution? Have you used it before?

Gil.

Quietguy

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #1 on: May 26, 2015, 07:53:42 PM »
Some years ago I was active at the state level ARES/RACES and put up an 80m NVIS dipole antenna specifically for the state net.  It was made from #14 stranded THHN electrical wire from Home Depot, and was installed about 10ish feet off the ground to a nearby tree.  Total length was about 135ish feet, cut for resonance at the net frequency, with about half running along one side of my shop and about half in the air to the tree.  I was able to run the Washington state SSB net a couple of times, with the biggest problem being the attitude of some of the net members apparently triggered by my obvious inexperience as net control.  I had good contact through a large part of Washington state and only needed fill a few times.

I also put up a 135ish foot dipole up about 50 feet, which has NVIS properties on 80m and have had a number of successful daytime SSB contacts with friends out to a couple of hundred miles.  In one daytime test at maybe 150 miles or so I lowered my output power in a series of steps to see how low we could go.  I was down to 10 or 20 watts on my Icom 706MkIIg when my friend had to break off the test and take care of something.  That antenna worked better than my 10 foot high experiment, maybe because it was in the  clear rather than running through trees and along a building.

Another time, our county EC/RO and I decided to try an experiment with a temporary 80m NVIS dipole setup for the state net.  He was scheduled to run the Saturday net, so we headed up to the local radio club (on 5 acres of land) and set up a temporary antenna.  I had pre-cut more of the #14 THHN Home Depot wire to about 135ish feet and strung it up using surplus portable fiberglass mast sections available on ebay.  I think I used two sections of mast for each support, which put the wire about 6 feet in the air, with a piece of coax run through the clubhouse door.  He was able to cover pretty much the entire state of Washington, at least as far as there were stations participating in the net.  The biggest problem with that experiment was stability of the mast sections - I needed to use a better anchoring system.  It has been several years, but I think I held up the masts with metal fence stakes driven into the ground, but the weight of the wire wanted to pull them over.  Subsurface rocks kept me from getting as much penetration as I needed.  I ended up with some paracord guys and accepted sag in the wire.  Later, a permanent NVIS wire was installed at the club.

I have not had nearly as much luck with 40m NVIS, presumably because of my latitude.  40m NVIS tends to work much better in, say, Southern California than it does here in SW Washington state.  However, 80m here is Golden if you can put up a suitable antenna.

if you can find the out-of-print book Near Vertical Incidence Skywave Communications Theory, Techniques and Validation by LTC David M. Fiedler and Maj Edward J. Farmer grab it.  It contains a series of articles on military testing of NVIS techniques.  They had excellent results using military HF whips tilted horizontal.

Wally


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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #2 on: May 26, 2015, 08:12:04 PM »
OTP with message sent over packet seems the best "secure" comms solution to me, but it's hard to co-ordinate since both parties need them before an event.  No other encryption is safe.

Then again, if you are up against a foe with those capabilities... No TRANSMISSION is safe and you best comms are the MW blister packs we all poo-poo.

Their weakness is their strength. 

Then again, if I was on a patrol scenario... Visual comms is the only sure thing.

It's good mental exercise to think through a comm issue at various levels of avoidance.

The think about packet is your emission is reduced to a level that rules out fox hunting by normal HAM operator capabilities.

The biggest issue is setting up relations now with OPs in NVIS that you can use as a source of information. 

That's were AMMRON comes in.


- Jim
-Jim

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #3 on: May 26, 2015, 08:18:31 PM »
Quiteguy, how did the tilted verticals do?  I'm still considering that ezmilitary for this reason.


- Jim
-Jim

cockpitbob

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #4 on: May 26, 2015, 09:53:41 PM »
Gil makes good points about the range of distances we need to cover.  I lived one example.  I was 20 miles from Oakland, CA for the 1989 Loma Prieta earth quake.  ALL civilian communication went dark for the first 12hrs (I wasn't a ham so I can't speak to what they were doing).  None of the local TV or radio stations worked.  Most cable TV went dark.  Some phone lines worked, but they were so jammed you could rarely make a call.  I kept dialing my folks in CT to try to let them know I was OK.  On 2 occasions over 24 hrs I got through and they, in CT, had better information (from the New York City news stations) about my situation than I had, even though they were 2,500miles away.

So yes, depending on the SHTF scenario you need local comms for family, friends and resources, then you need regional for for semi-local information, then global for the big picture of what's going on.
« Last Edit: May 26, 2015, 09:55:20 PM by cockpitbob »

madball13

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #5 on: May 26, 2015, 10:00:52 PM »
I've used NVIS with success is I my area for field day and some emcomm exercises. I have some buddies who in the first skip zone I'm pushing to get set up

Quietguy

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #6 on: May 26, 2015, 10:03:24 PM »
Quiteguy, how did the tilted verticals do?  I'm still considering that ezmilitary for this reason.

One article in the book describes some USMC tests run in 1989 with Camp Lejeune NC as the hub station and out stations in Cherry Point NC, Oak Grove NC, and Norfolk VA.  Two Humvees headed out, north and south, stopped and ran tests every 25 miles and had reliable comms with the hub and other stations out to a range of about 150 miles from the hub.  They estimated a reliable range of 200 miles from the hub.

The mobile stations tested two antennas: a 32 foot military whip antenna (AT-1011) bent 90 degrees using a whip-tilt adapter and a 32 foot wire.  Both antennas extended horizontally to the rear of the vehicle.  The wire antenna was supported 4 feet off the ground at the end.  They had 100% reliability in all cases. 

According to the article:

Quote
Using either the 32 foot bent whip or the 32 foot wire produced identical circuit reliability results to the fixed station test over the many stops and checks that were conducted in the operational area.

They ran a second series of tests with the whip-tilt adapter on a different route after shortening the 32 foot whip to 16 feet.  They did both day and night tests, changing frequencies as appropriate.  The article says:

Quote
Heavy thunderstorms with considerable lightning were present during the test period.  Despite the lightning, the inefficient 16 foot antenna and the inherent high noise level of the frequency band being used 90% reliability for voice communications was still achieved.

They don't specify which frequencies were used, but I believe it's a safe assumption they were somewhere in the vicinity of 60 meters or longer.  They chose their frequencies to make sure they would get ionospheric reflection.

Wally

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #7 on: May 27, 2015, 04:39:23 AM »
Great information both of you!  Having SWL is the next step after 20mile I believe as it's the easier to obtain with minimal gear and anything major will make international news.

NVIS is high on my list but I'm definitely in need of good material on it and propagation in general.


- Jim
-Jim

NavySEAL

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #8 on: May 27, 2015, 11:43:17 AM »
When my Ham Buddy and I got our AmEx  tickets a couple years ago we each bought new antennas . His was a vert GAP......just an alumn pole sticking up but the dreaded HOA letter came and he had to take it down...........so he hinged it about 4 feet up and kept it down in daylight and cranked it up when it got dark........that got old quick so he started using it laid down............it worked just fine......he has used it as a horizontal vertical........he has worked all states.....South America.....Africa....Europe....Australia etc.........remember his antenna "height" is 4 feet.......local Hams have trouble believing him and stop in to see the lay down vertical and check his logs.......my conclusion is that there is still things to be learned about antennas.

gil

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #9 on: May 27, 2015, 01:05:34 PM »
Quote
.remember his antenna "height" is 4 feet.

It all depends on soil conductivity. My SF antenna manual suggests laying antennas on the ground on top of sand dunes...

Quote
OTP with message sent over packet seems the best "secure" comms solution to me

Yes, in theory. Maintaining and powering a computer in an emergency/TEOTWAWKI situation might not be as easy as it sounds. I could see a small EePC with a solar panel and charger, I have one, but still a very fragile proposition. I guess with the price of netbooks today, you could buy a few, but who else is going to do that? It would be a group proposition at best.

What we need is a tiny portable PSK31 rig with an LCD and USB port for a small flexible keyboard. It would be offered as a kit for a reasonable price. Actually, there is such a rig, sort of, the Warbler 80 by Dave Benson, who unfortunately is retired. He released that rig to the public domain. Maybe someone should pick it up, modernize it and reintroduce it...

Gil.

K7JLJ

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #10 on: May 27, 2015, 10:29:25 PM »
Survival Tech Nord is on the right path with android tablets I believe, I'm building my system as an almost direct mirror of his.


- Jim
-Jim

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #11 on: June 10, 2015, 08:40:52 PM »
This is something I've been very interested in ever since I learned about it. I'm in Northwest Arkansas and I'd like to get some hams within a 300 mile radius to try this with. After I get another 80m dipole built, of course. The first one didn't work so well. It was my first attempt, though.
So long, and thanks for all the fish

gil

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #12 on: June 10, 2015, 08:48:07 PM »
NVIS works on 40m too!

Gil.

BrassPounder

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #13 on: June 26, 2015, 09:26:38 AM »
I also make a few NVIS type contacts regularly on 30 meters.  Distances are typically 10 to 70 miles.  Signal strengths are good and steady.  Running 100W to an inverted V at 33 feet.
John WW4DX (formerly WN4OFT)

hank scorpio

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Re: The Importance of NVIS on 40/80m and a great antenna book.
« Reply #14 on: August 03, 2015, 06:44:21 AM »
Bravo on this thread! I play with QRP NVIS quite a bit for a couple of reasons...average joe can't listen in if he doesn't have the equipment, and it gives dedicated beyond LOS capability, reliably, with a good amount of practice. It definitely has a place within any communications package, second only to local 2m/70cm for local information and communications. Any dipole between 1/8 and 1/4 wave elevation will work for NVIS, and even better if you add a counterpoise wire. It relies mostly on the vertical takeoff angle, so any low mounted antenna ***should*** have NVIS properties.

We used NVIS a good amount in one of the units I served in, as an alternate to tac sat as our mission requirements were communications sometimes extending far beyond LOS for conventional forces. These lessons are what got me into Amateur Radio, and I thank God that I learned them. The handbook you mention sounds like the 18E(SF Commo) handbook. If you could duplicate that manual, do it, as they're hard to come by and contain great info.

Some resources that I've had come in handy were the USMC antenna handbook(a must have for a LRS RTO), FM 7-93, the ARRL antenna book, and the ARRL stealth antenna handbook.