General Category > Tactical Corner

The Importance of NVIS on 40/80m and a great antenna book.

(1/5) > >>

gil:
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:
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

K7JLJ:
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

K7JLJ:
Quiteguy, how did the tilted verticals do?  I'm still considering that ezmilitary for this reason.


- Jim

cockpitbob:
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.

Navigation

[0] Message Index

[#] Next page

Go to full version