Not clear on the concept.
If you or someone you know has an HF station with a goal of DX communications in disaster situations, I have a question: What are you really trying to do? What is the need for very long distance communications in a SHTF scenario? Save for a few narrow, exceptional circumstances, you probably cannot give a solid answer to the question. A more practical HF communications plan for disasters is NVIS, or near vertical incidence skywave.
Reality check.
In a disaster situation, talking to another continent will not have much utility. That’s why I don’t understand these survival and EMCOMM guys who invest time, money, and effort into HF stations expecting to run DX. A more useful and productive area of interest will be your immediate locale, extending out a few hundred miles or so. That’s where NVIS comes in.
VHF/UHF is good for about 25 miles/45 km, maybe fifty miles with a good antenna and favorable conditions. For intermediate ranges of 50-500 miles, VHF/UHF isn’t enough, and conventional HF is too much. NVIS is an underrated communications technique that fills the gap between local and DX.
How NVIS works.
High frequency DX works by “skipping” a signal across the ionosphere. For best results, the radio signal should hit the ionosphere at a sharp angle.
Imagine a rubber ball. When thrown straight forward (almost parallel to the ground), the ball will travel a pretty good linear distance and bounce several times. Now suppose we threw the ball again. This time, we throw the same ball with the same amount of force, but at a nearly right angle to the ground. The ball would bounce the same number of times but the linear horizontal distance it travels from where it was thrown would be relatively small.
GRAPHIC COURTESY RADIONERDS.COM
This is how NVIS works. By sending a signal out at a near vertical angle to the ionosphere, it will by nature of physics travel a shorter linear distance. In practical terms, this distance is 50-500 miles, a range that nicely fills the hole between local and DX communications.
Other benefits of NVIS.
If your antenna is set up to pitch a signal a limited distance, it naturally follows it will only receive from a limited distance as well. While this sounds like an undesirable trait, it actually works as an advantage. This “limitation” has a built-in ability to reduce noise and interference from distant signals.
Secondly, NVIS stations have no concern about common physical obstructions or antenna elevation. A DX operator will want their antenna as high as possible to get that acute angle. An NVIS operator is sending their signal nearly straight up, so all they need is a clear shot at the sky. Nearby mountains, buildings, trees, and similar barriers to radio coverage are not an issue.
Implementing NVIS.
Setting up an NVIS station is crazy-easy. An operator could do it with equipment they already own.
As far as radios go, there are no “special” NVIS rigs. Any HF radio will do. The action is all in the antenna. If you have a dipole or similar wire antenna, mount it so it is between 0.1 and 0.25 wavelengths above the ground. This translates into 13-23 feet (3.96-9.75 meters) on 40 meters. This should give your signal the near-vertical angle needed to make NVIS work.
Vertical antennas are not effective for NVIS. If you have no other option, mount your vertical antenna low to the ground. Beam antennas are also not a prime choice. If you’re giving it a go anyway, point it a few degrees off from straight up. Horizontally-oriented antennas such as dipoles are the most effective.
PHOTO 1. COURTESY HAMUNIVERSE.COM
On disadvantage of NVIS is that it generally works best under 10 mHz, so the choice of operating frequencies may be limited. It can be done at higher frequencies, but it’s less reliable and some trial and error may be needed.
NVIS is ideally suited for QRP operation. Since it’s not DX so you don’t need a big boomer powerhouse transmitter nor an elaborate antenna. And as we already mentioned, the nature of the antenna reduces noise and interference. That means weaker signals are easier to copy.
Sweating the details.
For SHTF purposes, ideally both you and the the station you are communicating with should be configured for NVIS. It’s not absolutely required, but it does give each side the best chance of hearing the other. Setting up a schedule on pre-determined frequencies is also a smart move.
If for some reason DX is essential to your survival communications, then the easy solution is to have one antenna for DX and one for near vertical incidence skywave. NVIS-specific antennas are simple, cheap, and take up very little weight and space for transportation. There aren’t very many good reasons not to have NVIS capability.
If you are part of an organized EMCOMM or disaster response group, there’s a strong chance that they do not even require members to have HF equipment. But if HF is in the mix, then it probably specifies NVIS.
When planning for disaster/survival situations, there will not be a pressing need to work DX. NVIS is more practical, can be done with minimal equipment, and lends itself well to QRP which will reduce the burden on your power source.
Resources.
Here is a lengthy but detailed technical document describing NVIS.
This in depth article has good info about NVIS-specific antennas.
Thanks for linking to my article!
One comment I would add:
“The action is all in the antenna.”
Choosing the right frequency for current conditions is most important, and it varies throughout the day / season / sunspot cycle, as well as with your latitude. So your plans need to be flexible. Sometimes we have needed to use 160m.
Once you pick the right frequency, simple antennas and low power can work well. But no antenna can make up for lack of ionospheric cooperation!
Hi Dale, thanks for your comment. Yes, NVIS can be finicky but with some experience it’s an effective communications strategy.
I plan to use the “inverted V” setup on my antenna. (NVIS) I have a mobile rig in the pickup truck and our scenario requires to have the truck as close as possible to the antenna. How close can the truck be to the “NVIS” and still get satisfactory performance? I have a EFHW 40 meter wire antenna. thanks-James Hollen
Hi James, a truck placed near the antenna could possibly affect the signal pattern from the antenna, but I doubt it would otherwise be a problem.
Due to all the variables in your situation (that will change as the mobile setup is moved around), there’s no conclusive way to know how your station will perform. Unfortunately some trial and error may be required.
Good article, Chris! You could probably do a whole series of articles about the misconceptions a lot of people have about using amateur radio for emergency communications. Some of the stuff I run into out there is pretty wild.
I used to run 2 main antennas at the house, a vertical that I used mostly for DX and a off center fed dipole hanging at about 12 feet off the ground that I used for local communications on 75 meters. With the OCFD I could reliably talk to just about anyone in an area ranging from local guys out to about 400 – 500 miles using 100 watts.
It seems a lot of people’s eyes glaze over whenever you start talking about antennas. More and more people just go to Amazon or DX Enginnering or wherever and drop $$$ on complicated antennas when in the long run a good, old fashiuoned, easy to make dipole, nothuing more than a bit of wire hanging in a tree, is all you need.
Alas, all I have up outside now is my QRO magloop. All of the trees came down around here over the last year or so due to the emerald ash borer and old age, including the ones I had my wire antennas strung to. Sigh… I need to get my dipoles back up again, damn it. I picked up some 16 foot pressure treated 4X4s that I need to get in the ground before the frost comes so I can get them back up. But my lot is too small for my 130 ft OCFD so I’m either going to have to make a shorter one that wont tune down to 75 meters or string it up in a sort of “L” arrangement to fit into the lot.
Hi Randy, yes an article about ham radio “urban legends” is a good idea but it might have to be a multi-part series since there are so many.
I find the lack of technical knowledge in ham radio disappointing, which is in part why I run this website.
Regarding antennas, I’ve never had any high end HF antennas. They’ve mostly been homemade wire antennas. The “fanciest” antenna I ever owned was a Hy-Gain vertical. I can’t remember if I bought it used or someone gave it to me, but I know it wasn’t new when I got it. I was a broke high school kid at the time.
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I have been using nvis in some form or another for a few years. Mainly because I didn’t want to put antennas way high up and all that comes with doing that.
I have done sota and some pota. With sota, all the gear neaded to do higher antennas is just added weight. While it is exciting for me to get international and even east coast contacts, that isn’t my main focus.
I also make my hf antennas, i have made two 40m dipoles and two 20m dipoles. One of each for portable and one of each for the house station. I have also made a cb dipole and a 10m dipole. I find it fun to complete something and get it working.
Hi Doug, yeah I have a pantheon of home brew antennas, and a few cobbled together from parts of commercially produced antennas. They all work pretty good. DIY antennas and projects are more fun than actual operating, I think.
Thank you for this article.
I have forwarded it to many who hopefully
will carefully process it and that action accordingly
for the potential risk(s) abatement benefit(s) of
their own (and their family and friends).
~ Rip.
Thank you for the support, Rip. Off Grid Ham has no paid promotion. The readership is built on hams spreading the word.
Skywarn and hurricane traffic occasionaly need a bit broader coverage, especially for health & welfare checks and relays. As the world has headed headlong into collapse, I have added some new considerations, such as EMP proofiing. All my equipment is in a large solid steel cabinet, and available by simply lifting a cover in front of each shelf, and sliding it into the cabinet. Otherwise, everything is shut up and sealed tight. There is a large drawer at the bottom of the cabinet for a bunch of smaller items. I’ve tested it against RF intrusion and leakage, and everything inside is a dead zone. Grounding for the equipment is the same as normal operation, but when it’s not being used, it’s all shut up, grounds are disconnected, as well as antenna lines and power lines. (each has a manifold for this.) The cabinet it’self is not grounded and isolated from all equipment, making it function as a faraday cage. A true SHTF situation requires that we have established local trusts and a network of like-minded hams. I’ve gotten some comments from less SHTF-minded Hams, who claim that when it gets that serious, they plan on tucking and hoping for a quick end to it all; and they trefuse to investigate what’s really going on around us, in my nation and every nation on the globe. As for me, I have been around for 73 years and have seen a lot. I plan on being that guy who helps people until that end occurs. That’s it for this side rant.
Hi Robert, a while back I did a two-part article on EMP shielding. You may want to check that out.
Thanks for your comment!