American Partisan: HF NVIS Antenna

NC Scout of American Partisan talks about the HF NVIS Antenna.  Also check out a follow up post here.

In the last Radio Contra I discussed a simple way of rigging up an antenna for NVIS HF use. Its a topic that gets a lot of attention, and in turn, a lot of confusion. But trust me, its simple. The whole point behind HF is creating regional communications- anything that’s beyond line of sight– and while you can spend a heck of a lot of money in a hurry and not get a lot, you can spend just a few bucks and with a little knowhow I’m about to impart here, have a great setup.

NVIS relies on sending as much of your radiated energy skyward as possible, with as close to a zero degree takeoff as possible. So, this means a horizontal antenna close to the ground. In case you’re wondering, the takeoff angle is perpendicular to the orientation of the antenna- so, if the antenna is vertical, you’ll have a very shallow takeoff angle, aka groundwave, if its horizontal, the radiation goes vertical. NVIS generally works best between 1.8-8mHz, with the higher frequencies working better during the day and the lower ones at night.

I’ll also add to this that the direction finding threat almost exclusively comes from groundwave. So on HF, NVIS is what you’re looking for. As little groundwave as possible.

So with that said, let’s talk about this antenna.

The first thing to know is that its built out of dirt cheap materials. 128ft 14AWG stranded wire, a Cobra Head, and ten plastic electric fence posts. Less than $25 or so.

For an 80M dipole antenna, each leg is going to be roughly 64ft long. You can make a loop or use a ring terminal to secure the wire to each end of the cobra head. Stretch it out- now you’ve got a dipole. Those plastic fence posts serve both as a suspension for the antenna and as an insulator. All you have to do is wrap the ends in a loop, and boom, you’re ready to rock and roll.

The antenna itself is roughly 2ft off the ground. This creates a high amount of reflectivity from the ground, sending your radiation almost completely vertical.

And with that, you’ve got a dirt cheap antenna that works pretty well. If you want to see how it works and get hands on building one, come out to class.

American Partisan: Home Brew – NVIS Antenna

Johny Mac at American Partisan has an article up at American Partisan on making your own NVIS antenna. A near-vertical incidence skywave (NVIS) antenna is used in the high frequency (HF) spectrum from around 1.8MHz to  10MHz in order to cover an approximate radius/range of 300 miles. HF is usually used for long distance communication around the world, and most antennas are built to maximize range, but those antennas can often cause the radio signal to skip or make a gap in the range from about fifty to three hundred miles.  The NVIS antenna redirects the radio signal in such a way that it covers that gap at the sacrifice of the longer distance contacts. This makes the NVIS antenna ideal for communications during a regional disaster.

If you do not know what a NVIS antenna is and why it is a great addition to your antenna farm, go and read these four articles. The first being from NC Scout titled, Near Vertical Incidence Skywave – Simplified, then NVIS I, NVIS II, and NVIS III authored by Key Pounder and published on NC Scout’s site, Brushbeater.

At a 100,000-foot level though, a NVIS antenna is a dipole antenna that has a steeper transmit angle than a traditional horizontal flat-top dipole antenna and is located closer to the ground at between 8 to 15-feet Vs. a traditional 1/4λ height; let’s say 30 to 60-feet over tera firma. This allows you to transmit to other stations closer to your AO that you may skip over using a flat-top horizontal dipole antenna. Think of the skip bounced off the ‘F’ layer of the ionosphere being 25 to 50-miles Vs. 200 to 300-miles. The German Army developed this procedure during WW II to communicate with troops via the airwaves closer to the transmitting station.

Traditional dipole antenna radiation Vs. NVIS radiation

I started out building my NVIS antenna using the directions posted on the DX Engineeringsite as my blue-prints for the project. Although you can buy a kit from DX Engineering, I had plenty of hardware around the redoubt so I decided to save a few bucks. Below is my Mise en place for this build. Hams are the true scroungers of this earth…

Click here to read the entire article at American Partisan.

Near Vertical Incidence Skywave (NVIS) Simplified

NC Scout has a new article up at American Partisan about NVIS radio communications. If you were confused by some of the terms used in our Suggested Radio Equipment for Community Security article, this has some additional explanation of line of sight and beyond line of sight communication as well as, of course, NVIS which was briefly mentioned in that article.

HF radio wave propagation can be shot nearly straight into the Ionosphere, hence the name Near Vertical Incidence. What goes up, must come down. Knowing that all things have equal and opposite reactions, the angle coming down is also nearly vertical. Then it goes back up, and down again, and over and over. Like this:

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Near Vertical Propagation

And it gives you a range Beyond Line of Sight, at the tactical level, like this (minus the TACSAT in the photo):

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Tactical Coverage of NVIS Comms

With a regional picture looking something like this (which goes along with Planning your Footprint) pictured below.

Washington DC
Regional Coverage Using NVIS

So what does this give us? We now have Beyond Line of Sight Communications that do not rely upon repeaters. Once operators are decently trained and have a good amount of time working in this method under their belt, it can be very reliable.

Important to note is that not all HF bands work well for this. Generally speaking, 160-40M work best due to the way the frequencies themselves refract off the ionosphere. Experience as a Radio Operator should tell you when to use which band based on noise level, the amount of heard traffic, and beacon propagation near your operating frequency will give you a good idea if your traffic will be successfully transmitted or not.

Click here to read the article in its entirety at American Partisan.

 

RELATED:

Near Vertical Incidence Skywave Communication: Theory, Techniques, and Validation by Fiedler and Farmer (pdf)