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.