Portable antennas have become the mainstay of my HF ham radio kit. That’s for a very good reason. Living in a residential neighborhood, I seem to be fighting a never ending battle of noise and interference. RF noise is what cooled my ham radio activity a dozen or so years ago, but the ability to get out, enjoy nature, and get on the air with Parks on the Air had reinvigorated me several years ago.
Which brings me to this week’s video. Alpha antennas offered to ship to me, on loan, their HexTenna system. I had heard good things about the HexTenna and have always had good luck with dipoles, so I took them up on the offer and put it through its paces. The result is this week’s video: Is this the BEST DX portable antenna? Alpha Antenna HexTenna https://youtu.be/B7xe8Wz0nwM
Alpha Antenna HexTenna Deluxe Kit: https://www.alphaantenna.com/product/best-portable-hf-dipole-vertical-yagi-antenna-vhf-uhf-hf-hextenna-alpha/?aff=48
Is the HexTenna an NVIS Antenna?
Short answer, no. NVIS or Near Vertical Skywave Propagation is an effect where the bulk of the RF energy transmitted goes straight up and is reflected back down by the ionosphere. NVIS antennas are great for regional communications within a circle of about 300-500 miles. One method to accentuate the NVIS properties of an HF signal is to mount the antenna very low, ideally at a height of about .1 wavelength.
The key is that the NVIS effect is the most noticeable with low band frequencies, 40 meters and below, and does not really exhibit itself with frequencies at or above the 20 meter band. KC2G has a great map that shows the current NVIS conditons: https://prop.kc2g.com/fof2/ This map is a representation of the highest frequency the F2 layer will vertically reflect. Unless you are located along the equater, the viable NVIS frequencies will be below 10 MHz.
The HexTenna in and of itself is a rigid dipole antenna. So it should have the same expectations and performance as a wire dipole antenna. The main difference is that the HexTenna is designed to operate from a ground mounted tripod, while with conventional portable wire dipoles you elevate them 20-30 feet in the air.
My point being, any dipole tuned for 20 meters and above, no matter how low to the ground it is, will still have an element of forward RF takeoff. And at any latitude above 30 degrees, will not see effective vertical reflections of a 20 meter or higher frequency. You’ll still experience skywave and short skip propagation, but the effect is no where near what true NVIS can deliver.
The HexTenna, with it’s tripod mounted base at about 5 feet is going to show a significant amount of skywave propagation. By virtue of being a dipole, with no transformers or other energy sucking components, it should efficiently radiate whatever energy is given to it.
According to Alpha Antenna, in their testing they found the difference between a 20 meter dipole 5 feet off the ground vs a wire antenna at 20 feet is about 3dB or 1/2 of an S-Unit. Personally, I can live with those numbers for the convenience of not having to deploy a wire antenna.
Wire antennas do have their place. A 20 meter dipole hung at least 35 feet in the air will give your the best DX properties as it’s RF takeoff angle will be the lowest. Running the dipole 5 – 20 feet, common for most POTA activators, will exhibit a more moderate takeoff angle of 20 degrees or so.
But it will be an NVIS antenna on 40 meters
The 40 meter band is a different story. Adding the extension wires to the HexTenna allows it to function on 40 meters as an M shaped dipole. The bulk of the antenna’s element is in that .1 wavelengh zone, so you can expect it to radiate vertically. You’ll get some Skywave, but a significant part of the RF energy will go straight in the air.
Below are two charts from a weekend long Parks on the air Activation at US-1445, Hartman Creek State Park in Wisconsin. The first is with the PERformer antenna, that is, a 25 foot whip with the Chamleon M25-Coil and elevated counterpoise. This chart show a significant of contacts outside the NVIS circle as the vertical antenna allowed a fair amount of skywave propgation to develop.
Compare to that the map of the HexTenna 40 meter contacts done the next day at about the same time of day. More of the contacts are closer to the originating station, suggesting a greater NVIS effect is exhibited. Both of these tests were done on FT8 at 25 watts.
These two maps aren’t a conclusive test, but a demonstration showing how two different antennas, in similar locations, behave differently. If 40 meter DX is your target, bear in mind that the HexTenna won’t be your best choice. You’ll have to either elevate a wire dipole, or work towards developing a good forward RF path with a vertical radiator. (and take advantage of good low band night propagation).
Finishing up
I’ll have the HexTenna for a few more weeks before I have to send it back. I will certainly get it on the air again (I want to check out the vertical orientation of the antenna). Is there anything you’d like to see or know about the HexTenna? Let me know and I’ll put it through its paces.
I hope to get you in the log soon,
MIchael
KB9VBR
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