80-meter NVIS antenna

Posted by Fred C. (W6BSD) on Apr 27 2020

I live in the middle of Silicon Valley in a suburban setting with small front and back yards. My property does not have tall trees to take advantage of when installing an antenna. Given these conditions, my best option was to use a vertical antenna. I got a Hustler 4BTV resonant on the bands from 40-meter to 10-meter. The radial network is not optimal but I could still fit more than 30 radials that are the same physical size as the antenna. Vertical antennas work wonders. Their low takeoff angle makes up for the lack of gain and makes them an inexpensive option for DX contacts.

This setup makes it easier for me to make a QSO with Georgia or Virginia than with my friends a hundred or two hundred miles away from my QTH. It is why I have decided to install an NVIS1 antenna for 80-meter and 40-meter.

NVIS communications

NVIS radiation pattern (ARRL)

Near Vertical Incidence Skywave (NVIS) are antennas with a high radiation angle, 90 degrees up, or more. The signal directed straight up bounces back on the ionosphere with a low incidence angle. This method allows reliable communications within a range of 600 km (400 miles). It bridges the gap between VHF/UHF communications and longer distance skip on HF. The most suitable frequencies for this mode of communication are 160, 80-meter, and 40-meter during the day.

An NVIS antenna is simply a horizontally polarized at a height ranging from 1/20th to 1/4 wavelength above the ground. Optimum height is about 1/4 wavelength.

Antenna installation

Since I wanted to be able to work 80 and 40-meter, I have decided to use a two bands trapped dipole antenna. A trap is as an open circuit on the higher frequency (40m), but it is a load on the lower frequency (80m). It has the effect of lowering the overall length of the antenna, allowing me to fit the entire span of the antenna on the side fence. The drawback of a load is reducing the bandwidth of the antenna. It means I will have to use a [ATU]2 for the edges of the band.

For reasons of space and also aesthetics, I have decided to "hide" the antenna on top of my fence. The fence is at 3m height (9 feet), which is not the optimal height for an antenna, but it should work ok for local communications around northern California.

Diamond antenna feed point

For this project, I am using a Diamond W735. This antenna is made of high-quality materials, easy to assemble, and easy to tune.

Before permanently installing the antenna on top of the fence, I tested it with an inverted V configuration at 25 feet. It was working as specified on the Diamond booklet. The SWR was 1.2:1 on 80-meter and 1.4:1 on 40-meter. I am sure I could have had a better SWR if the antenna was higher.

After installing the antenna on top of the fence, the best SWR I could get was 1.7:1 on 80-meter. But this is normal: at 3m height (9 feet), the antenna is too close to the ground. At that height, the antenna is inductive.

Soon I will optimize this setup. I will replace the Diamond balun with a homebrew one that will include a capacitor to make the antenna purely resistive instead of reactive. It will improve the SWR, and the resonant frequency will be closer to what I want.

Testing

To test my antenna installation, I ran WSPR with 500 milli-Watts. The test showed that my setup is definitively not for DX. My goal was to be able to work stations outside the San Francisco Bay Area, and I have reached this goal.

The following graphics use data from WSPR, taken from the site DXplorer, and I wrote a python software to show the distances distribution.

Maximum Distance

Distance distribution showing outliers

Best chance of contact / distance

Contacts map

Contacts direction and density

Conclusion

The graphs show the antenna behaves as an NVIS setting. Most of the contacts are in the range of 700 Km. This is the perfect setup for emergency communications, to talk to ham friends in your area, or to participate in local nets.

For a long time, I didn't install an 80-meter antenna because of its length and the height necessary to make it a good DX antenna. But to stay in touch with your local community, having a wire on a fence around your property line is a perfectly valid option. A dipole is easy to build and can be a fun project. The antenna doesn't even have to be in a straight line. It can go around a corner or zig-zag.

I hope to talk to you soon on 80.


  1. Near Vertical Incidence Skywave 

  2. Antenna Tuning Unit 

Tags:  Antenna