NASWA Journal Columns · Technical Topics, April 1997

Technical Topics, April 1997

The Rotary Beverage Antenna

We interrupt our series on mobile SW listening this month to bring you some hot news from the NASWA Research Lab (NRL). The NRL always can be counted on to come up with some new innovation to enhance your radio listening. It does puzzle me, however, that they always seem to announce their most amazing developments at this time of year. Maybe it has something to do with those cold winter nights at their facility on the banks of the Potomac. The mobile SW listening series will continue next time with a description of several antenna options available to the mobile SWL.

The Beverage antenna has long been recognized as the best for Dxing the tropical bands. The Beverage provides directionality, reducing noise and interference arriving from undesired directions. Yes, the Beverage does require a good bit of real estate. A decent Beverage for the 90 meter SW band (3.2-3.4 MHz) should be at least 450 meters (or about 1500 feet) long.

The problem the NRL addressed in their research is a simple one, “How do you receive signals from different areas of the world without putting up a separate Beverage for each direction?”. Listeners on the USA East Coast might need a separate antenna for each of Europe and the Middle East, Africa, Central Asia, South America, South Pacific and Eastern Australia, and the Far East. That’s six different antennas adding up to more than a mile of antenna wire. The NRL’s new invention allows reception from any direction with only one length of antenna.

Our NRL engineers developed a Beverage antenna that can be rotated to point in any desired direction. A prototype is shown in the picture. The antenna is actually a large-diameter pipe that rotates around a central coupling device. This coupling contains a magnetically coupled matching transformer to which the feedline and a water supply connects. The feedline is direct burial coax similar to the RG-6 used for satellite receivers. The antenna is supported on triangular frames that in turn ride on rubber-tired wheels to provide insulation.

Rotation is accomplished by water-driven hydraulic motors. The antenna is actually a large diameter pipe through which water flows at high pressure. The water sprays out along the length of the antenna through small turbines to provide the basic motive force for each of the wheels. The water spray falls harmlessly to the ground where the moisture increases the soil conductivity and thus the efficiency of the antenna.

Moving water was chosen as the power source to eliminate a source of noise that would be present if electric or gasoline motors had been chosen. Water consumption only occurs while the antenna is moving. The antenna should only need to rotate once per day as the sunrise/sunset line moves around the earth. Cost of water will vary depending on your location.

NASWA’s engineers found that spraying salt water considerably enhances the ground conductivity compared to fresh water, but good sources of salt water are hard to find in many areas. The Environmental Protection Agency has also raised objections to salt water runoff from the NRL antenna farm. The equipment has been designed to withstand the corrosive effects of salt water for ten years of normal use. There is also no reason liquid fertilizers could not be injected into the water supply to permit crops to be grown under the antenna.

Any Beverage antenna should be terminated at the far end in a 500 ohm resistor connected to ground. The termination absorbs energy arriving from the back of the antenna. Maximum front-to-back gain ratio can be achieved by careful adjustment of the terminating resistor value. The rotary Beverage uses a wire dragging along the ground at the far end. The contact with the ground is not very good but the resistance is in series with the terminating resistor so a few more ohms won’t hurt much. The terminating resistance can be reduced to compensate for ground contact resistance. The purist could drive several stakes into the ground to permit a better connection at each favored direction but connection to the stakes would require a hike to the antenna each time the direction is changed.

This antenna is now being field tested at many rural locations. You might just happen upon one of our test sites the next time you are out driving in the country. If you do, stop the car and go over and take a close look. On second thought that might not be advisable. I have received word that due to budget cuts at NRL, they have been doing some of this research under contract to the government. Many of the receiving sites you will see are in fact government receiving sites disguised to look like working farms. We all know how nervous those feds get when you go snooping around their facilities. In fact tests are in process at many locations to phase together multiple rotary Beverage antennas into large phased arrays with correspondingly higher gain and directivity. Keep a lookout for these sites in your travels.

The design plans for building your own rotary Beverage antenna should be available from the NASWA store. Contact Kris Field at the address on the back cover of this magazine for pricing and delivery information. The store also sells a critical parts kit including pre-assembled hydraulic turbines, matching transformers, gear trains, rubber-tired wheels, irrigation pipe and direct burial coax. Sorry, this offer is only available to NASWA members. If you are reading this article on the ANARC web site, why not consider joining NASWA so you can help fund its many activities designed to enhance your enjoyment of SW radio.

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