The Cantenna

18 January 2014

 

The rod antenna on a shortwave receiver might be good for 2% of what the receiver can potentially get from around the world. If a 100-foot "longwire" gets 100%, the Cantenna rates about 80. It's light, portable, and cheap - mainly made from scrap. The central element came to me full of pineapple chunks.

I scoured the web looking for something like this and finally had to dope it out myself. Here it is, in standard and "take-down" versions, for people looking for the world outside their particular propaganda box.

I'm a news junkie, not a radio guy, but if I have the theory right, incoming broadcast energy induces a voltage in the perimeter wire, which creates a magnetic field inside the perimeter, which then induces a voltage in the pineapple can. 10 feet or so of speaker wire carries the signal to the receiver's rod antenna, where it connects via alligator clip.

The perimeter wire can be insulated or not, but it's important the only path for the voltage be to the rod antenna. For vehicle mounting I zip-tied one rod to a CB magnet-mount with the whip removed. This is for stationary use! A 40 mph gust (65 kph) has blown the thing over, a 50 mph gust (80 kph) has blown it right off the roof. It landed upright on the ground and kept right on working.

The pineapple can looks like galvanized steel. Dimensions: diameter 3.5 inches (8.9 cm), depth 1.875 inches (4.76 cm). There's no physical connection between the can and the perimeter wire except the non-conducting rods. Drill 4 holes in the can for the rods - 1 for the speaker wire - done.

The rods I bought at a kite store in Sacramento, California - two 4-footers, about $20 I could've saved with a little scrounging. The perimeter is automotive wiring 1/8th-inch diameter (3.17 mm) including the insulation. I drilled the ends of the rods to hold it in position.

I was amazed the thing works, let alone works so well. With a $60 Grundig G8 pocket receiver, it compares favorably with a 1990s Sangean ATS-909 (aka DX-398) and longwire. Somehow it even gets good reception on clear-channel AM stations through the rod antenna with the receiver enclosed in a steel-body cargo van.

A month after making it I came across the rags of an abandoned pop-tent, and scavenged the shock-corded fiberglass tent poles to make a take-down version for biking and backpacking trips. It uses 22-gauge wire for the perimeter. I've hung it up in a creosote bush for 12 hours of 40-mph wind, and it was still there ready to work the early morning news.

It also works fine lying horizontally on an insulating mat. The rod sections come apart to a 21-inch (53 cm) length. Weight is negligible compared to the radio, or even to 3 AA batteries.

From the Mojave Desert, 35°N, 114°W, I get the BBC at 250 kW from Thailand, about 7200 nautical miles away - New Zealand, 100 kW, 5900 miles - Australia, 100 kW, 6600 miles - and even CBC1, Vancouver, B.C. 890 miles on what I'm told is 10 kW on 6160 kHz.

AM stations received include Salt Lake City, Utah - Albuquerque, New Mexico - and San Francisco and LA. These are 50 kW overnight. So far I've failed to get Denver, Colorado, and Seattle, Washington, which I used to get on the Sangean - but it's been too cold and windy so far to get outside the van!

There you have it. Comments are welcome - joeperalta at hotmail dot com. Have fun!