On 6/27/2014 6:30 AM, Don Wilhelm wrote:
if you can couple RF into it, it will radiate.
Yes. There are many ways to feed a vertical dipole. I developed a method that uses the outside of the coax as half of the dipole, and uses a ferrite choke to form the "end insulator." The choke is positioned a quarter wave down the coax from the center insulator.
I published this method about six years ago, after seeing a similar suggestion ftrom Rudy Severns, N6LF. Rudy used a coil of coax, forming only an inductor. My contribution was the ferrite choke, which makes the antenna insensitive to feedline length.
The end of a dipole is a high voltage point, so there's considerable voltage across the choke if you're running much power. I tested this concept around 2008 on 40M with 1.5kW, and found that I needed two chokes in series, each of which was 5,000 ohms. A single choke would work quite well at 100W or less.
Note that electrically, this dipole is center fed -- there's a quarter wave wire connected to the coax that goes to an end insulator that can be suspended in a tree, then the coax hangs down, and the choke is a quarter wave down from the wire. The impedance of a vertical dipole like this is about 70 ohms at resonance, which makes it a good match for 75 ohm coax. Remember that SWR in a system is determined by the match between the antenna and the line, so losses will be lower with 75 ohm coax. Also remember that the velocity factor of coax is for signals INSIDE the coax. For signals OUTSIDE the coax, the velocity factor is like any other insulated wire of comparable size, roughly 0.98.
The choke should be wound using the guidelines in my Cookbook. 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
