[Elecraft] RF in the Trees

[Jim Brown]

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