F4DRH wrote:
Hello Harbin

The bottom of the J-Pole antenna is a 1/4 wave length to transform the
high impedance of the 1/2 wave length above into a lower impedance
(close to 50 ohms). The impedance of that 1/4WL sould be Z = SQUARE ( Z
coax X Z antenna) .... in that case Z antenna is the impedance of the
1/2 WL (usually several hundreds ohms)
That's why the space between elements as well as there diameter is
important.

You certainly know the Topfkreis ... which is the same antenna ... 1/2
WL + 1/4 WL ... but with another technical realisation.

Hoping it helps.

Regards

Jean-Marc
F4DRH

Jean-Marc

Thanks for mentioning the Topfkreis. It seems like it should work much
better, and is so obvious once you see it. I will have to make one.

tom
K0TAR