Saturday, July 31, 2010

Can you tell me how one figures the spacing for the matching section of a J antenna? ...

 Q Howard, KK7KL, asks, “Can you tell me how one figures the spacing for the matching section of a J antenna? Is there a rule of thumb? Any assistance you could pass along would be greatly appreciated.”

A The J pole is an interesting antenna. It is essentially an endfed half-wavelength antenna, and the transmission line is used as a transmission-line transformer to transform the high feedpoint impedance to 50 Ω for your radio. The relationship between all of the elements is complex. The high impedance of the end-fed radiator is determined by its length and diameter and, to a smaller degree, the presence of nearby dielectric insulators.

The impedance of the matching section is determined by its conductor diameter and spacing. This is not very critical in most J pole designs. The transmission line transformer is usually cut to a quarter wavelength, shorted at one end. It is then tapped at the point that corresponds to an impedance of 50 Ω. Minor variations in the length of the radiating element and the transmission-line transformer can be compensated for by either changing the length of one or both elements slightly, or by changing the tap point on the transformer. 

Most J poles are either designed by trial and error, or by
modeling them on a computer. You can start with the half-quarter wavelength dimensions (don’t forget the approximately 0.97 velocity factor on the transmission line section), then adjust the tap point and length of the radiating element. If you have access to an impedance bridge, such as the MFJ-259B or Autek VHF Analyst, you can adjust the tap position for 50 Ω resistive and the length of the radiating element to get the reactance down to zero.

Of course, there is nothing magical about the half-/quarter wavelength combination. I recently modeled a J pole whose dimensions are much shorter than the norm, and it, too, gives 50 Ω at the feed point.

From QST September 2000