The AM Forum

Watch all the action on Twitter as I, along with Circuit Girl (Jeri Ellsworth), and a host of others, will try and put an end fed 20 meter 8JK up at Quartzfest.  The fun begins tomorrow.



Matt

OM,

The online literature is sparse: but I gather that the 8JK has a "DC to Daylight" bandwidth. Please provide pointers to any information you know of that will tell me what I need to know in order to try one out for myself.

Thanks!

Bill, W4EWH

The 8JK is a rather narrowband antenna since is a close spaced (0.125 wavelength) array of two dipoles (the "classic" design was a pair of half-waves in phase or a full wavelength dipole but these lengths were not required). The arrangement was developed and published by John Krauss, W8JK. The first QST article on the subject was in January of 1938 in an article by Krauss entitled, "Directional Antennas with Closely-Spaced Elements."  The array is bidirectional.

Many years later, W9BRD, Rod Newkirk who for a long time was the DX editor for the ARRL and QST did an article on how he end-fed a W8JK array. His article in the June 1990 QST, "BRD Zapper, The: A Quick, Cheap and Easy "ZL Special" Antenna" used a feed system that made the array unidirectional, and effectively an end-fed ZL-special.

https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/radcom/1990/09/page31c/index.html

It's not necessary, or always desirable, to stick with Dr Kraus' close spacing for an 8JK concept antenna.  My 80m version of the 8JK is made with a pair of 80m half wave dipoles, spaced about 65 feet apart on a Phillystran "boom" between two large pines.  The wide spacing keeps the element impedance somewhere around 35 ohms.  I feed each dipole with a quarter wave of open wire, transforming them up to about 1200 ohms, joining them at a common point, but twisting one of the 180 degrees., for a constant 180 phase shift, then to the shack with more open wire, and to a balanced network
The result is a bidirectional low angle radiator.  At my height of about 80 feet, maximum radiation is around 35 degrees above the horizon, with an overhead null, and plenty of radiation above and below 35 degrees.  It competes with good 4-squares on long paths but, in the first hundred miles or so, it's only so-so.  Driving both elements prevents them from "seeing" much mutual impedance with the ground image.  It's the only way I know for a relatively low horizontal element to get good gain at lower angles.. With an 8JK like this, along with a plain dipole for local work, the bases are covered well. A bonus is that it maintains the clean bidirectional pattern on 40m and 30m.  At 20m and above it gets cloverleafy, but still works out well.  It's a forgiving antenna.  My friend Joe K1RQG(sk) had one nearer to 50 feet above ground and his signal into Europe, the Middle East, and across North America was about the same as mine.
Dr Kraus came up with a winner when he designed the original 8JK.  And it's easier for low angle stuff on 80m than building a radial field out in the grass.