The W8JK bidirectional array

Want a little antenna gain but in both directions? For instance if you want to cover up and down a straight coast. Or maybe long and short paths simultaneously for DX.  If these apply a W8JK may be worth building.  

Very much an experimenters antenna, the W8JK dates from the 1930s. It's named after the inventor's callsign. I don't know of any commercial versions of it so if you want one you'll have to make it yourself. You will also need to be willing to use open wire feedline. However in return you can load it up on multiple bands. 

What is a W8JK? It's basically two centre fed dipole elements spaced closer than what you'd normally find in a 2 element yagi. 1/8 wavelength spacing is typical. Unlike a yagi both are the same length. And both are driven with the feedline connected to them. Although, as you can see in the twist above, the connections to one element are reversed. That provides a 180 degree phase shift. If you've ever looked closely at a log periodic TV antenna you will see the same technique used there. 

The connections between the elements to the feedpoint are typically made from (now scarce) open wire TV feedline. Or you can use ladder line such as sold by the better amateur radio outlets. More ladder line extends from the feedline to the shack. A balanced antenna coupler is used to allow the antenna to operate over a wide frequency range. Because this looks after impedance mismatches, no trimming of the antenna is required.

As mentioned before, the W8JK is bidirectional broadside to the elements. There is a significant null off the sides that can help reduce interference pick-up. And if you move its feedpoint along towards one of the elements you can obtain some directivity. More on that tomorrow. In the meantime, enjoy these articles. 

* W8JK by ZR6TXA
* K1OJ W8JK wire antennas
* K5LJ multiband beam based on W8JK
* AF6SA modified W8JK beam
* F5RDS' disappointments with the W8JK

Have you had experience with W8JKs? If so please let us know in the comments below.

PS: Want to get more from low power amateur radio? Or even do better with your 100 watts? If so Minimum QRP can help. Available as paperback or ebook thousands have been sold. Find out more here.