A few years back, Eric (WG3J) wanted more gain on 40m at his QTH in Maryland. He presented me an idea for an antenna that he had been playing around with called a Bobtail Curtain.
A conventional bobtail curtain (right) is fed in the middle with an LC circuit and requires a ground grid below the antenna. The antenna provides a bidirectional, end-fire pattern at low angle.
Eric's approach is comprised of 3 quarter wavelength elements separated by half wavelengths, making this antenna a full wavelength long. The antenna is coax fed in the back upper corner with a vertical element connected to the coax center conductor, while the top rail and remaining vertical elements were soldered together and connected to the coax shield. The center insulator is typically use is a Budwig HQ-1.
His initial attempts provided a great SWR curve and excellent signal reports, but what was this antenna really doing?
At first we thought this antenna as had an end-fire pattern, although after importing his initial figures into EZ-NEC, we quickly discovered this variation's gain had 60 degree lobes on either side, in the shape of butterfly wings. Below are the initial plots of our 3 element, 40m Butterfly Bobtail Curtain.
Having 4.5db of gain, 4db front to back with a 20 degree take-off angle was nice, but what would happen if we added a 4th element?
The antenna now starts to become more directional. Our forward gain doubles to 10.6db, the pattern gets tighter and front-to-back increases to 10.8db. Take-off angle remains at 20 degrees.
If we then add a 5th element, we reach the point of diminishing returns. The antenna is now 2 full wavelengths long, with a 2db increase in forward gain, while front-to-back increases another 3db.
This antenna has become quite the flame thrower on 40m at the WG3J contest station and has served him well. With 4 elements, his primary lobes are directed into Asia and the South Pacific, while the rear lobes are directed towards Europe and South Africa. I've also utilized this antenna in my small city lot (100ftx80ft) on 12m and 17m favoring Europe with great results. Below are plots on 10, 15 and 20m with 3-5 element configurations.