Modified W8JK Beam
I was considering building and installing a compact 2el W8JK 20M to 10M beam on my roof. Looking for more information about multiband performance and impedance matching, I found a very interesting article, "A Compact and Efficient Multiband Beam Based on a Modified W8JK Array” by K5LJ. Moving the feed point slightly away from the middle will give you directional radiation pattern on the lowest band.
I wanted to build this beam with ready available or easy obtainable parts. You can get 6ft tubing easily online. Home depot has 1in OD 8ft length AL tube, which was used for the boom. Each half element is made from telescoping 6ft 1/2" and 5/8" OD tubes with 4" overlap. Elements are insulated from the boom with 3/4” PVC split pipe and mounted with 1” pipe U bolts. To bring down the resonance and support the elements a length of 1.8M 18AWG stranded house wire, insulator and a piece of Dacron rope is used on each side. Download 4nec2 file: W8jk_20m-Vfolded 20m.NEC
Phasing and feeder was made of stranded 450 ladder line. Feed point was 11cm toward “front element". About 8.5M long feed line and with AT-600ProII tuner mounted in the attic will allow me to operate on all 20M to 10M bands. Tested with 500W output power without problems. Keep the ladder line about 3” away from other metal structures.
Compared to my 80M to 10M Windom (both antennas 10M above the ground) this beam was always better on RX and I can get about 1S unit better reports. Sometimes I can get 2-3 S units signal increase, showing that my Windom has deep nulls in some directions.