Stacking Tri Banders the easy way
Dick McDuffie NY1E
Knowing that the bands (20, 15, and 10) are harmonically related, I sat down with a calculator and by trial and error found a length of coax that is close to an odd multiple of a quarter wave on the three bands (.66 velocity factor): 20 meters five electrical quarter waves = 58'. 15 meters seven electrical quarter waves = 54' and 10 meters nine electrical quarter waves = 52.2'.
I made two RG-11 cables 54' long and fed them with a tee into two Cantenna dummy loads and had a flat SWR across all three bands (and had a high power dummy load!). So I decided it was worth picking up another tribander, if only to satisfy my curiosity. I use a Dow Key 6 position relay with separate coils for each position so I can run either antenna individually, or both at once. Leaving the 75 ohm coax in line when using only one antenna only moved the resonant frequency a little. I am now running a pair of TH6-DXXs at 45 and 90 feet on separate rotors with upper, lower, both in phase and both out of phase switching. The phase switching is accomplished with a DPDT relay between the balun and the driven element of one antenna wired so as to cross the wires, as in reversing polarity. This puts the antennas out of phase without regard to frequency.
As can be seen by the SWR charts taken at 100 Khz points, the SWR is less than 1.8:1 across the bands feeding both antennas, with a maximum of 2.4:1 feeding a single beam with the 75 ohm coax in place. While I have found the radiation angle diversity and occasional gain worth the trouble, the best use is running two directions at once in my favorite contests. In the 10 meter contest leaving one on europe and one stateside keeps the rate up, in SS I leave one on the last multiplier I need, normally VY1 or KP2.
I have tried
different heights for the lower antenna. 30' played better on 10 meters
but worse on the other bands. So a third antenna would be nice, but I guess
it can wait until the next cycle peak.