[TowerTalk] 10/15/20 M yagi and 12/17 M yagi minimum stacking distance on a common mast

[rwhitetexas at verizon.net]

I have a CL-33 (18 ft. boom weighing 42 lbs.) up on a HDX555 and want to stack on the same mast a 12/17 meter 3 element yagi above it. The 12/17 meter yagi is a Cushcraft A3WS with a 12 ft. boom weighing 23 lbs. I would like to know the minimum distance I can mount the 12/17 yagi above the CL-33 before I start to adversely degrade the performance of both yagis. I am not looking for peak overall performance, but do not want to have a situation of too small a distance that will adversely effect both yagis performance. My two favorite bands of interest are 15 and 20 meters once the sun spots cooperate in a few years.

Is 1/2 the boom length of the higher freq antenna (12/2 = 6 ft. ) the correct distance? I want to minimize the length (weight) of the steel mast on top of the tower, so if I can have a shorter distance than 6 ft. between the yagis and still have reasonable gain, then I would go with a shorter distance.

For a 6 ft. distance between the yagis, the mast would be 10 feet approximately (with 4 ft. from rotator to top of tower). That is about the most I want on top. I usually leave the tower at 50 ft. high to have more overlap than what it would have at 55 ft., thus giving me a little more safety margin to handle the high winds we have in our North Texas thunderstorms.

Roger W5RD
Murphy, Texas