Stacking different band yagis on a single tower The traditional approach for stacking yagis of different bands on the same mast sharing one rotator (aka christmas tree) shows very severe problems of interaction among the different bands if the antennas are not properly distant. Even with spacings of 3 meters a serious degradation of the radiation pattern is observed mainly on the upper bands. This effect has been discussed on the amateur technical literature such as Leeson W6QHS ‘Physical Design of Yagi Antennas’ or the ARRL’s ‘Antenna Handbook’ on various editions. The big boys approach to this problems is to have a very large rotatable tower that permits antenna spacings of more than 1/2 wavelengths. Or even better; to set in place a different tower for each band (well, perhaps when I grow-up). Being limited on my QTH to one single 24 meter high tower and lacking the necessary vertical real state for that optimum separation among antennas some other solutions had to be sought. A very advantageous solution for stacking two different band yagis is to place fhe higher frequency yagi just in front of the lower frequency yagi. This configuration not only eliminates destructive interaction but also enhances the performance of both antennas. This was the solution adopted for the 10 and 15 meter yagis on my QTH. Force 12 manufactures the Magnum 415-410 which consists on two different yagis on the same boom. with 4 elements on 15m and 4 elements on 10m. 15 meter 4 el yagi isolated   Gain = 8.7 dBi and F/B= 17 dB 15 meter 4 el yagi behind 10m 4 el yagi        Gain = 9.1 dBi and F/B= 23 dB Force 12 Magnum 415-410  10 meter 4 el yagi isolated    Gain = 7.6 dBi and F/B= 20.5 dB 10 meter 4 el yagi in front of 15m 4 el yagi            Gain = 7.6 dBi and F/B= 25.5 dB Both yagis are fed separately and a special caution was taken on the length of coax feeding the 10 meter yagi in order to avoid any resonance that the resulting stub on the center of the 10m driver could result on 15 meters. Nevertheless, this solution was not very practical when trying to add the 20m and 40m beams to the stack. It is certainly not easy to install a 20 plus meter long boom to acomodate 3 yagis and much less to maintain up in the air for a reasonable amount of time. First attempts were to place the 40m/20m yagi just 3 meters below the 15m/10m yagis, but the 15 meter yagi showed a poor front to back and the SWR was seriously degraded. After many simulations with EZNEC the solution came by turning 90º the booms of the stacked yagis. SWR on 15 meters came back to 1:1 at the middle of the band like the rest of yagis and all showed a nice F/B. Conclusions Whenever feasible try to: 1.- Stack the yagis horizontally on the same boom, lower band stern, higher band ahead. An increase of performance will result. Or... 2- Separate the yagis as much as possible on the mast and then turn booms 90 degrees.   Force 12 Magnum 415-410 at 27 meters o.g.l at EA5DY/4.  Home        Shack       Antennas       Maritime Mobile       Propagation        Gallery          QSL info