Tuning 3 Elements two stacked Yagi – effects of feed line to a tuned antenna

Two stacked 3 elements Yagi

Stacking two antennas – and effects of feed line to a properly tuned antenna

Stacking antenna is done to achieve additional gain ideally a 3dB additional gain is targeted but may not be achievable in real world due to losses introduced by additional cables, you need to make sure that the phasing harness is of the same length and construction. If your cable is not the same length then the signal from those won’t reach the antennas at the same time. The differences may be small but it is enough to create phasing problems, when your signal get to the antenna at two different times they don’t result in a much stronger signal in fact in extreme cases if the signal are exactly 180┬░ degrees out of phase they would cancel each other and you’ll get nothing.

Basic Stacking Requirements

1. Two antenna’s with similar characteristics in terms of
Gain: 8.89 dBi
Center Frequency: 145Mhz
Matching: Gamma Match (Tuning stub)
Impedance: 50 ohms
Beamwidth: 84┬░ vertical 58┬░horizontal
Front/Back ratio: 11dB
SWR: 1:1 @ center frequency



2. Phasing harness – must be of the same length Velocity Factor of coaxial cable accounted for, use two basically identical cables. If the cable types are different, or if the connectors are different, you can have the same phasing problems.

phasing-harness

3. Tune the antenna to the lowest possible VSWR match, identically the same response is ideal but a slight mismatch or mis-alignment is acceptable but not much. Your antenna analyzer can help you check this before stacking.

Tuning two stacked Yagi Antenna

Conclusion

A properly matched single antenna, combined with a similar antenna to achieve stacking gain will perform much better than a single antenna, however care must be taken to achieve a good stacking practice. The result of a combined antenna when tested with a good antenna analyzer will result in very little deviation in its Center Frequency , VSWR curve, S11 curve, and Impedance even if different lengths of feed lines are used to test the antenna system.