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The image above is a Google Earth mock-up of my current antenna setup. The red one is the HF dipole, the blue one is the VHF/UHF vertical and the green lines are the two feedlines.
The HF antenna is a an off-center fed dipole (OCFD) 41.8 meters long; this makes it resonate - when properly installed - on 3.5MHz, but on all it's harmonics also (7MHz, 10.5MHz, 14MHz, 17.5MHz, 21Mhz, 24.5Mhz, 28MHz etc). We usually feed dipoles at the center but due to the current/voltage distribution across the resonant wire, it will present a low (close to 50ohm) impedance only at the uneven harmonics (3.5Mhz, 10.5Mhz, 17.5Mhz, 24.5Mhz etc), while at the even harmonics (7MHz, 14Mhz, 21Mhz, 28Mhz etc) it will present a very large (theoretically infinite, practically a few thousand ohms) feed impedance, wich is very far from out 50ohm feedlines making the antenna unusable due to high feed mismatch.