Multiband Mini Dipoles with Capacitance Hats
The mini dipoles presented in the single band HF antennas menu item can be stacked and fed via a common feed point. In this way, mini fan dipoles with cap-hats or multi mini dipoles with cap-hats (if you prefer such name) can be created. They all feature small length - about 1/8 wavelength of the lowest band.
3-band mini dipole with square cap-hats
Modeling 3-band version (14-21-28 MHz) did not pose a big problem. I chose the mini dipoles with square capacitance hats, and my model looked like that:
The AGT = 0.2...0.3 dB I achieved in this model means that although the model could be improved, the simulation results should be regarded as quite acceptable approximation. And here they are:
The antenna feed point impedance is 12.5 ohms. Overall dimensions: 250 x 137 x 137 cm. Longest spreader (half-diagonal): 97 cm. If need be, it should not be difficult to hide this antenna in the attic.
When I tried to add more bands, I observed strong interaction between different band dipoles. I think it would be difficult to tune a 4-band or 5-band version in the real world. Also, bandwidth decreased. Moreover, the AGT was going up, and an extra effort would be required to improve the NEC-2 models.
Click here to download the antenna models.
5-band mini dipole with triangle cap-hats
Finally, I managed to design a 5-band mini dipole with quite acceptable bandwidth and gain. I took the mini dipole version with Triangle No. 1 hats and created something that looked like a crab to me.
It can also be turned upside down, and then it will fit under a roof in an attic.
The overall antenna dimensions are X=3.14 m, Y=2.65 m Z=1.33 m.
Find below the performance figures. The gain is referred to a dipole and expressed in dBd. If you want it in dBi - just add 2.15. Feed point impedance is 12.5 ohms.
Would building the Crab be easy is another question. Perhaps this antenna is too complex to be practical? But I wanted to share the good news that it is feasible to combine 5 mini dipoles into one multiband antenna with quite reasonable performance.
Click here to download this antenna models.