Eznec can be downloaded from http://www.eznec.com/. The free demo version suits well for this kind of simple antennas.
|The Eznec model for the vertical dipole for the 7MHz band.|
|Horizontal dipole for 7MHz, at a height of 10m above ground.|
|Horizontal dipole for 7MHz, at a height of 20m above ground.|
What about 21MHz?
This antenna tunes easily into 21MHz, because it's exactly 1,5 wavelengths on that band.
As seen in the elevation diagram in the picture above, there are two lobes, at angles of 11 grades and 42 grades. The 11 grade lobe is not bad (and might explain my DX QSOs on that band), but it's 7db weaker than the 42 grade lobe. What if I put up a real half-wave dipole for 21MHz, so that the top is at around 22 meters? Here is it according to the Eznec theory:
Wow, a nice 9 grade lobe! At least in theory. Yes, why not, since at these heights we are quite many wavelenghts above the ground. I'll certainly climb up in the three and try that some day! Why not install similar antennas for each band? Do they interfere badly with each other? Hopefully not. I have other tall trees in that case :-)
|The Eznec model for the vertical dipole for the 21MHz band.|
|For comparison, the elevation diagram for a real half-wave dipole for 21MHz, where the top is at a height of 22 meters|