Thursday, 28 April 2011

Looking at Eznec models of my 7/21 MHz vertical dipole

My vertical dipole for 7 MHz appears to work well for both 7Mhz and 21Mhz. (ref. My earlier posting). For 21MHz the dipole is a 1,5 wavelength dipole and that made me wonder how the elevation diagram looks least in theory.

Eznec can be downloaded from The free demo version suits well for this kind of simple antennas.
The Eznec model for the vertical dipole for the 7MHz band.
The picture above shows the Eznec model for the vertical dipole, using it on the, initially intended, 7MHz band. The elevation diagram is quite ok...If I could raise the antenna higher, the elevation angle would decrease, and the antenna would be better suited for DX working. As comparison I checked, how the antenna would perform, if it would be installed horisontally instead, keeping the feeding point at the same height:
Horizontal dipole for 7MHz, at a height of 10m above ground.
Well...not at all suitable for my DX intentions! What if I could raise the horizontal dipole to 20m?
Horizontal dipole for 7MHz, at a height of 20m above ground.
Well...better, and but the elevation angle is still higher than for the vertical dipole. And I'm interested in DX QSOs. Note that I don't consider directions (azimuth diagrams) here; vertical antennas radiate in all directions and horizontal antennas have direction-specific gains. My interest is to get antennas that radiate almost everywhere. North directions are not that interesting, which makes me think of slopers in my next experiments.

What about 21MHz?
This antenna tunes easily into 21MHz, because it's exactly 1,5 wavelengths on that band.
The Eznec model for the vertical dipole for the 21MHz 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:

For comparison, the elevation diagram for a real half-wave dipole for 21MHz, where the top is at a height of 22 meters
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 :-)

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