Electrical Downtilt

[pstuckey]

I was looking at omni antennas with electrical downtilt, and I was wondering how exactly the electrical downtilt is realized in the antenna? I'm also curious what are the limitations. It seems the range is typically 2-10 degree downtilt, is 10 deg a practical limit? Thanks.

Peter

 

[VEBill]

Often the antenna is simply a co-linear arrangement of dipoles. The relative phasing (typically implemented in the phasing harness) is such that the pattern shifts downward to provide better close-in coverage.

You can confirm this by comparing the total physical length of the antenna to the wavelength. Two half-wave dipoles in such an arrangement would be just a shade over (due to the gap) one lambda in total length. Four would be 2 lambda. 8 is pretty rare and 'non 2^n' co-linear combinations are even rarer.

Stand-by in case someone else knows of different examples.

 

[fmradio]

Vertical element spacing closer to 1 wavelength also is common, to increase directivity in the horizontal plane. But in any case where beam tilt is effected by relative phasing of the power in each element, the total aperture of the antenna in numbers of wavelengths is not an indicator that beam tilt is employed. Element spacing can be the same regardless of the amount of beam tilt, including zero.

 

[Higgler]

The small downtilt angle hints a printed circuit serial divider. Spacing antennas for best VSWR in a serial divider (where all power divisions are identical) requires phasing that's just slightly different (less or more ) than 360 degrees from element to element. This slight difference from 360 degrees produces a beam that's tilted off the horizon. If you changed frequency on this antenna the beam would move up and down, so it's typically a narrow band antenna design.
The practical limit for this type antenna is probably greater than 10 degrees and would depend on your operating bandwidth.

For normal arrays, there is no scan limit for good antenna engineers with good software. If you just guess at the antenna design, antenna pattern blindspots (due to mutual coupling) can occur when you scan off broadside at angles beyond ?say 30 or 40 degrees. The further out you scan, the more likely you get ugly blind spots. It's hard to scan out to 60, 70 or 80 degrees over a wide frequency range without some real ugly antenna patterns. We recently designed and built a large printed circuit antenna array that scans from broadside to endfire.

kch