Questions about tilted antennas

[EAFrizzle]

In the Sti-co thread it's mentioned about the performance sacrifice of the tilted element. I know you guys will know all the numbers and charts, and have real-world experience with installs, so let me check my thinking with y'all.

I'm of the idea that the tilt may or may not affect the pattern as viewed from overhead, but that it does alter the take-off angle as viewed from the side by elevating the TOA on the more obtuse side of the angle. This could be OK or good in a valley with repeaters on the ridge, but in a place with broad, rolling hills could cause you to miss a repeater.

I'm sure there are other drawbacks as well with impedance and such, but am I thinking along the right lines? And I'm asking specifically about VHF and above. I can already make weird stuff radiate HF. 😆

 

[prcguy]

A dipole in free space tilted exactly 45 degrees will be 3dB down when looking at the sides where you see the 45 deg angle. A monopole tilted over a ground plane will probable not be affected as much.

 

[EAFrizzle]

I was thinking primarily about pattern skewing, hadn't considered any polarization issues.

 

[dlwtrunked]

I was thinking primarily about pattern skewing, hadn't considered any polarization issues.

I am not certain you cannot talk about pattern skewing without considering the receiving antenna and then polarization is a factor.

 

[EAFrizzle]

Ok, let me rephrase, hopefully more accurately. Compared to a 90° vertical, what effect will tilt (such as the sti-co) do to the az/el patterns. That's what I meant to ask.

 

[freddaniel]

As prcguy has already said, a tilted antenna will not be affected much. But to give you something more specific, I have attached a chart of a typical 1/4 wave omni antenna radiation pattern. You will note the loss is less than 6 dB up to 60 degrees tilt. Keep in mind, most signals arrive indirectly, in other words, not perfectly vertical polarization. This is because unless you are within visual distance, the signal is most likely a bounce or multi-path in nature. Unless the signal is line-of-sight, you also have variable phase cancellation effects as well as polarization issues. Note the incredible deep loss when the polarization is close to 90 degrees, such as 75 to 105 degrees.

The cellular industry gave up on trying to maintain vertical polarization long ago. They started with dual vertical antennas [and receivers], spaced several wavelengths apart. Then they found polarization diversity worked far better, with the two dipole antennas inches apart, at 90 degree angles to each other, like an X. Most cellular antennas today are dual 4-bay dipole panel antennas.

If you really want peak performance, then diversity receive antennas on the base or repeater is the answer, as it will typically yield 12 dB improvement. However, if you need omnidirectional, then you are likely stuck using space diversity.