cross-polarization loss measurments

[daldesign]

We are trying to predict the cross-polarization pattern of 4-element and 6-element Yagi antennas at around 220MHz. The vendors have co-polar pattern data but not cross-polar patterns. Does anyone have any measured data on typical Yagis? The Yagis in question are mounted with elements horizontal but our signal is coming from a vertically polarized omni. We are considering renting a 10m semi-anechoic chamber and doing the measurements there. Not sure if the typical reflective ground plane would be a problem.
Thanks

 

[VEBill]

Because it's a nulling characteristic, the depth of the null depends critically on small details (feed point wiring, environmental reflections). A typical (bore sight) cross polarization loss is about 30dB. But it would be sensitive to variation of details.

Are you trying to reuse a frequency and you're predicting the rejection? Otherwise you'd just install the Yagis vertically if you wanted to receive the vertical signal.

 

[daldesign]

Good day, VE1BLL. I am curious to know if the cross-pole pattern follows the horizontal pattern, at say 80 to 100 deg azimuth. If there is a null at 90 degrees in the co-polar pattern, is there a similar null in the cross-polar pattern?
Some background: here in the US but not in Canada, one can buy spectrum at 217-218,and at 219-220 MHz for LMR usage. This spectrum all falls in the (N+1) adjacent channel to US TV channel 13 which is now used by DTV stations. The DTV transmissions are mostly horizontally polarized. However the LMR operators wish to use vertical polarization as usual. In many cases, the interference contour of the LMR base station falls inside the service contour of a DTV station...
Regards, N7DAL

 

[VEBill]

One sign that you're asking a very specific and specialized question is when a Google search of the applicable keywords (yagi cross polarization pattern) finds this exact Eng-Tips thread as the second hit. LOL.

I don't have any first hand experience that exactly fits your question. I wonder if anyone does?

In general, deep nulls depend upon cancellation and are typically very sensitive to details. So, for your example, if the vertical source was slightly above or below the horizontal plane of the Yagi, then the symmetry is broken and the null wouldn't be very deep. If you assumed that the vertical source was in the plane of the Yagi, your results would incomplete and thus invalid. In the worst case (e.g. mobile parked three stories below Yagi), the polarity loss might be essentially zero - due to the geometry.

Does the FCC even allow for polarization diversity from licensee to another? I'll bet that they might for certain fixed installations; but mobile would be too unpredictable.

 

[Higgler]

One simple way to estimate crosspole would be if the Yagi antenna elements drooped X degrees in angle. Or were tilted.

The polariation vector of each element would provide crosspole of 20*log10(Tan (angle droop)), so 1 degree droop is -35 dB crosspole level.
With a 5 degree angular droop (1 inch over 13 inch angle), that would make 20log10(tan 5) = -21 db cross pole.

similar calculations if you don't orient your antenna perfectly horizontal or vertical.