Crossed Yagi antenna gain

[vo1ks]

Thought question... Not meant to warp anyone worse than they already are.

I'm planning to use a crossed yagi antenna on 2-meters. Instead of one set of elements being for vertical and the other for horizontal I want to mount the antenna in an X configuration with each driven element at +/- 45 degrees.
By feeding them with either 0 or 180 phase difference, the antenna will produce horizontal or vertical polarization. With 90 or 270 degrees difference it will be circular left or right hand rotation.

If both elements are driven to create circular polarity, the calculated ERP in either plane is reduced by 3dB. However if both are driven to create a linear polarity, what effect will it have upon the gain?

The antenna gain spec is 13dBd for each polarity. I would expect it to be 10dBd while operating CP when compared to a linear dipole in either orientation.

What would you expect for rated gain when operating both elements to produce a linear signal? Would it return to 13dBd, increase to 16dBd, or become some other value? (Let's ignore cable losses and so on)

No guessing allowed. Some form of explanation or reasoning required...

 

[vo1ks]

Here's a picture of what I want to build.
Supplier of Quad Stacked Antenna from New Delhi,Delhi,India,ID: 1307032212

Each linear Yagi has a gain spec of 12.6dBd In CP I would expect 9.6dBd
For all four as pictured gain should be 15.6dBd when operating CP.

By changing phase to create a linear vertical or horizontal signal, what would the expected gain of a single crossed yagi become? From there, the overall gain of the array should increase by about 6dB.

 

[Road Squawker]

it doesn't cover your gain question, but, here is an interesting read. he also has some other neat stuff on his home page

Polarizations

 

[vo1ks]

That's essentially what I want to build with the four Yagis.
The relays K1 through K4 will be a little different in that they will have four terminals. It will require only two DowKey 402-320132 or similar. The Y cable length will essentially be the switch itself. I'll have to custom build the 90 and 180 cables with a network analyzer.
There will be a 2-way divider with 7-16DIN input and Type-N outputs. Each output will feed a 4-way divider. V and H elements will connect to separate dividers, and the 90/180 phase will be at their inputs.

Nothing showing there about what to expect for gain in either polarization...

 

[W5LZ]

It'll be interesting to find out what/how that ends up. Anything I said would only be a guess, so I won't say anything. I think I remember how to do that calculating but I'm way to lazy to do it... gives me a headache anymore, that thinking you know...
- 'Doc

 

[Captain Kilowatt]

No guessing allowed. Some form of explanation or reasoning required...

I worked last night and have been up since 4:00pm YESTERDAY. It is now 9:30 pm. I am beyond any explanation and all reasoning. Initial thought says the gain would be restored to that of a single plane yagi of 13 dB however my thought process at the moment leaves a little bit to be desired.

 

[Radio Tech]

I worked last night and have been up since 4:00pm YESTERDAY. It is now 9:30 pm. I am beyond any explanation and all reasoning. Initial thought says the gain would be restored to that of a single plane yagi of 13 dB however my thought process at the moment leaves a little bit to be desired.

I think your thought process is pretty close to being spot on .
If a remember correct there is a -3db loss per polarity due to not being separated by a wave length.

Been a while since I modeled a cross yagi.

 

[vo1ks]

I'm leaning that way also.

Half power is in each angle which is then combined to restore the single linear polarity. More thought and reasoning needed.

If the two yagis were separated by some distance and operated in the same plane their gain would increase by 3dB.

Operating both polarities simultaneously does not increase the linear polarity gain. It creates flexibility to easily select either of four polarities and doubles the power handling capacity over a single yagi.

I worked last night and have been up since 4:00pm YESTERDAY. It is now 9:30 pm. I am beyond any explanation and all reasoning. Initial thought says the gain would be restored to that of a single plane yagi of 13 dB however my thought process at the moment leaves a little bit to be desired.

 

[Shockwave]

Lets follow the thought process out to get to the answer. We are still splitting the feedline to drive two elements. We are still feeding elements that are neither in the horizontal plane or vertical plane. It will take both sets of elements to form circular, horizontal or vertical polarization. When we change the phase shift or delay between driven elements we are not changing the power applied to them, only the phase angle changes.

The only way we could prevent a loss in gain or add to gain would be if both sets of elements were in the same plane. I'm almost sure at the 45 degree cross polarization point between signals, there is a 6 db drop. This is the point where the drop in signal starts becoming very noticeable, hitting 20 db down by the time the polarization is off by 90 degrees. Two antennas that are 6 db down still give a total gain 3 db down when combined or in this case 10 dbd.

What you have done is remove most of the distortion in the vertical and circular polarization modes that would have been caused by the tower or mast being inline with the vertical elements. That's because these polarizations are formed outside of the near field and away from the tower.

 

[ghz24]

Cross polarization loss

I know I'm a little late to this party but I can't resist.
Hi Shockwave
Your logic seams dead on to me but...

I'm almost sure at the 45 degree cross polarization point between signals, there is a 6 db drop.

It's actually 3 dB loss. The formula is Cosine theta squared.
So Cos (45)= 0.7071..... , squared= 0.5 as in half the signal or 3 dB.

Using the same formula the loss from 90 difference in polarization is infinite
Cos 90 =0 squared it is still 0 (
Reflections and other real world effects cause only a 20 dB loss when measured. (on earth)

With that small correction following shockwaves logic there should be no difference in the gain produced regardless of polarizatilon produced.

That is except that horizontal gets more of a boost from the ground than does vertical .

vo1ks : cool project I hope you keep us updated.
The only imperfection I'd point to is due to the ground your circular polarization will be slightly eliptical, slewed some toward horizontal gain due to the ground favoring the horizontal component.