Crosshair - 10 dbic Circularly polarized antenna tutorial!

[SENTRY]

Hi Sentry,

What kind of range have you gotten out of your Crosshair?

I turned around at 8 miles. Basically perfect video.

[coil]

Thanks for the answer, it loos like you have tried RV micro diversity, i have read that it takes about 5s to switch between video sources... Is that true ?

Or it is better to use your DIY version and buy video splitter - amplifier, because i need two outputs.

Edit: I have lawmate 1.2 RX with narrow SAW filter, i suppose it is good.

[Pollux]

@Pollux - What are the tolerances on the PCB material? How thick copper plating can you make it and how thin can you get the PCB? I think this is very possible. The problem is that I cannot simulate flat wire in my software. Flat wire has different inductive quailties than round wire. However, If you can make a PCB, I would be happy to test it for you.

-Alex

@IBCrazy: sorry for not answering sooner, I've just seeen your answer...
I can't make PCB myself (especially for hyperfrequencies. I tried to make a directionnal coupler for 5.8 GHz, epic fail)... some prototyping company could do this job.
For the material, I was thinking 0.8 mm FR4 35µ up to 2.4 GHz, and 0.4 mm 35µ FR4 for 5.8 GHz.
we could try Rogers material, but I'm not sure it's worth the price for our applications.

Sadly, I can't design this myself, I've not the necessary knowledge. I'm a noob, I just begin to understand some of your designs and I have no experience with any antenna simulation.
That was just an idea thrown up in the air... I will fully try to understand the crosshair by reading again this thread, then I'll try to figure out how to make a pcb version... but I seriously doubt to get any result.

[IBCrazy]

Thanks for the answer, it loos like you have tried RV micro diversity, i have read that it takes about 5s to switch between video sources... Is that true ?

Or it is better to use your DIY version and buy video splitter - amplifier, because i need two outputs.

Edit: I have lawmate 1.2 RX with narrow SAW filter, i suppose it is good.

I'm not entirely certain how long it takes to change, but I know it takes a while. This is a really good thing for CP. The lack of sensitivity of the RV controller allows it to more accurately select the proper antenna. Since Lawmate scales RSSI pretty accurately, you will have a respectable result. The only problem I forsee is lack of scaling. I scale my RSSI on a gain controller which increases the accuracy of the RSSI reading. See my DIY diversity controller on RCG for an example.

-Alex

[adamchicago]

CP and diversity...I saw some posts saying video diversity doesn't work well with CP, can someone explain why? I read some guesses that for some reason there's a high level of switching that overwhelms the controller, but I haven't heard anything technically conclusive yet. It seems to work with my helicals and SPW at least out to 1.5miles...so am not sure where the problem is.

I use 3 1280mhz antenna on two diversity controllers: SPW and 9.5dbi helical on Oracle and 9.5dbi helical on EagleEyes.

Thanks for any clarity!

[luzaban]

Hey Guys i just finished my first crosshair build. I was aiming for 2370mhz. Here is an swr plot which i did with the diy swr meter.



as 2370 is the lowest freq available for this tx i am not sure if i hit the center frequnecy. Should i cut down the elements or leave it like that with an swr of 1:1.47 ?

[MASHTON1138]

Leave it alone. Thats ~96% efficient and in reality 1.25 is the best you could hope for on this antenna anyway. Besides if you cut too much you will throw off the circularity and turn it into a dual LP antenna. Also the DIY SWR meter is a little off +/- so it is likely it is a little better or just a little worse depending on your directional coupler. I wouldn't mess with it personally.

Marc

[MASHTON1138]

CP and diversity...I saw some posts saying video diversity doesn't work well with CP, can someone explain why? I read some guesses that for some reason there's a high level of switching that overwhelms the controller, but I haven't heard anything technically conclusive yet. It seems to work with my helicals and SPW at least out to 1.5miles...so am not sure where the problem is.

I use 3 1280mhz antenna on two diversity controllers: SPW and 9.5dbi helical on Oracle and 9.5dbi helical on EagleEyes.

Thanks for any clarity!

I think the issue is more the sensitivity of the diversity controllers. Some people have no issues (like you) and others have major issues with the same setup. Alex has done a lot of testing so his knowledge of it is way beyond my scope but through talks he and I have that is what I really take from it. But basically a huge number of the DCs switch way to much with CP because of the sensitivity of the controllers. It is basically seeing two good (enough to use) signals and rapidly switching between them. With CP the signal fades and with linear it drops fairly quick (not instant but quicker than CP) and so the controller is able to distinguish between the two signals better. Most people find their problem goes away when the fly without the diversity in the system.

I could be way off here but that is how I understand it. And I have never found a need to fly diversity so I never tried it personally.

Marc

Marc

[adamchicago]

Marc, thanks for the great explanation, that makes sense.

Adam

I think the issue is more the sensitivity of the diversity controllers. Some people have no issues (like you) and others have major issues with the same setup. Alex has done a lot of testing so his knowledge of it is way beyond my scope but through talks he and I have that is what I really take from it. But basically a huge number of the DCs switch way to much with CP because of the sensitivity of the controllers. It is basically seeing two good (enough to use) signals and rapidly switching between them. With CP the signal fades and with linear it drops fairly quick (not instant but quicker than CP) and so the controller is able to distinguish between the two signals better. Most people find their problem goes away when the fly without the diversity in the system.

I could be way off here but that is how I understand it. And I have never found a need to fly diversity so I never tried it personally.

Marc

Marc

[IBCrazy]

Hey Guys i just finished my first crosshair build. I was aiming for 2370mhz. Here is an swr plot which i did with the diy swr meter.

Attachment 8352

as 2370 is the lowest freq available for this tx i am not sure if i hit the center frequnecy. Should i cut down the elements or leave it like that with an swr of 1:1.47 ?

Chances are your antenna is perfect.

However, there might be a small issue, as well. Looking at the SWR (assuming it is accurate) I would suggest that your short elements might be a bit too short or your feed gap could be a bit much. Tough to say without knowing the details of the build, but the trend is a bit unusual as it should be almost flat through a +/-60 MHz band.

If you can, please post the details of your build (wire size and height over reflector specifically) and take some photos of it with a close up of a measuring device streched across the elements in both the long sides and the short sides. I have found many people mistake how the feedpoint should be soldered as it is not trivial.

This antenna is very precise to get perfection, but it really delivers big and you don't need measuring equipment like you do with a helical.

-Alex