Crosshair - 10 dbic Circularly polarized antenna tutorial!

[IBCrazy]

Ok I'm done teasing with photos. Here it is, the VAS/True RC Crosshair!

The Crosshair antenna is the newest FPV RX antenna. Developed as a joint effort between IBCrazy (Video Aerial Systems) and HugeOne (True RC), the Crosshair delivers a nearly perfect axial ratio in the front and boasts an 8.5dbic forward gain in a nice compact package or a full 10dbic in a larger panel. Unlike the helical, the Crosshair needs no tuning and is much more compact. The bandwidth is fairly large, but circularity drops off very fast as you get off of the center frequency and thus it must be made for a specific frequency for best results.

In comparison to the helical, the Crosshair gives three distinct advantages: it is compact in size, is 98% efficient, and does not require tuning. It isn’t perfect, though. While the circularity is great right out front, it loses its axial ratio to the peripherals (sides) faster than the helical antenna. Also, while the bandwidth is quite large, the circularity suffers greatly as you get away from the center frequency.

Extra care must be taken to ensure the antenna is made very precisely. Additionally, moisture detunes the antenna. Thus it was designed for ~70% humidity. Excessive moisture (or lack of it) will detune the antenna just a bit. It also is nearly impossible to make for 5.8GHz. Hugo and I are still working on that one, so stay tuned (excuse the bad pun).

I give this tutorial to all hobbyists who wish to try their hand at constructing one. However, I ask that you not produce these for commercial sale. Making one for a friend or something is certainly ok, though.

Copyright C. Alex Greve Feb 1, 2012
Please ask permission before copying

[IBCrazy]

IBCrazy’s build method:


Materials needed:
1 sheet of copper or galvanized steel plate (I used galvanized steel)
1 Copper tube either ¼” or 3/8” (if using a balun build method)
1 coaxial cable
2 pieces of MIG welding wire or Memory wire.
4 nylon bolts
4 nylon nuts

Calculations:
Wire diameter in inches = 42/f in MHz to 60/f in MHz
Height over reflector in inches = 2350/f in MHz
Length of copper tube = 2933/f in MHz
Size of reflector panel in inches= 8800/f in MHz to 17500/f in MHz (larger reflector increases gain and F/B ratio)
Short leg length in inches = 2490/f in MHz
Long leg length in inches = 2945/fin MHz


You can find a complete listing of the dimensions for commonly used frequencies here: https://docs.google.com/spreadsheet/...0b0UxMXc#gid=0


For this tutorial I am building a 1280MHz Crosshair. I am using .035" Welding wire and RG316 cable.

Copyright C. Alex Greve Feb 1, 2012
Obtain permission before copying

[IBCrazy]

Building the antenna: Making the reflector

Start out by building the reflector. The side should be between .75 and 1.5 wavelengths. Larger reflectors increase the gain. 1.5 wavelengths yields 10dbic with no sidelobes and .75 wavelengths yields 8.5dbic with a few small side and rear lobes.


Make a cross across the panel. Drill the center out on solder in your copper tube such that only the height calculation protrudes (2350/f in MHz) if you plan on using a balun. If not using a balun, just drill it out so your coaxial cable can fit through.

Now drill 4 holes on the “X” to accommodate your nylon bolts. These will support your elements so place them so it will support somewhere between the center and edge of your element. If you must have a calculation, I placed mine at 1800/f in MHz away from the center, but this is not critical. Run your nylon bolts through the reflector and secure them with the nylon nuts. Now cut the ends off such that the height is equal to your height calculation.



Building the antenna: Making the balun

Uh-oh. A balun? Yes. While not necessary, it does increase the axial ratio significantly, so I recommend you make this. It’s pretty easy. Basically you are making ¼ wavelength of air insulated coaxial cable waveguide. First you need to prep your cable. Strip enough insulation off the end of the cable so that you have enough to comfortably solder your elements to. Now measure down from the outside INSULATION ¼ wavelength (2933/f in MHz) and remove the outer covering at that point. Tin this area of your cable with solder being careful not to overheat the center conductor.


Install the copper tube so it protrudes through the back of the panel. Because the tube is longer than your element height it must protrude through the back some. The measurement from the top of the tube to the base of the reflector should be about 1/16” less than your height so your elements can clear it. Now heat up the base of the copper tube and get it to accept a dab of solder. Once it does, insert the coaxial cable into the tube and solder the area where you removed the shield to the inside of the copper tube.

Now you must isolate the cable in the center. I use cheap nitro fuel line over the coaxial cable to keep it centered but hot glue works fine. DO NOT FILL THE TUBE! It should be mostly air or it will not work properly.

Copyright Feb 1, 2012: C. Alex Greve
Obtain permission before copying

[IBCrazy]

Building the antenna: Making the elements


Trim down your coaxial cable so only about 1/2" or less is protruding from your balun. Split your coaxial cable shield and center element. Pull the shield out to the side and twist it up. I personally cut off about ½ of the shielded strands to make this easier, then tin the remaining strands. Now strip as much insulation off the center element as possible without it shorting out to the shield. You should have about 1/16” of insulation sticking out of the cable at this point. Tin both the center element and the shield. Trim the shield back very short so that only about 1/8” or less is sticking out the side. You want this to be as clean as possible. This will make or break the antenna.





Now take your element wires and bend them into a sharp 90 degree bend. Solder one bend to the center element and one to the shield as close to the feedpoint as possible without shorting them out.





Glue the wires to the securing posts and let the glue dry before proceeding to the next step.

Copyright C. Alex Greve Feb 2012
Obtain permission before copying

[IBCrazy]

Building the antenna: Tuning

Now you must trim your elements to the proper size. BE ABSOLUTELY CERTAIN TO DO THIS STEP VERY PRECISELY!!! The orientation of the elements and their length will determine SWR and circularity.

Circularity:

Circularity is determined by the rotational vectors of the elements. The short section of the element seems capacitive (leading) while the long section of the element seems inductive (lagging). A signal rotating counter clockwise is LHCP. A signal rotating clockwise is RHCP. Since we are looking from the front of the antenna, this is reversed as we are looking at the opposite direction in which the wave propogates. Thus if looking at the element, if you rotate from long to short in a clockwise direction, you have RHCP. If you rotate from long to short counter clockwise, you have LHCP. Thus the signal “travels” around from short to long on each element.

IN other words the signal follows the below pattern.

Long (active) -> Short (active) -> Long (Shield) -> Short (Shield)

If you trace in a clockwise direction, you have RHCP. If you trace in a counter clockwise direction, you have LHCP.



Construction:

Starting with the element connected to the center of the coaxial cable, measure out your short leg length on one side and your long leg length on the other side of the element making certain of proper phase rotation. Trim this off exactly. Measure with a caliper from the edge of the wire to the center of the coaxial cable to ensure the proper length. THIS IS CRITICAL!

Now move on to the element connected to the shield. This is measured from the point where the shield touches the insulation of the center element on the coaxial cable. Again, pay attention to phase rotation. The short element should be opposite the short element on the other element. In other words they should be 180 degrees apart.

Your antenna is now ready for flight. Do not use any extra supports or it will throw off the circularity of the antenna. Be certain the elements do not touch the center tube (balun).

Copyright C. Alex Greve Feb 1, 2012
Obtain permission before copying

[IBCrazy]

Like many great things it was a bit of a mistake. I was looking to build a 5-6dbic antenna using phase shift vectoring. I already had designed a turnstile antenna and figured that over a reflector would increase gain by 3 db. However, when I put the elements over the reflector, it threw off the phase shifts. I played with the height and element sizes and got it close before I called Hugo at True RC and asked for help.

Hugo threw my model into his modeling program and refined it. He came up with 9dbic! Oops. Slight miscalculation on my part, huh? SWR was showing 1.3 on the simulation, so I built it and tried it. I got 1.6 in my build. Hugo got 1.3. Still not perfect, but we flew them anyway.

The initial tests were good, but not good enough for the two perfectionists designing them. I asked Hugo to enlarge the element diameter of one element and see if that made it better. It didn’t. It made it worse, so he then instinctively reduced the diameter until it had nearly perfect SWR. He then tweaked it for axial ratio. I was the first to build it. Actually, I built two for 1280MHz and immediately flew them on my Oracle diversity controller. They worked incredibly well! Hugo made his only a few hours later for 2.4GHz and reported excellent results as well. Thus, the Crosshair was born!

And now the crosshair is here for you to build, fly, and enjoy. I encourage anyone with time and soldering skill to build this antenna. All I ask is that you give credit where it is due. HugeOne and I work very diligently to bring you only the best FPV experience we can. We sincerely hope you enjoy this design. As always, feel free to ask any questions you might have.


-Alex

[IBCrazy]

Q&A:

Q: Is this antenna better than the helical?
A: Yes and no. It is certainly smaller and easier to build precisely without any measuring equipment. I personally prefer the helical when I can deal with the larger size and maximum efficiency is not required.

Q: Can I add supports to the elements?
A: You can, but it will have an adverse effect on the antenna. For example: A PVC pipe cut to the proper height will harm axial ratio.

Q: Does the bolt diameter matter?
A: Not much, but technically speaking you want it as narrow as possible for best circularity.

Q: Can I paint this antenna?
A: Yes.


Q: Why won't my antenna converge to a perfect 1.0 SWR?
A: This antenna actually does not converge to a perfect 50 OHms and 1.0 SWR without significant drop in axial ratio. Thus, moving it closer to the reflector does reduce SWR, but it does so at the cost of axial ratio. If you get 1.3SWR, you did very well. Perfection is achieved when you get about 1.15.


Q: Copyright? Really?
A: Yes, sadly. This belongs to Hugo and I. While we freely give this away, we do have a business to run. We have seen our tutorials reposted all over the place, often times incorrectly, which leads to improper copy cat antennas that are seldom built well. We want you to enjoy this as a hobbyist, but PLEASE DO NOT PRODUCE THIS AND SELL IT COMMERCIALLY.


-Alex

[IBCrazy]

Ok, time for me to step back and let you try this out. Let me know how it goes. I'm here to answer questions as always!

-Alex
KJ4RIV

[Joe90]

Big Big thanks for sharing, I wish the world be more like this.
If you need anything ask, we never know, maybe I could help.

[Tracks]

Well, the big question is: When will they be available on your sites?