Circularly Polarized Helical antenna 7.5 -13 dbi - New: Wave Trap match!
The community has recently seen a few FPVer’s including myself gravitating toward circular polarization. Ham’s have been using it for many years for ATV and satellite communications. Old Man Mike has proven the incredible benefits of using a CP system on his quad copter in its ability to reject multipath interference. I have seen its benefits in never losing my signal due to banking the plane. The problem with CP is mainly in that the antennas are generally not easy, nor well understood. It is my belief that CP is the way of the future of FPV and thus, this tutorial gives you a fairly easy to make directional CP antenna that can be mounted to an antenna tracker: the helical.
Advantages - Can be made to virtually any gain you desire. Easy to make and truly circularly polarized.
Disadvantage - Efficiency drops with added turns. Maximum efficiency is about 80% with a 3-4 turn helical and drops to about 60% at 12 turns.
The helical antenna when in axial mode is incredibly wide band which makes it great to receive our wide band analog signals as well as making it great for construction because building tolerances are incredibly loose. Thus, if you want an easy to construct antenna that will keep signal in a bank, this is the one for you. I know the impedance match section looks cumbersome. It’s not. Don’t over think this one. It’s pretty easy.
I will be making two types of helicals: a former wound helical, and a cross supported helical. The former wound unit is much easier to build, but the proper size former may not be available. The cross supported helical is more efficient and can be made at any frequency, but is harder to make. It is your choice.
My helicals will be built for 1280MHz. You can scale the design to fit any frequency by the simple equation:
1280/Your frequency * My dimension = Your dimension
Or you can simply calculate it with the equations given in the second post.
For an example of a 2.4GHz helix, you can see Jason Hecker’s design here: http://www.wireless.org.au/~jhecker/helix/
First, a couple of thank you's
Before I write this, I need to give thanks to a few people for their help. It’s people like these that make this hobby great:
A special thanks to Dave Albert (Dalbert02) for taking the time to do extensive testing of these antennas. Without him, this would not have been possible.
I also want to thank:
Old Man Mike – For his great explanations on CP systems
Nigel (devonboy) – For introducing the skew planar wheel and its feasibility which make CP a reality for FPV
Blupp – For his ideas, support, and independent research
Steven Katz (WB2WIK) – It was his idea for the RG62 impedance match (match method #2).
Juan Nurmela (OH5NXO) – For recognizing a more measurable microstrip match than conventional methods (match method #3).
Jason Hecker - For his incredibly easy to make impedance transformer (match method #1)
The materials are subject to what you have available, but here are the basics:
Support brace – PVC pipe, cardboard poster tube or 1X4 wooden board
Wire – the thicker, the better. 14AWG is fine. 12 or 10 AWG is better. 1/8" or 3/16" copper tubing is supreme!
Former – A short length of any kind of pipe to wind your coil
Reflector – A piece of solderable metal. Galvanized steel or copper
Feedline – 50 Ohm coaxial cable
Matching stub – 93 Ohm Coax, 75 Ohm coax, ¼” tubing, or a strip of galvanized steel
To translate into millimeters, simply multiply all dimensions by 25.4
Gain ~ 4.5 + number of turns (3 turn minimum, 12 turn max)
3 turns = 7 dbi
4 turns = 8.5dbi
5 turns = 9.5 dbi
6 turns = 10.5 dbi
7 turns = 11.25 dbi
12 turns = 12.5 dbi
PVC former wound helix:
PVC pipe former outer diameter range in inches = 3000/f in MHZ – to – 4600/f in MHz
Coil spacing in inches: 2790/f in MHz
PVC pipe length = number of turns * coil spacing
Reflector plate size = 11732/f in MHz
For my 1280MHz antenna, I found that 2.5” PVC conduit fits my needed former size almost perfectly (out diameter is about 2.75”). I cut it 12” long for a 5 turn antenna. A little long is fine.
Cross insulated Helix (a paper or cardboard round support would follow these equations):
Cross width in inches: 3600/f in MHz (+/- 10% tolerance)
Coil spacing in inches = 2900/f in MHz
Cross length = number of turns * coils spacing
Reflector plate size = 11732/f in MHz
Why the different calculations? The PVC former is a dielectric and shifts the frequency down 5% or so. Also, the former selection is limited, so a range is specified for the diameter where the wooden cross can be cut to size. The beauty of this antenna is that it has such a wide bandwidth, there is a good range of error we can have and still make a good antenna!
If you are wondering where these numbers come from, they are just factors of wavelength scaled to measurable units. Coil spacing should be ~ 1/4 wavelength (a little less for the PVC former since it shifts the frequency down), and coil length of 1 turn is between 80% and 130% of the wavelength. The reflector is simply 1 wavelength. Larger reflectors will result in larger sidelobes and slightly less forward gain, but it really affects performance very little.
Building the antenna: Winding the coil
Polarization: Before you start making the coil, consider your polarization. If you are flying with a linearly polarized antenna, then polarization doesn’t matter. If you are flying with a circularly polarized antenna on your plane, then you need to make your helix match the same polarization. The polarization of the helix is simple. If the thread is the same as a bolt it is RHCP (Right Hand). If it is reverse thread of a bolt it is LHCP. In this tutorial, all antennas are RHCP.
The first thing you should do is mark your support for proper coil spacing. For the PVC, I drew 2 lines on either side of my PVC pipe and made a cross where the coil would overlap it.
So for my 1280 MHz, my measurements on one side were:
0, 2.125, 4.25, 6.375, 8.5, 10.625”
On the back side:
1.0625, 3.1875, 5.3125, 7.4375, 9.5625”
For my wooden former, I marked the following on one side:
0, 2.1875, 4.375, 6.5625, 8.75, 10.9375”
1.09375, 3.28, 5.46, 7.66, 9.84"
I highly recommend winding the coil on a small former before trying to install it on your antenna structure. This will allow it to be installed much more easily. Copper wire will spring back about 15% or so, therefore find a former with a diameter about 15% less than the diameter of your antenna support. Since this size likely does not exist, just find something close.
Wind your coil up the temporary former keeping the coil spacing a little bit closer than your calculation above. I used tape to secure the bottom of the wire turn so it wouldn’t come undone while winding.
For my 1280MHz antenna, I used 2”PVC pipe and coiled it to a separation of 2” per coil. It sprung back to nearly the perfect size to fit. Not perfect, but close enough.
Now install the coil by sliding it over your support. Move the wire up and down so that it crosses over your marks. Glue this in place. I used hot glue, but any glue works. You can also wrap the whole coil in tape to hold it in place as well if you prefer. For the wood former, you may decide to notch it slightly. Just keep it from moving on you.
Building the antenna – Installing the Reflector
Ideally, the reflector should be round and not square. However I found this to be difficult, so I simply trimmed the corners from my square pattern. Really this makes little difference whether it’s round or square, but it keeps me from getting cut on the sharp edges anyway.
For my 1280MHz my reflector is 9-1/8” square
Now that your helix is wound, it’s time to mount it to your reflector. I chose to glue mine down with contact adhesive. It’s cheap and it works. You can also screw it down provided you use short screws as long screws will attenuate the signal. Secure the helix to the center of your reflector.
Matching the antenna - Do not over think this. Use what seems easiest!
Choose only one method described here. Use whatever method appears easiest as they all work. Your helix is almost done. Now to match it. The biggest problem with the helical antenna is that the impedance isn’t a nice 50 ohms as we would like. It’s theoretically about 140 ohms, But I am measuring 100-105 Ohms. Thus we need to make an impedance transformer to match it to our 50 ohm coaxial line.
Uh-oh! Impedance transformer! Sounds really difficult… It’s not. It’s actually easy! For this match, the ¼ wave impedance transformer is perhaps the easiest. We need a quarter wave “stub transmission line” of 70 Ohm impedance. I will describe 4 different methods below along with their advantages. Any of these methods will work. Choose one that best fits you.
Method one – The Hecker transformer
Advantage: Easiest to make
Disadvantage: Attenuates the signal and causes narrower bandwidth (least effective match)
For the PVC formed helical, I used the “Hecker transformer”, designed by Jason Hecker. This is nothing more than a triangular piece of metal (I used galvanized steel) wrapped halfway around the bottom of the former.
It’s dimensions are:
L = 5450/f in MHz (Twice your coil spacing)
H = 1360/F in MHz (½ your coil spacing)
Method 2 – 75 Ohm (or 93 Ohm) Coaxial cable
Advantage – Very easy
Disadvantage – Not a perfect match. Best VSWR will be 1.25 (which is still very good).
Another method to match is to use a ¼ wave section of 75 Ohm coaxial cable at the input of the helix. Loss is fairly low since the section is so short. You install this by connecting one end to the helix (center conductor connects to the helix and the shield connects to the ground plane) and the other end to your feedline (shield to shield and center to center). Try to keep the stub/feedline connection as short as possible without shorting it. I use hot glue as an insulator for added insurance.
You will need to look up the velocity factor (Vf) of the cable you choose. I chose to use RG62 cable because it’s cheap and easy to find. The length of this stub is:
Length of stub in inches = 2900*Vf/F in MHz
For my RG62 cable, the velocity factor is .84, so my length is 1.90”
75 Ohm coaxial cable also works better in my experiments. VSWR will be about 1.25 with a 75 Ohm stub, which is actually pretty good. The great thing is 75 Ohm coaxial cable can be found almost anywhere
Method 3 – The IBCrazy microstrip
Advantage: Provides a good match and maintains bandwidth
Disadvantage: Slightly more difficult than other methods above
Yet another way to match the helix to the line is to make a ¼ turn micro strip line. This method is briefly explained in the ARRL antenna book based on Gerald Brown’s (K5OE’s) satellite dish design using a tapered line. I chose a parallel line rather than a tapered line for ease of calculation. Why ¼ turn? Because a full turn of the coil is one wavelength, ¼ turn is ¼ wavelength. The strip is made to the following equations:
Length – 2775/f in MHz (same as your coil spacing)
Width = 1.25 * height
This is very dynamic! You can make the sub whatever width/height you want. I would keep the width as small as possible that you can measure accurately. I made mine out of galvanized steel and 5/16” wide. Height is ~1/4” above the reflector. Alternately the K5OE design calls for a 6mm strip starting at 1.2mm and running up to 3mm. I find this match is not as good as a parallel match as outlined here, but it does work. The strip doesn’t need to be curved and length isn’t terribly critical. Use the strip as the first 1/4 turn of the helix. DO NOT TAPER IT. Leave it parallel to the reflector. Your first turn spacing will be a little bit off due to this. Just be sure the end of the first turn is still 2775/f in MHz above your reflector and the spacing is maintained the rest of the way up.
Method 4 – The IBCrazy conductor match
Advantage: Obtains and excellent match and fairly simple to make
Disadvantage: Hard to measure accurately
The final way to match your helix is to use a round conductor. You can use anything from a thick copper wire to copper (or brass) tubing. The diameter of that tube/wire is up to you. It’s dimensions are given by the following equations:
Length in inches = 2800/f in MHz
Diameter = 2 X Height
Again the diameter is up to you. Just keep it as small as possible that you can measure the height accurately. I chose a ¼” copper tube and placed it 1/8” away from my ground plane. I heated my tube with a torch to make it bend more easily, but it’s probably better just to leave it straight. Use this stub as the first 1/4 turn of your helix. Again, do not taper this. Leave it parallel to the ground plane. This will cause the first turn spacing to be a bit off, but that's ok. Just be sure that the first full turn ends at the proper height above your reflector.
This method is not supported in any reference I know of, but experimentally it works incredibly well. If you used a nice thick wire for your helix winding, you don't even need to add this, but just adjust your first 1/4 turn!
Method 5 - IBCrazy's tapered match
Advantage - Provides the best match and maintains the antenna integrity. Also tolerant of manufacturing errors.
Disadvantage: Hardest to make.
I came up with this one experimenting with my directional Wattmeter. It has been the best method for me. You simply make a metal strip that is tapered so that the width is equal to the height over the reflector and follow the normal helix winding.
Width at wide point in inches = 750/f in MHz
Width at starting point - 1.25 *height above reflector (this is usually just a bit less the diameter of the coaxial cable, so of in doubt use the diameter of the coax)
Use this as the first 1/4 turn of your helix making it follow the natural helix winding spacing. The shape keeps the signal from being attenuated and also allows for some manufacturing error. This match is not supported by any reference I know of. It is something I made up by a tapered transmission line calculation and empirical measurement.
Method #6 - IBCrazy's Wavetrap!
Advantages: Incredibly easy to make, Increases antenna bandwidth substantially
Since going commercial with these antennas I needed an easier match. While searching for an easier way to match this yet another time, I came up with this one which is so easy it's incredible. All you need to do is cut a rectangle where one side is 1/8 wavelength and the other side is 1/16 wavelength. Put this in the middle of your first quarter turn of the helix. All it does is follow the helix.
Length in inches = 1460/f in MHz
Length in cm = 3720/f in MHz
Width in inches = 730/f in MHz
Length in cm = 1860/f in MHz
Start point distance away from coaxial feed in inches = 730/f in MHz
Simply solder this to your helix. The center of the match should be the center of your first 1/4 turn. Very simple and it works!
Installing the cable
With your reflector mounted (whether screwed down, glued, or other method) drill a small hole right next to the beginning of your antenna just large enough to pass the center conductor and it’s insulation through the hole.
If you used a matching strip or Hecker transformer:
Strip a short length of the jacket off of the coaxial cable and flair out the shield. Pass the center conductor through the hole. Solder your shield to the bottom of your reflector. Solder the center element to the beginning of the matching stub.
If you used a coaxial transformer match:
Install the stub at the hole. Solder the shield to the reflector and the center element to the base of the coil. Now connect your feedline to the other end of the stub. Solder the center elements together and solder the shields together. This can be messy, just don’t let the shield and center conductors touch. I insulated mine with hot glue for added insurance.
If you used the stub match- Simply use the stub as the first 1/4 turn of the helix and feed the cable through the base to the stub.
You now have a high gain, wide band circularly polarized antenna!
A Final Word
To get the best performance from this antenna you must tune it. This involves shifting the matching stub (first 1/4 turn). You can build your own SWR meter from my instructions found here: http://fpvlab.com/forums/showthread....urself-project
This antenna does very well when combined with the Skew Planar Wheel, cloverleaf or the TrueRC windmill.
I write this and all of my tutorials in the name of Jesus Christ. It is with His heart, and God’s blessing that I freely give this away to the FPV community. Without them, this tutorial and others that I have written would never have existed.
Q: Will you be building these?
A: Yes. I now sell these through a website: www.videoaerialsystems.com
Q: Will these work with a linear antenna such as the Vee?
A: Certainly. However you will have a 3dbi polarization loss. You also los the antennas greatest ability: multipath rejection.
Q: How will this work with the Cloverleaf/Skew-wheel/virevent antennas?
A: Incredibly well. The best video you will ever have.
Q: What TX antenna do you recommend with this?
A: The Cloverleaf is the best match for this antenna. The ske-planar wheel is a close second.
Q: Should I use this or a circularly polarized patch?
A: This can be made to fit your needed gain. Use a CP patch if you already have one and the gain is acceptable or if this one is too big. Otherwise, you might prefer this antenna.
Q: How do you recommend I match the impedance?
A: It’s up to you, but for 5.8 GHz, I would use a Wavetrap match or a Hecker transformer. For 2.4 GHz it’s a toss-up, but I would probably opt for the Wavetrap. For 1.2/1.3 GHz and 900MHz, I’d opt for the tapered microstrip. However, the final design is up to you. They all work pretty well.
Q: What’s with the religion thing?
A: Call it what you like, but my faith is a big part of my life. Without the blessings of God and the guidance of the Holy Spirit, I would not even consider posting these tutorials. I’d just be some arrogant idiot who didn’t care. Let's face it, I didn't have a choice in my ability to understand this stuff. That's Gods blessing to me. I wish to honor Him by sharing it with you.
Thanks for posting this Alex. And thanks for being open about the beliefs that this country was founded upon. Too bad folks have forgotten that aspect. "There is no other name by which we must be saved."
One question; How does the beamwidth of this antenna compare to the Bi-quad ?
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