Materials

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
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

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”
Opposite side:
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.

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.

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
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
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
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.
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
or
Length in cm = 3720/f in MHz
Width in inches = 730/f in MHz
or
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!

Views: 3277
RG62 matching stub

·

Views: 3317
Connected to the coaxial cable

·

Views: 3447
Detail of solder joint for RG62 match

·

Views: 3095
Conductor matching stub

·