Forward: I have always wanted to make my own hi-gain antenna for "acquiring" WiFi from across the street. Searching all across the net I found the most direct antenna I could make was either a parabolic dish or a helical antenna. I determined making a parabolic dish was to difficult to make a dish that was accurate enough to use and I couldn't find a used SAT dish for sale locally for a decent price. Because that idea was thrown out the only good choice for a directional antenna was a helical design.
Screw the imperial system of measuring. I refuse to use it as much as possible so when ever I give measurements I will try to use the base 10 metric system as much as possible.
Math:
I had to determine the ideal dimensions for a helical antenna.
The average frequency (ƒ) in 11 channel 802.11g is 2437MHz.
That converts to a wavelength (λ) of 0123.10mm
Using measurements from here I determined the ideal antenna to have these properties:
Antenna Diameter (D) = λ*.331 ≈ 0040.75mm
Plate Reflector Diameter (R) = λ*.95 ≈ 0116.95mm
Coil circumference (C) = D*pi ≈ 0128.02mm
Vertical Distance Between Coils (d) = C*.25 ≈ 0032.00mm
Number of Turns (T) = 19
Estimated Gain (G) ≈ 18.9
Length of Wire Needed = C*T-(λ/4) = 2401.605mm ≈ 7.88'
Parts:
Now with those measurement I wanted to find a antenna rod to wrap the wire around. I originally thought of using 31.75mm (1.25") PVC pipe, the outside diameter of the pipe would be slighty smaller than what I needed. Unfortunately, after some searching on the net I found that PVC has a high dielectric constant. So after looking locally, I found a Styrofoam rod (dielectric constant of 1.1) from a hobby store.
Styrofoam Rod Length ≈ 0609.60mm = (24") = (2")
Styrofoam Rod Diameter ≈ 0038.10mm = (1.5")
This rod has a slightly smaller diameter (thus a smaller radius) than that of the ideal antenna. The difference in the radius is 1.325mm. At this point I could either get a solid core wire with a radius with an exact radius of 1.325mm or get a thicker wire and gouge out a small portion in the rod. I figured that if I gouge out a portion it would leave a path that would allow a guide for a wire. A 4AGW wire has a diameter of 5.189mm (radius 2.95mm). The radius of the wire plus the radius of the rod is .3mm greater than that of the ideal antenna, so a very small grove is required.
Also while I was at the hobby store I found Styrofoams discs and bought two of them.
Styrofoam Disk Diameter ≈ 0149mm = (5.875")
Styrofoam Disk Depth ≈ 0031.1625mm = (1.1875")
as well as a Very Large copper plate I will cut down to size for a reflector plate and still have quite a lot to use in case of mistakes.
Copper Plate ≈ 0304.8mm * 457.2mm * 0.406mm = 12" * 18" * .016"
Instead of just Gorilla gluing the antenna rod to the reflector plate and stabilizer discs I bought a wood screw with the biggest teeth I could find at my local hardware store.
As for the actual WiFi receiver I pick up this nifty little WiFi USB stick with a RPSMA antenna connector
Still To Purchase
N Female Panel Mount
RP-SMA Male to N Male Adapter
4 AWG Wire
Impedance Matching:
The output impedance (Z1) is 50Ω and the input impedance (Z2) is measured as 137 * (C/λ) or 142.48Ω. The formula for impedance matching is Zs = √(Z1*Z2) ≈ 84.402. The ARRL handbook recommends a quarter wave strip of metal parallel to the reflector plane would be the best. I plan on using the same 0.406mm thick copper sheet for the matching strip. Using TraceSim, the matching strip would have to be place 8.03mm (1/4 distance between coils) above terminal lead, ≈9mm wide, and the arc length between where the Type N connector and Antenna wire meet has to be 30.775mm (λ/4).
Construction
First I drew the guide for the wires out on printer paper
Then I copied the pattern 10 times and taped them to the Styrofoam rod
After which I trace the guide with a fine tipped felt marker making sure it bled through onto the Styrofoam
Still To Do:
Purchase wire and connectors
Wind wire around rod
Cut out reflector plate from sheet
Drill holes in reflector plate for connector and center screw
Cut out matching strip from sheet
Cut out holes out of Styrofoam discs for Styrofoam rod
Cut out area in one disc for type N connector and (λ/4) matching strip
Actually assemble the antenna
Figure out how to mount it
I will continue working on this project. If you have question or comments, feel free to post. Any guidance I can get with this project would be appreciated as all the information I have gathered on this project has been scattered all across the net so if you see a problem or inconsistency with my math please speak up.
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