DIY WIFI Helical Antenna

[AllAgainstPaul]

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.

[mpilchfamily]

Actually the dish is easier then you may think. Have a look at the one i built and the resourses i found for it.
http://www.xcpus.com/forums/other-mo...apter-mod.html

What you have is a long list of materials which translates into higher cost. A dish can be made from anything from anm old Wok to pasta strainer. Here are some good examples of cheap dishes.
http://www.usbwifi.orconhosting.net.nz/

My project would be displayed on that site but i havn't done any real testing to demonstraight the benifites i've gotten form the dish.

[AllAgainstPaul]

Actually the dish is easier then you may think. Have a look at the one i built and the resourses i found for it.
http://www.xcpus.com/forums/other-mo...apter-mod.html

What you have is a long list of materials which translates into higher cost. A dish can be made from anything from anm old Wok to pasta strainer. Here are some good examples of cheap dishes.
http://www.usbwifi.orconhosting.net.nz/

My project would be displayed on that site but i havn't done any real testing to demonstraight the benifites i've gotten form the dish.

I have carefully thought over those WokFi modifications and the big problem is the fact that NOWHERE locally can I find a Asian mesh frying pans, the walmart/kmart/shopko/target where I live never heard of anything like them even after I showed them a picture. Also there are no Asian markets or World Stores where I live. Even though where I live is of a decent size it has almost no diversity, the only demographic that stores pander to is the redneck farmer. As for strainers/cookware: I could not find one that was either large enough for any use or too deep to a parabolic.

[dengyong]

What's the maximum cable length to the antenna ?

[STEvil]

a steel mixing bowl will work as well.

If you want to make it work a little better then cutting 4 slits in it can allow you to spread it out a bit.

Wire mesh can be cut an joined as needed to create your own "wok" as well.

EDIT

what is the 4 AWG wire for? Thats going to be almost the same size in thickness as the USB wireless adapter is wide alone... did you mean 40 AWG?

[dengyong]

I should have said how long can the antenna cable be ?

[SocketMan]

Thats what I've been using:
7$ for the antenna plus whatever for the cookies
Also 802.11b has a longer range so
switch your cards to "B",if the signal is not good.

[AllAgainstPaul]

I should have said how long can the antenna cable be ?

Thank you for the question, I totally left out number of turns vs gain and antenna length.

There are two different ways of interpreting that question. 1: you're asking how much bare wire the actual helix is going to use, or 2: you're asking how long the coax cable will be before it reaches the WiFi adapter.

1. 2401.605mm
2. Ideally, none. I will have the USB stick connected directly to the RPSMA connector and a 5m USB cable connecting the WiFi USB stick to my computer.

what is the 4 AWG wire for? Thats going to be almost the same size in thickness as the USB wireless adapter is wide alone... did you mean 40 AWG?

The Solid Core 4 AWG wire is going to be used for the Helix. Mathematically it was recommended that I use Solid Core 12 AWG, but as soon as I finish making this antenna I will attempt to make a 5 watt amplifier for it. This would significantly increase the maximum distance I can transmit data but at a slight cost of accuracy, the thicker 4 AWG wire will compensate for it.

[dengyong]

What's the maximum distance the computer can be from the antenna before you get signal degradation ?

[STEvil]

Depends on the quality of the usb cable and the usb adaptor.

[dengyong]

At best, what would you say ?

[little_scrapper]

Yes. Look for a very good low loss cable, some ptfe dielectric or something like that.

18dB ehh? Dont hold that thing next to your sac for very long.

[mpilchfamily]

Why are you pilling the updates into the first post? That will get very big very quickly. It may be better to add a post and continue the Work log. Thats typicly how you do these write ups. Unless you've already completed the project and are just now getting around to posting everything about it. Even then you want to break it up among several posts.