Enter your can diameter in decimal inches. This is the critical dimension from which all others are taken. The can should ideally be around 90 mm, 3.5 inch O.D. (overall diameter), but any can between 3 and 4 inches will work. Outside of this range is not so good. You can leave the Channel at 6, mid-band, in the Calculator input.

Click on the
calculate button. 802.11 WiFi networking equipment operates in the frequency band 2.412 GHz to 2.462 GHz, with 11 channels. Ideally, with your can size, the TE11 cut-off frequency should be lower than 2.412 and the TM01 cut-off should be higher than 2.462. It would be good, also, if your can is equal to or longer than the 3/4 Guide Wavelength. Cans of non-optimum diameter work, but not as well. If the either of the Warning boxes fill, this applies to your can; but those of only a few millimetres outside the min & max still work well. Forget about giant cans of 6" diameter (150 mm) or bigger even 4" is slightly too large.

With a really big can, though, you could use a different design: the driven elements would be a dipole, or a quad, on the back face of the can. We'll look at that another time.


What You Need To Know

The 1/4 (Wave)Guide Wavelength is the dimension you seek: the standoff distance from the back of the can, for the driven element (the positive, or centre pole, active element). For the 98 mm can it is 44 mm. This dimension changes with can OD (not length).

The Calculator gives results in inches;
multiply by 25.4 to get an answer in mm.


The Driven Element

Driven element length never, ever, changes, and is always 31 mm. You will see many instances of online advice that say different; they're wrong. To optimise it for Channel 1, you can make it 0.5 mm longer; to optimise for Channel 11, then 0.5 mm shorter. Otherwise, leave well alone. For practical purposes, it is always left at centre band optimum length, which is around 31mm. In fact you could get it absolutely spot-on at mid-band by calculating the 1/4-wave length with a slightly more accurate formula for real aerials than the theoretical free-space method, which is actually wrong for this purpose; then trimming it to length to an accuracy of + or - 0.1 mm on a lathe; then attaching a microwave VSWR meter and checking the result. But er it's just a can, man.


Can Length

There is also an optimum length for the can at any diameter. Ideally it will be longer than 3/4 of the standing wave in the can (which is not the same as the SW in free space). For the 98 mm diameter 430 gm baby milk can (or the US equivalent, a Nalley beef stew can), the ideal min. length is 133 mm, which is 3/4 SW length (178 mm). At 125 mm, the baby milk can is a whisker short, but not enough to worry about. The length may be profitably extended by fitting an angled horn extension, and even 4" here will give an antenna gain improvement. You could make it as large as you can handle: but it must be angled out at 30 degrees. An extension to the baby milk can will ideally take the overall length to 2 standing wavelengths, which is 356 mm (14"). This should give an additional gain of 3 db: well worth having.
You will need to calculate the rear standoff distance for the driven element, if your can is not the Baby Milk type we advocate.


Can Diameter: inches.  Channel: (leave at 6)

Warning
Cutoff Frequency for TE11 (desired) mode GHz 2.400 GHz max
Cutoff Frequency for TM01 (undesired) mode GHz 2.475 GHz min
Operating Frequency for this channel GHz
Free-space wavelength Inches
1/4 Free-space Wavelength (element length) Inches
In-Guide Wavelength inches
# 1/4 Guide W/L (rear standoff) # inches
3/4 Guide W/L (optimum can length) inches
This Calculator script is used by kind permission of Seattle Wireless










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