## Pringles Can Antenna

The PRINGLES CAN antenna is quite popular for 802.11 applications. The deviation of this design appears to be from the COFFEE CAN antenna that is popular for feeding homebrew parabolic dishes in the Ham Radio world. It got it’s name since the 1″ Coffee Can is about the correct

dimensions for this antenna. It was popularised by the book Building Community Networks.

According to the Satellite Experimenter’s Handbook by Martin Davidoff, K2UBC, the correct dimensions for this antenna are

Diameter | ^{3} / 4 * λ |

Length | 1.0 – 1.5 λ |

Feedpoint from end | ^{3} / 8 * λ |

Feedpoint Length | ^{1} / 4 * λ |

Of course other dimensions will work, and may work well. Unfortunately they are likely not to work as well. Despite appearances, this antenna is linearly polarized. At other dimensions though there could be a tendency for the antenna to change polarization, or decrease in gain as the antenna attempts to transmit left and right circular polarization at the same time.

**More Analysis from the NoCat List**

James Roe [jroe@jamesroe.com] analyzed the performance of the pringles antenna and posted the result on nocat@pez.oreillynet.com

I’m new to the list and I’m not sure this is the one that was interested in the coffee can antenna (I’m traveling and away from my home computer archives), but I’ve been dusting off microwave skills I walked away from a long time ago and I’ve come up with some info that may be helpful.

For the frequencies of interest near 2400 MHz, the minimum can diameter is something like 74 mm (say, 3 in) and the maximum is about 90 mm (say, 3.5 in) before higher order modes set up in the waveguide which can mess up the radiated pattern. Note that larger cans WILL radiate, but the results will not repeat too well from example

to example due to small differences in feed element, etc.

A ‘bare’ can of 90 mm diameter will have a maximum gain of about 6.5 dBi, but this can be increased considerably by forming a ‘horn’ that would flare up from the 90 mm diameter to a larger one. There are optimal flare angles and total lengths depending upon the starting and ending diameters, and I have calculated a couple that might be of interest.

Can Dia | Final Dia | Length | Gain | |

90 mm | 192 mm | 14.9 mm | 11 dBi | |

90 mm | 305 mm | 141 mm | 15 dBi |

This last is about a foot in diameter but just less than 6 in long. The horn can be made by making a paper template to cut out an arc of copper sheet that can be rolled up and soldered to make the horn. The template can be constructed by drawing two concentric arcs of length 218 deg with one radius at 74.3 mm and the larger at 252 mm.

This idea can be pushed on up in gain but the horns get pretty big such that a parabolic reflector probably would serve better – a 20 dBi

horn would be about 21 inches in diameter and 25 inches long.

Q:What is the optimum length for the can?

Can LENGTH is really not important as long as it isn’t too short (say less that a couple of wavelengths). Higher order modes will be launched at the

feed, but will die out quickly if the can diameter is too small for them to propagate. The 1/4, or 3/4 (or any odd multiple of 1/4) wavelength refers to the placement of the feed element from the back, conducting wall of the can (short circuits at these distances appear to be an open circuit

to the feed – hence not there).

On Horn Waveguides

The purpose of the horn is to match the characteristic impedance of the circular waveguide to that of free space (the air). The impedance of free space is a constant, but the

characteristic impedance of the circular waveguide depends upon the frequency and the diameter, so different diameter cans will need differently sized horns to achieve the same result (ie, gain). The final diameter of the horn will usually be the same for a given gain, but the angle of the cone and overall length will be different to get the impedance match right. I know of no simple geometrical procedure to calculate these parameters.

The effect of a mis-match of the impedances is to cause reflections (on both the transmit and receive sides) which means less power transmitted or

received – effectively a loss of gain. The difference between an ‘optimal’ design and one slapped together may not be too great, but the effort to construct is the same – so why not squeeze the design for all it’s worth?

And yes, the horn must be soldered to the end of the can for a good electrical connection.

Jim Roe

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