Ops chatter can

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Operation: Chatter Can #

Intro to building a cantenna #

I have always wanted to build a super duper working cantenna, and I have actually done so several times, but I never really possessed the need to test them out for use. Eventually, these sculptures of geekery fell to the wayside and were either trashed, given away, or were burried amongst everything else. Most recently, I possessed the where-with-all to purchase some ductwork from the local hardware store in the hopes of building a cantenna with some serious gain for long range. I remember at first I had chosen a 6" diameter cylindrical duct for the project, but when this proved to be too large in diameter, I returned it for a 4". This size is most optimal for the 2.4GHZ wavelength, and is sure to out perform pringles cans and soup tins.

Assembling the "can" #

Depending on availability, a 4" piece of ducting may or may not come pre-assembled. An unassembled sheet of ducting is easier to cut precisely, but assemblage can be a frustrating experience, as the sheet wants to maintain it's original form. Regardless, working with ducting does pose considerable safety risks, as unprotected hands and razor sharp metal are a combination that leads to many accidents. So, with gloves, assembling is a matter of bringing the two ends together, and sliding the seam together. Once you have the seam started, the remaining length is considerably easier to assemble. After interlocking the seam, it is time to practice your duct tape skills, so run a strip of tape the length of the seam to finish assembly.

End caps can be purchased, and are highly reccommended over other capping methods. What is referred to as a "horn" is actually a 4" - 6" ducting adapter. Both can be purchased at the same store you purchased the ducting from.

Different designs, different results #

The two most popular designs for a cantenna are relatively the same, the only thing differentiating the two is whether a bell shaped horn is attached to the front or not. The design with the horn attached posseses a greater gain, for this reason we will incorporate a horn into the new cantenna. If for some reason I suspect a good amount of ducting will be removed from the cylindrical duct, I might even consider producing a double barreled cantenna.

Obviously, the addition or subtraction of the horn effects what calculations need to be performed to get optimum performance. This is something where attention needs to be paid, because using the wrong calculations could result in a hunk of useless scrapmetal.

Note: 4" = 101.6mm dB loss with 20ft of coax = 1.51-3.38 dB

Legend Dia = Diameter Unit = Unit of Measurement CO TE11 = Cut Off Freq. for TE11 Mode CO TM01 = Cut Off Freq. for TM01 Mode wlg = Waveguide Wavelength Dtr = Distance to Rear of Can LC = Overall Length of Can FS = FreeSpace before opening Len Mono = Length of Monopole Dia Mono = Diameter of Monopole

Coaxil and Connectors #

Ask anyone if they know the difference between cat6 cable and cat4, and they will respond a difference of 2 cats. Ask them the difference between LMR400 and RG56, and most will have a very puzzled look on their face. The reality is selection of coaxil type is very crucial to acquiring optimum performance for antennas and rf devices. The plethora of connectors and variations of those connectors really compound what is already a complex subject matter. So, without getting too far over our heads, let’s take a look at the varieties of coaxil cabling and the connectors that... connect them.

To thy coaxil be true #

With anything regarding current traveling through a wire there are two important things to remember. The longer the wire, the greater the voltage drop, and interference happens. To avoid interference, you need good shielding, and plenty of it. So, since copper is more conductive than steel, we can conclude copper shielding is more protective than steel shielding. We also can very easy deduce that two layers of shielding is better than one, and usually braided shielding is more effective than foil shielding.

Below is a table listing the different coaxil types encountered in our project, along with the loss per ten feet, and shielding varieties.

Connector fool me not #

There are many different connectors for rf devices, and unlike what many would assume, a male connector does not equate to a male connection. In fact, just about every connector, regardless of designation, can posseses either a male or female connection, this is referred to a reverse polarity. Some of the most commonly seen connectors are.

• SMA
• N-TYPE
• BNC
• F-TYPE
• RP-SMA
• TNC
• UHF

Troubled Shots #

Although somewhat similar in appearance, there is a big difference between an N-TYPE connector and a UHF connector. N-TYPE connectors are standardized and rated with an impedence of 75 OHMS, where the impedence of UHF connectors variates due to never achieving a standardized specification, not even for military application. It is due to this variance for which UHF connectors are being phased out of use. The UHF connector also provides no weather protection.

This was the error commited on our first cantenna build, which incorporated a UHF bulkhead instead of the reccommended N-Type bulkhead, which version 2.0 now uses. Although, it might seem quite feasible, almost practical, to skip the N-Type and go directly to an SMA bulkhead instead, it will not provide the same high performance as the N-Type connector. This is mostly due to the relatively thick diameter of wire that will be used as the collector in the wave guide antenna. The N-Type bulkhead provides the appropriate dimensioned sleeve to solder the collector thoroughly to the bulkhead. Standard SMA bulkheads do not possess a sleeve in any form, thus requires the solder to completely support the connector, which is ensured to eventually break off.

Another revision made to the bulkhead type used in the second version, we opted for a threaded bulkhead over the traditional square plate with four screw holes type. The reason for this choice was not only a matter of easier installation, but to prevent attaching the bulkhead from altering the tubular shape of the duct work. Having gone through the assembly process previously, it was observed how on the traditional plated bulkhead the proximity of the four holes to each other weakened the strength of the ducting.

Reference Links #

• KB9VBR Antennas