Vertical antennas "with gain" are usually pretty tough to build utilizing tools found around the average home. Hard to construct matching coils are normally needed as well as hardware and tubing which can be difficult to find.
The Coaxial Collinear is rarely mentioned in radio circles, odd....since many commercial antennas are based on it's design. Appreciable gain can be had from them if one wishes to spend a little extra time consructing additional elements. It's design is super simple and since it needs no radials, it takes up little horizontal space; its weight is negligible. Best of all, it is made from coaxial cable.
The only tools needed are a soldering iron with solder, a sharp cutting tool and an optional plumber's tubing cutter. A little tape or shrink tube to seal it up and an old antenna tube or piece of PVC pipe to mount it in.
First, determine what the working frequency will be. Though this antenna design will work on any frequency, versions for frequencies below about 90 MHz will tend to be quite long.
Divide 2757 by the working frequency to obtain 1/4 wavelength through air (1/4 WLA). Many people use 492 and get 1/2 wavelength in feet, then convert usually giving them a dimension much too long. I use 2757 with excellent results. Next, select the type of coax to be used. If Teflon insulated type is used, multiply 1/4 WLA by 0.88; the resulting figure will be 1/4 Wavelength of Teflon coax. Preferably you will decide on polyethylene coax (the cheap stuff); the 0.66 Velocity Factor will result in much shorter and easier to work with antennas. For polyethylene coax, multiply 1/4 WLA by 0.66 to obtain the length of a 1/4 wave section of polyethylene coax.
Teflon coax, also referred to as "low loss" or "foam" type, is usually marked as such. If not, it can be identified by its very soft white insulation between the center conductor and braiding.
**Note: Either RG-58/U or RG-8/U will work. RG-58/U is ok for receive only or for transmitting under 20 watts.**
Step#1 The top section is made from #12 or #10 copper wire. The bottom "balun stub" is made from a peice of #14 wire or equivalent.
Step#2A Cut pieces utilizing the above formulas, plus 2-1/4".
Step#2B Cut 1-1/8" of black outer insulation from each end.
Step#2C Tin copper braiding with solder.
Step#2D Using a sharp cutting tool or tubing cutter, cut 5/8" of tinned braiding off each section end.
Step#2E Cut 1/2" of insulation from center conductor, leaving 1/8" between center conductor and braiding.
After soldering all sections and balun stub together, test SWR with an antenna analyzer or transmitter and SWR meter. If formulas were followed, all should be OK. If not, trim or add to the 1/2 wave section(s). The more 1/2 wave sections you use, the more gain you get. About 6dB gain with 8 halfwave sections and 9dB with 16 halfwave sections.
When you are happy with the SWR, cover each joint with tape or shrink tubing for insulation and strength. Insert entire assembly into a piece of PVC tubing or a hollow fiberglass antenna tube, such as used with marine antennas. PVC pipe is sometimes made with materials containing metal. This type of pipe will now work with this antenna. I test PVC by putting it in the microwave oven. If it gets warm, I don't use it for antennas.
For down tilt, but antenna sections 2% shorter than the calculated length and adjust VSWR with the balun.
Mount antenna as high as possible away from any power carrying wires.
Special thanks to Leonard/WB3AYW, without whom this article would not be possible.