Hello. My name's Don, AC2RS, and this is a guest article. I am a math geek, computer nerd, and recovering civil servant. I have come up with a trio of webbased calculators which should help you design and build your own wire antennas  no need to buy expensive kits when the wire you need is available at the local hardware store for a fraction of the price!
I hereby make these calculators available for public use and redistribution under the GNU General Public License v3.0. I will post the download link here soon.
Thanks to Janis for allowing me to use her soapbox for a bit.
I hope you find my calculators useful. Enjoy! ;)
OK, the first calculator is pretty intuitive. Just put in the frequency you want the antenna to be resonant on, choose the type of antenna you're making, and whether you want to use insulated or bare wire. Then click [Calculate] and Bob's your uncle. I would suggest adding a little bit to the length you get, as real life is never as neat as mathematics. And shortening is much easier than lengthening. ^_^
 
This calculator is also simple enough. It allows you to test the resonant frequency of the antenna for lowest SWR and zero reactance, and adjust the length to the exact frequency you want, in just one step, rather than a lengthy process of cut and try. The only problem is, what to do if you don't know the current length of your wire antenna? Instead of taking it down just to measure it, you can use the Wire Antenna Length Calculator above to estimate it. Since we're (probably) talking about very short lengths relative to the full antenna, this should be close enough.
 
This final calculator can be useful if you are using a ¼ wave coaxial matching line to couple your 50Ω transmitter to a full wave loop. It gives you the appproximate length of, for instance, 75Ω RG11/U coax or whatever you want to use; again, make the initial cut a bit longer than you need, and shorten it after an initial test with an antenna analyzer. The tricky part is that it needs to know your coax's Velocity Factor. If you know it, or have the original spool the coax came on, you should be fine. If not, I found a couple of places (see below) to look up the VF based on the coax type. If the markings have rubbed off your coax, I even found a calculator to let you get the VF of a piece of coax using an antenna analyzer. Thanks, W6LSN! :)
 
To look up your coax's VF by cable type:And to calculate the VF: 
