What I'm about to share will probably sound like Mrs. Noah telling Mr. Noah all about the big flood they had. Anyway, this is for newbies. Some where down the road you are going to see something that tells you that you need to cut a particular length of coax for something that you are about to build or install. An example would be co-phasing two antennas into one coax. This is usually done when you want two identical beam antennas tied to one coaxial feedline. They might say that you need to cut a 1/2 wave length of 75 ohm coax to feed each antenna and then use a TEE connector to feed the single 50 Ohm coax. Hmmmmm….. Nobody mentioned if its supposed to be a physical 1/2 wave or an Electrical 1/2 wave. OK, when talking about coax, you will have to assume that they mean an electrical 1/2 wave. Then you'll need to know the velocity factor of the coax that you're using. RG-58 usually has a velocity factor of about 0.66 but high quality, low loss coax can and usually does have a velocity factor of something like 0.88. What does that mean? It means that if you want to cut a 1/2 wave length of physical coax and end up with an Electrical 1/2 wave, then you have to multiply the velocity factor to the physical length and the answer will be the Electrical length. For example: If a Physical length is 10 feet long and the Velocity Factor is. 0.66, then the length you need to end up with is 6.6 feet. If the velocity factor is 0.88, then you'd need to end up with a length of 8.8 feet. I struggled with that for a long time because they never tell you its an Electrical wavelength that you need. An old timer finally filled me in on it when he said that if they don't tell you, then you must assume its an electrical wavelength that they want. Now, when it comes to cutting antennas, then a wavelength is a wavelength and you do not need to apply the velocity factor. However, If you are building an antenna out of wire, you just cut it to length and you're pretty much finished. If you are using a very large diameter tube, let's say 1 foot diameter, you will end up with an antenna that actually is a bit too long and will need to be trimmed back to a little shorter length. Why would anybody build an antenna out of a metal tube that was 1 foot in diameter? Well, its because big fat antennas are more broad banded. I hope that didn't irritate too many who already knew this and I hope it helps those who didn't.
