Expert: Cezar L. Palconet Date: 6/18/2008 Subject: Velocity factor RG6
Question Sir
I am building a phased vertical 20 meter antenna that I plan to feed in phase and be space at 1/2 wave length apart. Both antennas will have an approximately ohm rating of 50ohms both being a total of 100 ohms, and I plan to feed it with 50 RG8x coax. I have used the formula square root of (50X100) which equals 71.71 ohms which would make RG6 75 ohm cable a good choice to use as a match feed line however to correctly determine the length using (300m/sec/freq x VF x .25 for quarter wave) I need to determine the VF of the 75 ohm cable. I do not have access to an antenna analyzer if I did I would measure a length at 144 MHz to determine the specific VF. So here is my question: I have research many different manufactures of RG6 cable and have found that the VF ranges from .78 to .82 so by using the .82 and the constant for the above formula I hope to achieve satisfactory results however if the SWR and match is not with in the range that I hope to achieve I intend to cut back the 75 ohm cable to achieve the match that I am looking for. Am I on the right path, or is the .78 to .82 range nominal enough not to matter. The center frequency that I building these antennas at will be 14.250 MHz
David
Answer Hi David.
First of all, I would like you to know that you are on the right track however I would like to know the type and configuration of the phased vertical, that you arrived at 100 ohms, as a simple rule to phasing harnesses, you should get them to be odd multiples of your quaterwave, the manufacturers data for the VF is correct and they holds true for most serious applications.
keep in mind tough that a high SWR does not mean that your antenna is bad or not working, it only tells you that there is a reflection on the feed point, if you use a 50 ohm unit to measure a 75 ohm line you will definitely have a significant amount of reflection, that also applies for the connector type and impedance.
I do not understand why you would manually measure the VF at 144 Mhz. when you are running 14 Mhz.
