Expert: Cezar L. Palconet Date: 6/24/2008 Subject: Velocity factor RG6
Question 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.
good luck and best regards.
---------- FOLLOW-UP ----------
QUESTION: The VHF was a typo and should have read HF. I arrived at the 100 ohms figure by assuming that two quarter wave antenna built to the specific frequency which operated in the 50 ohm world would be 100 ohms combined. I have already moved forward in this project a have achieved a match for the phasing harness by, applying a zero ohms accost the cable and cutting to the best SWR. Using the length found I have assembled two quarter wave verticals placing them at 1/2 wave length apart and fed them with 100 feet of RG8X coax to a T connector to the RG6 75 ohm cable which is cut to 11'-4'' then to 10 more feet of RG8X cable. Both antenna are mounted at 4" above the ground and have a radial plate connected to the shied with 30 radials on each antenna. The SWR that I read is 1:23 to 1:11 at 100 watts accost the 20 meter band from 14.000 to 14.500.
73
David
AG4RO
Answer Hi there David,
If I got it straight, you are making 2 pcs. vertical ¼ wave antenna to operate at 14.250 Mhz. frequency band, that you plan to feed in phase, which is horizontally spaced at ½ wave length apart.
Parallel feed antennas have a reduce impedance as seen by the generator, Feeding them in phase using RG6 coaxial cables as matching harnesses, will reduce the bandwidth response of the circuit, making it respond to a very narrow area of the operating frequency. This is largely due to the radiator resonant frequency and the harness, each acting like a filter in series, well their basic circuit closely resembles a lumped filter circuit where air represent the capacitance.
If it is about gain and directivity, the same could be attained using ¼ wave and ½ wave feed of the same RG8 50 ohms coaxial cable from the tee for each one of the individual vertical ¼ wave radiators which is ½ wave length apart.
Manipulation of voltages and current on a radiator will only change the directionality or shape of the lobe, but not the power contained in the envelope it self, impedance transition harness tend to reduce the amount of power delivered at the radiator feed. In most application where this is used, it is implemented very close to the feed point of the radiator.
Keep in mind that the SWR only tells about the reflected voltage towards the generator and not the impedance at the feed point from the generator or transmitter, and voltage null’s occur along the feed line, and it certainly do not indicate any relationship to the radiation characteristics of the antenna or radiator.
A vertically stacked phased array has a much better radiation lobe shape than a horizontally spaced phased array, and is more compact and performance wise, more predictable. The only limiting factors are the physical size, mechanical complexity and material strength specially in the lower frequency band.
If you are working similar to a beam forming array you are on the right track, but I think you will need more land to accommodate antennas especially in the 20 meter band.
73’s Best regards and good luck
DU1AZ, portable HZ7
