rainman wrote:
Again just my opinion. I don't use rg58 I use rg8x. It is Belden brand and I buy it in 500' rolls. I go through 1-2 rolls per year. It is 97 % percent shielded but I use this brand for price, not because of the heavy shielding. I measure the swr with an MFJ analyzer but don't remember the model #. 259 or259b, not the one that goes to 440 mhz. Anyway, when I check the match, I am happy with a match of 1.5 or lower and an impedance of 40-60 ohms. A friend of mine is a factory trained cb technician servicing Ranger and other brands. He likes multiples of 12' for mobile installations. He doesn't use the same coax I use. It probably has to do with the velocity factors of the differing cables but I don't like the match I get with my coax at 12' multiples. My cables match well with good impedance at 9 and 18'. For my use those lengths work well so if it aint broke don't fix it. Using the 12' lengths, I can get a respectable match if I trim a lot of the whip, or wire coil at the top of a fiberglass antenna like a firestik etc. I don't see the point in making an antenna shorter for no reason. 73 KZ7L
I talked with a radio technician that I work with about this because after reading your post I had a lot of questions. From the extreme dumbed down version that I could wrap my brain around, we are both right. In a perfect world, cable length doesn't matter. He went into detail about transmitted power traveling through the center conductor and getting reflected into the outer conductor which should carry no power (after typing that I realize he was talking about SWR!). He told me that when that happens the coax basically becomes part of the antenna and that by coiling the excess you are creating a RF choke point that helps to mitigate the problem. He also told me that the choke point should be as close to the antenna as possible because the area between the antenna and the choke point is still acting as part of the antenna and is in essence wasted power. He told me that cable length doesn't really matter so long as you have between 6 and 8 coils. He also told me that the problem doesn't always present itself which is why some installations (Phil's for instance) can get away without coils and still not have the high SWR.
With that, I want to say thank you. I understand antenna and coax theroy a little better now than I did before and never would have asked the questions if it hadn't been for this discussion.
Edit: Just found something that explains it better than I can. Have a look if you like:
http://www.signalengineering.com/ultima ... asics.htmlI also found that I still really don't understand the concepts and get lost in the technicals of it all.