'J.D.',
It sounds like you aren't making allowences for the velocity factor of the coax being used. Becaus of how coax is made it's length isn't figured the same way as the length of an antenna. An 'electrical' length of coax is typically shorter than the same length of an antenna (1/4, 1/2, 5/8 wave etc.). The number typically used to find a 1/3 wave length is 246. For coax, multiply that number by the coax's velocity factor, giving you the correct 'number' to use in figuring coax length. If the coax's VF is .66, then 246 times .66 is 162. Divide that by the design frequency (eg:ch-19 = 27.185) and you get the length of a 1/4 wave of coax, 162/27.185 = 5.9 feet, not the typically qouted 9 feet. Substitute the right 'number' for '246' for the other lengths.
"Hey, that ain't gonna be long enough to reach.", he says, and he's right. So, you wanna extend that short length to a usable length by adding enough coax. BUT, that additional coax has to be 'invisible' to your signal, like it isn't really there. There just happens to be a 'magic' length of coax that has those qualities! [Sound the 'fan-fare', flash the lights, and all that other B.S.!] The 'magic' length is an electrical 1/2 wave length of coax at the frequencys you are using. [Substitute '492' for that '246' to find an electrical 1/2 wave length (or just multiply the 1/4W coax by 2).] One section of this 'magic' length goes in each antenna's lead before the place where you make the connection going to the radio.
The 'Catch'...
No coax cable is perfect. It's actual impedance can change a bit (not the 'typical' impedance though) from one end of a spool to the other. A 10% change in impedance isn't all that unusual. So, having some way of measureing the cable's 'real' impedance isn't a bad (or cheap) idea. One particular 'chunk' of coax may not have the same impedance as the next 'chunk' of the same coax from the same spool. In general, the change isn't all that much and usually doesn't make much difference except to the person reading the meter who wants everything to be 'perfect'. "Close" does count though. The biggest difference in typical performance is with the expectations of the operator, not the signal.
Having said all that, you have to decide how much you should believe any of it. I'm not trying to sell you anything, I don't get a commision from any sales, and I'm too lazy to tell you something that isn't true to the best of my knowledge ('sides, I can make up better stories than that!).
Have fun...
- 'Doc
It sounds like you aren't making allowences for the velocity factor of the coax being used. Becaus of how coax is made it's length isn't figured the same way as the length of an antenna. An 'electrical' length of coax is typically shorter than the same length of an antenna (1/4, 1/2, 5/8 wave etc.). The number typically used to find a 1/3 wave length is 246. For coax, multiply that number by the coax's velocity factor, giving you the correct 'number' to use in figuring coax length. If the coax's VF is .66, then 246 times .66 is 162. Divide that by the design frequency (eg:ch-19 = 27.185) and you get the length of a 1/4 wave of coax, 162/27.185 = 5.9 feet, not the typically qouted 9 feet. Substitute the right 'number' for '246' for the other lengths.
"Hey, that ain't gonna be long enough to reach.", he says, and he's right. So, you wanna extend that short length to a usable length by adding enough coax. BUT, that additional coax has to be 'invisible' to your signal, like it isn't really there. There just happens to be a 'magic' length of coax that has those qualities! [Sound the 'fan-fare', flash the lights, and all that other B.S.!] The 'magic' length is an electrical 1/2 wave length of coax at the frequencys you are using. [Substitute '492' for that '246' to find an electrical 1/2 wave length (or just multiply the 1/4W coax by 2).] One section of this 'magic' length goes in each antenna's lead before the place where you make the connection going to the radio.
The 'Catch'...
No coax cable is perfect. It's actual impedance can change a bit (not the 'typical' impedance though) from one end of a spool to the other. A 10% change in impedance isn't all that unusual. So, having some way of measureing the cable's 'real' impedance isn't a bad (or cheap) idea. One particular 'chunk' of coax may not have the same impedance as the next 'chunk' of the same coax from the same spool. In general, the change isn't all that much and usually doesn't make much difference except to the person reading the meter who wants everything to be 'perfect'. "Close" does count though. The biggest difference in typical performance is with the expectations of the operator, not the signal.
Having said all that, you have to decide how much you should believe any of it. I'm not trying to sell you anything, I don't get a commision from any sales, and I'm too lazy to tell you something that isn't true to the best of my knowledge ('sides, I can make up better stories than that!).
Have fun...
- 'Doc
