Is there an ideal length for coaxial cable in a CB installation?
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JK Newbie
Joined: May 2011
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From: Royal Oaks, California
Is there an ideal length for coaxial cable in a CB installation?
I have lost count of the number well meaning, but misinformed individuals that have offered advice pertaining to the correct length of coaxial cable in a CB radio installation, and promising dire consequences if the cable were other than 18 feet in length.
How did the concept of a “correct length” come about? Is there a correct length, in fact?
I will answer these questions as best I can, using professionally derived information with link references as I deem needed. I would also encourage discussion as there will certainly be some doubts about what I write since it contradicts the current internet forum wisdom(s). We will not discuss antenna theory as that is above the pay grade of the intended audience.
The first answer, as to how the magic 18 foot length came about is just speculation that the obvious reason is that 18 feet is ½ wavelength of 27 MHZ. 36 feet is, indeed, the wavelength in a vacuum. *** The average radio enthusiast might be aware of this fact and because of factors involved in troubleshooting a high VSWR problem in his installation and discovering that trimming or adding length to the cable lowered (or raised) his observed VSWR, decided that the cable length was significant.*
Now, to aid understanding in what comes next, I must illuminate parts of that last paragraph.
36 feet (let's avoid metric for now) is the wavelength of 27 MHZ in vacuum. The speed of radio propagation in vacuum is the speed of light, approximately. This is called Vp or velocity factor **** and has an assigned value of 1. Radio frequency propagates slower through a coaxial cable with possible Vp between .6 to .8, or between 60 to 80 percent of the speed of light through vacuum. The signal slows down! Because it slows down, the wavelength inside the cable decreases! This makes the electrical length of the coaxial cable shorter! **, ******
That may take some thought.
Coaxial cable is commonly available with Vp between .8 and .6, with most common Vp of .66. Simple math shows that instead of ½ wavelength of 18 feet, the electrical equivalent of a coax with a Vp of .6 will be 10.94 feet! Put another way, a coaxial cable cut to ½ wavelength at 27 MHZ will be 10.94 feet in length. ****** A coaxial cable with a Vp of .9 will be 16.4 feet long. Obviously the 18 foot length legend of internet lore is false and there is no correct length other than what it takes to connect to the antenna.
And what about that trimming of the coax that changes the VSWR? There are layers to this also. A blunt statement that is sure to rankle is... the only place you can reliably, accurately, measure VSWR is at the antenna base. ***** If you measure at the back of the radio, and there is reflected power from the antenna, the length of the coax may affect the reading and can, in fact, completely hide the presence of a standing wave. HAMS should be familiar with this since this property of transmission lines allows one to match the radio to an antenna that has less than ideal properties. No antenna theory beyond this point.
This transforming function is why all professional radio techs were dispatched with a minimum of three random lengths of “patch cables” to use between the inline power meter and the radio during antenna installations and subsequent adjustments while monitoring reflected power readings.
It is important to know that adjusting the length of the cable to reduce/eliminate the VSWR (reflected power) reading does not change the system VSWR. The antenna is still reflecting power, and the power is being dissipated as heat through resistive losses during the multiple passes through the cable. You just cant see it at the back of the radio.
This will be posted at various other forums.
Phil
FREQUENCY & WAVELENGTH CALCULATOR ***
Coax Physical/Electrical Length **
Ham Radio Coaxial Cable Power Handling and Loss Specifications - Velocity Factors of Feedlines *****
Velocity factor - Wikipedia, the free encyclopedia ****
http://www.arrl.org/files/file/Techn...f/q1106037.pdf *
Coax Velocity Factor | Coaxial Cable Velocity Factor | Tutorial ******
How did the concept of a “correct length” come about? Is there a correct length, in fact?
I will answer these questions as best I can, using professionally derived information with link references as I deem needed. I would also encourage discussion as there will certainly be some doubts about what I write since it contradicts the current internet forum wisdom(s). We will not discuss antenna theory as that is above the pay grade of the intended audience.
The first answer, as to how the magic 18 foot length came about is just speculation that the obvious reason is that 18 feet is ½ wavelength of 27 MHZ. 36 feet is, indeed, the wavelength in a vacuum. *** The average radio enthusiast might be aware of this fact and because of factors involved in troubleshooting a high VSWR problem in his installation and discovering that trimming or adding length to the cable lowered (or raised) his observed VSWR, decided that the cable length was significant.*
Now, to aid understanding in what comes next, I must illuminate parts of that last paragraph.
36 feet (let's avoid metric for now) is the wavelength of 27 MHZ in vacuum. The speed of radio propagation in vacuum is the speed of light, approximately. This is called Vp or velocity factor **** and has an assigned value of 1. Radio frequency propagates slower through a coaxial cable with possible Vp between .6 to .8, or between 60 to 80 percent of the speed of light through vacuum. The signal slows down! Because it slows down, the wavelength inside the cable decreases! This makes the electrical length of the coaxial cable shorter! **, ******
That may take some thought.
Coaxial cable is commonly available with Vp between .8 and .6, with most common Vp of .66. Simple math shows that instead of ½ wavelength of 18 feet, the electrical equivalent of a coax with a Vp of .6 will be 10.94 feet! Put another way, a coaxial cable cut to ½ wavelength at 27 MHZ will be 10.94 feet in length. ****** A coaxial cable with a Vp of .9 will be 16.4 feet long. Obviously the 18 foot length legend of internet lore is false and there is no correct length other than what it takes to connect to the antenna.
And what about that trimming of the coax that changes the VSWR? There are layers to this also. A blunt statement that is sure to rankle is... the only place you can reliably, accurately, measure VSWR is at the antenna base. ***** If you measure at the back of the radio, and there is reflected power from the antenna, the length of the coax may affect the reading and can, in fact, completely hide the presence of a standing wave. HAMS should be familiar with this since this property of transmission lines allows one to match the radio to an antenna that has less than ideal properties. No antenna theory beyond this point.
This transforming function is why all professional radio techs were dispatched with a minimum of three random lengths of “patch cables” to use between the inline power meter and the radio during antenna installations and subsequent adjustments while monitoring reflected power readings.
It is important to know that adjusting the length of the cable to reduce/eliminate the VSWR (reflected power) reading does not change the system VSWR. The antenna is still reflecting power, and the power is being dissipated as heat through resistive losses during the multiple passes through the cable. You just cant see it at the back of the radio.
This will be posted at various other forums.
Phil
FREQUENCY & WAVELENGTH CALCULATOR ***
Coax Physical/Electrical Length **
Ham Radio Coaxial Cable Power Handling and Loss Specifications - Velocity Factors of Feedlines *****
Velocity factor - Wikipedia, the free encyclopedia ****
http://www.arrl.org/files/file/Techn...f/q1106037.pdf *
Coax Velocity Factor | Coaxial Cable Velocity Factor | Tutorial ******
