mobil coax help

[1iwilly]

hello to all
ok today i got my coax belden 9913f
now from some of the reading i've done here a lots of guys say they always use 18ft to the antenna
then others claim to use the velocity factor of the coax to determine the actual right lenght to the frequency of operation.???

question#2
right now i'm running rg8x mini vf=.66 on my antenna mount is there going to be any conflict if the rest of my jumpers are belden 9913f vf=.84?? this is on a temp basis until i get my new ball mounts.??
to run my 55r-4

 

[kswitchdr]

Use whatever length will get you to where you need to be. It doesn't have to be 18 feet . As far as the jumpers go it doesn't matter on the types of coax used.

 

[BOOTY MONSTER]

the half wave length velocity factor thing is only to make cheap meters (like mine) give a more accurate vswr reading . it does not affect the true vswr of the antenna . just tune for the lowest vswr with what you got.........unless youre a geek like me . hehehe

 

[freecell]

"now from some of the reading i've done here a lots of guys say they always use 18ft to the antenna........"

that physical length is capable of providing the proper input impedance at the transmitter (line-input matching) when the input impedance at the feedpoint of the antenna is in the neighborhood of roughly 36 ohms. the majority of mobile antenna installations fall into this category. it's all explained here:

Coax Theory

this is the reason this length has the effect that it does under the approximate conditions above. saavy operators who understand the value of this realize that (in solid state gear particularly) what the transmitter is looking into is far more important than what the other end sees at the load. all power delivered to the load is absorbed by it, regardless of the vswr.

http://www.firecommunications.com/mismatch.txt

"then others claim to use the velocity factor of the coax to determine the actual right lenght to the frequency of operation.???"

the velocity factor is a ratio of free-space travel to conductor travel. we are not "tuning" the line so much as we are utilizing the impedance of the line at a particular length to "match" two slightly dissimiliar impedances and in the procedure known as line input matching we are basically concerned with the match between the transmitter output and the feedline input, with little concern whatsoever with the match at the feedpoint. the two commonly used and referenced lines are the electrical half-wave line and the quarter wave line. the first one has the ability to repeat impedances from one end of the line to the other while the latter has the ability to "invert" impedances.

http://www.firecommunications.com/light.txt

"right now i'm running rg8x mini vf=.66........"

unless there's a polyethylene jacket surrounding the center conductor the velocity of your rg8x (usually foam) is .78, not .66.

 

[W5LZ]

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