mobile coax advice needed.??

[1iwilly]

hello to all
after talking with a tech few days ago what he said seems to make sense how it started was that i run belden 9913f which has a velocity factor of .84 coax in my mobilee. after doing the math using the 468 formula i was suppose to cut the antenna coax to 14ft.5 but instead i have no idea where my mind was
and i cut to 12 1/2 ft he said that since i have my bird meter in line which is 4' long and a 6ft jumper from box to meter so that lenght = 6ft.4
+my 12ft 1/2 for antenna coax = 18ft.4 1/2 total lenght insted of the 14.5 ok so does that mean, that now my 12ft 1/2 wire needs to be cut to 8ft.1 to give me my 14.5 total antenna coax lenght.???
are you with me so far a little confusing i know but does this makes sense or should i leave it the way it is and balls to the wall.
i like my **Censored** to be right to avoid future problems when i make more upgrades to bigger mule from the 2x8. thanks

['Doc]

Tune your antenna correctly and it won't matter what the length of that feed line is. The only time the length of that feed line makes any difference is when the antenna isn't 50 ohms and resonant. Or, you are using more than one antenna (phasing/timing).
Velocity factor does play a part if you are doing any phasing/timing, or need a particular 'electrical' length of feed line for some reason. The 'size' of that velocity factor means very little in it's self. A VF of 84% isn't any better, or worse, than a VF of 66%.
- 'Doc

[1iwilly]

Tune your antenna correctly and it won't matter what the length of that feed line is. The only time the length of that feed line makes any difference is when the antenna isn't 50 ohms and resonant. Or, you are using more than one antenna (phasing/timing).
Velocity factor does play a part if you are doing any phasing/timing, or need a particular 'electrical' length of feed line for some reason. The 'size' of that velocity factor means very little in it's self. A VF of 84% isn't any better, or worse, than a VF of 66%.
- 'Doc

yes i'm running 2 antanna set up back grounded front hot

['Doc]

Then what's happening is that the input impedance of that 'hot' antenna is being lowered by that reflector element. That means that to get that input impedance back up to something close to 50 ohms, you are using the feed line as an impedance matching device, a tuner. In that sort of situation velocity factors definitely do come into play because of using that coax as the matching device.
The 'catch' to using coax in that way is that there are some very high voltages or currents involved anytime you start transforming impedances. After some point, those voltages/currents are large enough to destroy or change the characteristics of the coax being used. That's just not the best idea in the world. So, there are other ways of going about getting that input impedance closer to where you want it for a lower SWR without destroying anything. All that depends to some extent on the amount of power you plan to feed that thing. It doesn't take 1000's of watts to mess up coax, or even 100's of watts. And it's not a 'yes/no' sort of thingy, that messing up part, there are various 'levels' of being 'messed up'. So why not use another type of 'matching device' that doesn't care about what you feed it (or maybe not as much)? Seems reasonable to me. What you basically have is a two element 'beam'. What kinds of impedance matching devices have you ever seen on a two element beam??
Think about it...
- 'Doc

[1iwilly]

Then what's happening is that the input impedance of that 'hot' antenna is being lowered by that reflector element. That means that to get that input impedance back up to something close to 50 ohms, you are using the feed line as an impedance matching device, a tuner. In that sort of situation velocity factors definitely do come into play because of using that coax as the matching device.
The 'catch' to using coax in that way is that there are some very high voltages or currents involved anytime you start transforming impedances. After some point, those voltages/currents are large enough to destroy or change the characteristics of the coax being used. That's just not the best idea in the world. So, there are other ways of going about getting that input impedance closer to where you want it for a lower SWR without destroying anything. All that depends to some extent on the amount of power you plan to feed that thing. It doesn't take 1000's of watts to mess up coax, or even 100's of watts. And it's not a 'yes/no' sort of thingy, that messing up part, there are various 'levels' of being 'messed up'. So why not use another type of 'matching device' that doesn't care about what you feed it (or maybe not as much)? Seems reasonable to me. What you basically have is a two element 'beam'. What kinds of impedance matching devices have you ever seen on a two element beam??
Think about it...
- 'Doc

doc the way it is now running 2 r4 from 55 at 600watts i have 5 watts of reflect not bad or wrong but i just want to make sure it is 100% efficient last time i check with a friends mfj it was like 39 ohms
so what i came up with is using the 468 formula for 9913f v.84 at 27.205=14ft .5in so what i did was made antenna coax to 12ft even made a 2ft jumpper my bird meter from end to end is 5 in so my toatal lenght should be 14ft.5in is this correct i haven't tested it yet raining here in fl.

[1iwilly]

HERE IS AN UPDATE
while i had the headliner down i check the ground wire i had for the reflector i was using
a piece of copper wire cut from house electrical wire . so today while i had a nice piece from the coax i had trim, i decided to use that instead. well now my reflect went down from 5 watts to 3 watts
and when i switch from the 6ft jumper to the new 2ft jumper nothing changes at all reflect and dead key remains the same. so either my 2x8 is tune real good or my antennas are tune rite.

['Doc]

What was the 'X' value when you saw that 39 ohms?
- 'Doc