mobile coax help

[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 shit to be right to avoid future problems when i make more upgrades to bigger mule from the 2x8. thanks

 

[mackmobile43]

As long as you've assembled your connectors correctly there should not be any problems with the coax the way it is, velocity factors go flying out the window when you add connectors, especially if you have any exposed(unshielded) center lead inside the so-239s'.

 

[W5LZ]

If you correctly tune your antenna for a 50 ohm impedance match and make it resonant, the length of feed line will make no difference at all. It's just that simple.
The only time you need worry about any particular length of feed line is when you have some need of phasing/timing adjustments, or if you are using that feed line as an impedance adjusting device (which is a really dumb way of going about it, it's always destructive of that feed line). That's also the only time the velocity factor of the feed line ever has anything to do with things. The 'size' of that velocity factor has no significance at all. A VF of 84% is no better, or worse, than a VF of 66%.
- 'Doc

 

[1iwilly]

If you correctly tune your antenna for a 50 ohm impedance match and make it resonant, the length of feed line will make no difference at all. It's just that simple.
The only time you need worry about any particular length of feed line is when you have some need of phasing/timing adjustments, or if you are using that feed line as an impedance adjusting device (which is a really dumb way of going about it, it's always destructive of that feed line). That's also the only time the velocity factor of the feed line ever has anything to do with things. The 'size' of that velocity factor has no significance at all. A VF of 84% is no better, or worse, than a VF of 66%.
- 'Doc

ok i'm running 2 r4 from 55 rear grounded front hot a 2x8 box that on 14.volts is doing 600watts 5 watts of reflect the reason i want to fix it right is
accourding to the 468 math i need 14ft.5in coaxt to antenna but with 12ft now +6ft jumper +the 5in from the bird meter that would be a total of=18ft.5
right.???? so what i did was 12ft from antenna made a 2ft jumper +5in from bird meter=14ft.5in i haven't tested it yet raining here in fl.with the old set up
the best reflect i can do tunning 1 antenna is 10 watts won't go any lower than that.


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.

 

[packrat]

The 468 formula has nothing to do with your coax length. ( and is incorrect for most antennas, anyway)

 

[1iwilly]

The 468 formula has nothing to do with your coax length. ( and is incorrect for most antennas, anyway)

so what is the correct formula.

 

[Robb]

Using coax with a high velocity factor means better efficiency overall as well. Less resistance, capacitance, inductance means less loss. Not to mention, that cable has better shielding and power handling capacity as well.

Not that a short run like that would sap much power from the radio anyway. I would do as Doc said; just run the length you need and go with it.

 

[BOOTY MONSTER]

"ok i'm running 2 r4 from 55 rear grounded front hot"

so youre running a bounce back or something like that . im not up on those setups but their popular among the comp guys and guys that like that stuff . i think there is a optimal spacing for those setups and optimum coax length . im not into that so i dont know . try contacting stroker amps and ask for 577 . hes a good guy and actually runs a pair of fatboy antennas on his truck .

STROKER AMPLIFIERS - Contact Us

BTW , to figgure the electrical half wavelength of coax divide 492 by the freguency or center of the band you want to use (27.205 on the cb band) and then multiply that by the velocity factor of your coax . 9913F7 has VF or ,85 so a EHw/v will be 15 ft 4 1/2 inches on channel 20 on the cb band .

 

[packrat]

All that velocity factor is absolutly unimportant for ANYTHING we do. Period. Didnt exist til rather recently, and is a buzzword to try and look important and sell expensive coax. I challenge anyone to show me where, in real world use, it makes a damn bit of difference. That is like figuring the velocity factor of light before buying light bulbs. You can, but why?
Coax length makes not one whit of difference in most applications, except in really long runs, or very high frequency, and that is only because of losses.:bdh:
Forget all that crap, use good quality 8x or 8u and have a great day.

 

[BOOTY MONSTER]

so what is the correct formula.

sorry if my answer annoyed you packrat , but i was just answering the mans question .

i agree it doesnt change or affect anything on our cb antennas . i do plan to get a new section of coax 2 electrical wavelengths long for my new antenna install and cut my current 50 ft section or 8x into 3 electrical half wavelengths for jumpers . but only because it can help meters read the vswr more accurately and itll give me a warm fuzzy feeling . and yep i know it wont make or break and contacts on the air either .

 

[packrat]

No,
Sorry if that came out wrong.

You didn't annoy me. I am always annoyed about something.

I just hate seeing the same question coming up over and over and over.

I hate repetition!

Hell, I hate everything!

 

[W5LZ]

I agree to a point, velocity factor isn't something to be obsessed with. But it's also something very useful to know about, or at least be aware of. I can think of several instances where you would only be guessing without knowing about velocity factors. If you do that guessing long enough, you can certainly find the 'right' guess. Knowing what that velocity factor is, and how it works can certainly cut down on the number of those guesses. It's all about timing.
Don't want to be concerned with it, or know about it at all? Fine, then ignore it. But don't think that it's worthless, cuz you'd be wrong. Suit yourself.
- 'Doc

 

[jazzsinger]

If you correctly tune your antenna for a 50 ohm impedance match and make it resonant, the length of feed line will make no difference at all. It's just that simple.

- 'Doc

Certainly is that simple if you only transmit on one channel,seeing as most cb'ers tend to transmit anywhere between 26 and 28 mhz these days,sort of complicates matters slightly,as resonance is only found at one frequency,which means there will be standing waves on the cable,and there will be x amount of impedance transformation going on of growing amounts the further you move either side of that resonant frequency and depending on what fraction of a wavelength/multiple thereof you cut that random cable too.

Incidentally packrat,rg8u is no longer a US military specification and is mainly used to flog poor quality coax to the unsuspecting buyer who hasn't kept up with modern military specifications,it was replaced years ago by rg213/u which used to be classified under Mil-c-17e,then Mil-c-17f and now M17/74.

As for rg8x,you will very rarely find that in the same sentence as good quality coax.

You'll notice in the link i've provided neither are listed as mil spec cables,I wonder why? If it ain't good enough for the US military,it certainly ain't good enough for a serious cb'er/amateur.

M17 / MIL-C-17 Coaxial Cable from Allied Wire & Cable - Manufacturer of Custom Cables and Coaxial Cable Assemblies


If it don't have those markings on it you can be pretty certain it is substandard imported junk being sold as premium cable,usually at premium prices.It is the single most problematic reason why people have so many difficulties with coax,especially when running high power.Next biggest problem area is junk pl259 connectors,insist on amphenol 83-1SP,you can find a list of certified dealers on amphenols website.

You guys have some of the best coaxial cables and uhf plugs on the market,at very reasonable prices,right on your doorstep,why accept cheap junk?

 

[jazzsinger]

Using coax with a high velocity factor means better efficiency overall as well. Less resistance, capacitance, inductance means less loss. Not to mention, that cable has better shielding and power handling capacity as well.

Not that a short run like that would sap much power from the radio anyway. I would do as Doc said; just run the length you need and go with it.

velocity factor has nothing to do with efficiency,velocity factor is a result of the dielectric material the copper conductor is wrapped in.

capcitance and inductance have little if anything to do with losses,capacitance is due to the spacing of the conductors and the type of dielectric used,inductance is down to the diametres of the inner and outer conductors.

resistance due to quality/type of copper/plating and quantity of conductor surface area do.most loss is caused through resistance,

next most loss is caused by dielectric losses given of as heat,dielectric losses are least prevalent at lower frequency,rising with increased frequency to a point where they can equal or overtake resistive losses at ultra high frequencies.

least loss is caused by radiated loss,unless cable has piss poor shielding.

Coax with high velocity factors tend to be foam dielectric,which when new may well be less lossy,but foam readily absorbs moisture and as it ages losses increase,which means efficiency decreases,for outdoor use its much better to use solid polyethylene dielectric,as it doesn't absorb water as readily and stays more consistant with respect to dielectric losses.

Shielding is totally independent of velocity factor,sheilding is there to prevent rf leakage,provide a return path and to prevent rf pick up.

power handling again is nothing directly to do with velocity factor,its to do with the quantity/quality of the conductors mainly the centre conductor and the type of dielectric used,as heating in the centre conductor from resistance and heating in the dielectric are what determines power handling,the velocity factor is only a result of the type of dielectric used,which is only part of the factoring of power handling capabilities.coax used in climates with high temperatures will handle less power as the same coax used in low temperature climates,as it can't dissipate heat as quickly.

to say a cable with high velocity factor is better shielded,handles more power,is more efficient and less lossy,is far from the whole story.

 

[Captain Kilowatt]

Using coax with a high velocity factor means better efficiency overall as well. Less resistance, capacitance, inductance means less loss. Not to mention, that cable has better shielding and power handling capacity as well.

Not that a short run like that would sap much power from the radio anyway. I would do as Doc said; just run the length you need and go with it.

velocity factor has nothing to do with efficiency,velocity factor is a result of the dielectric material the copper conductor is wrapped in.




Shielding is totally independent of velocity factor,sheilding is there to prevent rf leakage,provide a return path and to prevent rf pick up.

power handling again is nothing directly to do with velocity factor,its to do with the quantity/quality of the conductors mainly the centre conductor and the type of dielectric used,as heating in the centre conductor from resistance and heating in the dielectric are what determines power handling,the velocity factor is only a result of the type of dielectric used,which is only part of the factoring of power handling capabilities.coax used in climates with high temperatures will handle less power as the same coax used in low temperature climates,as it can't dissipate heat as quickly.

to say a cable with high velocity factor is better shielded,handles more power,is more efficient and less lossy,is far from the whole story.

Have to agree with jazzsinger here Rob. RG-214 has a Vf of 0.66 while RG-8X has a Vf of 0.84. Which is the better cable? Which has lower loss? From looking at a coaxial cable spec chart I can see many examples of where your way of thinking about coaxial Vf is wrong.