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doughboy
01-31-2012, 10:13 AM
There is a misconception among CB operators that changing the length of your coax can "tune" your antenna system. This is absolutely untrue! This short discussion will detail why.
The coax cable is a means for transferring your RF signal to the radiating portion of the radio system. The cable, in theory, is meant to be a contained, non-radiating link. Because it does not radiate and serves only to transfer RF between two components of the transmitter system, it's performance in terms of efficiency is affected by length, but only in terms of overall resistance. In other words, using a long run of coax will reduce the total amount of signal at the antenna, but only because of loss due to resistance and NOT because of standing waves.
Ideally, you want to check the SWR of your antenna at the antenna feedpoint. In a perfect world, this is the best way. However, we all know that this is ludicrous to expect in a standard base antenna installation. Unless a remote SWR meter head is incorporated, we usually use the standard SWR meter located at the radio. The drawback is that resistance and slight impedance mismatch of the coax affects the overall SWR reading.
Because radio waves are tuned wavelengths of energy, we have to take into account the coax cable length. A typical 11-meter signal has a basic wavelength of 36 feet/wave. "Tuning" the coax for the exact full wavelength tends to throw off the SWR meter by not allowing any standing waves to return to the meter. Excess RF on the coax has been given an ideal medium by which to "hide" electrically from your SWR meter. That is not to say that the excess RF is not returning to the radio, you just can't see it on your meter.
What we want to do is create an environment where any excess RF (standing waves) are rendered as visible as possible to the meter. This is effectively done by using multiples of the 1/2-wavelength of the radiated signal. One half wave for the 11-meter band is 18 feet. However, this is not the length that you will cut your coax. There is another factor that affects the length. This is Velocity Factor. The velocity factor is basically a term for how fast the signal moves through the coax. This factor affects the overall electrical performance of the coax and thus needs to be accounted for when determining the true half wave length
Here are the velocity factors of the various Belden coaxial cables:
RG-59 .66

RG-59/U (foam) .79

RG-58 .66

RG-58/U (foam) .79

RG-8A/U .66

RG-8/U (foam) .80 9913 .84

RG-213/U .66
Here is how to figure out your true 1/2-wave:
492 x (Velocity Factor) / Frequency (MHz)
For example, I want to figure out the true half wave coax length for RG-59/U (foam) on my home channel (ch. 33 - 27.335):
492 x .79 / 27.335 = 14.219 feet
Now add 14.219 to itself to determine your 1/2 wave multiples. Remember to use every other number. See the example below:
14.219 feet 1/2-wave multiple, 28.438 feet 1-wave multiple, 42.657 feet 1/2-wave multiple, 56.876 feet 1-wave multiple, 71.095 feet 1/2-wave multiple, 85.314 feet 1-wave multiple
and so on . . . .
Use only the lengths that fall on the 1/2-wave multiples and you will be all set.
Now in order to get the true SWR of the system, you have to throw away that 3-foot jumper cable for now. The SWR meter has to fall on a 1/2-wave point on the coax run. Using the example above, you need a 14.219 foot jumper from the radio to the SWR meter, and a 1/2-wave multiple length from the SWR meter to the antenna. If my antenna is 65 feet away from my radio, I need a 14.219 foot jumper from the radio to the SWR meter, and a 71.095 foot length between the SWR meter and the antenna.
In laymans terms, the coax length fools your SWR meter into thinking that your SWR is different to what it actually is !
The ONLY way to know what your SWR actually is, is to use a correct length of coax for the operating frequency

Mr.5150cbrn
01-31-2012, 04:33 PM
thanks Doughboy , thats easy enough to understand!

but im sure someone will chime in to contradict that explanation
as they always do.......

BOOTY MONSTER
01-31-2012, 05:58 PM
"Here is how to figure out your true 1/2-wave:
492 x (Velocity Factor) / Frequency (MHz)
For example, I want to figure out the true half wave coax length for RG-59/U (foam) on my home channel (ch. 33 - 27.335):
492 x .79 / 27.335 = 14.219 feet
Now add 14.219 to itself to determine your 1/2 wave multiples. Remember to use every other number. See the example below:
14.219 feet 1/2-wave multiple, 28.438 feet 1-wave multiple, 42.657 feet 1/2-wave multiple, 56.876 feet 1-wave multiple, 71.095 feet 1/2-wave multiple, 85.314 feet 1-wave multiple
and so on . . . .
Use only the lengths that fall on the 1/2-wave multiples and you will be all set.
"

if im understanding what he's saying ... hes saying 1 or 2 or 3 electrical wavelengths don't work to get the effect he's talking about , but 1/2 , 1 1/2 , 2 1/2 etc. will . it's my understanding that a electrical half wavelength or any multiple of that length for that frequency will work .

doughboy , can you supply a link to the article ?
thanks :)

doughboy
01-31-2012, 06:17 PM
I believe that to be correct booty because you want a half cyle through the coax
Take a look at this link might help
http://signalengineering.com/ultimate/coax_basics.html

Mr.5150cbrn
01-31-2012, 07:15 PM
now is that what they mean by resonating the coax ?
to the given frequency to obtain the length , without using the math end of the equation .


and also does that also apply to a mobile installations too?

BOOTY MONSTER
01-31-2012, 08:03 PM
i didn't have any issue with the resignating coax article itself once i figured out what they were trying to achieve , although they definitely took the long hard way to do it just to avoid two simple math equations . i took issue with the following post suggesting 3 s-units of gain from doing it .

pure CB bullchit worthy of being posted as fact over at the mule droppings forum .

Mr.5150cbrn
01-31-2012, 08:42 PM
well if your not tx ing a full wattage to antenna and the antenna sys isnt resonant at ??>???? frequency ,
then by resonating the coax to get everything in check , then yes one might see a 3db gain ?

BOOTY MONSTER
01-31-2012, 10:19 PM
if the antenna is not resonate absolutely no length of coax is going to change that .
tune the antenna ...... not the coax .

there is some loss associated with high vswr , but it would have to be high to reduce a signal by 18db .

Mr.5150cbrn
02-01-2012, 08:51 AM
if the antenna is not resonate absolutely no length of coax is going to change that .
tune the antenna ...... not the coax .

there is some loss associated with high vswr , but it would have to be high to reduce a signal by 18db .

ok , but explain how you might do / pull that off with a 102 -108 ss whip? which is set usually @ 36 ohm

BOOTY MONSTER
02-01-2012, 12:18 PM
it's "usually @ 36 ohms" because of the shape of the ground element (which is the vehicle) so that will vary from vehicle to vehicle as well as where its placed on that vehicle . FWIU , a 1/4 wave ground plane antenna (which is basically what %99 of all mobile antennas are) needs ground elements sloped down at a certain angle to tune to 50 ohms .... along with the proper length radiator .

here's a link that explains it without me doing a bunch of typing .

http://www.comportco.com/~w5alt/antennas/notes/ant-notes.php?pg=20

B (http://www.comportco.com/~w5alt/antennas/notes/ant-notes.php?pg=20)TW a 36 ohm antenna is about a 1.3 vswr which is completely usable .

BOOTY MONSTER
02-06-2012, 11:23 PM
if anyone is interested ............

http://www.dx-antennas.com/Coax.htm (http://www.dx-antennas.com/Coax.htm)

"WHAT LENGTH?
As mentioned the best length is the shortest, in this way the loss due to the cable is the lowest.
There can be one advantage by extending the length of the cable:.
A HALF WAVE coax cable or ANY multiple (1, 1, 2 wavelength) length of this will show you: "exactly the same input resistance at both ends of cable". In normal words:
You can measure the exact SWR from the antenna with these lengths of coax cable.If your antenna is truly 1:1 it doesn't matter, each SWR measurement anywhere on the line and you will find 1:1.
Only when your SWR isn't 1:1 but higher your measurements will change according to length, this is when it can come in handy to keep the line a half wave long (or any multiple). "



"COAX AND VELOCITY FACTORThe shielding and the core of a coax form a capacitor. Through the use of insulation material, the value of the capacitor is bigger then it would be in air, and this has the effect of slowing down the signal. This can be of great importance in some applications, although for many purposes it does not needed this information ;-)Velocity factor
The speed at which the signal travels is normally given the abbreviation VF and this is the fraction of the speed at which the signal travels when compared to a signal traveling in free space. So VF for a signal traveling at the speed of light would be 1.0, and for one traveling at half the speed of light it would be 0.5.

The velocity factor of the coax cable is found by VF = 1 / SQRT (dielectric constant)Coax cable electrical length
One important factor of a coax cable in some applications is the wavelength of the signals traveling in it. In the same way that the wavelength of a signal is the speed of light divided by the frequency for free space, the same is also true in any other medium. As the speed of the wave has been reduced, so too is the wavelength reduced by the same factor. So if the velocity factor of the coax cable is 0,66, then the wavelength is 0,66 times the wavelength in free space.

In some instances lengths of coax cable are cut to a specific length to act as an impedance transformed or a resonant circuit, then this needs to be taken into consideration when determining the required length of coax cable.
The advantage of using a coax cable with a low velocity factor is that the length of coax cable required for the resonant length is shorter than if it had a figure approaching one. For example, 1/4 lambda 75ohm coax (VF * 1/4 lambda length) for transform the impedance from 125ohm to 50ohm. "

Mr.5150cbrn
04-03-2012, 09:38 PM
if the antenna is not resonate absolutely no length of coax is going to change that .
tune the antenna ...... not the coax .

there is some loss associated with high vswr , but it would have to be high to reduce a signal by 18db .

well over the last weekend , i had been having issues with a 102 whip and swr readings !
and my truck .
when the usual antenna is mounted on cab roof adjusting swr isnt a major deal as running a half wave on the coax tunned fairly well
but when running a 102 behind cab on bed rail and running coax in multiples , swr is way out of spec.
pulled my hair out , but he goes; 102 ss whip 36 ohm reading taken at base of antenna @ pl-259 with a mfj259b analyzer
now i resonated my coax (rg-8x ) vf ".78 to 27.200 giving me a coax length of 28 ft 2.250 in. for a full wave in length netting me a true 1.3;1 swr
not tuning antenna by trimming stinger , but more by tuning the coax to the setup
final measurements on the analyzer freq. 27.200 , swr-1.3, R=40, X=6 .( R &X readings @given freqency ) which equals a 1.1 swr even though the swr bridge reads a swr of 1.3

so in laymens terms yes adjusting length of coax can change the caricaturization of a antenna system in a mobile installation in regards to a 102 whip,

now with a base type of installation , and antennas having adjust-ability though gamma , beta matches , tuning rings etc and the ability to obtain a proper rf ground plane , things are some what different with regards to antenna height ground soil composition (salinity) capacitive reactance and magnetic inductance and a myriad of other variables along with coaxial length and vf's

BOOTY MONSTER
04-03-2012, 10:16 PM
"but when running a 102 behind cab on bed rail and running coax in multiples , swr is way out of spec. "

so your reading probally went up because of reflection from the cab and you were already using coax that was a multiple of a electrical half wavelength ? correct ?



"now i resonated my coax (rg-8x ) vf ".78 to 27.200 giving me a coax length of 28 ft 2.250 in. for a full wave in length netting me a true 1.3;1 swr "

so you changed to another section of coax that was one full electrical wavelength and your reading got better ?
is it possible one of the coax sections has been damaged or that a 259 needs some attention ?


"so in laymens terms yes adjusting length of coax can change the caricaturization of a antenna system in a mobile installation in regards to a 102 whip, "

i'll be sticking with coax length not changing the tuning of a antenna , just what a meter reads .


....

Mr.5150cbrn
04-03-2012, 10:41 PM
"but when running a 102 behind cab on bed rail and running coax in multiples , swr is way out of spec. "

so your reading probally went up because of reflection from the cab and you were already using coax that was a multiple of a electrical half wavelength ? correct ?
well its not exactly behind cab . 10 in. to side of cab and 10 in. rearward of cab , bed is a commercial utility bed box
original coax length 18ftwhen ant. was mounted on top of cab



"now i resonated my coax (rg-8x ) vf ".78 to 27.200 giving me a coax length of 28 ft 2.250 in. for a full wave in length netting me a true 1.3;1 swr "

so you changed to another section of coax that was one full electrical wavelength and your reading got better ? actually added section /s to come up with a full wave length as using 3/4 wave length gave undesirable results
is it possible one of the coax sections has been damaged or that a 259 needs some attention ?
no as i thoroughly inspected the coax and pl-259's


"so in laymens terms yes adjusting length of coax can change the caricaturization of a antenna system in a mobile installation in regards to a 102 whip, "

i'll be sticking with coax length not changing the tuning of a antenna , just what a meter reads .
thats your opinion and like wise your entitled to your opinion , but it doesnt change the fact , that it dispels your theories or hypothesis. - using a simple swr bridge in conjunction with a analyzer and a 50 ohm dummy load shows a different side of the story


....
and i did this with my own ability without using another link to disprove another's result!

BOOTY MONSTER
04-03-2012, 11:02 PM
18 ft of coax is a physical half wavelength , not a electrical half wavelength .
you had 36 ohms and went to 40 ohms .

you still didn't change any thing at the antenna , changing the length of the coax changed what the meter/259 was seeing to display it's readings .


believe what you want ............

doughboy
04-04-2012, 07:16 AM
Its basically just like a ham tuner i can tune a hammer and talk on it But the hammer is still not resonant there is no doubt coax length can change Swr at the load but the antenna stays resonant where ever it was in the begining Also dont forget vf is not always 100% accurate and may vary by manufacturer even Though both may be rg-8 or whatever

doughboy
04-04-2012, 07:28 AM
In the case of the steel whip they are normally about 36 to 40 ohms at resonace but some cheap equipment probably wont play well with that so yes most people adjust their coax to show the equipment the load it wants most radio stations antennas run as low as 28 ohms at resonance so they just run a large matching network they lose some power through the match but the antenna performs much better because is resonant

Mr.5150cbrn
04-04-2012, 09:31 PM
18 ft of coax is a physical half wavelength , not a electrical half wavelength .
you had 36 ohms and went to 40 ohms .

you still didn't change any thing at the antenna , changing the length of the coax changed what the meter/259 was seeing to display it's readings .


believe what you want ............oh its all understood , but booty
# 1. ya dont have to be a dick about it!
#2. go lick your self !

it wasnt about believing anything , i was just stating my findings .
in response of you getting discombobulated on every forum known on the world wide web.

by the way , booty did you tell everyone here , how you cried on the phone to lonewolf - 4 to five times a day
because you tried to TROLL some members and got asshurt from the flames from HELL !

now its time to knock it off!
keep it in the appropiate areas , or keep it to your self

BOOTY MONSTER
04-04-2012, 09:58 PM
LMAO !!
i just explained the cause of your findings , your assumption that you were tuning the antenna is wrong .
changing coax length doesn't change the tuning of the antenna . but it can skew the reading on a meter .
my post here were on topic and non flaming , so take your own advice .

Mr.5150cbrn
04-05-2012, 05:07 PM
LMAO !!
i just explained the cause of your findings , your assumption that you were tuning the antenna is wrong .
changing coax length doesn't change the tuning of the antenna . but it can skew the reading on a meter .
my post here were on topic and non flaming , so take your own advice .it wasnt an assumption , you made that.
i just made a log of my findings that coincide with the original post here- that meters can be fooled and or give false readings, i wasnt flaming untill you lit the flame ...... now its on you !

BOOTY MONSTER
04-05-2012, 06:47 PM
you made a post with a quote from me . i replied commenting on your post and offering my thoughts on what was happening .
where did i flame ???? LMAO

Mr.5150cbrn
04-06-2012, 10:49 PM
no i answered a post that you carried from place to place to stir the pot with .
where as it wasnt a flame , as was your intent was .
same thing that you did elsewhere

funny booty , you cant help your self from replying to what you deem a justifiable response
to what you feel is rite in your mind

BOOTY MONSTER
04-06-2012, 11:32 PM
quit your fucking crying you stupid ass sissy .

nino
08-16-2012, 07:16 PM
but how does this info stack up against the no ground plane coax used in ngp auto systems?

doughboy
08-17-2012, 05:38 AM
Ngp system is a last resort basically the coax braid becomes part of the antenna
The system becomes similar to a dipole antenna the coax braid would need to be a specific length in relation to the antenna length this can give you a good match but the overall performance of the antenna takes a big hit i would only use this if the whole vehicle is fiberglass like a boat corvette etc even a corvette has a metal frame so you probably wouldnt even need it there.
there are even ways around using it on a boat but its easier to just use it in that situation