Tuning jumpers and coax length

[Cheech]

How do you figure out the length a tunning jumper should be? I have heard to use 3 and 6 foot jumper for CB but where did this idea come from? Is there some formula or something. I guess this also aplies to coax length from the antenna to the meter to. I know I have heard it both ways but if the length does matter what is the formula? I know I probly just stated another 3-4 page thread :roll: Anyway I added a poll to see what people think. Thanks!

 

[W5LZ]

Cheech,
You're right, more answers/comments than you'll want to see! LOL

The 'real' question is, "Could it/should it make a difference?", and the answer is yes it can, and no, it shouldn't. Thinking in terms of efficiency and good design, the feed line (jumpers) should not make any difference because the inputs and outputs should all be the same impedance as well as the feed line's. If the input/output (or the impedance on both ends of the feed line or jumper) are the same then there is no difference in what the length of the feed line is.

A difference in impedance between the input and output means there is some reactance in the feed line and that means a reduced efficiency. Power isn't developed in a reactance (or transfered) and that means you want as little as possible. There will always be some reactance, but not much, hopefully.

Any feed line has a characteristic reactance, some value of capacitance and inductance, and those 'values' depend on the frequency of use. In a well tuned/designed system they are kept to a minimum. Reactance isn't always 'bad', but when dealing with the transfer of power it is.
- 'Doc

PS - Is there a formula for determining the length of jumpers/feed lines? Not as such, but yes, you can figure whats the 'best' length to use by knowing and using the accumulated characteristic capacitance and inductance of the jumper's or feed line's length. I'm too lazy, I ain't going there...

 

[Switch Kit]

IM to lazy .....I ain't go'in there !! ......if it works ? weather I know the truth or not ? ......I'll leave well enough alone ! especially in my motor car !! ...in the beginning of this one (motor car) I used 10ft mini 8 from a Wilson 5000 drill throw with a 3ft jump from radio to amp matches were 1.1 SWR...my amps relays stuck on the high side .....I went from 3ft jump to 6 ,9 and then 12 and no more stick on the amps high side .....but )-: ...for some reason with the 12ft jump the numbers just were not looking right ? Huh ? !! ......I was losing major swing from the amp !! Huh ?? ......Not good enough !! I ripped my head liner down and re-soldered 17ft mini 8 to the connector ....BAD MATCH !!! Huh !! ?? so I did some math (for what good that truly was?) it all seemed to work with the 12ft jump and the original 10 ft from the antenna (even though there was a major lose in swing from the amplifier with the 12ft'er) ...I added it up to 22ft of mini 8 ...so I re-soldered that to the antenna connector and went back to a 3ft jumper !!! .......Auh Yes !!!!! 1.1 SWR match around the band /no stick on the highside / no lose in the amplifiers swing ....it was working and working well for me !!! .....OK ...Is it perfect ? .....I truly doubt it ....but then again I really don't know ? .....I only know the match is good through the amp and the radio and visa versa and I see no lose of power !! ....I have never had any fancy meter on my system either ....so I guess there could be some lose there ? ....if that should be the case ? ...I haven't a clue what that might be ? ....and to be perfectly honest with you ,I really don't want to know !! It works and it works well and has been for the last year or so with the system IM using ......So like the gentleman said .....IM to lazy .....I ani't going there. does coax make a differance ? ....Yes and no and maybe so...but in vehicals I think it does over the years .......weather or not Einstiens theroy of realativity has anything to do with it or not ? If it works after all I went through .....I need not fix it.

 

[Cheech]

Dang 50/50 :shock: Ow well Ill figure this mess out ine day :roll:

 

[Switch Kit]

You know what Cheech ......some times the really deep stuff on how and why can really screw it all up for the simple stuff that seems to work for other folks ......if it has worked for you over a period of time without any complaints or problems and you yourself are happy with the performance of it all ....I often think that's all that really matters ......sure we want whats best but one must ask himself is it truly worth all the time and effort and money (if one can truly afford it ? ) to make sure all the I's are dotted to ones liking ?I would agree that it's all apart of the hobby it's just that how deep does one really want to go with it ? .....A perfectionist IM not )-: ....I tend to drive myself crazy that way. There's a old saying ...If it works ......don't fix it. (-: I think W5LZ answered your question very well. Peace

 

[Cheech]

Yep I supose your right.

 

[Chuck Bravo]

Length Matters

The characteristic impedance of a given feedline is repeated only at half-wave multiples that is determined by operating frequency and velocity factor of the feedline. You should avoid quarter-wave multiples as they will always be reactive. In pinch, you can tune out the reactance with tuning stubs that act as a matching transformer.

 

[freecell]

no, IT IS NOT the characteristic impedance of the feedline that is repeated but the input impedance present at the feedpoint when other than 1:1. in response to the original question from Cheech, for best results all jumpers or patchcords should be kept to 1/20th of a wavelength or less at the frequency in use. this is done to minimize changes in current and voltage phase angle relationships between equipment and limit the maximum phase angle excursion to approximately 18 degrees. when working with RG8 Mini-Foam for example, this works out to approximately 16.95".

 

[Chuck Bravo]

Hi freecell.
I stand corrected. Thanks. Feedline length doesn't matter at unity.

 

[freecell]

here's a question. why does it say you can delete your posts in this forum and you can't?

 

[freecell]

exactly. when no reflection exists feedline length is not a factor.

 

[Cheech]

Okay Im kinda geting this. So when your cuting a legth of coax it should not be quarter wave multiples? Also when I had a length of mini 8 made at a shop I was told odd wave lengths. I wish I would have payed more attention! My coax is in the rage of 44 foot can someone explain this to me if its right.

 

[FL Native]

Okay Im kinda geting this. So when your cuting a legth of coax it should not be quarter wave multiples? Also when I had a length of mini 8 made at a shop I was told odd wave lengths. I wish I would have payed more attention! My coax is in the rage of 44 foot can someone explain this to me if its right.

well what type of coax (velocity factor)and what your trying to do with the feedline.

if your antenna is allready tuned and showing as close to 50 ohms. length will not make a difference.

but they say to use multiples of 1/2 waves. (shrugs)

www.freewebs.com/skunkworks/coax1.htm

that link will explain it in detail and either confuse you more or the light will go off and you will finaly get it, and then have more questions LOL

 

[Cheech]

LOL thanks!

 

[2at124]

feedline

cool i wasn wondering on ho wto do the math for this too . That link helped me out alot