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Message |
    
Couchpotatoe
Junior Member
Username: Couchpotatoe
Post Number: 20
Registered: 6-2005
| | Posted on Tuesday, June 14, 2005 - 8:27 pm: | 
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WHERE DO PEOPLE GET THESE FIGURES ON LENGTHS OF COAX TO USE? 3FT,6FT,9FT,18FT,ETC. COAX IS COAX. ITS RESISTANCE UNDER RF LOAD IS DEPENDENT UPON THE DIAMETER OF THE CENTER CONDUCTOR TO THE DIA. OF THE SHIELD. RG-8,RG-8X,RG-58,RG-213 ARE ALL 50 OHM COAX NO MATTER WHAT LENGTH YOU RUN. NOW BACK TO MY QUESTION... WHAT DIFFERENCE DOES IT MAKE ON ME RUNNING 1FT OR 100FT? TAKE IN MIND I DO REALIZE YOU CAN MAKE BALUNS WITH CERTAIN LENGTHS ON CERTAIN FREQUENCIES. IF IT IS 50 OHM COAX NO MATTER WHAT LENGTH, WHY MUST I RUN 18 FT OF COAX TO MY ANTENNA BECAUSE IT HAPPENS TO BE 1/2 WAVE LENGTH? ITS PROFIT FOR THE CB SHOP!!
I'VE ALWAYS WANTED TO BARK ABOUT THAT AND NOW THAT I HAVE I FEEL MUCH BETTER. THANK YOU FOR ALLOWING ME TO RANT AND RAVE!! MY IRE ON THIS HAS SETTLED. |
Rob5mike
Intermediate Member
Username: Rob5mike
Post Number: 162
Registered: 7-2002
| | Posted on Tuesday, June 14, 2005 - 8:39 pm: |
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I don't have an answere for you and have read many arguments about it on this forum.I will tell you that I have run jumpers between radio and amp as short as 10 inches and have no problems and I run enough coax to make it to the antenna and pay no attention to the 3,6,9,18ft rule. |
Tech291
Moderator
Username: Tech291
Post Number: 198
Registered: 11-2004
| | Posted on Tuesday, June 14, 2005 - 8:42 pm: |
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personally,the only time i get out the tape measure for cutting coax is when making baluns,phasing harnesses or delay-lines.for general runs what ever it takes to get from point A to point B is what i use.If lengh was really a critical issue,how accurate is the tape measure gonna be at satellite frequencies when the if I.F. line will vary several HUNDRED megahertz?
tech291
CEF#291
kc8zpj |
Couchpotatoe
Junior Member
Username: Couchpotatoe
Post Number: 23
Registered: 6-2005
| | Posted on Tuesday, June 14, 2005 - 9:56 pm: |
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i'm glad ya'll see where i'm coming from |
Bruce
Senior Member
Username: Bruce
Post Number: 2860
Registered: 9-2003
| | Posted on Tuesday, June 14, 2005 - 10:49 pm: |
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Coax length is
L = 1/2 wave in inches =(5540/f ) * VF (velocity factor ) of coax
So for each 1/2 wave of say RG-8 foam at 27 it would be .....
L =(5540/27) * .8
L ( in inches ) =197
as long as you know the VF of the coax and the 2 most common are .67 and .8 you can easly cut the coax to what you need.
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Hollowpoint445
Intermediate Member
Username: Hollowpoint445
Post Number: 396
Registered: 6-2004
| | Posted on Wednesday, June 15, 2005 - 6:46 am: |
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"I DO REALIZE YOU CAN MAKE BALUNS WITH CERTAIN LENGTHS ON CERTAIN FREQUENCIES."
The reason you can do this is because radio waves propogate through coaxial cable just like they do through an antenna or the Aether. This allows you to use coax as a matching device.
At odd 1/4 wavelength points the impedance is inverted, the current is maximum, and the voltage is minimum. Taking a VSWR reading at one of these points will not indicate the antenna's actual VSWR.
At 1/2 wavelength points the voltage is maximum, the current is minimum, and the impedance is the same as the antenna's. When you take a VSWR reading there you will get a true indication of the antenna's match.
That's why some folks measure their coax. Is it necessary? No. Is it useful? Sure - to the extent that an accurate VSWR reading is useful.
Criticizing someone for making an effort to get the best information about their antenna system possible makes no logical sense. It's like criticizing someone for looking at their fuel gauge to see how much fuel remains in their vehicle's tank.
About mobile antennas and coax length - Most mobile antennas probably come packaged with 18' of coax because that length will work for most installations.
There is also the school of thought that it's 1/2 wavelength to act as a counterpoise for vehicles that don't have enough metal to offer the antenna a proper counterpoise. This may or may not be true, but you can tell if your coax is acting as a counterpoise by coiling up the excess and checking the VSWR before and after. If the reading changes then your coax is acting as a counterpoise. |
Pig040
Advanced Member
Username: Pig040
Post Number: 812
Registered: 7-2003
| | Posted on Wednesday, June 15, 2005 - 11:55 am: |
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Good post hollowpoint. Hey couch potato, even some manufacturers suggest coax length cuts in their owners manuals, for my pdl2, it said cut coax in multiples of 4 feet!
Rich |
Road_warrior
Advanced Member
Username: Road_warrior
Post Number: 663
Registered: 7-2004
| | Posted on Wednesday, June 15, 2005 - 12:17 pm: |
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I also run coax from A to B and as of
yet have never had a problem with
achieving a good Swr. on my antennas.
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Bigbob
Senior Member
Username: Bigbob
Post Number: 1978
Registered: 12-2001
| | Posted on Saturday, June 18, 2005 - 8:48 am: |
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And now from mister-know-it-all,everybody is right,a half wave length is an impedence repeater,to see what the swr is at the antenna without attaching a meter at the antenna,a matcher is just an infinitely variable length of coax to get your desk meter to read what you want,it doesn't change swr at antenna,so instead of racking your brain buy a matcher and run any length of coax OR tune your antenna to the freq. you most use,it would be great for someone to make a spiral drive motor and gear to attach to gamma-match to make it adjustable from bench,or even spiral drives on elements to say set it for 6 meters one day and lengthen the elements from the bench for ten meters the next,spiral drive motors are small and have a worm and gear drive the gear is nylon and has a spiral cut in the center in which a long twisted rod is inserted,when rod is inserted and held so as not to rotate,the motor when activated will draw the rod in or push it out,used on automatic blinds,sashes,any place where rotory motion must be converted to linear motion,just need someone to take care of the logistics,hmmm.Bigbob |
Pig040
Advanced Member
Username: Pig040
Post Number: 816
Registered: 7-2003
| | Posted on Saturday, June 18, 2005 - 10:22 am: |
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That is a great idea, Bigbob, once you put it on the tower, never have to take it down again, that would be very cool! |
Hollowpoint445
Intermediate Member
Username: Hollowpoint445
Post Number: 411
Registered: 6-2004
| | Posted on Saturday, June 18, 2005 - 7:07 pm: |
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Bigbob - Screwdriver antennas basically do just that for mobiles, and with an appropriate counterpoise I see no reason why they couldn't be used for base antenna installations as well.
I think trying to use the same antenna for more than one band would be quite a compromise on the lower frequencies, but it would work. A more reasonable demand would be using one for 10, 11, and 12 meters, but there already are antennas that easily cover that bandwidth with a reasonable VSWR. |
