| Author |
Message |
    
Paper765
New member
Username: Paper765
Post Number: 9
Registered: 12-2004
| | Posted on Sunday, January 09, 2005 - 8:12 pm: | 
|
On to the next issue, coax length. A particular friend of mine who is pretty good with cb otherwise, swears that coax length matters. He has that drilled in his head, inside out, upside down. However ARRL books and other things I've seen on the internet says thats an old wives tale, that trimming your coax is not the way to adjust SWR issues.
What do the knowledgable members on this message base say? Is trimming the coax to adjust for SWR issues a lot of nonsense, or is there something to it?
Thanks goes to Kid Vivacious, Bruce, and Tech833 for helping me out with the homebrew antenna dilema |
Patzerozero
Intermediate Member
Username: Patzerozero
Post Number: 327
Registered: 7-2004
| | Posted on Sunday, January 09, 2005 - 9:40 pm: |
|
cutting coax does not change swr's. if they do, there's a problem with connector or something is big time wrong. antenna adjustments are what changes swr's.
HOWEVER, your coax OVERALL length should be odd multiples of 1/2 wavelength of freq being used, in cb's case, odd multiples of approx 17 feet X 1 or x 3 or x 5, etc, THEN multiplied by velocity factor of coax, which could be 66% or 78% or 84%.
supposedly, if you were to use an even multiple of 1/2 wavelength, or 2/2-34 feet (multiplied by velocity factor) and hooked your swr meter at back of radio as usual, you'd get a significantly different reading then if you hooked up swr meter at other end of coax-at antenna. which one is right or if it even works that way-don't know, but have used that theory for 30 years without fail. |
Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 121
Registered: 9-2004
| | Posted on Sunday, January 09, 2005 - 9:52 pm: |
|
kid vivacious? i dont even know how to take that one! LOL
your best bet is to do a search on this forum for "coax length". this has been discussed many times before. (no surprise there!)
here are some basics.
the entire theory behind coaxial (two axis) cable is that it DOESNT radiate your signal out into the air. for this reason alone, coax length should not affect the antenna tuning. its not supposed to be part of the antenna.
in reality, coax does radiate A LITTLE bit. if the coax is cut to one wavelength at 27 mhz or a multiple of this length, then it can throw off the reading on your swr meter. NOW HERE IS THE IMPORTANT PART! this false reading is rarely more than .5 off from the actual swr. thats not much and the swr is not higher, only the reading on the meter is. make sense? if you were to put the swr meter at the antenna end of the coax you would get the actual swr reading. this is where the confusion sets in. there are certain lengths of coax that will give you a true reading at both ends of the coax. i cant remember what they are right now, i'll go home and check tonite, or maybe someone else on here knows them.
so, in conclusion, coax length will not affect the swr of your antenna. certain lengths can give you a false reading at the radio end of the coax.
if your cb buddies are cutting their coax and it is changing their swr, then you can tell them that they actually have a problem with their antenna installation that is causing the coax to radiate as part of the antenna.
hope this clears it up a bit.
matt |
Paper765
Junior Member
Username: Paper765
Post Number: 11
Registered: 12-2004
| | Posted on Sunday, January 09, 2005 - 11:33 pm: |
|
Kid Vicious  Yes, thanks, that makes sense. So what your telling me is that the true SWR isn't changing, the meter's reading is becoming more accurate at certain lengths of cable. Hmm, i don't know that reassures me. I'd rather have the meter accurately tell me there is SWR then have an innaccurate (but happy reading).
Ok, any thoughts on my planned usage of 50 ft of a belden 9913 clone? |
Dx431
Advanced Member
Username: Dx431
Post Number: 970
Registered: 2-2002
| | Posted on Monday, January 10, 2005 - 12:20 am: |
|
Kid_vicious pretty much said it.Only thing I can add is, forget what your friends say and run enough coax to go from the ant. to your transmitter, key the mic and have fun! |
Pig040
Advanced Member
Username: Pig040
Post Number: 631
Registered: 7-2003
| | Posted on Monday, January 10, 2005 - 11:16 am: |
|
Some antenna manufacturers state coax cuts in their manuals for their antennas. My pdl II clearly stated in the manual to cut the coax in 4 ft increments. Most of the old hams swear by 3' increments, Myself, I put the mfj on it, and go by what it says, it has never failed me yet!
Rich |
Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 128
Registered: 9-2004
| | Posted on Tuesday, January 11, 2005 - 12:04 am: |
|
uh oh, here we go again!
let me see if i can put this in a way that doesnt incite anyone to convey their stories of coax cutting "magic".
if you buy a good quality antenna, put it together correctly, mount it securely, and ground it. if the swr meter says 1.5 or lower, then rejoice; you have an antenna system that will provide everything it was designed to.
if you guys still need proof; ask one of the techs on here what the wattage output difference is between a station with a 1.5 to 1 swr and a station with a 2.5 to 1 swr. the answer will shock you, and some of you just flat wont believe it.
matt |
Bruce
Senior Member
Username: Bruce
Post Number: 2060
Registered: 9-2003
| | Posted on Tuesday, January 11, 2005 - 8:06 am: |
|
Kid ....
This is the loss for VSWR of ....
1.5/1 = 4%
2.0/1 = 10%
3.0/1 = 25% |
Patzerozero
Intermediate Member
Username: Patzerozero
Post Number: 335
Registered: 7-2004
| | Posted on Tuesday, January 11, 2005 - 7:53 pm: |
|
but when you have 500w going out & 125w of it coming back into the amp...rumor has it the transistors won't be too happy for very long
|
Tech291
Moderator
Username: Tech291
Post Number: 40
Registered: 11-2004
| | Posted on Tuesday, January 11, 2005 - 8:38 pm: |
|
Pat-0-0,you sounded like you had some "happy transistors "Sunday! |
Troublemaker
Member
Username: Troublemaker
Post Number: 57
Registered: 2-2004
| | Posted on Tuesday, January 11, 2005 - 11:02 pm: |
|
I second that!!SMOKIN!!!! |
Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 132
Registered: 9-2004
| | Posted on Tuesday, January 11, 2005 - 11:51 pm: |
|
pat, while that is true, it is not the point i was making. what im trying to convey is that the coax length is not going to make or break your station. if your swr is 3.0 to 1, then go tune your antenna, not your coax. if you cant get the swr below 2.0 to 1, then go fix or reinstall your antenna, not your coax. if your swr is 1.5 to 1, then leave it alone.
if your coax is older than, say, five years, or 50,000 miles, then replace it.
the only time i would suspect the coax as the weak link would be if i hadnt soldered the ends on myself.
matt |
Pig040
Advanced Member
Username: Pig040
Post Number: 634
Registered: 7-2003
| | Posted on Wednesday, January 12, 2005 - 10:29 am: |
|
Yeah Kid, BUT,
Your coax is an integral part of your antenna system, when you are checking your swr you are normally doing it with your coax attached, and checking at the feed point where it goes into your shack, so it is a part of your system. I always tune my antennas with the amount of coax I am going to use attached, to do it any other way would not make sense to me.
Rich |
Bruce
Senior Member
Username: Bruce
Post Number: 2073
Registered: 9-2003
| | Posted on Wednesday, January 12, 2005 - 12:07 pm: |
|
Hummmm ..... 3/1 match smoked your amp .....
Back in the good old days of 6 and 2 meter AM we used 807's and 829b tubes .... the 829b was cheep so we had a way of tuning them ..... simply we watched the white hot grid glowing through the red hot plate...... 3/1 match was no problem! |
Patzerozero
Intermediate Member
Username: Patzerozero
Post Number: 339
Registered: 7-2004
| | Posted on Wednesday, January 12, 2005 - 5:15 pm: |
|
no, kid, i was commenting on bruces' loss comment. and i agree coax length shouldn't be a problem & swr's need to be lowered by manipulating antenna length at all adjustable points to get swr's under 1.5:1. mobile coax i usually have to replace because it gets too short from having to replace ends because of removing radio/antenna frequently, base is over 6 years old do have new cable & some plumbing pipe to hopefully get my feedpoint closer to 30' then the 15' it currently is.
291, what joy my transistors in my mobile see from reflected power of MAYBE a couple of watts is negated by the force feeding with a bit too much input! keeps me happy though! |
Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 135
Registered: 9-2004
| | Posted on Wednesday, January 12, 2005 - 10:02 pm: |
|
sorry pat, i misundrstood. man i've been mouthy lately! must be the change in the earth's rotation.
matt |
Dogfacedsoldier
Junior Member
Username: Dogfacedsoldier
Post Number: 29
Registered: 10-2004
| | Posted on Friday, January 14, 2005 - 2:48 am: |
|
or if u dont wanna be a rocket eng to understand that lol this might make it easier...http://www.timesmicrowave.com/cgi-bin/calculate.pl |
Kj7gs
New member
Username: Kj7gs
Post Number: 7
Registered: 12-2004
| | Posted on Tuesday, January 25, 2005 - 9:13 pm: |
|
Paper765, if your friend isn't going to budge on whether coax length matters, you can let him save a little bit of face by saying that it does matter in certain circumstances. When you're phasing antennas, coax length becomes critical to provide the signal pattern desired (along with proper antenna spacing, of course). Half-wavelength multiples repeat impedances which makes all transmission lines -- not just coax -- critical for measuring antenna impedance, and this property comes in handy for providing a match between a resonant antenna and say, 300-ohm twin lead as your transmission line. For the majority of CB applications though, any 50-ohm load connected to 50-ohm coax will give you a 1:1 SWR on the line. And that makes you right, coax length doesn't really matter. |
