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Message |
    
Hotwire
Junior Member
Username: Hotwire
Post Number: 39
Registered: 1-2005
| | Posted on Tuesday, February 08, 2005 - 12:28 pm: | 
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What would be an ideal length of coax between a radio and amplifier. Space not being an issue. |
1861
Member
Username: 1861
Post Number: 66
Registered: 2-2004
| | Posted on Tuesday, February 08, 2005 - 1:34 pm: |
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WHATEVER LENGTH NEATLY CONNECTS THEM |
Rover
Member
Username: Rover
Post Number: 82
Registered: 2-2004
| | Posted on Tuesday, February 08, 2005 - 6:51 pm: |
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LOL! Honestly, WHERE does that coax length thing COME from? Some fellas will argue and argue that it matters, while others will tell you what 1861 told you! And he IS correct. In a normal situation where grounds are sufficient and correct, coax has whatever to do with it (and I CAN prove it). Now if you are dealing with.....say, a non-metal vehicle body, yes, coax *can* be used as a counterpoise. Antenna resonance is done AT the antenna itself and NOT by "fooling" the meter, coiling or cutting coax to a "certain" length! So, as long as you don't have to run your feedline out to the back pasture
(and catch a taxi back home!), don't sweat the coax length. Keep it as short as possible, prune the ANTENNA to resonance (lengthen or shorten the radiating elements), and you'll do fine. "Trimming" coax only masks the REAL problem (if any) and does nothing to correct anything at all. |
Ashtray
Junior Member
Username: Ashtray
Post Number: 21
Registered: 1-2005
| | Posted on Wednesday, February 09, 2005 - 2:12 am: |
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11 feet.
and that's seriously from an article I've read that says any measure of coax should be done in 11 feet.
need 50 feet? make it 55. need 60 feet? make it 66.
but then again...who knows? Everybody has a different opinion and they all think they're right. The article I read semms to make the most sense to me though. |
Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Wednesday, February 09, 2005 - 7:28 am: |
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The only reason to ever use a specific length of coax is to have the VSWR show correctly at the radio's end of the coax.
There should be no reason to use a specific length of coax between an amplifier and a radio. If the amplifier is poorly made it's possible that the coax length would matter, but that's another story.
11 feet is about right for a 1/2 wavelength of coax with a .66 velocity factor at 27MHz. If the coax had a different velocity factor - RG8X is about .78 - you would need to recalculate the length. But again, you would only want to do this so the VSWR would show accurately at the radio end of the coax. |
Bruce
Senior Member
Username: Bruce
Post Number: 2189
Registered: 9-2003
| | Posted on Wednesday, February 09, 2005 - 7:43 am: |
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Coax is a way to move power ...... the only time it matters is if you are tring to phase something otherwise it's happy no matter how long or short it is. |
Chad
Intermediate Member
Username: Chad
Post Number: 134
Registered: 11-2004
| | Posted on Wednesday, February 09, 2005 - 10:08 am: |
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1861
I have a very complex formula that i like to use when calculating coax length  I think you will like it and use it often!
A=Velocity factor of the coax(X)Pi
B=Melting point of the dielectric in Kelvin.
C=Distance from the comsoutta to the guzinta
D=The RMS of your lattitude (Lattitude(X).707)
E=The mean humidity of your climate squared.
F=The amount of cable used inside each connector.
And.....
X=The correct length of your Coax
So here we go, got your calculators?....
X=F(x)2+C
Or with simple addition.......
X=F+F+C
Chad |
Hotwire
Junior Member
Username: Hotwire
Post Number: 40
Registered: 1-2005
| | Posted on Wednesday, February 09, 2005 - 12:26 pm: |
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I know that coax length does not matter for swr. I was just wondering about linear performance.I know that 20 ft between the radio and linear is ridiclous but will yhe linear still perform correctly with that much? 1861 you said whatever length neatl connects them and I agree. Do you think a linear will work say in the trunk and the radio up front on the dash. I have install space limits. |
Rover
Member
Username: Rover
Post Number: 84
Registered: 2-2004
| | Posted on Wednesday, February 09, 2005 - 1:27 pm: |
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It's sort of like tilting at windmills. If you want to fret over how long your coax should, go right ahead. But, you know what blows the CB gurus minds? When you start explaining/showing how a screwdriver multiband antenna that covers everything from 75 thru 6 Meters works, and how you'd have to have a multitude of coaxes cut to specific lengths for it to work, THEN they get befuddled and won't even TRY to explain it. Or they come up with some feeble explanation that won't hold water. Being that I can smoothely resonate my antenna on ANY frequency within its specified range, if all that CB legend were true, then I'd have to have about 10 different lengths of coax ranging from 60 FEET to about 11 feet for the antenna to work!!!!!! HAHAHA! Now, in a mobile, WHERE am I going to put 60 feet, 30 feet, 26 feet, 19 feet, and 11 feet, etc, etc, etc. There's not ROOM for all that feedline!!
Coax length is mostly smoke and mirrors. It sounds good to the un=-initiated, and, yes, it *can* work and make the SWR appear to be good. Still you are merely forcing the meter to read what you WANT it to read, and you may be masking a REAL problem. Resonate the A N T E N N A. Use a random piece of coax. When you measure the SWR at the radio end AND at the antenna, the reading will be just about the same. It simply doesn't matter! Use whatever length is convenient. Try to keep it short to minimize losses, and don't sweat it! Jinking around with coax and.or trying to get from 1.4 to 1.1 will NOT make a bit of difference in your signal! |
Hotwire
Junior Member
Username: Hotwire
Post Number: 46
Registered: 1-2005
| | Posted on Wednesday, February 09, 2005 - 1:32 pm: |
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10-4 Bruce, thank you sir. Thanks everybody |
1861
Member
Username: 1861
Post Number: 68
Registered: 2-2004
| | Posted on Wednesday, February 09, 2005 - 2:11 pm: |
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I,M WITH COAX ,LIKE I AM WITH A WATER HOSE -- IF IT ISN,T LONG ENOUGH TO REACH , IT IS NO GOOD -- IF IT IS TOO LONG , IT IS JUST EXCESS MESS , AND YES , HOTWIRE , AN AMP WILL WORK FINE IN THE TRUNK WITH RADIO IN DASH . IT IS GOOD TO HAVE A REMOTE SWITCH THOUGH |
Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Wednesday, February 09, 2005 - 2:54 pm: |
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Radio waves propogate through coax just like they do through the air. At different lengths you will have a different relationship between the voltage and the current. IF you are terribly particular and work only one band (Does that sound like any CBer that you know?) you can calculate the exact length of coax that will put a 1/2 wave interval at the radio - which will show your radio the same VSWR as the antenna.
BTW - The actual formula to find the 1/2 wavelength of coax for a particular frequency is:
(492 / Frequency in MHz) * coax velocity factor = 1/2 wavelength of coax in feet
Do I cut my coax to particular lengths? Life is far too short for me to waste my time that way.
Hotwire - If you're going to have 20' of coax between the radio and amp you should use the absolute best coax you can afford for the jumper. The longer the jumper the more likely that RF could leak into the coax and cause the amp to stay keyed even after you unkey the radio. |
Bruce
Senior Member
Username: Bruce
Post Number: 2191
Registered: 9-2003
| | Posted on Wednesday, February 09, 2005 - 3:13 pm: |
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If your coax has a 50 ohm load on each end it should not care ....
Life is too short to cut coax ...... |
Twa77
Intermediate Member
Username: Twa77
Post Number: 134
Registered: 1-2003
| | Posted on Wednesday, February 09, 2005 - 3:38 pm: |
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nice one chad. |
Rover
Member
Username: Rover
Post Number: 85
Registered: 2-2004
| | Posted on Wednesday, February 09, 2005 - 5:52 pm: |
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Actually, it is 468/frequency in mhz. 492 is a half wave length in free space. Since a solid medium conducts a tad slower than free space (resistance) the divider is 468. By using 468 you eliminate all that VF stuff! ;) All VF is a measure of conductivity (how fast a signal travels thru a solid medium. Even YOU have a "VF"! LOL! Since most coax used in CB and ham applications is 50 ohms anyway, the VF isn't a factor; if you don't mind, it don't matter! |
Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Wednesday, February 09, 2005 - 7:14 pm: |
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Actually it is 492. The velocity factor of the coax is what corrects for the material. 468/F is an approximation to make a 1/2 wavelength antenna out of wire. |
Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Wednesday, February 09, 2005 - 7:24 pm: |
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Velocity factor is not resistance. It's also not impedance. It's mostly about the dielectric material. That's why RG8 has a velocity factor of .66 and RG8X has a velocity factor of .78. They both have 50 ohm impedance, yet they have different velocity factors. Even the same type of coax can have slightly different velocity factors when made by different companies. |
Red_devil
Junior Member
Username: Red_devil
Post Number: 42
Registered: 10-2004
| | Posted on Wednesday, February 09, 2005 - 8:24 pm: |
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I LOVE to watch the coax length debate :-).
I just like to watch.
Some places lenth matters, some places is dosen't.
:-)
:-) |
Tech808
Moderator
Username: Tech808
Post Number: 4783
Registered: 8-2002
| | Posted on Wednesday, February 09, 2005 - 8:44 pm: |
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Just a Suggestion,
Use just what you need to connect from your equipment to your antenna and you will have no problems.
Been doing it for 40+ years and NEVER measured it yet.
Run the amount of Coax you need and adjust your antenna to get the Lowest SWR reading you can achieve and you are all set. 
Lon
Tech808
CEF808
N9OSN
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Marconi
Intermediate Member
Username: Marconi
Post Number: 360
Registered: 11-2001
| | Posted on Wednesday, February 09, 2005 - 9:39 pm: |
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I have to agree with Racer here if we are talking about making a resonant line. Using 468 to start will cut you short. |
Tech808
Moderator
Username: Tech808
Post Number: 4784
Registered: 8-2002
| | Posted on Wednesday, February 09, 2005 - 11:44 pm: |
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Hmmmmm,
I thought the original question was:
What would be an ideal length of coax between a radio and amplifier. Space not being an issue.
In most setups I have seen and personally use for home or mobile use, the amplifier has been located within 1 to 4 feet of the radio, which makes for easy adjustments of the amplifier and radio.
And most generally a 3 or 6 foot jumper will handle 98% of most setups.
Any further makes it difficult to adjust / operate the amplifier and radio when changing frequencies / bands / modes.
It is just no fun when you are talking on AM to have to get up and move 4 or 5 feet to switch the amp to SSB or re-load it when you change frequency/bands/modes.
The question is not asking about the distance between the Amplifier and Antenna BUT between the Radio and the Amplifier.
The Closer the Better, and whatever length you need to use to make operation of BOTH the Radio and Amplifier easy for the user.
Simple Question = Simple Answer.
Just my nickels worth,
Lon
Tech808
CEF808
N9OSN 
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Marconi
Intermediate Member
Username: Marconi
Post Number: 361
Registered: 11-2001
| | Posted on Thursday, February 10, 2005 - 1:00 am: |
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Yea 808, I think we got off on another subject at some point. There are so many ways of doing this, no wonder there is not just one way to do it.
It's getting late! |
Rover
Member
Username: Rover
Post Number: 87
Registered: 2-2004
| | Posted on Thursday, February 10, 2005 - 1:07 pm: |
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Just a Suggestion,
Use just what you need to connect from your equipment to your antenna and you will have no problems.
Been doing it for 40+ years and NEVER measured it yet.
Run the amount of Coax you need and adjust your antenna to get the Lowest SWR reading you can achieve and you are all set.
>Q<
*************************************************
THAT is what I have been saying all along. Whether you believe it is 492, 1005 (full wave), or 468 which is what the handbook uses, using 468 and adjusting the ANTENNA will get what you want. Anything else is wasted effort!
When I was first introduced to radio and antennas, not ONCE did anyone EVER mention coax length, only LOSS caused by excessive runs of feedline. In 40 years of installing, resonating, and BUILDING antennas, I never even HEARD of coax length--- only from the CB "experts" that insisted that you just GOTTA have a "certain" length--and even THAT varied from person to person. And, btw, VF is a measurement of how fast a wave travels thru a medium and has no real bearing on "length" calculations. That is all it is. Any difference is taken care of by resonating the ANTENNA itself.
So the bottom line is STILL what Tech 808 said. It simply doesn't matter! ;) |
Hatchet
Junior Member
Username: Hatchet
Post Number: 13
Registered: 7-2004
| | Posted on Thursday, February 10, 2005 - 5:27 pm: |
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Actually sometimes it does matter on length between the amp and radio. Sometimes the amp wont key up or it will stay keyed when you unkey the mic....I have seen it happen before a few different times
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Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Thursday, February 10, 2005 - 6:15 pm: |
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Velocity factor is the ratio of the electrical length of the coax to it's physical length. It's actually determined by measurement. If you have an MFJ-259 there are instructions in the manual that tell you how to do it.
There are reasons to cut coax to a specific length. I've personally used coax to match a collapsable 2m Yagi-Uda made of PVC with tape measure elements. I've also used a short length of 75 ohm coax as an impedance transformer to make the match of a dipole to 50 ohm coax nearly perfect. And yes, you can measure and cut coax to length so that you can see a nearly perfect representation of your antenna's VSWR at the radio's end of the coax.
Coax length doesn't change the antenna's match. It does change the VSWR reading at the radio's end. If you don't believe that it does then find a frequency where the VSWR is above 1.5:1, add a 6' coax jumper and take the VSWR again. It'll be different.
I personally don't bother with measuring coax, but I can see why some would. It's not that big a deal, and I don't know why some folks have such a problem with just accepting that there is a reason to trim coax, no matter how small. |
Tech808
Moderator
Username: Tech808
Post Number: 4800
Registered: 8-2002
| | Posted on Thursday, February 10, 2005 - 6:32 pm: |
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The Question asked and the Topic is NOT asking about the coax between the radio or amp and the antenna.
What would be an ideal length of coax between a radio and amplifier. Space not being an issue.
I may be wrong here but I do not think the MFJ-259 Antenna/SWR Analyzer can tell you what length you need to make a jumper between a radio and meter or radio and amp.
Lon
Tech808
CEF808
N9OSN 
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Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Thursday, February 10, 2005 - 7:21 pm: |
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Lon - Why not just be direct and say what you want rather than relying on an animation for expression? |
Tech808
Moderator
Username: Tech808
Post Number: 4803
Registered: 8-2002
| | Posted on Thursday, February 10, 2005 - 8:26 pm: |
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Well, I was as direct as I could possibly be in my above post on this topic and it did not seem to work.
*************************************************
Posted on Wednesday, February 09, 2005 - 11:44 pm:
--------------------------------------------------------------------------------
Hmmmmm,
I thought the original question was:
What would be an ideal length of coax between a radio and amplifier. Space not being an issue.
In most setups I have seen and personally use for home or mobile use, the amplifier has been located within 1 to 4 feet of the radio, which makes for easy adjustments of the amplifier and radio.
And most generally a 3 or 6 foot jumper will handle 98% of most setups.
Any further makes it difficult to adjust / operate the amplifier and radio when changing frequencies / bands / modes.
It is just no fun when you are talking on AM to have to get up and move 4 or 5 feet to switch the amp to SSB or re-load it when you change frequency/bands/modes.
The question is not asking about the distance between the Amplifier and Antenna BUT between the Radio and the Amplifier.
The Closer the Better, and whatever length you need to use to make operation of BOTH the Radio and Amplifier easy for the user.
Simple Question = Simple Answer.
Just my nickels worth,
Lon
Tech808
CEF808
N9OSN
*************************************************
So then I figured it would help to draw attention to the Actual Question that was asked again.
Somehow it must have been overlooked so I had an idea. 
The question had nothing to do with, velocity factor, electrical length, antennas, resistance, impedance or VSWR.
I guess it worked.
Lon
Tech808
CEF808
N9OSN
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Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Thursday, February 10, 2005 - 9:00 pm: |
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The specific question was correctly answered by the very first responce "WHATEVER LENGTH NEATLY CONNECTS THEM", and then the topic of discussion was skewed to coaxial cable properties and the value of using specific lengths of coax. Some things were said that needed to be redressed and the discussion was fun. There was even a follow up question asked and answered in the mix. So why? You're not interested? |
Bc910
Intermediate Member
Username: Bc910
Post Number: 276
Registered: 7-2004
| | Posted on Thursday, February 10, 2005 - 11:46 pm: |
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duh....coax?
You mean I need something to connect my coat hanger to the radio, MAN!
I wish someone would'a told me that earlier
BC |
Tech808
Moderator
Username: Tech808
Post Number: 4805
Registered: 8-2002
| | Posted on Thursday, February 10, 2005 - 11:51 pm: |
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BC,
That's a good one.
Lon
Tech808
CEF808
N9OSN |
Hotwire
Junior Member
Username: Hotwire
Post Number: 47
Registered: 1-2005
| | Posted on Friday, February 11, 2005 - 11:28 am: |
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OK tech 808 my Question was answered very informative. Just the info I was looking for. I use a 1 foot jumper now. Amp operates very well atop my radio Your correct and all is well. Yes thank you very helpful sir. Didnt know I was going to start a thread of debate. I believe we have a crew of knowledgable people here. I am thankful for everyones kindness and the education found here at CEF. 73
hotwire
CEF491
Franklin,Indiana |
