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which one is the right formula.?????
- Thread starter 1iwilly
- Start date
The only one I use is:
Take coax length to be enough from antenna to your rig plus some for extra radio movement around place is mounted.
Always worked fine.
Mike
What are you trying to ask?
Use as much coax as you need to reach your antenna?
-Richard-
I use 492 x VF / frequency in order to cut coax close to a resonant 1/2 wavelength. I might use 468 / frequency to figure close to the 1/2 wavelength close to a resonant wire dipole, but I usually cut the wire a little long in case of error.
If you have some particular need for an electrical 1/2 wave length of coax, then use 492 times the velocity factor of that coax and divide by the frequency in Mhz, the answer will be in feet. Will that give you a guarantied electrical 1/2 wave length exactly? No, it won't. But it will certainly get you very close. Want an electrical 1/4 wave length? Cut that electrical 1/2 wave length in half and you got two of them.
Why won't they come out exactly? Because there are too many errors possible in manufacturing, cutting, and just plain human error.
And as already said, if there isn't a particularly good reason to use an 'electrical' length, then there's no need to use one.
- 'Doc
PS - The reason you see that '468' number for 1/2 wave lengths is because it's the result of 492 x VF of common wire, which is about 95%(0.95). It typically ends up a tad longer than necessary, but that's better than too short.
Why won't they come out exactly? Because there are too many errors possible in manufacturing, cutting, and just plain human error.
And as already said, if there isn't a particularly good reason to use an 'electrical' length, then there's no need to use one.
- 'Doc
PS - The reason you see that '468' number for 1/2 wave lengths is because it's the result of 492 x VF of common wire, which is about 95%(0.95). It typically ends up a tad longer than necessary, but that's better than too short.
When building an antenna to figure iut length such as a wire or dipole you would use 468 / frequency where you want it to work. Such as if you us channel 38 you would do this 468 / 27.385 = 17 feet is what you get.
As far as coax Ive heard so many theories like its gotta be in increments of 3 feet or 25 feet but yet I have used 2 foot if not shorter pieces and made jumpers with no trouble so there is indeed a mystery in there somewhere I just havent found it or experienced it.
As far as coax Ive heard so many theories like its gotta be in increments of 3 feet or 25 feet but yet I have used 2 foot if not shorter pieces and made jumpers with no trouble so there is indeed a mystery in there somewhere I just havent found it or experienced it.
Set the antenna with an antenna analyzer, and cut the coax the length you need plus a bit.
If you are trying to set the "SWR of the system" with different coax lengths using competition style transistor amplifiers, like I think you are, then you'll be pulling your hair out soon.
I think I remember you asking this same question a month or two ago, and it appears that you are still fiddling with it.
BREAK OUT THE SOLDEING IRON
If you are trying to set the "SWR of the system" with different coax lengths using competition style transistor amplifiers, like I think you are, then you'll be pulling your hair out soon.
I think I remember you asking this same question a month or two ago, and it appears that you are still fiddling with it.
BREAK OUT THE SOLDEING IRON
Cutting coax to change SWR = pure snake oil.
It's been disproven thousands of times.
Use the highest quality coax you can afford, with a size and flexibility that fits your installation.
-Richard-
It's been disproven thousands of times.
Use the highest quality coax you can afford, with a size and flexibility that fits your installation.
-Richard-
That's right, on the base with tube stuff, I never have a problem with coax length.
In the truck I run a G/L and a 1x4 XForce. I set the Wilson with my MFJ-259b and run it. When I look at the SWR running that amp it scrares me, so I just don't look at it
no my main concern was just from antena to box. i'm using belden 9913f vf=.84. right now i have 12ft of the 9913f and after some reading if i do it by the formula it should be 14ft for the 468 and 15ft for the 492. i was told to set the swr with a barefoot radio to 1.1 if it goes up with amp on to play with the jumper lengh to bring it down??? also if i'm using the 12ft to bird meter then a 6 ft jumper from meter to box does that mean that i have 18ft.4' of coax??and when you add the 6ft from radio to box is that now 24ft.4' or does it stay 18ft 4' or 12ft. sorry for the dumm question just curious.
Honestly? I'd use a short piece of coax to set the antenna using my MFJ-259b. After that I'd cut the coax the lenght that I needed to reach between the radio, boxes and antenna and call it a day.
Unless you want to go and add filtering and feedback circuits to the amp/amps in question, then that's a different thread.
HERE Here is what you are trying to do. You are trying to find a low voltage area to place your meter, when nothing else in the system has changed.
Unless you want to go and add filtering and feedback circuits to the amp/amps in question, then that's a different thread.
HERE Here is what you are trying to do. You are trying to find a low voltage area to place your meter, when nothing else in the system has changed.
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That's right, on the base with tube stuff, I never have a problem with coax length.
In the truck I run a G/L and a 1x4 XForce. I set the Wilson with my MFJ-259b and run it. When I look at the SWR running that amp it scrares me, so I just don't look at it
All my transistor amps show only .1 higher swr reading than the radio that is txing through them, well the one's I've pruchased but the one's I've built and tuned myself have the same swr reading as the txer does no matter what length coax I use unless there is not sufficient counterpoise as in my old chevy pickup with the antenna mounted to the far rear bed corner but when mounted center of the roof line there is no difference.
It's a fairly simple thing to understand. You want to get something from point 'A' to point 'B'. That 'something' has a certain 'size' and 'shape'. So, the path from 'A' to 'B' has to be a certain size and shape to let that 'something' pass along it without a lot of trimming of that 'something' to fit the path, or without a lot of work changing that path. That's reasonable, isn't it?
Once that 'something' gets to that point 'B', the door in point 'B' has to be of the right size and shape so that 'something' can pass easily into point 'B' without having to be changed to fit that door. Or the door has to be worked on so that the 'something' can pass through it. That's reasonable too, right?
One of the requirements is that the 'something' going from point 'A' to point 'B' can't or shouldn't change size or shape. If it does change then you wasted all that work in making that 'something' into the size and shape you wanted, it isn't 'perfect' anymore, real bummer! So, that means you should have a "yardstick" (measuring device) that can tell you if the 'path' you want the 'something' to travel is the right shape and size. That "yardstick" can also tell you if the door into point 'B' is the right size and shape. So that means that you'd better know how to use that "yardstick" (measuring device), right?
So what happens if that door, for instance, is the right size and shape, but it's rotated 90 degrees? That 'something' will still fit through it if you tilt the 'something' sideways, right? Yeah, but then all the 'stuff' inside that 'something' will fall out, so you can't do that! RATS, another bummer!
So you do all the measuring you should, make sure things are the right size and shape for that 'something' to get from point 'A' to point 'B'. Things is fine and dandy! Ain't no bummers in sight! [Bare-foot radio feed line and antenna.]
Uh oh, but now there's a stop along the path where the 'something' is made prettier in some way (insert one amplifier along that path, or switch box, or whatever). The first thing to do is to get that "yardstick" out and do some measuring where needed, right? That usually means the doors into and out of that stop along the path. So the door into that stop along the way ought'a be the same size shape as the path and door into point 'B' is, right? Yep, so make sure. The next thing to use that "yardstick" on is the door out of that stop along the path. That exit door has to be of the right size shape so that we don't scrape any of the new paint off of that 'something', or bend the new tailfins. Okay, so measure it. It really isn't that difficult an idea is it?
So what happens if one of those doors into or out of the stop along the path is the wrong size? Do you modify the size and shape of that 'something', or modify the doors? [Input and output impedances of the amplifier.]
Are you following this so far? So what would you do? Would you change the path going to the stop along the path so that the path reshapes the 'something' to fit the doors going into that stop along the way? Or, would you have the door fixed to fit the 'something'? Do you want to work out of your wallet to fix that door or path, or would you rather the contractor who built the 'stop along the way' use his wallet to do it right to start with? Same thing holds true for the door out of that stop along the way, or your wallet get's emptied to fix the path and door into that point 'B'.
Again, what would you do?
Seems like an easy question to me (and my wallet)!
- 'Doc
Once that 'something' gets to that point 'B', the door in point 'B' has to be of the right size and shape so that 'something' can pass easily into point 'B' without having to be changed to fit that door. Or the door has to be worked on so that the 'something' can pass through it. That's reasonable too, right?
One of the requirements is that the 'something' going from point 'A' to point 'B' can't or shouldn't change size or shape. If it does change then you wasted all that work in making that 'something' into the size and shape you wanted, it isn't 'perfect' anymore, real bummer! So, that means you should have a "yardstick" (measuring device) that can tell you if the 'path' you want the 'something' to travel is the right shape and size. That "yardstick" can also tell you if the door into point 'B' is the right size and shape. So that means that you'd better know how to use that "yardstick" (measuring device), right?
So what happens if that door, for instance, is the right size and shape, but it's rotated 90 degrees? That 'something' will still fit through it if you tilt the 'something' sideways, right? Yeah, but then all the 'stuff' inside that 'something' will fall out, so you can't do that! RATS, another bummer!
So you do all the measuring you should, make sure things are the right size and shape for that 'something' to get from point 'A' to point 'B'. Things is fine and dandy! Ain't no bummers in sight! [Bare-foot radio feed line and antenna.]
Uh oh, but now there's a stop along the path where the 'something' is made prettier in some way (insert one amplifier along that path, or switch box, or whatever). The first thing to do is to get that "yardstick" out and do some measuring where needed, right? That usually means the doors into and out of that stop along the path. So the door into that stop along the way ought'a be the same size shape as the path and door into point 'B' is, right? Yep, so make sure. The next thing to use that "yardstick" on is the door out of that stop along the path. That exit door has to be of the right size shape so that we don't scrape any of the new paint off of that 'something', or bend the new tailfins. Okay, so measure it. It really isn't that difficult an idea is it?
So what happens if one of those doors into or out of the stop along the path is the wrong size? Do you modify the size and shape of that 'something', or modify the doors? [Input and output impedances of the amplifier.]
Are you following this so far? So what would you do? Would you change the path going to the stop along the path so that the path reshapes the 'something' to fit the doors going into that stop along the way? Or, would you have the door fixed to fit the 'something'? Do you want to work out of your wallet to fix that door or path, or would you rather the contractor who built the 'stop along the way' use his wallet to do it right to start with? Same thing holds true for the door out of that stop along the way, or your wallet get's emptied to fix the path and door into that point 'B'.
Again, what would you do?
Seems like an easy question to me (and my wallet)!
- 'Doc
If your coax velocity factor is .84 and you want your 1/2 wave multiples/antenna centred on 27.185 mhz then your looking at 15.2feet or 15' 2 1/2 inches.
if you want to repeat the antenna swr to the meter,then repeat that swr back to the amp,s output,and also repeat the amps input impedance back to the radio the safest policy is to use 3x15' 21/2" leads.the meter isn't a load its a pass through device so shouldn't affect your swr readings,
the linear input is a load so
if after that the swr is high at the radio then the input swr of the amp is poor,due to poor design of the amp.a good amp should have variable input tuning,most cb transistor ones don't.
you could mess around with the lead between the amp and radio to fix that but it won't have any effect on antenna swr,only what the radio see's,if the radio has high swr protection circuits it will cut power output if it doesn't see a good swr,another way you could do it is keep the leads the same size and use a matcher at the radio end which would give you a conjugate match if set right and prevent the radio dropping power.which is a similar effect to messing with the lead length.
if you have poor grounding you could wrap both leads between radio and amp and amp and swr meter into a coil to form an rf choke which will reduce common mode current.but the best solution when running power is a good grounded antenna.
bottom line is nothing you do with coax length will change the ANTENNA swr,but it can affect readings elsewhere,especially at the radio end which is undesireable as it could trigger the radios protection circuits and reduce output power.some radios are more forgiving than others in that dept,i've had radios that would tx into very high swr with minimal power reduction and others like the ts50 that shut down tx completely above 2.0:1 swr.
if you want to repeat the antenna swr to the meter,then repeat that swr back to the amp,s output,and also repeat the amps input impedance back to the radio the safest policy is to use 3x15' 21/2" leads.the meter isn't a load its a pass through device so shouldn't affect your swr readings,
the linear input is a load so
if after that the swr is high at the radio then the input swr of the amp is poor,due to poor design of the amp.a good amp should have variable input tuning,most cb transistor ones don't.
you could mess around with the lead between the amp and radio to fix that but it won't have any effect on antenna swr,only what the radio see's,if the radio has high swr protection circuits it will cut power output if it doesn't see a good swr,another way you could do it is keep the leads the same size and use a matcher at the radio end which would give you a conjugate match if set right and prevent the radio dropping power.which is a similar effect to messing with the lead length.
if you have poor grounding you could wrap both leads between radio and amp and amp and swr meter into a coil to form an rf choke which will reduce common mode current.but the best solution when running power is a good grounded antenna.
bottom line is nothing you do with coax length will change the ANTENNA swr,but it can affect readings elsewhere,especially at the radio end which is undesireable as it could trigger the radios protection circuits and reduce output power.some radios are more forgiving than others in that dept,i've had radios that would tx into very high swr with minimal power reduction and others like the ts50 that shut down tx completely above 2.0:1 swr.