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high swr with co-phased 102" steel whips
- Thread starter kamikaze701st
- Start date
well i just took off the mounts themselves and made sure everything was grounded good. swr is still at 2:1 across the board, so im pretty much stumped. the next step is to try the springs, but i didnt want to put springs in there because the 102's bounce around enough and springs are not gonna help at all with them moving around. any other suggestions?
701, before you change things, move your frequency up towards 10 meters and see if your match gets better. If it does then you just need to make the radiators longer. If it doesn't, then your ground is still not good enough to resonante the setup on the frequency you want. That is why I suggested you check each antenna on each mount individually, before trying to get the cophase to work. All you found out was you didn't find out anything by making your last change.
got the paint off the grounding areas, but that didnt even make a difference at all in the swr, still 2:1 across the board. i thought about trying springs, but i am reluctant as the antennas already move quite a bit. also thought about using the extensions off of some wilson 2000 trucker antennas i have. would raising the antennas up make a big difference?
now im also having another issue. i have an astatic mobile max echo box in my mobile and since i switched from my wilson 5000 (permanent mount) to the 102's, it squeals whenever i turn the amplifier on. i tried to ground the astatic box the the radio (from bracket knob to bracket knob) and it didnt help. the astatic box is inline with my microphone, as in the box plugs into the mic jack on the radio, and the mic plugs into the box.
kamikaze701st,
Do yourself a favor, take it one step at a time. Otherwise, you'll drive yourself nuts trying to do it all at once.
Antenna length.
The 'antenna' starts where the coax stops. Which typically means that 102" whip is 102" plus whatever the height of the mount is. An inch or two can make -some- difference as far as resonance goes, but not really all that much. If the antenna is resonant on one channel, it's going to be close enough to resonance on channels 8 - 10 away for it not to make any practical difference. You've got some 'skootch' room to play with.
Phasing harnesses are one of those thingys that are more important than others. They do two things, 'split' the signal so each antenna gets the proper amount, and make an impedance transformer to make matching the two antennas easier (not easy, just easier). They have to be a certain length determined by the velocity factor of the coax used, that's for the 'timing'. (A 'yardstick' 1/4 wave is not the same as an electrical 1/4 wave length, ain't that dandy?) They have to have some greater than 50 ohms imdepance so that when all those impedances are added together, they are at least close to 50 ohms. That almost never happens, so don't expect a '1:1' SWR, a 1.5:1 is nice. If you can get it closer than that, great. If not, don't worry about it.
Since you've "already been there" considering the equipment you have/had, I figure you know about the grounding stuff. It's just more of the same.
I would suggest just using the radio to get things set up, then worry about the amplifier, that one step at a time thingy. If it's 'good' at low power, it'll probably be just fine at higher power too.
That phasing harness is only what goes between the two antennas, nothing else. From the phasing harness to the radio is the feed line. You know that, I know, but making sure we're on the right page is nice. If that 12 foot phasing harness won't fit/reach between the two antennas, you've got a bigger truck than I'm familiar with! If that phasing harness really is 18 feet between antennas, it's just not going to work, it's too long. Take it back, throw it away, whatever. (I would also suggest NOT doing what you'd like to do with it. First, it won't fit, and you go to jail for that sort of stuff.)
Don't know where to go from here, so good luck.
- 'Doc
Do yourself a favor, take it one step at a time. Otherwise, you'll drive yourself nuts trying to do it all at once.
Antenna length.
The 'antenna' starts where the coax stops. Which typically means that 102" whip is 102" plus whatever the height of the mount is. An inch or two can make -some- difference as far as resonance goes, but not really all that much. If the antenna is resonant on one channel, it's going to be close enough to resonance on channels 8 - 10 away for it not to make any practical difference. You've got some 'skootch' room to play with.
Phasing harnesses are one of those thingys that are more important than others. They do two things, 'split' the signal so each antenna gets the proper amount, and make an impedance transformer to make matching the two antennas easier (not easy, just easier). They have to be a certain length determined by the velocity factor of the coax used, that's for the 'timing'. (A 'yardstick' 1/4 wave is not the same as an electrical 1/4 wave length, ain't that dandy?) They have to have some greater than 50 ohms imdepance so that when all those impedances are added together, they are at least close to 50 ohms. That almost never happens, so don't expect a '1:1' SWR, a 1.5:1 is nice. If you can get it closer than that, great. If not, don't worry about it.
Since you've "already been there" considering the equipment you have/had, I figure you know about the grounding stuff. It's just more of the same.
I would suggest just using the radio to get things set up, then worry about the amplifier, that one step at a time thingy. If it's 'good' at low power, it'll probably be just fine at higher power too.
That phasing harness is only what goes between the two antennas, nothing else. From the phasing harness to the radio is the feed line. You know that, I know, but making sure we're on the right page is nice. If that 12 foot phasing harness won't fit/reach between the two antennas, you've got a bigger truck than I'm familiar with! If that phasing harness really is 18 feet between antennas, it's just not going to work, it's too long. Take it back, throw it away, whatever. (I would also suggest NOT doing what you'd like to do with it. First, it won't fit, and you go to jail for that sort of stuff.)
Don't know where to go from here, so good luck.
- 'Doc
Does thee system work w/o the amp in line? Like Doc said, you are trying to make everything happen at once. Slow down and just get the radio working with the antenna - first. Eliminate the amp for now, and get it back in line AFTER everything is working with the co-phase.
Do you have a 3 foot jumper for the amp?
What length is it?
Do you have a 3 foot jumper for the amp?
What length is it?
with the amp in line but the amp off, it has a 2:1 swr. with the amp inline and on, the match drops to 1.5:1 on the low side and with the amp on high it drops to almost dead flat. the amp is attached with a 3 ft jumper from the radio to the amp.
W5LZ, as far as the harness is concerned, it is a pilot truck stop harness. the way it is built is that there is no "t" connection anywhere. it is two seperate pieces. one piece comes from each antenna, and they do not meet until the connector that goes to the radio, that is the only spot that the two pieces of coax join. as far as the 18 foot length of the harness, it is just long enough to have some room to move things a little, but a 12 ft harness wouldnt reach from the antennas to the radio because my truck is a crew cab. the 18 ft reaches when i run it down the middle of the cab (inside) to the radio. i havent even put it under the floor liner yet. so i know the 12 ft wouldnt work.
i tried a 6" shaft from my wilson 2000 trucker antenna on one side of the truck so that one of the 102's was higher, and it dropped from being a 2.2:1 to about a 1.7:1. if i add a shaft on the other side, it should drop the swr even more in theory right?
I'm not sure I understood what you are saying.
Did you add 6 inches of length to the 102 inch whip - or?
If that is the case, then adding a 6 inch spring may be the right course.
I'm assuming that you want to tune your antenna for 11 meters - then the 108 inch length is preferred.
Right?
Did you add 6 inches of length to the 102 inch whip - or?
If that is the case, then adding a 6 inch spring may be the right course.
I'm assuming that you want to tune your antenna for 11 meters - then the 108 inch length is preferred.
Right?
Rob, if you had ever worked with a 1/4 wave GP base type antenna with radials, you would know that the 108" length is not a rule, some how set in concrete as a magic length. In mobiles the radiator length depends a lot on the particular auto and how the antenna location sees the GP side of the setup.
Such preconceived ideas are likely why 701, is having problems with his co-phase setup, where he makes several changes at one time and hopes for some good results.
The 102" should be close enough to work the co-phase just fine if his ground plane is working right. He may find that he can change the length to a better resonance for where he wants to talk later after he gets the duals to working, but not before. If he has an easy way to adjust the length then do it, but say consistent with the length until he sees some good results with his duals first. The problems 701 is having are not tuning problems.
Such preconceived ideas are likely why 701, is having problems with his co-phase setup, where he makes several changes at one time and hopes for some good results.
The 102" should be close enough to work the co-phase just fine if his ground plane is working right. He may find that he can change the length to a better resonance for where he wants to talk later after he gets the duals to working, but not before. If he has an easy way to adjust the length then do it, but say consistent with the length until he sees some good results with his duals first. The problems 701 is having are not tuning problems.
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Yeah that's the standard co phase harness that you can buy from a lot of radio shops/truck stops. The 18' is still the wrong length to make this work properly. The coax is going into a 50 Ohm connector, plugged into a 50ohm output of the radio. That's like having about a 3 inch piece of 50 ohm coax attached. What you need to do is forget about amp for a minute and run enough 50 ohm coax from the radio until the 12' harness (you really should calculate the exact length, but the 12' probably is the right one) will reach. Then you can just use a barrel connector to attach the 12' harness to the other coax. Once you're satisfied with that SWR reading, then disconnect it from the radio, plug it into the back of the amp, and attach the amp with your jumper.
If you can't get back to the truck stop to get the 12' cophase harness, send me a PM...I can get them for you, or you can find somewhere else and order it online.
kamikaze701s,
I'm glad you took the time to describe that harness to me. It made me realize that I've been making an assumption I shouldn't have been making, and might have to apologize to who ever made that harness. Nasty thought, huh? It takes a little explaining.
The typical cophasing harness is a section of 75 ohm coax going to each antenna, then meeting in the middle where they join usually in a 'T' connection. From that 'T' you run ordinary 50 ohm coax to the radio. Because of how phasing works that combination of two 75 ohm impedances works out close to 50 ohms that the radio wants/expects to see from whatever is on the other end of that feed line. It isn't exactly 50 ohms but it's close enough to keep the radio happy if not jumping up and down with joy. Swr typically ends up somewhere around 1.5:1 or so, which is very good considering the "conjugate" impedance matching that's being/been done.
That 'matching section', commonly called a 'Q' section, one going to each antenna, has to be a certain length, an electrical 1/4 wave length long. That 'electrical' thingy is in there because a physical 1/4 wave length they measure with a yardstick is NOT the same as a 1/4 wave length that an electron travels in coax during 1/4th of a cycle. That has to do with the speed of an electron through stuff other than a vacuum, which is called a 'velocity factor'. An electron is faster in a vacuum than when going through something other than a vacuum, air, feed lines, wire, wood, whatever. So, an electrical 1/4 wave length of typical 75 ohm coax has a velocity factor (VF) of around 66%, an electrical 1/4 wave length is only 66% of the length of a 1/4 wave length meashured by a yardstick. So at 27 Mhz, instead of about 9 feet, it's really only about 6 feet long. So, that harness should be almost 12 feet from end to end, with a connector half way down it.
And now where I wasn't paying attention.
A characteristic of an electrical 1/4 wave length is that if the impedance is 'high' on one end of it, it'll be 'low' at the other end of it. That's what makes all this phasing stuff work, so that's good. But, another characteristic is that if you put two of those electrical 1/4 wave length end to end, what it sees at one end is what you will get at the other end. That's an electrical 1/2 wave length, by the way. They are nice because it's almost like it wasn't there at all, invisible sort of. so if one electrical 1/4 wave length isn't long enough to get from here to there, you can add two more electrical 1/4 wave lengths (or one electricl 1/2 wave length) to it and it's like that extra 12 feet or so wasn't there, and it now reaches from here to there. 'Odd' numbers of electrical 1/4 wave lengths changes things. 'Even' numbers of electrical 1/4 wave lengths do NOT change things. Makes things easier or more practical sometimes, right?
So, if each 'leg' of your harness is 18 feet of RG-59 coax, it's the equivalent of only 6 feet of RG-59 coax, which is just what is needed. @#$, it ought'a work.
Now for the 'catches', and there's always going to be a 'catch' or two.
If it's man-made, it ain't gonna be perfect. May only be 'off' just a tiny bit, but when you add all those 'tiny bits' up, they can get sort of big. Doing things 'right' always means a little more work and/or money. May still have a few of them 'tiny bits' wrong with it, but not near as many, sort of. ("Perfect" requires a huge amount of money/work!)
It seems there are enough of those 'tiny bits' in your harness that it ain't as close to 'right' as it ought to be. Saving a few dollars by not having that 'T' connector ~may~ be one of those 'tiny bits'. That coax NOT being as close to 75 ohms as it's thought to be is a much more likely 'catch'! That can vary like you wouldn't believe in the same roll of coax, much less between different rolls. the only sure way of telling is by measuring it each and every time, and that's just not real likely to happen.
So what does all that mess mean? It means your set up still ain't working, and that I screwed up in my thinking that it would never work. Still doesn't work, but I was wrong in the 'why' of it not working.
An awful 'wordy' excuse, right? Yeah, but it's how I had to think it through to see my mistake. Being 'slow' isn't always bad, but it's embarrassing. Sorry 'bout that.
- 'Doc
The Ultimate Resort!
Beat the @#$ thing to death with a hammer and take up knitting!
- A. Einstien
I'm glad you took the time to describe that harness to me. It made me realize that I've been making an assumption I shouldn't have been making, and might have to apologize to who ever made that harness. Nasty thought, huh? It takes a little explaining.
The typical cophasing harness is a section of 75 ohm coax going to each antenna, then meeting in the middle where they join usually in a 'T' connection. From that 'T' you run ordinary 50 ohm coax to the radio. Because of how phasing works that combination of two 75 ohm impedances works out close to 50 ohms that the radio wants/expects to see from whatever is on the other end of that feed line. It isn't exactly 50 ohms but it's close enough to keep the radio happy if not jumping up and down with joy. Swr typically ends up somewhere around 1.5:1 or so, which is very good considering the "conjugate" impedance matching that's being/been done.
That 'matching section', commonly called a 'Q' section, one going to each antenna, has to be a certain length, an electrical 1/4 wave length long. That 'electrical' thingy is in there because a physical 1/4 wave length they measure with a yardstick is NOT the same as a 1/4 wave length that an electron travels in coax during 1/4th of a cycle. That has to do with the speed of an electron through stuff other than a vacuum, which is called a 'velocity factor'. An electron is faster in a vacuum than when going through something other than a vacuum, air, feed lines, wire, wood, whatever. So, an electrical 1/4 wave length of typical 75 ohm coax has a velocity factor (VF) of around 66%, an electrical 1/4 wave length is only 66% of the length of a 1/4 wave length meashured by a yardstick. So at 27 Mhz, instead of about 9 feet, it's really only about 6 feet long. So, that harness should be almost 12 feet from end to end, with a connector half way down it.
And now where I wasn't paying attention.
A characteristic of an electrical 1/4 wave length is that if the impedance is 'high' on one end of it, it'll be 'low' at the other end of it. That's what makes all this phasing stuff work, so that's good. But, another characteristic is that if you put two of those electrical 1/4 wave length end to end, what it sees at one end is what you will get at the other end. That's an electrical 1/2 wave length, by the way. They are nice because it's almost like it wasn't there at all, invisible sort of. so if one electrical 1/4 wave length isn't long enough to get from here to there, you can add two more electrical 1/4 wave lengths (or one electricl 1/2 wave length) to it and it's like that extra 12 feet or so wasn't there, and it now reaches from here to there. 'Odd' numbers of electrical 1/4 wave lengths changes things. 'Even' numbers of electrical 1/4 wave lengths do NOT change things. Makes things easier or more practical sometimes, right?
So, if each 'leg' of your harness is 18 feet of RG-59 coax, it's the equivalent of only 6 feet of RG-59 coax, which is just what is needed. @#$, it ought'a work.
Now for the 'catches', and there's always going to be a 'catch' or two.
If it's man-made, it ain't gonna be perfect. May only be 'off' just a tiny bit, but when you add all those 'tiny bits' up, they can get sort of big. Doing things 'right' always means a little more work and/or money. May still have a few of them 'tiny bits' wrong with it, but not near as many, sort of. ("Perfect" requires a huge amount of money/work!)
It seems there are enough of those 'tiny bits' in your harness that it ain't as close to 'right' as it ought to be. Saving a few dollars by not having that 'T' connector ~may~ be one of those 'tiny bits'. That coax NOT being as close to 75 ohms as it's thought to be is a much more likely 'catch'! That can vary like you wouldn't believe in the same roll of coax, much less between different rolls. the only sure way of telling is by measuring it each and every time, and that's just not real likely to happen.
So what does all that mess mean? It means your set up still ain't working, and that I screwed up in my thinking that it would never work. Still doesn't work, but I was wrong in the 'why' of it not working.
An awful 'wordy' excuse, right? Yeah, but it's how I had to think it through to see my mistake. Being 'slow' isn't always bad, but it's embarrassing. Sorry 'bout that.
- 'Doc
The Ultimate Resort!
Beat the @#$ thing to death with a hammer and take up knitting!
- A. Einstien
KAMIKAZE -
If you are using the SWR meter in the radio to check this out, having the linear in-line will affect the readings even when the linear is not turned on. If you don't have an external meter to use then remove the linear and its jumper when doing the SWR testing. To check for problems with the harness, disconnect the antennas and connect a 50 ohm dummy load to each end of the harness and measure SWR. It should be 1.2:1 or less. If it's not then there is a harness problem. If SWR is OK with the dummy loads, then mount one of the antennas and leave the dummy load in place on the other line. Measure SWR. Then do this with the second antenna. The SWR readings should be very close to being the same. This will tell you how well the antennas match up with each other.
Also, when the antennas are mounted to the corners of the bed, they are probably very close to the cab and there could be some capacitive coupling there. This will affect SWR. Try mounting the antennas to the bed rails a foot or so further back from the cab. Just hold them in place with a vice grip or a clamp. If SWR goes down, then antenna-to-cab capacitive coupling is probably the problem.
BTW...a quote from Lou Franklin in " UNDERSSTANDING AND REPAIRING CB RADIOS FOR THE PROFESSIONAL TECHNICIAN" (page 329)
" I never recommend dual antennas except on big trucks or buses, where a distance close to the correct 1/2 wavelength spacing of 17' can be realized. On a smaller vehicle they're a total waste of money. "
- 399
If you are using the SWR meter in the radio to check this out, having the linear in-line will affect the readings even when the linear is not turned on. If you don't have an external meter to use then remove the linear and its jumper when doing the SWR testing. To check for problems with the harness, disconnect the antennas and connect a 50 ohm dummy load to each end of the harness and measure SWR. It should be 1.2:1 or less. If it's not then there is a harness problem. If SWR is OK with the dummy loads, then mount one of the antennas and leave the dummy load in place on the other line. Measure SWR. Then do this with the second antenna. The SWR readings should be very close to being the same. This will tell you how well the antennas match up with each other.
Also, when the antennas are mounted to the corners of the bed, they are probably very close to the cab and there could be some capacitive coupling there. This will affect SWR. Try mounting the antennas to the bed rails a foot or so further back from the cab. Just hold them in place with a vice grip or a clamp. If SWR goes down, then antenna-to-cab capacitive coupling is probably the problem.
BTW...a quote from Lou Franklin in " UNDERSSTANDING AND REPAIRING CB RADIOS FOR THE PROFESSIONAL TECHNICIAN" (page 329)
" I never recommend dual antennas except on big trucks or buses, where a distance close to the correct 1/2 wavelength spacing of 17' can be realized. On a smaller vehicle they're a total waste of money. "
- 399