Go Back   WorldwideDX Amateur Radio Forums > Citizens Band Related > CB Band and CB Radios


Amateur Callsign Lookup
Enter Callsign:

Antenna Feed Line Length

Reply
 
LinkBack Thread Tools Display Modes
  #9  
Old 12-11-2006, 09:30 PM
Senior Member
iTrader: (0)
 
Join Date: Mar 2006
Location: Hermiston, OR
Posts: 320
Hamin' X is on a distinguished road
Default


Just as an example of what I have been saying about coax length, here is what happened to me several years ago.

I installed a screwdriver antenna on my pickup bed rail and used it with my 706m2g. Everything was fine 80-10 except for 7250-7300kHz on 40 meters. Now I check into a net on 7268.5 every day, so this situation was not acceptable. The internal SWR meter on the 706 and the FS meter that I tune the screwdriver by both agreed, things went south above 7250. Back to the shack to grab the Bird 43 and a 12" jumper for a closer look at the antenna feedpoint. Hooked up the 43 and jumper cable and WTF, it tunes perfectly. Back and forth a few times, clean the grounds, check connections, no change. With jumper and meter in-line , the antenna worked perfectly. I replaced the feedline with one about 16 inches longer and it still works perfectly.

Moral of the story: If it works, use it. If you want it to work every time, figure out why it worked. In this case, there was obviously something wrong with the installation, but the coax change made the radio happy and the signal reports are great, so it is not worth my time to delve any deeper into the problem. As far as I am concerned, problem solved.

Rich
__________________
Life is hard, let's not make it even harder by being stupid.

Reply With Quote
  #10  
Old 12-12-2006, 11:19 AM
BANNED
iTrader: (1)
 
Join Date: Apr 2005
Location: non-cartesian cyberspace
Posts: 1,450
freecell is an unknown quantity at this point
Default

SWR has virtually nothing to do with antenna radiation efficiency. once again this is demonstrated best by terminating the load end of the feedline with a value of resistance that is identical to the Zo of the line, as in the case of an rf "dummy" load. it's further demonstrated by the fact that free space has an intrinsic impedance value of approximately 377 ohms. it's funny to see you all pontificating over your myths when the real problem presented by SWR in any system is the reduction in the amount of power delivered to the load, a problem that is easily addressed and remedied once it is realized that whenever the load does not equal the Zo (characteristic impedance) of the line that the line impedance is determined by a widely varying range of values of E/I all along the line which can be used to restore full power output from the (transmitter) source to the load. that's the real problem.

antenna efficiency is determined by BIRP. Balance, Impedance, Resonance and Pattern, only one of which has anything remotely to do with SWR. what has escaped most of you is the fact that once the impedance of the line becomes anything but Zo and if the values of E/I along the line are known that those points can be used to terminate the line and provide the necessary conjugate interference to match the output of the transmitter to a line input impedance that is anything but 50 ohms. this solves the problem created by the load mismatch presented at the other end of the line.

an SWR meter doesn't measure antenna efficiency any more than a flat SWR is a guarantee of optimum antenna performance. if you're feeding a balanced antenna with coaxial feedline (which destroys the pattern) and the antenna is not resonant at the frequency of choice then achieving a flat swr is meaningless. by the same token, antenna systems are floating around in space with swr present that would make any cb'er (and some hams) squeamish while the transmitters feeding them are delivering over 90+% of their available power to those same mismatched antennas.

the principles at work here are the same ones that require the francis line of antennas (under 8') to be used with specific lengths of differing types of feedline for best performance (and lowest swr) when used singly and the same principles involved in a situation that arose recently when one or two customers called me complaining that their PowerStikII antennas (which matched well in their previous tractor rigs) were not seeing the usual low swr in their new Freightliner XL tractors with the original 18' lines. quite the contrary, the swr has risen to 3:1 when relocated to the new tractors. one of the drivers with which i have spent time on several occasions explaining to him how antennas and feedlines operate, particularly under mismatch conditions, decided to take my advice and replace the 18' RG8M lines with 9' RG58 lines and was happy to see that the swr (as measured from the line input) dropped from over 3:1 to just over 1:1 when the 9' lines of RG58 were installed.

in a line with a Zo of 50 ohms and a load that represents anything but 50 ohms the differing readings that are seen using the swr meter when placed in various spots in the line are only fooling you if you don't know what they're trying to tell you and specifically what that is is that since VSWR=EMAX/EMIN what it's trying to tell you is that the impedance along the line varies with the placement location of the bridge in the line, which makes absolute sense since in an unmatched line impedance (Z=E/I) changes from one spot in the line to the next.

the only time a feedline with a Zo of 50 ohms is exactly that occurs only under two sets of conditions: either the transmitter, (source) the antenna (load) and the feedline are all presenting an impedance of 50 ohms or the feedline is unterminated at either end in which case the characteristic impedance of the feedline is solely dependent on its physical characteristics only, length not being one of them. whenever a source and a load are attached where either deviates from the Zo of the line, then and only then does the range of dynamic impedance values appear on the line.

Z (Impedance) = E (Voltage) / (Divided By) I (Current)
for example, given that E and I measured at a particular spot in the line are both equal to 50 volts and 1 amp respectively, the impedance at that point in the line is 50 ohms. i can assure you this is only the case where both the source and the load are both equal to the Zo (characteristic impedance) of the line.

where it is otherwise the line reverts to a rather wide ranging set of values that are easily visible at *multiple spots along the line, especially if that line is *longer than 180 degrees at the operating frequency in question. real resistance values are known to be present minimally anywhere from 33 - 78 ohms wholly dependent on the length of the line for absolute terminating values.

as to the question of the tuned 1/2 wave line. do you want to use it? if you do then know this. when you do the exact values of resistance and reactance that are present at the feedpoint of the antenna will be mirrored almost exactly at the transmitter input to the line. that's not a bad idea if you can measure and know that the resistance at the feedpoint is + or - 50 ohms and the reactance is near a zero net value. if they're say 33 ohms and j-15 ohms capacitively reactive you have to ask yourself a question. do you really want those exact conditions to exist at the transmitter? only if the transmitter is equipped with its own adjustable PA matching circuit. with a fixed 50 ohm transmitter output and 33 j-15 at the antenna the last thing you want to use is a tuned 1/2 wave line, unless you want reduced transmitter power output or maybe you just want to be able to see the conditions at the antenna feedpoint as it may be inconvenient to make measurements at the antenna for one reason or another. if the transmitter is fixed 50 ohm output then you may want to consider the fact that what's going on with the match between the transmitter and the line might be of equal if not somewhat more concern since it can be managed from either end of the line anyway. that's what the AT in the nomenclature in TS850SAT does. it's not really an antenna tuner per se since what it really does is makes sure that the transmitter is always working into as close to a 50 ohm line input as possible at all times, regardless of the mismatch at the load
up to a certain point. ranges of matching values achieved by these "antenna tuners" are anywhere from 16 - 150 ohms and wider. the same thing to a lesser extent (33 - 78) can easily be done by knowing where these values of impedance can be found along an unmatched line under similar conditions.

every length of feedline is a matching transformer....Cebik.

Reply With Quote
  #11  
Old 12-12-2006, 01:24 PM
Senior Member
iTrader: (0)
 
Join Date: Mar 2005
Location: england
Posts: 2,104
bob85 is on a distinguished road
Default

thanks freecell, now how much pain am i in?? lol.
__________________

tom bearden taught me that quantum entanglement allows thought to travell faster than the speed of light, this explains why at first you can think somebody is smart but if you wait a while you will see they are not very bright at all.


Reply With Quote
  #12  
Old 12-14-2006, 07:26 AM
Junior Member
iTrader: (0)
 
Join Date: Dec 2006
Location: Alexandria, Virginia
Posts: 19
Paris is on a distinguished road
Default Death, Taxes, and Coax Woes

In an ideal system, the length of the coax antenna feedline should not make any difference to either the antennaís SWR or its efficiency, see ARRL Handbook, 2006 Edition, Sec 21.1. However, as others have pointed, out its length is often critical real-world mobile installations. I suggest (as Hamin' X did) that this can only happen if the coax shield is radiating RF energy. In order for the coax antenna lead to work as an ideal feedline, the current carried on the outer braid must be equal and opposite from the transmitted signal traveling along its core; otherwise, the signal attempting to radiate from the core cannot be cancelled out. The ideal feedline does not radiate any of the transmitted signal, and only the antenna forms the resistive load. However, if the feedline does leak, it must then be considered part of the antenna, and itís length will certainly effect both efficiency and SWR.

When you think about it, it seems inevitable that you will not be able to achieve an ideal ground in your car or truck. Almost certainly the chassis to which the coax shield is grounded will carry stray RF from whatever electronic control systems are found in modern cars, including the ignition, the CPU/ECU, the fuel injectors, the fuel pump, among others; not only this, but chassis itself is probably segmented, and unable to act as an efficient ground plane. The result will be common-mode currents in the coax shield not be present in the core, which will, in turn, allow the transmitted RF to escape from the core. In short, your feedline in a mobile installation will almost certainly become part of your antenna.

Since the nature of the stray current introduced onto the outer braid of the coax feedline cannot be predicted, it would follow that it is impossible to predict any specific length of feedline necessary to achieve a low SWR. Thus, the proper length of your feedline will have to be determined by experimentation on a case by case basis, but itís important to understand that it will make a difference, nevertheless.

Perhaps one solution is to coil the feedline so that it acts as an RF choke, as Iíve seen suggested in some articles, but then other writers advise strongly against doing that. Has anyone tried this?

Reply With Quote
  #13  
Old 12-14-2006, 09:45 AM
Senior Member
iTrader: (0)
 
Join Date: Jul 2006
Location: 6000 feet above the Mojave Desert, Tehachapi, Ca
Posts: 481
Toll_Free is on a distinguished road
Default

Quote:
Originally Posted by Hamin' X
Ah Grasshopper, you have discovered one of the great mysteries and your BS filter is working well. These people that tell you to use a specific length of coax, don't have a clue. I was reading the back of a packaged Francis 5.5 Hotrod antenna, the other day. It was saying to use different lengths of RG-58, RG-8X, or RG-59 coax, depending upon your installation. While it is true that when you co-phase two 50 ohm antennas, that they be fed with 75 ohm coax, of odd 1/4 wave electrical multiples, you have to know the velocity factor of the coax used. Just because it is RG-59, tells you nothing. Depending upon the construction of the coax, you could have a velocity factor of .66 to .95. A big difference when calculating electrical lengths. It is the same with all coax.

The long and short of it is: With a single antenna, the correct length of coax is the amount it takes to reach the antenna. If you are having to use a specific length, then the coax has become part of the antenna and you don't want any part of it in a mobile installation. Anybody that tries to tell you different is just perpetuating a myth.

Rich

And the long of it is, if you use a piece of coax that is not a multiple of a quarter wave, electrically, and you don't measure your VSWR at the FEEDPOINT of the antenna, you don't have a friggin clue as to what your real feedpoint match is.

Don't buy into this. The proper way, is to use an electrical quarter wave to match your system. Then, use as long a piece as necessary, if you want to. I stick with the electrical quarter waves. Works for me, and I KNOW my measurements are accurate, not reading the capacitance values in the coaxial cable itself, and not reading a skewed reading from the feedpoint reactance.

--Toll_Free

Reply With Quote
  #14  
Old 12-14-2006, 09:49 AM
Senior Member
iTrader: (0)
 
Join Date: Jul 2006
Location: 6000 feet above the Mojave Desert, Tehachapi, Ca
Posts: 481
Toll_Free is on a distinguished road
Default

Quote:
Originally Posted by Hamin' X
Just as an example of what I have been saying about coax length, here is what happened to me several years ago.

I installed a screwdriver antenna on my pickup bed rail and used it with my 706m2g. Everything was fine 80-10 except for 7250-7300kHz on 40 meters. Now I check into a net on 7268.5 every day, so this situation was not acceptable. The internal SWR meter on the 706 and the FS meter that I tune the screwdriver by both agreed, things went south above 7250. Back to the shack to grab the Bird 43 and a 12" jumper for a closer look at the antenna feedpoint. Hooked up the 43 and jumper cable and WTF, it tunes perfectly. Back and forth a few times, clean the grounds, check connections, no change. With jumper and meter in-line , the antenna worked perfectly. I replaced the feedline with one about 16 inches longer and it still works perfectly.

Moral of the story: If it works, use it. If you want it to work every time, figure out why it worked. In this case, there was obviously something wrong with the installation, but the coax change made the radio happy and the signal reports are great, so it is not worth my time to delve any deeper into the problem. As far as I am concerned, problem solved.

Rich
Well, that's one way to look at it.

I guess the other way is that coaxial cable length can play into account when your trying to get a flat match, and don't know what the complex impedance is on the other side, huh?

Let me get this straight. When you measured as close to the feedpoint as possible, it worked OK, and you got a smaller amount of reflected power.

But, when you put a piece on that was "convenient", your radio saw a worse match.

And you think that your coaxial cable isn't being used as a radiator? As part of the counterpoise of your antenna system?

Please, I beg you. Explain this? How a good match at your feedpoint can be repeated as a bad match at the 'OTHER' end of the coaxial cable, but it really doesn't mean anything, other than the foldback circuit in the radio will cut your power back, you won't see max power transfer, and you can see hotspots on the coax, the radio, etc.

Not to mention, this causes more stray RF around.

--Toll_Free



--Toll_Free

Reply With Quote
  #15  
Old 12-14-2006, 09:50 AM
Senior Member
iTrader: (0)
 
Join Date: Jul 2006
Location: 6000 feet above the Mojave Desert, Tehachapi, Ca
Posts: 481
Toll_Free is on a distinguished road
Default

Quote:
Originally Posted by freecell
SWR has virtually nothing to do with antenna radiation efficiency. once again this is demonstrated best by terminating the load end of the feedline with a value of resistance that is identical to the Zo of the line, as in the case of an rf "dummy" load. it's further demonstrated by the fact that free space has an intrinsic impedance value of approximately 377 ohms. it's funny to see you all pontificating over your myths when the real problem presented by SWR in any system is the reduction in the amount of power delivered to the load, a problem that is easily addressed and remedied once it is realized that whenever the load does not equal the Zo (characteristic impedance) of the line that the line impedance is determined by a widely varying range of values of E/I all along the line which can be used to restore full power output from the (transmitter) source to the load. that's the real problem.

antenna efficiency is determined by BIRP. Balance, Impedance, Resonance and Pattern, only one of which has anything remotely to do with SWR. what has escaped most of you is the fact that once the impedance of the line becomes anything but Zo and if the values of E/I along the line are known that those points can be used to terminate the line and provide the necessary conjugate interference to match the output of the transmitter to a line input impedance that is anything but 50 ohms. this solves the problem created by the load mismatch presented at the other end of the line.

an SWR meter doesn't measure antenna efficiency any more than a flat SWR is a guarantee of optimum antenna performance. if you're feeding a balanced antenna with coaxial feedline (which destroys the pattern) and the antenna is not resonant at the frequency of choice then achieving a flat swr is meaningless. by the same token, antenna systems are floating around in space with swr present that would make any cb'er (and some hams) squeamish while the transmitters feeding them are delivering over 90+% of their available power to those same mismatched antennas.

the principles at work here are the same ones that require the francis line of antennas (under 8') to be used with specific lengths of differing types of feedline for best performance (and lowest swr) when used singly and the same principles involved in a situation that arose recently when one or two customers called me complaining that their PowerStikII antennas (which matched well in their previous tractor rigs) were not seeing the usual low swr in their new Freightliner XL tractors with the original 18' lines. quite the contrary, the swr has risen to 3:1 when relocated to the new tractors. one of the drivers with which i have spent time on several occasions explaining to him how antennas and feedlines operate, particularly under mismatch conditions, decided to take my advice and replace the 18' RG8M lines with 9' RG58 lines and was happy to see that the swr (as measured from the line input) dropped from over 3:1 to just over 1:1 when the 9' lines of RG58 were installed.

in a line with a Zo of 50 ohms and a load that represents anything but 50 ohms the differing readings that are seen using the swr meter when placed in various spots in the line are only fooling you if you don't know what they're trying to tell you and specifically what that is is that since VSWR=EMAX/EMIN what it's trying to tell you is that the impedance along the line varies with the placement location of the bridge in the line, which makes absolute sense since in an unmatched line impedance (Z=E/I) changes from one spot in the line to the next.

the only time a feedline with a Zo of 50 ohms is exactly that occurs only under two sets of conditions: either the transmitter, (source) the antenna (load) and the feedline are all presenting an impedance of 50 ohms or the feedline is unterminated at either end in which case the characteristic impedance of the feedline is solely dependent on its physical characteristics only, length not being one of them. whenever a source and a load are attached where either deviates from the Zo of the line, then and only then does the range of dynamic impedance values appear on the line.

Z (Impedance) = E (Voltage) / (Divided By) I (Current)
for example, given that E and I measured at a particular spot in the line are both equal to 50 volts and 1 amp respectively, the impedance at that point in the line is 50 ohms. i can assure you this is only the case where both the source and the load are both equal to the Zo (characteristic impedance) of the line.

where it is otherwise the line reverts to a rather wide ranging set of values that are easily visible at *multiple spots along the line, especially if that line is *longer than 180 degrees at the operating frequency in question. real resistance values are known to be present minimally anywhere from 33 - 78 ohms wholly dependent on the length of the line for absolute terminating values.

as to the question of the tuned 1/2 wave line. do you want to use it? if you do then know this. when you do the exact values of resistance and reactance that are present at the feedpoint of the antenna will be mirrored almost exactly at the transmitter input to the line. that's not a bad idea if you can measure and know that the resistance at the feedpoint is + or - 50 ohms and the reactance is near a zero net value. if they're say 33 ohms and j-15 ohms capacitively reactive you have to ask yourself a question. do you really want those exact conditions to exist at the transmitter? only if the transmitter is equipped with its own adjustable PA matching circuit. with a fixed 50 ohm transmitter output and 33 j-15 at the antenna the last thing you want to use is a tuned 1/2 wave line, unless you want reduced transmitter power output or maybe you just want to be able to see the conditions at the antenna feedpoint as it may be inconvenient to make measurements at the antenna for one reason or another. if the transmitter is fixed 50 ohm output then you may want to consider the fact that what's going on with the match between the transmitter and the line might be of equal if not somewhat more concern since it can be managed from either end of the line anyway. that's what the AT in the nomenclature in TS850SAT does. it's not really an antenna tuner per se since what it really does is makes sure that the transmitter is always working into as close to a 50 ohm line input as possible at all times, regardless of the mismatch at the load
up to a certain point. ranges of matching values achieved by these "antenna tuners" are anywhere from 16 - 150 ohms and wider. the same thing to a lesser extent (33 - 78) can easily be done by knowing where these values of impedance can be found along an unmatched line under similar conditions.

every length of feedline is a matching transformer....Cebik.
Wow. Someone that actually reads, comprehends, and gets it.

And you don't post with an amateur radio callsign? Howd you get so smart

--Toll_Free

Reply With Quote
  #16  
Old 12-14-2006, 09:54 AM
Senior Member
iTrader: (0)
 
Join Date: Jul 2006
Location: 6000 feet above the Mojave Desert, Tehachapi, Ca
Posts: 481
Toll_Free is on a distinguished road
Default Re: Death, Taxes, and Coax Woes

Quote:
Originally Posted by Paris
In an ideal system, the length of the coax antenna feedline should not make any difference to either the antenna’s SWR or its efficiency, see ARRL Handbook, 2006 Edition, Sec 21.1. However, as others have pointed, out its length is often critical real-world mobile installations. I suggest (as Hamin' X did) that this can only happen if the coax shield is radiating RF energy. In order for the coax antenna lead to work as an ideal feedline, the current carried on the outer braid must be equal and opposite from the transmitted signal traveling along its core; otherwise, the signal attempting to radiate from the core cannot be cancelled out. The ideal feedline does not radiate any of the transmitted signal, and only the antenna forms the resistive load. However, if the feedline does leak, it must then be considered part of the antenna, and it’s length will certainly effect both efficiency and SWR.

When you think about it, it seems inevitable that you will not be able to achieve an ideal ground in your car or truck. Almost certainly the chassis to which the coax shield is grounded will carry stray RF from whatever electronic control systems are found in modern cars, including the ignition, the CPU/ECU, the fuel injectors, the fuel pump, among others; not only this, but chassis itself is probably segmented, and unable to act as an efficient ground plane. The result will be common-mode currents in the coax shield not be present in the core, which will, in turn, allow the transmitted RF to escape from the core. In short, your feedline in a mobile installation will almost certainly become part of your antenna.

Since the nature of the stray current introduced onto the outer braid of the coax feedline cannot be predicted, it would follow that it is impossible to predict any specific length of feedline necessary to achieve a low SWR. Thus, the proper length of your feedline will have to be determined by experimentation on a case by case basis, but it’s important to understand that it will make a difference, nevertheless.

Perhaps one solution is to coil the feedline so that it acts as an RF choke, as I’ve seen suggested in some articles, but then other writers advise strongly against doing that. Has anyone tried this?
Yes. It works.

However, the length necessary to choke off antenna currents, and prevent the thing from becoming an antenna itself (the feedline) is usually long enough to reach the amplifier itself.

Your 100 percent right, too. You need to choke off the feedpoint. You need to get the feedpoint to 50 ohms, which is next to impossible on a mobile install. Remember, most vertical antennas DON'T present 50ohm-0j. Anywhere.

The choke works. Ferrite cores work even better, just because of the ease of use. I prefer a coaxial choke, however, and have noticed better performance out of them when used on my base station.

They do, however, need to be wound for a single frequency.

Your coax is not balanced, though. The shield is supposed to be "cold" as far as RF is concerned.

--Toll_Free

Reply With Quote
Reply

Bookmarks
Worldwide Radio Forum



Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -7. The time now is 01:15 AM.


Powered by vBulletin®
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Search Engine Optimization by vBSEO 3.6.0
Worldwide Radio Forum