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Q about balanced feed line
How does one check the balance between two parallel feed wires into a
doublet antenna. Neon bulbs or some sort of meter gizmo? tnx d |
Q about balanced feed line
That topic was discussed in some detail on this newsgroup just a few
weeks ago. Do a search in groups.google.com of this newsgroup for the topic "unbalance indicator". Roy Lewallen, W7EL Big Endian wrote: How does one check the balance between two parallel feed wires into a doublet antenna. Neon bulbs or some sort of meter gizmo? tnx d |
Q about balanced feed line
How does one check the balance between two parallel feed wires into
a doublet antenna. =========================================== Just go out into the garden and look up at the antenna. If each half of the doublet is of about the same length, and both halves are about the same height above ground, then the parallel pair of wires are balanced. ---- Reg. |
Q about balanced feed line
Big Endian wrote:
How does one check the balance between two parallel feed wires into a doublet antenna. Neon bulbs or some sort of meter gizmo? tnx d YEP!! An RF Ammeter will do just fine |
Q about balanced feed line
Amos Keag wrote:
YEP!! An RF Ammeter will do just fine Balance requires that the currents in the two wires be equal in magnitude and opposite in phase (or direction). How do you connect an RF ammeter to determine this? Roy Lewallen, W7EL |
Q about balanced feed line
Wouldn't the clamp-on type ammeter as discussed previously provide that
information (provided a section of twin lead small enough to fit the clamp were used? Chuck Roy Lewallen wrote: Amos Keag wrote: YEP!! An RF Ammeter will do just fine Balance requires that the currents in the two wires be equal in magnitude and opposite in phase (or direction). How do you connect an RF ammeter to determine this? Roy Lewallen, W7EL |
Q about balanced feed line
"Roy Lewallen" wrote Balance requires that the currents in the two wires be equal in magnitude and opposite in phase (or direction). How do you connect an RF ammeter to determine this? ===================================== It's unbelievably easy Roy, you just pass both wires together through a clamp-on ammeter. ---- Reg. |
Q about balanced feed line
Reg Edwards wrote:
How does one check the balance between two parallel feed wires into a doublet antenna. =========================================== Just go out into the garden and look up at the antenna. If each half of the doublet is of about the same length, and both halves are about the same height above ground, then the parallel pair of wires are balanced. In your opinion. But the original questioner wants to measure what's *really* happening. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Q about balanced feed line
Big Endian wrote:
How does one check the balance between two parallel feed wires into a doublet antenna. Neon bulbs or some sort of meter gizmo? The easiest way is to use a large sampling toroid such that the balanced line can be fed through the toroid. A ten-turn sampling coil will indicate any unbalance. Note that the balanced line needs to be centered with respect to the toroid and needs to be moved along the feedline to obtain maximum accuracy. Ideally, the toroid needs to be located at a standing wave current maximum point. Since my choke-balun is always located at a current maximum point, I can easily monitor my feedline balance. You can also use smaller toroids on each line and then superpose the two readings. But the two toroids need to be identical which is no small requirement. -- 73, Cecil http://www.qsl.net/w5dxp |
Q about balanced feed line
Reg Edwards wrote:
If each half of the doublet is of about the same length, and both halves are about the same height above ground, then the parallel pair of wires are balanced. What if one is broken and you don't know it? -- 73, Cecil http://www.qsl.net/w5dxp |
Q about balanced feed line
Amos Keag wrote:
An RF Ammeter will do just fine What if the first current is one amp at zero degrees and the other current is one amp at 45 degrees. Is that balanced? -- 73, Cecil http://www.qsl.net/w5dxp |
Q about balanced feed line
Dot wrote:
I one side of a dipole gets loose, you'll know all about it... the swr is likely to go to 6 or higher. Your receiver would be mysteriously quiet and you wouldn't be getting normal power from your transmitter. In the worst case it could damage your equipment. Would you believe that an SGC-230 will match that mismatch condition and the only clue that you will have is that you are not making as many contacts as before? -- 73, Cecil http://www.qsl.net/w5dxp |
Q about balanced feed line
Sorry, when I read "RF ammeter", I thought only of the old type which
resembles a panel meter and has two terminals to connect in line with a single conductor. A clamp-on type would indeed do the job, as we discussed here not long ago. Roy Lewallen, W7EL chuck wrote: Wouldn't the clamp-on type ammeter as discussed previously provide that information (provided a section of twin lead small enough to fit the clamp were used? Chuck Roy Lewallen wrote: Amos Keag wrote: YEP!! An RF Ammeter will do just fine Balance requires that the currents in the two wires be equal in magnitude and opposite in phase (or direction). How do you connect an RF ammeter to determine this? Roy Lewallen, W7EL |
Q about balanced feed line
In article ,
Cecil Moore wrote: Big Endian wrote: How does one check the balance between two parallel feed wires into a doublet antenna. Neon bulbs or some sort of meter gizmo? The easiest way is to use a large sampling toroid such that the balanced line can be fed through the toroid. A ten-turn sampling coil will indicate any unbalance. Note that the balanced line needs to be centered with respect to the toroid and needs to be moved along the feedline to obtain maximum accuracy. Ideally, the toroid needs to be located at a standing wave current maximum point. Since my choke-balun is always located at a current maximum point, I can easily monitor my feedline balance. You can also use smaller toroids on each line and then superpose the two readings. But the two toroids need to be identical which is no small requirement. An unbalanced condition would have a meter indication, like current flow? Balanced the meter needle would not move? I want to monitor this condition on the feedline in the shack from the Johnson KW matchbox. |
Q about balanced feed line
Big Endian wrote:
An unbalanced condition would have a meter indication, like current flow? Balanced the meter needle would not move? Yes, for a balanced condition, the meter needle should not move. -- 73, Cecil http://www.qsl.net/w5dxp |
Q about balanced feed line
Hi Roy,
I have a couple of RF current meters with freq. response from 10KHz to 100MHZ that are the types used for FCC part 15, DO-160E testing etc. and have a transfer impedance of 0dB-ohm. They have an opening of about 1.6" dia. and I sqeeze the feedline together (spacing about 1.5") temporarily and clamp over both conductors for measurement #1 and clamp over just 1 of the conductors for measurement #2. I read the output of the current probe with an RF power meter. In my case the current in a single conductor was approximately 15dB above the differential current which seemed to be pretty well balanced. Is there anything wrong with this approach? Larry Benko, W0QE Roy Lewallen wrote: Amos Keag wrote: YEP!! An RF Ammeter will do just fine Balance requires that the currents in the two wires be equal in magnitude and opposite in phase (or direction). How do you connect an RF ammeter to determine this? Roy Lewallen, W7EL |
Q about balanced feed line
Roy,
It is amazing how a simple concept such as impedance can be made obscure but most current probes are spec'd this way by the manufacturers and test procedures for DO-160E (FAA aircraft testing) call out probes the same way. By saying that a current probe has a transfer impedance of 0dB-ohm means 0dB relative to 1 ohm, but R = V/I so 0dB-ohm means that for 1A of current thru the primary of the probe produces 1V across a 50 ohm load. Similarly a -20db-ohm transfer means that 1A produces .1V across a 50 ohm load. For most of us, this means a 1 turn primary and a 50 turn secondary which yields a theoretical primary impedance or 0.02 ohms. One of the probes I have says the primary impedance is less than ..1 ohm. Larry Benko, W0QE Roy Lewallen wrote: Larry Benko wrote: Hi Roy, I have a couple of RF current meters with freq. response from 10KHz to 100MHZ that are the types used for FCC part 15, DO-160E testing etc. and have a transfer impedance of 0dB-ohm. They have an opening of about 1.6" dia. and I sqeeze the feedline together (spacing about 1.5") temporarily and clamp over both conductors for measurement #1 and clamp over just 1 of the conductors for measurement #2. I read the output of the current probe with an RF power meter. In my case the current in a single conductor was approximately 15dB above the differential current which seemed to be pretty well balanced. Is there anything wrong with this approach? Larry Benko, W0QE Not that I can see. In my opinion it's the best way to make the measurement. A homebrew version of this is entirely adequate, though, as described in the earlier thread. It's very important to terminate the secondary with a fairly low impedance so the transformer doesn't present a significant impedance to the line. I assume your meter does this internally -- maybe that information is in the transfer impedance you mentioned and which I don't really understand. Roy Lewallen, W7EL |
Q about balanced feed line
Dot wrote:
SNIPPED Which, of course, explains my continuing reluctance to use transmatches. Yes it limits my range of antenna usage (and experience) but curse it all, I'd really like to know when something goes wrong. You will know that also with a transmatch!!!! I've used a transmatch for almost 45 years out of 50+ years in Ham Radio. If my antenna has a problem the settings for a 'match' [in quotes] change!! That indicates something has happened. The system SWR increase can be seen in the changes transmatch settings. |
Q about balanced feed line
Amos Keag wrote:
I've used a transmatch for almost 45 years out of 50+ years in Ham Radio. If my antenna has a problem the settings for a 'match' [in quotes] change!! How do you know when the settings for an SGC-230 autotuner change? -- 73, Cecil http://www.qsl.net/w5dxp |
Q about balanced feed line
Larry Benko wrote:
Roy, It is amazing how a simple concept such as impedance can be made obscure but most current probes are spec'd this way by the manufacturers and test procedures for DO-160E (FAA aircraft testing) call out probes the same way. By saying that a current probe has a transfer impedance of 0dB-ohm means 0dB relative to 1 ohm, but R = V/I so 0dB-ohm means that for 1A of current thru the primary of the probe produces 1V across a 50 ohm load. Similarly a -20db-ohm transfer means that 1A produces .1V across a 50 ohm load. For most of us, this means a 1 turn primary and a 50 turn secondary which yields a theoretical primary impedance or 0.02 ohms. One of the probes I have says the primary impedance is less than .1 ohm. Larry Benko, W0QE Thanks for the explanation. My concern is with the insertion impedance, which at 0.02 or even 0.1 ohm, is certainly adequately low for this device -- as long as it's properly terminated. Roy Lewallen, W7EL |
Q about balanced feed line
Roy Lewallen wrote:
Larry Benko wrote: Roy, It is amazing how a simple concept such as impedance can be made obscure but most current probes are spec'd this way by the manufacturers and test procedures for DO-160E (FAA aircraft testing) call out probes the same way. By saying that a current probe has a transfer impedance of 0dB-ohm means 0dB relative to 1 ohm, but R = V/I so 0dB-ohm means that for 1A of current thru the primary of the probe produces 1V across a 50 ohm load. Similarly a -20db-ohm transfer means that 1A produces .1V across a 50 ohm load. For most of us, this means a 1 turn primary and a 50 turn secondary which yields a theoretical primary impedance or 0.02 ohms. One of the probes I have says the primary impedance is less than .1 ohm. Larry Benko, W0QE Thanks for the explanation. My concern is with the insertion impedance, which at 0.02 or even 0.1 ohm, is certainly adequately low for this device -- as long as it's properly terminated. Roy Lewallen, W7EL That's why these probes which are about the size of a small donut cost over $1000. Since they are used to qualify EMI emissions from other equipment they are priced like test equipment and certified to be accurate. Generally the frequency response if terminated properly is flat to within a fraction of a dB. Occasionally they show up on eBay and usually go for less than $100. With no parts to wear out they never go bad unless they have been used to test equipment for EMI susceptability where you transmit RF into them, sometimes at high power levels. Larry, W0QE |
Q about balanced feed line
Larry Benko wrote:
Roy Lewallen wrote: Larry Benko wrote: Roy, It is amazing how a simple concept such as impedance can be made obscure but most current probes are spec'd this way by the manufacturers and test procedures for DO-160E (FAA aircraft testing) call out probes the same way. By saying that a current probe has a transfer impedance of 0dB-ohm means 0dB relative to 1 ohm, but R = V/I so 0dB-ohm means that for 1A of current thru the primary of the probe produces 1V across a 50 ohm load. Similarly a -20db-ohm transfer means that 1A produces .1V across a 50 ohm load. For most of us, this means a 1 turn primary and a 50 turn secondary which yields a theoretical primary impedance or 0.02 ohms. One of the probes I have says the primary impedance is less than .1 ohm. Larry Benko, W0QE Thanks for the explanation. My concern is with the insertion impedance, which at 0.02 or even 0.1 ohm, is certainly adequately low for this device -- as long as it's properly terminated. Thanks, Larry. "dB-ohm" was a new one on me, too. That's why these probes which are about the size of a small donut cost over $1000. Since they are used to qualify EMI emissions from other equipment they are priced like test equipment and certified to be accurate. Generally the frequency response if terminated properly is flat to within a fraction of a dB. Occasionally they show up on eBay and usually go for less than $100. With no parts to wear out they never go bad unless they have been used to test equipment for EMI susceptability where you transmit RF into them, sometimes at high power levels. They also come in larger sizes than donuts. A friend who works in radiation protection uses them to measure RF currents in the wrists and ankles of workers, eg operators of machines for RF welding of plastics. But all these things have to be tested first... which is how he found *himself* with one of these big toroids clamped around his ankle, standing outdoors on the chicken-wire groundplane of an HF monopole, in bare feet, in November. All in the name of "Occupational Health"... but definitely not his own. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Q about balanced feed line
"Ian White wrote But the original questioner wants to measure what's *really* happening. ======================================== But the original questioner can tell what's *really* happening just by inspecting the antenna and feedline. He doesn't need any electrical measuring instruments. Of course, it may be in the middle of the night, in which case it would be best to wait till the sun comes up. Or he could rely on his memory of how he installed the antenna. The trouble with you experts is that you overcomplicate matters and make quite simple, obvious things appear to be mysterious. But you have to justify your existence in one way or another. smiley ---- Reg. |
Q about balanced feed line
In article ,
"Reg Edwards" wrote: "Ian White wrote But the original questioner wants to measure what's *really* happening. ======================================== But the original questioner can tell what's *really* happening just by inspecting the antenna and feedline. He doesn't need any electrical measuring instruments. Of course, it may be in the middle of the night, in which case it would be best to wait till the sun comes up. Or he could rely on his memory of how he installed the antenna. The trouble with you experts is that you overcomplicate matters and make quite simple, obvious things appear to be mysterious. But you have to justify your existence in one way or another. smiley ---- Reg. Actually I want to monitor the antenna because I have a splice in the feedline and the feedpoint is subject to wx, snow ice and rain. Some times the feedline itself is covered with snow and ice. I would find it interesting to see what is happening when I see my SWR shoot up and retuning my matchbox. I also have a strange thing happen when I apply high power, I get a slight bump up in SWR suddenly. I often wonder if one of my end insulators are breaking down under 1KW of RF or if there is some imbalance on one leg due to ground or trees swaying in the wind. |
Q about balanced feed line
On Sat, 25 Feb 2006 18:18:35 GMT, Big Endian
wrote: "Ian White wrote But the original questioner wants to measure what's *really* happening. ======================================== But the original questioner can tell what's *really* happening just by inspecting the antenna and feedline. He doesn't need any electrical measuring instruments. Actually I want to monitor the antenna because I have a splice in the feedline and the feedpoint is subject to wx, snow ice and rain. Some times the feedline itself is covered with snow and ice. I would find it interesting to see what is happening when I see my SWR shoot up and retuning my matchbox. I also have a strange thing happen when I apply high power, I get a slight bump up in SWR suddenly. I often wonder if one of my end insulators are breaking down under 1KW of RF or if there is some imbalance on one leg due to ground or trees swaying in the wind. Hi OM, Reggie is particularly obtuse to this matter. Simple observation of a dipole that is symmetrical to earth does not guarantee balance. When that twin lead arrives at your gear, you can easily wipe that out (the presumed balance) through an inappropriate connection to ground (through any number of paths that are commonly overlooked in the shack). Snow and ice on the line are not likely to impart a common mode current, but as revealed by bench test by contributors here, the characteristic Z of the line can change drastically, which then upsets the tune, where it follows SWR begins to change - noticeably. If your feedline is coaxial, then the ice is of no consequence, but that does not remove the jeopardy of common mode current. There you need to look at how you isolate the line from the drivepoint. 73's Richard Clark, KB7QHC |
Q about balanced feed line
"Dot" wrote in message ... On Fri, 24 Feb 2006 19:39:31 GMT, Cecil Moore wrote: What if one is broken and you don't know it? I one side of a dipole gets loose, you'll know all about it... the swr is likely to go to 6 or higher. Your receiver would be mysteriously quiet and you wouldn't be getting normal power from your transmitter. In the worst case it could damage your equipment. Antenna failure is not trivial. ========================================== I quite agree. Of course, if there's anything the matter with the antenna then you will be aware of it long before you discover whether or not there's any need to get out your measuring instruments to find how much your feedline is unbalanced. Just go out into your back yard and see whether your antenna is still up in the air or not. Many years back I once had a neighbor who sabataged my inverted-L at 1am in the morning while I was on the air. I did NOT measure unbalance on the feedline - I was not equipped to do it - I just called the police! It turned out that my neighbor had very sensitive ears and could hear my microphone voice through the walls of our adjoining houses. ---- Reg. |
Q about balanced feed line
"Big Endian" wrote Actually I want to monitor the antenna because I have a splice in the feedline and the feedpoint is subject to wx, snow ice and rain. Some times the feedline itself is covered with snow and ice. I would find it interesting to see what is happening when I see my SWR shoot up and retuning my matchbox. I also have a strange thing happen when I apply high power, I get a slight bump up in SWR suddenly. I often wonder if one of my end insulators are breaking down under 1KW of RF or if there is some imbalance on one leg due to ground or trees swaying in the wind. ======================================= Perhaps you should have mentioned all this in your original enquiry. The people you expect replies from are not mind readers! ---- Reg. |
Q about balanced feed line
Reg Edwards wrote:
"Big Endian" wrote Actually I want to monitor the antenna because I have a splice in the feedline and the feedpoint is subject to wx, snow ice and rain. Some times the feedline itself is covered with snow and ice. I would find it interesting to see what is happening when I see my SWR shoot up and retuning my matchbox. I also have a strange thing happen when I apply high power, I get a slight bump up in SWR suddenly. I often wonder if one of my end insulators are breaking down under 1KW of RF or if there is some imbalance on one leg due to ground or trees swaying in the wind. ======================================= Perhaps you should have mentioned all this in your original enquiry. The people you expect replies from are not mind readers! It was clear enough to everyone else that he wanted to know how to make measurements. It was also clear enough for you to declare that measurements are not necessary. Maybe this is a good time to remind you of your own favourite quotation: "When you can measure what you are speaking about and express it in numbers you know something about it. But when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind. It may be the beginning of knowledge but you have scarcely in your thoughts advanced to the state of science." William Thomson, Lord Kelvin, 1824-1907. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Q about balanced feed line
Reg Edwards wrote:
. . . The trouble with you experts is that you overcomplicate matters and make quite simple, obvious things appear to be mysterious. . . . I recently finished reading Richard Hofstadter's _Anti-Intellectualism in American Life_ (written in 1962, won a Pulitzer prize in 1964), where the author shows in great detail that a general disdain for education, sophistication, and complexity is a very deeply seated American tradition. That general outlook was virtually a basis of the founding of a number of now-mainstream religions, and has had lasting impacts on our educational and political systems. In short, it's a long and dearly held American tradition. So, Reg, that was a very American thing to say. You'd fit right in here. Roy Lewallen, W7EL |
Q about balanced feed line
Ian White GM3SEK wrote:
It was clear enough to everyone else that he wanted to know how to make measurements. It was also clear enough for you to declare that measurements are not necessary. Maybe this is a good time to remind you of your own favourite quotation: "When you can measure what you are speaking about and express it in numbers you know something about it. But when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind. It may be the beginning of knowledge but you have scarcely in your thoughts advanced to the state of science." William Thomson, Lord Kelvin, 1824-1907. I recently came across some other notable quotes from Lord Kelvin: "Radio has no future." "Heavier than air flying machines are impossible." "X-rays will prove to be a hoax." -- quoted in _Unweaving the Rainbow: Science, Delusion, and the Appetite for Wonder_, by noted British writer Richard Dawkins. Dawkins also notes that "William Thompson, first Lord Kelvin, was one of the most distinguished and influential of nineteenth-century British physicists. He was a thorn in Darwin's side because he 'proved', with massive authority but, as we now know, even more massive error, that the earth was too young for evolution to have occurred." The value of Kelvin's contributions is unquestionable. But even he didn't get it right all the time. Roy Lewallen, W7EL |
Q about balanced feed line
Roy Lewallen wrote:
The value of Kelvin's contributions is unquestionable. But even he didn't get it right all the time. And Roy, allow me to point out, in that respect, you don't get it right all the time either. I can prove to you that reflected waves contain joules but you simply refuse to listen and have "ploinked" me. What really aggravates me is that when an internet guru, such as yourself (or W8JI) appears to be on the verge of losing an argument, you simply kill-file the opponent and refuse to continue the thread. -- 73, Cecil http://www.qsl.net/w5dxp |
Q about balanced feed line
On Sat, 25 Feb 2006 21:31:17 GMT, The Benevolent dbu
wrote: I am also concerned that the feedline may be causing unbalance condition because it is routed straight down perhaps 30 feet from the feedpoint then it makes a sharp turn to the shack and at that point it is parallel with one leg of the antenna for about 30 feet. Hi Al, Too much worry can come from too little problem. You need only ask yourself does the line inappropriately inject receive signal from the nulls of your antenna; or do you get RF in the shack? If neither are a concern, there is certainly nothing worse to worry about. It's the only way I can do it. I have thought of discarding the multiband doublet for a plain dipole with 52 ohm coax. I do like the doublet and my Johnson matchbox, besides I can use this antenna on 160 when I route the feedline via a knife switch over to my MN2700 and configure it to a inverted L. Nothing here suggests you change anything. 73's Richard Clark, KB7QHC |
Q about balanced feed line
If the antenna system can be satisfactorily matched to the transmitter
output impedance, it would seem to matter little whether the feed line currents are balanced, or, if they are not, by how much. If there are no obvious common mode currents causing problems in the shack, then of what consequence would an imbalance be, other than to modify the radiation pattern, perhaps even beneficially? Chuck, NT3G Cecil Moore wrote: Big Endian wrote: An unbalanced condition would have a meter indication, like current flow? Balanced the meter needle would not move? Yes, for a balanced condition, the meter needle should not move. |
Q about balanced feed line
chuck wrote:
If the antenna system can be satisfactorily matched to the transmitter output impedance, it would seem to matter little whether the feed line currents are balanced, or, if they are not, by how much. If there are no obvious common mode currents causing problems in the shack, then of what consequence would an imbalance be, other than to modify the radiation pattern, perhaps even beneficially? You have to remember that the common mode feedline current doesn't stop at the rig. It continues to the Earth by whatever means are available. So your house wiring, appliance cords, and other odd conductors often become part of your antenna system. These aren't likely to be very efficient radiators. People often go to a lot of trouble to put their antennas high and in the clear. That doesn't make much sense if you're going to have your house do a good part of the radiating. But you can still talk to lots of stations even if some of your radiating is being done by your feedline and house wiring. And that's enough for a lot of folks. Roy Lewallen, W7EL |
Q about balanced feed line
If the antenna system can be satisfactorily matched to the
transmitter output impedance, it would seem to matter little whether the feed line currents are balanced, or, if they are not, by how much. If there are no obvious common mode currents causing problems in the shack, then of what consequence would an imbalance be, other than to modify the radiation pattern, perhaps even beneficially? Chuck, NT3G ========================================== I agree with what you say. But antenna (or line) unbalance and line-to-antenna impedance mismatch are not entirely independent of each other. An unbalanced line or antenna causes a small impedance mismatch. Because it is small is the reason why it very often happens it doesn't matter very much whether or not a balanced feedline is used. For example, a coax line can be used quite successfully to feed a balanced dipole. And, in practice, no antenna is perfectly balanced about ground. ---- Reg. |
Q about balanced feed line
chuck wrote:
If there are no obvious common mode currents causing problems in the shack, then of what consequence would an imbalance be, other than to modify the radiation pattern, perhaps even beneficially? The purpose of the antenna is to radiate. The purpose of the transmission line is to transfer the energy from the transmitter to the antenna with as little loss as feasible. How much an antenna system is allowed to deviate from its purpose is up to the individual. When I was in high school, I didn't much care about the purpose of an antenna system and burned a hole in my lip. -- 73, Cecil http://www.qsl.net/w5dxp |
Q about balanced feed line
In article ,
Roy Lewallen wrote: chuck wrote: If the antenna system can be satisfactorily matched to the transmitter output impedance, it would seem to matter little whether the feed line currents are balanced, or, if they are not, by how much. If there are no obvious common mode currents causing problems in the shack, then of what consequence would an imbalance be, other than to modify the radiation pattern, perhaps even beneficially? You have to remember that the common mode feedline current doesn't stop at the rig. It continues to the Earth by whatever means are available. So your house wiring, appliance cords, and other odd conductors often become part of your antenna system. These aren't likely to be very efficient radiators. People often go to a lot of trouble to put their antennas high and in the clear. That doesn't make much sense if you're going to have your house do a good part of the radiating. But you can still talk to lots of stations even if some of your radiating is being done by your feedline and house wiring. And that's enough for a lot of folks. Roy Lewallen, W7EL I'm always concerned about TVI and telephone I. and that is another reason I hope to have my feeders balanced, so they radiate minimum RF. I was using a field strength meter while I had the transmitter going the other day and working the antenna along some table lamps and the electrical wire I found the F.S. meter would jump up quite a bit. So what you say about re-radiating house wiring is surely true. |
Q about balanced feed line
Thanks for the responses.
Put differently, I guess the purpose of the entire station is to radiate and intercept radiated waves containing information. I was trying to explore the benefits and consequences of preventing transmission line radiation as they relate to this more global purpose. While it is clear that maintaining balance in the line will prevent radiation from the line, some (maybe all) of the radiation prevented might have contributed to the global purpose of the station. For example, radiation from common-mode currents in a vertical transmission line could produce beneficial low-angle, omnidirectional radiation, as has been pointed out often on the group. It would seem that for the common-mode transmission line currents the antenna system would look like a top-loaded vertical, ignoring the balanced line currents and their interaction with the horizontal portion of the antenna. The efficiency of the vertically polarized radiation from the line would depend heavily on the station's RF ground system. But basically we would have (for the common-mode currents) a conventional vertical antenna with all its attendant plusses and minuses. To the extent the AC grounding conductor in the house presents a lower impedance than the station's RF grounding system, we would expect to see displacement currents in the AC system, just as with an "ordinary" vertical using a poor RF ground. Coupling to the telephone wires also would seem to be a consequence of a vertical radiator with a poor RF ground, rather than a consequence of transmission line imbalance (which I understand is the cause of the radiation in the first place). So here is my main question: do we object to the vertical radiation per se (i.e., if we wanted vertically polarized radiation, we would have put up a vertical in the first place), or is radiation from an unbalanced line somehow more insidious in that it causes other problems that "ordinary" verticals do not cause? In other words, why do we really care about imbalance? Reg has called attention to another of my shortcomings: I have no idea how the common-mode line currents that enter a link in the tuner are seen and "processed" by the tuner. It would seem that the link appears as one plate of a capacitor for those currents. Thanks again for everyone's patience. Chuck, NT3G Cecil Moore wrote: chuck wrote: If there are no obvious common mode currents causing problems in the shack, then of what consequence would an imbalance be, other than to modify the radiation pattern, perhaps even beneficially? The purpose of the antenna is to radiate. The purpose of the transmission line is to transfer the energy from the transmitter to the antenna with as little loss as feasible. How much an antenna system is allowed to deviate from its purpose is up to the individual. When I was in high school, I didn't much care about the purpose of an antenna system and burned a hole in my lip. |
Q about balanced feed line
Please change my reference to a "link" in the tuner to the symmetrical
taps on the inductor in the tuner. Sorry. I still have the same conceptual problem with common-mode currents entering that inductor. Chuck, NT3G |
Q about balanced feed line
chuck wrote:
So here is my main question: do we object to the vertical radiation per se ...? Maybe an example would help. I model a dipole until I am happy with the results predicted by EZNEC. The take-off-angle is perfect for my schedule to AZ. EZNEC assumes no feedline radiation. I am sloppy about putting my dipole in the air and lots of feedline radiation results which goes off in directions not predicted by EZNEC. Murphey's Law predicts that the unknown directions will be bad (entropy never decreases). Besides, half of my feedline is routed horizontal under the eaves of my house. So would you rather deal with the devil you know or trust the angel that you don't know? Does the angel that you are trusting really understand entropy? -- 73, Cecil http://www.qsl.net/w5dxp |
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