|
Non-radiating Feedlines?
Post below from May 20, 2004
Wouldn't a "non-radiating" feedline in the field of a radiator also distort the patterns of that radiator? Any conductor can do that, even if it is not a feedline. NEC-2 models of FM broadcast transmit elements (and test range patterns) show this clearly. Paper 6 at http://rfry.org shows the free space patterns that the element arms of a rototiller FM broadcast transmit antenna develop if they could be driven from internal power sources -- and then the effects of adding the element stem, mounts, feedline, and some nearby tower structure. The patterns can get very skewed, even though the only radiators getting power via a metallic path from the tx are the element arms themselves. RF ____________ "Roy Lewallen" wrote With a typical ground plane antenna, the feedline can radiate significantly, distorting the pattern. This effect could easily be different for the different antennas. Modeling indicates that two baluns are often needed to suppress the current on the outside of the feedline. A model which includes the feedline might give some insights as to why the antennas behave so differently. |
No.
A "non-radiating" feedline is one on which there is no net current (i.e., no common mode current). In the case of coax, this translates to zero current on the outside of the shield; for twinlead feedlines, it means that the currents in the two conductors are exactly equal in magnitude and opposite in direction. Pattern distortion is caused by current being induced in a conductor by an impinging field. That current, in turn, creates a field which adds to the impinging field, resulting in pattern distortion. (This is, in fact, the way a Yagi functions.) If there is current induced in a feedline by the field, it's a radiating conductor (because the current causes radiation which interferes with the impinging field). If there is no current induced in the feedline by the impinging field, it creates no field of its own (i.e., it's non-radiating) and therefore causes no interference. A transmission line placed symmetrically with respect to a dipole won't have any current induced in it, although current can be conducted via a direct connection. A transmission line asymmetrically placed will have current induced in it and will distort the pattern. The feedline of a ground plane or J-Pole likewise has induced current which distorts the pattern. The amount of common mode current flowing in a transmission line can be reduced by introducing an impedance to the common mode current. It's desirable to do this without disturbing the differential mode transmission line operation. That's the function of a balun. The amount of induced current depends strongly on the orientation and length of the parasitic conductor, and might be large or small in a particular case. Roy Lewallen, W7EL Richard Fry wrote: Post below from May 20, 2004 Wouldn't a "non-radiating" feedline in the field of a radiator also distort the patterns of that radiator? Any conductor can do that, even if it is not a feedline. NEC-2 models of FM broadcast transmit elements (and test range patterns) show this clearly. Paper 6 at http://rfry.org shows the free space patterns that the element arms of a rototiller FM broadcast transmit antenna develop if they could be driven from internal power sources -- and then the effects of adding the element stem, mounts, feedline, and some nearby tower structure. The patterns can get very skewed, even though the only radiators getting power via a metallic path from the tx are the element arms themselves. RF ____________ "Roy Lewallen" wrote With a typical ground plane antenna, the feedline can radiate significantly, distorting the pattern. This effect could easily be different for the different antennas. Modeling indicates that two baluns are often needed to suppress the current on the outside of the feedline. A model which includes the feedline might give some insights as to why the antennas behave so differently. |
A balun can reduce common mode feedline currents due to the power supplied
to the line by the transmitter, but if the feedline is immersed in an asymmetric radiated field, how does the balun reduce/remove the resulting, unpredictable differential current on the feedline, and its contribution toward producing the net radiated pattern? RF _____________ "Roy Lewallen" wrote: A "non-radiating" feedline is one on which there is no net current (i.e., no common mode current). In the case of coax, this translates to zero current on the outside of the shield; for twinlead feedlines, it means that the currents in the two conductors are exactly equal in magnitude and opposite in direction. A transmission line placed symmetrically with respect to a dipole won't have any current induced in it, although current can be conducted via a direct connection. The amount of common mode current flowing in a transmission line can be reduced by introducing an impedance to the common mode current. It's desirable to do this without disturbing the differential mode transmission line operation. That's the function of a balun. |
Richard Fry wrote:
A balun can reduce common mode feedline currents due to the power supplied to the line by the transmitter, but if the feedline is immersed in an asymmetric radiated field, how does the balun reduce/remove the resulting, unpredictable differential current on the feedline, and its contribution toward producing the net radiated pattern? It simply provides an impedance to the common-mode currents. Whether that impedance is high enough to be effective depends upon the system parameters and configuration. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
The radiated field in which a feedline is immersed produces a common
mode, not differential, current on the feedline(*). The balun creates a high impedance to the flow of common mode current. The result is that much less common mode current is induced than would be the case if the balun were absent. AM broadcasters put insulators periodically in tower guy wires. Baluns have the same effect, although they're of course not as perfect as the fully open circuit created by the insulator. I'd also like to add that the induced current isn't unpredictable, as you stated. It has to follow rules like all other physical phenomena, so it's entirely predictable. I have to qualify this, though, by noting that in many or most amateur installations, the path from the antenna along the outside of the feedline to the rig and from there to the Earth is often not well known. And precise predictions of feedline current can't be made without knowing this path. This effect is easily observed in models, as well as being experimentally verifiable with a small amount of effort. For example, make a simple wire Yagi with wire elements. Slip a few ferrite cores over one of the elements and see what happens to the pattern. What you've done is to reduce the current in the parasitic element, which is immersed in the field from the driven and other elements, thereby reducing its contribution to the total field. Cutting the element (analogous to the broadcasters' insertion of an insulator) has the same effect, although it'll be even more profound than the high but finite impedance of the cores. Quanitative measurements aren't hard to make, either. You can find details of current measurement devices in Chapter 8 of the ARRL Antenna book and other references. When placed over a coax feedline, or over both conductors of a twinlead feedline, they'll measure the common mode current. (*) I'm assuming here that we're dealing either with coax, or with twinlead whose wire spacing is small in terms of wavelength and whose spacing from the antenna is large compared to the wire spacing. If you put twinlead in a field such that the field seen by one conductor is significantly different than the field seen by the other, you will get an induced differential, as well as common mode current. This won't happen with coax. Unlike common mode current, an induced differential current will affect the properties of the line as a transmission line (that is, change its apparent Z0 and velocity factor). But this would be an unusual condition in an amateur installation, and I'm not considering it here. Roy Lewallen, W7EL Richard Fry wrote: A balun can reduce common mode feedline currents due to the power supplied to the line by the transmitter, but if the feedline is immersed in an asymmetric radiated field, how does the balun reduce/remove the resulting, unpredictable differential current on the feedline, and its contribution toward producing the net radiated pattern? RF _____________ "Roy Lewallen" wrote: A "non-radiating" feedline is one on which there is no net current (i.e., no common mode current). In the case of coax, this translates to zero current on the outside of the shield; for twinlead feedlines, it means that the currents in the two conductors are exactly equal in magnitude and opposite in direction. A transmission line placed symmetrically with respect to a dipole won't have any current induced in it, although current can be conducted via a direct connection. The amount of common mode current flowing in a transmission line can be reduced by introducing an impedance to the common mode current. It's desirable to do this without disturbing the differential mode transmission line operation. That's the function of a balun. |
Yes, it distorts the pattern, radiating or not.
How much depends upon the situation, frequencies, spacing, size of feedline. It is a conductive surface so it will distort pattern. Side mounted on a tower changes pattern quite a lot depending upon spacing from the tower, you can tune the pattern that way. dB Products used to have a good Catalogue that showed this. I would not expect much pattern distortion from a feedline very small, but would from a tower. "Richard Fry" wrote in message ... Post below from May 20, 2004 Wouldn't a "non-radiating" feedline in the field of a radiator also distort the patterns of that radiator? Any conductor can do that, even if it is not a feedline. NEC-2 models of FM broadcast transmit elements (and test range patterns) show this clearly. Paper 6 at http://rfry.org shows the free space patterns that the element arms of a rototiller FM broadcast transmit antenna develop if they could be driven from internal power sources -- and then the effects of adding the element stem, mounts, feedline, and some nearby tower structure. The patterns can get very skewed, even though the only radiators getting power via a metallic path from the tx are the element arms themselves. RF ____________ "Roy Lewallen" wrote With a typical ground plane antenna, the feedline can radiate significantly, distorting the pattern. This effect could easily be different for the different antennas. Modeling indicates that two baluns are often needed to suppress the current on the outside of the feedline. A model which includes the feedline might give some insights as to why the antennas behave so differently. |
As a discussion point, consider 1/2-wave parasitic radiators sometimes
positioned within a foot or two of FM broadcast transmit elements, to "shape" their patterns. This is the common technique used in "sidemount" antennas that must meet FCC requirements for directional FM broadcast assignments. A parasitic itself is suspended mechanically in space by a non-metallic support. It has no direct connection to the transmitter, and no conductive physical path to any part of the antenna, its feedline, mounts, or supporting structure. Such parasitics do affect the net radiation pattern(s) of the array. Isn't a "non-radiating" feedline with a balun just an arbitrary length of conductor, but now with a metallically conductive path to the driven element(s), as well? The feedline (and other metallic structures) adjacent to an FM broadcast transmit antenna will affect the radiation patterns of the antenna even though the measured match between the feedline and antenna input is extremely good (even 1:1 SWR) -- in which case the line should have no differential current to produce such an effect. What is the explanation for that, please? RF ______________ "Roy Lewallen" wrote in message ... The radiated field in which a feedline is immersed produces a common mode, not differential, current on the feedline(*). ETC |
Richard Fry wrote:
As a discussion point, consider 1/2-wave parasitic radiators sometimes positioned within a foot or two of FM broadcast transmit elements, to "shape" their patterns. This is the common technique used in "sidemount" antennas that must meet FCC requirements for directional FM broadcast assignments. A parasitic itself is suspended mechanically in space by a non-metallic support. It has no direct connection to the transmitter, and no conductive physical path to any part of the antenna, its feedline, mounts, or supporting structure. Such parasitics do affect the net radiation pattern(s) of the array. Isn't a "non-radiating" feedline with a balun just an arbitrary length of conductor, but now with a metallically conductive path to the driven element(s), as well? No. A "non-radiating" feedline is one which has no significant amount of common mode current. This can be accomplished by making the feedline a length such that the induced current is minimal; by inserting a balun or baluns; and/or by placing the feedline symmetrically with respect to the antenna. I thought I had explained this -- I don't seem to be communicating well. The feedline (and other metallic structures) adjacent to an FM broadcast transmit antenna will affect the radiation patterns of the antenna even though the measured match between the feedline and antenna input is extremely good (even 1:1 SWR) -- in which case the line should have no differential current to produce such an effect. What is the explanation for that, please? We've been down this path before, and you've shown that you won't accept the fact that SWR has nothing to do with whether or not common mode current exists on a feedline, and there's nothing I've been able to do to convince you otherwise. You also either haven't read or won't believe that it's common mode, not differential, current that causes a line to radiate and thereby contribute to the overall pattern. But hopefully other readers have learned from this exchange. Once the basic principles are grasped, these phenomena lose their mystery, and they're no longer "unpredictable", but readily measured, modeled, and understood. Based on past experience, nothing I say will sway you from the way you've chosen to interpret observed phenomena. And I believe I've done enough explaining so that any other readers, who are open to learning some fundamentals, can come away with a better understanding. So that's enough for now. Roy Lewallen, W7EL RF ______________ "Roy Lewallen" wrote in message ... The radiated field in which a feedline is immersed produces a common mode, not differential, current on the feedline(*). ETC |
Just curious, Roy. Have you used EZNEC to model an FM broadcast transmit
array, feedlines and adjacent tower -- as in the models in Paper 6 at http://rfry.org ? If so, and your pattern results are appreciably different than those, for those same scenarios -- I'd truly appreciate learning from you the reason(s) you might pose for that. Maybe I'm doing it wrong. I'll be glad to send you the intermediate results: radiators alone, complete elements with feedlines, complete elements with feedlines and tower. RF |
Roy Lewallen wrote:
I'd also like to add that the induced current isn't unpredictable, as you stated. It has to follow rules like all other physical phenomena, so it's entirely predictable. The key to reducing the unwanted feedline current is that knowledge that "it has to follow rules". We can't change the rules, but we *can* change the antenna/feedline configuration so that the rules will work in our favor. These are the well-known physical rules about the way that voltages and currents distribute themselves on wires, for example: * Antenna wires carry standing waves of voltage and current, and also so do feedlines if they carry net (common-mode or surface) RF currents. * A voltage maximum is located at a current minimum, and vice versa. * Maxima and minima are a quarter-wavelength apart. * Current goes to zero at the physical end of a wire; voltage goes to maximum. Whenever you change the antenna/feedline configuration, the rules will then force the voltage and current to rearrange themselves into a new configuration also. So how can we "play the rules" to reduce unwanted feedline currents? For the moment, let's talk about coax feedline and unwanted surface currents, because it's easier to insert chokes and also easier to understand the results. The unwanted surface current on the feedline has two components: * Conducted current, launched onto the feedline from the hard-wired connection at the feedpoint * Induced current, caused by the feedline being placed in an EM field. Wherever you insert a feedline choke along the feedline (by winding the coax into a coil, or using ferrite loading) then you are creating a high impedance which *forces* a current minimum at that point. The rules will then force the current distribution along the feedline to change, so that it meet this new additional requirement. A choke balun at the feedpoint is the single most effective change you can make, because it almost completely prevents conducted currents from being launched onto the feedline. That leaves the induced currents to be dealt with. These are typically less of a problem than the conducted current, but are very hard to kill. It's rather like trying to squeeze the air out of a long inflated tube. Squeeze in one place by forcing a current minimum, and a new current maximum will pop up a quarter-wavelength away. Your feedpoint choke has defined a current minimum at the top of the feedline, but by squeezing there you have created an induced current maximum a quarter-wavelength down the feedline. So place another choke there too, and the rules will make it impossible for ay significant currents to build up between those two chokes. By making the rules work in your favor, you have effectively killed the feedline current between the two chokes. The current won't quite go to zero, and a current maximum will certainly try to pop up somewhere else, if the rules allow... but it probably won't be as large as before, and you can chase that one down too. There are many other ways to look at the problem, but I've found this viewpoint of "playing the rules" to be quite a helpful one. I hope it chimes with some other people too. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Roy Lewallen wrote:
The feedline (and other metallic structures) adjacent to an FM broadcast transmit antenna will affect the radiation patterns of the antenna even though the measured match between the feedline and antenna input is extremely good (even 1:1 SWR) -- in which case the line should have no differential current to produce such an effect. What is the explanation for that, please? We've been down this path before, and you've shown that you won't accept the fact that SWR has nothing to do with whether or not common mode current exists on a feedline, and there's nothing I've been able to do to convince you otherwise. You also either haven't read or won't believe that it's common mode, not differential, current that causes a line to radiate and thereby contribute to the overall pattern. I agree with Roy that SWR itself is not the *cause* of radiating feedlines. However, if the feedline is allowed to carry currents and radiate, it effectively becomes part of a new and different antenna configuration. That new configuration will have a different feedpoint impedance, so the SWR will change. But the SWR didn't cause the feedline current and radiation; in fact it was exactly the opposite. Also, Richard is assuming that unwanted feedline currents will always change the SWR for the worse. That's often true, but it doesn't have to be - sometimes the SWR gets worse when the feedline current is choked off. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Roy Lewallen wrote in message ...
No. A "non-radiating" feedline is one which has no significant amount of common mode current. This can be accomplished by making the feedline a length such that the induced current is minimal; by inserting a balun or baluns; and/or by placing the feedline symmetrically with respect to the antenna. I thought I had explained this -- I don't seem to be communicating well. What's up Roy? Long time no see! Ok, well, I'd like to discuss this a bit. So for most of the dipole based antennas (including Yagis), you can use 6 turns of 4" diameter coils in the coax, to make an inductive loop that is supposed to prevent current from moving down the outer braid (non-radiating). We've been down this path before, and you've shown that you won't accept the fact that SWR has nothing to do with whether or not common mode current exists on a feedline, and there's nothing I've been able to do to convince you otherwise. You also either haven't read or won't believe that it's common mode, not differential, current that causes a line to radiate and thereby contribute to the overall pattern. But hopefully other readers have learned from this exchange. Once the basic principles are grasped, these phenomena lose their mystery, and they're no longer "unpredictable", but readily measured, modeled, and understood. Ok, so I understand how the common-mode-rejection-ratio works with an audio amplifier that has an XLR cable input: signals in phase (common mode) will cancel each other out when they reach the input transformer (balun). And although the XLR cable is shielded, the two signal wires are more like a twin-lead transmission line instead of like coaxial cable. So i'm not sure how to ask this, but coxial cable is obviously a much different beast than twin lead, so the concept of common-mode currents radiating from the line is a bit strange because the outer braid completely encloses the inner radial. But this is weird because coaxial cable is unbalanced already, while twin-lead (or in the case of the audio XLR, shielded twin lead) is balanced. This is a discussion he http://lists.contesting.com/archives.../msg00484.html Slick |
Dr. Slick wrote:
So for most of the dipole based antennas (including Yagis), you can use 6 turns of 4" diameter coils in the coax, to make an inductive loop that is supposed to prevent current from moving down the outer braid (non-radiating). 6 turns is probably not enough inductance to do much choking on 160m or 80m. If the dipole is non-resonant, 6 turns of coax may have very little effect on any HF frequency. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Fri, 11 Jun 2004 15:39:59 -0700, Roy Lewallen
wrote: This can be accomplished by making the feedline a length such that the induced current is minimal; by inserting a balun or baluns; and/or by placing the feedline symmetrically with respect to the antenna. I thought I had explained this -- I don't seem to be communicating well. Hi Roy, Richard, Perhaps it is because of the naming convention vs. application. The term more appropriate to the Topic is RF Choke rather than BalUn (which performs the choking action by implication, and could easily fail that implication if the incorrect BalUn design is chosen). Further, as you have noted in past correspondence Roy, a second Choke (not BalUn) is often required for these VHF/UHF applications to completely isolate the line. 73's Richard Clark, KB7QHC |
|
Cecil Moore wrote in message ...
Dr. Slick wrote: So for most of the dipole based antennas (including Yagis), you can use 6 turns of 4" diameter coils in the coax, to make an inductive loop that is supposed to prevent current from moving down the outer braid (non-radiating). 6 turns is probably not enough inductance to do much choking on 160m or 80m. If the dipole is non-resonant, 6 turns of coax may have very little effect on any HF frequency. What's up Cecil? ok, you may be right about HF, but for VHF (30-300 MHz), 6 turns in the coax is enough. So have you made an inductive coil in coax for HF? If so, how many turns did you need, and what was the diameter of the turns? I may need to do this for an AM station one day, but maybe not because most AM antennas are some sort of end-fed random length wire with a tuner, where the wire length is greater than 1/4 wavelength. In other words, i haven't seen any dipole antennas for broadcast band AM....it would be too long! Slick |
Dr. Slick wrote:
So have you made an inductive coil in coax for HF? If so, how many turns did you need, and what was the diameter of the turns? I once made one out of a 2-liter pop bottle about 1/3 full of turns of RG-8X. It worked pretty well on 40m, my favorite band, but a thunderstorm destroyed it. I have no idea what the choking impedance was. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Sat, 12 Jun 2004 07:52:50 -0500, Cecil Moore
wrote: Dr. Slick wrote: So for most of the dipole based antennas (including Yagis), you can use 6 turns of 4" diameter coils in the coax, to make an inductive loop that is supposed to prevent current from moving down the outer braid (non-radiating). 6 turns is probably not enough inductance to do much choking on 160m or 80m. If the dipole is non-resonant, 6 turns of coax may have very little effect on any HF frequency. It took 27 turn here to get decent choking on 80m. But it finally worked 6" diamater selenoid wound. 73 Dave KC1DI P.S. feeding a OFCD cut for 3.6 MHZ. Thru a 4 to 1 Balun. |
Richard Clark wrote in message . ..
(Large paragraphs of Nonsensical, pyseudo-technobable snipped) The discussions in this group are often on shaky ground (pun intended) by lacking discussion of ground. The discussions in this group are often on that same tremulous substrate when the arguments run to Kirchhoff, and tectonic when Superposition is given its fanfare. It seems you are still spouting out words and concepts that you really don't understand, eh? If things are on shaky ground here, you certainly ain't helpin'! This is a reminder that ANYONE can post anything they want here. Slick Slick |
|
Perhaps I might be of assistance by providing some orders of magnitude. A 1/2-wave stub line, 1 metre (39.37 inches) long constructed of a pair of polished copper tubes, each 1/2-inch in diameter, centers spaced 3 inches apart, at 150 MHz (not very far from 2 meters) has the following characteristics - Zo = 299.8 ohms. Attenuation = 0.0024 dB. As a tuned circuit, Resonant Q = 5800. When short-circuited - Parallel input components of Zin - Rsc = 0.08 ohms, Xsc = -j*582 ohms. When open circuited - Series input components of Zin - Roc = 1.11 megohms, Xoc = j*38.6 ohms. These and many other interesting values can be very accurately and rapidly calculated, from 20 Hz to 1 GHz, by using program RJELINE2 available, free issue, from website below. Contributors to this newsgroup, who by no means are lacking in intelligence, may then be able to get their feet on the ground. ----- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
On Sat, 12 Jun 2004 08:35:42 +0100, "Ian White, G3SEK"
wrote: Roy Lewallen wrote: I'd also like to add that the induced current isn't unpredictable, as you stated. It has to follow rules like all other physical phenomena, so it's entirely predictable. Remainder deleted Three questions emerge from my foggy brain wrt to a current project. 1. What sort of choke loading on a feed line would be most effective at 70 cm, ferrite or coil wound from coax. 2. Could ferrite loading at a feed point take the place of say a 1/4 wave sleeve balun matching the unbalanced coax feed to a centre-fed dipole element. 3. What grade of ferrite bead would best be useful at 70cm. Thanks in advance. MikeN ZL1BNB |
MikeN wrote:
Three questions emerge from my foggy brain wrt to a current project. 1. What sort of choke loading on a feed line would be most effective at 70 cm, ferrite or coil wound from coax. Probably equally effective, although it might be tricky to get a coax coil to resonate at 70 cm, while a ferrite core choke, which is inherently broadband, would be easy. 2. Could ferrite loading at a feed point take the place of say a 1/4 wave sleeve balun matching the unbalanced coax feed to a centre-fed dipole element. Yes. A properly constructed sleeve balun can be made to have higher impedance, but in the application you describe, a ferrite core choke would be perfectly adequate. 3. What grade of ferrite bead would best be useful at 70cm. I'd probably use type 43. That's a Fair-Rite designation, but ferrites from other vendors with initial permeability of 700-800 or so would work equally well. Cores from that material are readily available in a wide variety of shapes and sizes. Some of the 60 series ferrites would also probably be adequate at that frequency. I suggest you visit the Fair-Rite website and take a look at the impedances of various cores at the frequency of interest, and choose ones that get you the impedance you need. (Unless you're running a lot of power, you don't need to worry about whether the impedance is resistive or reactive -- just look at its magnitude.) You can place cores on the outside of the coax and get an imedance that's the product of the number of cores and the impedance of one core. Or you can wind the coax in multiple turns on a single core and get N^2 times the single turn impedance, where N is the number of turns. 500 - 1000 ohms or so of impedance is adequate for most applications. If you have an antenna analyzer that operates at that frequency, you can measure it. Otherwise, just go by the manufacturer's stated value of impedance at the frequency of interest. Roy Lewallen, W7EL |
| All times are GMT +1. The time now is 07:35 PM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com