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Power radiated from feedlines
I am truly disappointed with the long-running discourse on balanced and unbalanced feedlines and the power radiated therefrom. It's been going on for years. Nobody, especially poor novices, has ever learned anything from it. 99% of it is bafflegab. Few of us understand what on Earth is being waffled about. And those who do, prefer not to waste their time by joining in. The reason I'm making this seemingly outrageous statement is that NOBODY HAS EVER QUANTIFIED, not even once, what they are waffling about. This demonstrates a great ignorance of the subject. (Remember what Lord Kelvin said about the ability to measure and quantify what it is you are gabbing about and how that ability is directly related to what you really know about it.) Perhaps somebody might be prepared to state the power actually radiated from feedlines in watts. At least it may create the impression you know what you are talking about. It might possibly be at such a low level that, in the great majority of cases, it's not worth all the megabytes of bandwidth which are wasted on it. As an unbiased World Citizen, I now find myself half-way down a bottle of Merlo, 2000, a produce of France. But I have in reserve some decent Californian stuff. May I say how saddened I am to learn about the terrible disastrous storm which has befallen some of the Southern states. ---- Reg, G4FGQ |
Reg Edwards wrote:
. . . The reason I'm making this seemingly outrageous statement is that NOBODY HAS EVER QUANTIFIED, not even once, what they are waffling about. This demonstrates a great ignorance of the subject. The last time you made a similar statement (not long ago) I referred you to my article on baluns, then and now available at my web site, which includes measurements of imbalance when various balun configurations are used. Have you looked at it yet? Perhaps somebody might be prepared to state the power actually radiated from feedlines in watts. At least it may create the impression you know what you are talking about. . . . Let's begin a little more simply. Can you tell us how many watts each portion of an inverted-L antenna radiates? How about each element of a Yagi? Or even, say, the outer 1/3 of a dipole? If you can't, may we assume that you don't know what you're talking about when you discuss antennas? Roy Lewallen, W7EL |
Roy, do everybody a favour, by stating, numerically, how much power is
radiated from feedlines. Then somebody might have some confidence in what you are bafflegabbing about. Are you still using your S-meter as the North American Standard? Found a corkscrew and I've just opened the Californian. I'd like to try some of your Oregon stuff - do you have any. ;o) ---- Reg. |
On Tue, 30 Aug 2005 22:35:43 +0000 (UTC), "Reg Edwards"
wrote: [Tired repetitive bafflegab snipped] As an unbiased World Citizen, I now find myself half-way down a bottle of Merlo, 2000, a produce of France. But I have in reserve some decent Californian stuff. Since you're drinking a French wine the least you can do it keep the "t" on the end of the French "Merlot." When you break out the reserve Californian vintage, then you can drop the "t" and just call it "delicious" May I say how saddened I am to learn about the terrible disastrous storm which has befallen some of the Southern states. Thank you for your kind thoughts. Do you suppose Britain or any of the other developed nations will be sending any relief aid? |
On Wed, 31 Aug 2005 00:08:30 +0000 (UTC), "Reg Edwards"
wrote: Roy, do everybody a favour, by stating, numerically, how much power is radiated from feedlines. Then somebody might have some confidence in what you are bafflegabbing about. 12.75 Watts +/- 6 dB. |
Reg Edwards wrote:
Roy, do everybody a favour, by stating, numerically, how much power is radiated from feedlines. Then somebody might have some confidence in what you are bafflegabbing about. Of the many things I've said you disagree with, which one are you now referring to as "bafflegabbing"? And, who are "everybody" and "somebody"? You? Roy Lewallen, W7EL Reg's Old Wife & Nit-Picker and, new title, Bafflegabber |
All,
Below is a link to a site that claims to model coax radiation from a dipole. http://www.smeter.net/feeding/feedpowr.php I have compared several of the models from this site with other models and they are close. I have tried several parameters and have gotten results from a very low level with a close match to over 90% of power radiated in the feedline when the dipole resonance is far from the transmit frequency. We may use this model for a couple of things. It is available and it predicts radiation. That gives the opportunity to create a test. I for one are willing to experiment. Dan Reg Edwards wrote: I am truly disappointed with the long-running discourse on balanced and unbalanced feedlines and the power radiated therefrom. It's been going on for years. Nobody, especially poor novices, has ever learned anything from it. 99% of it is bafflegab. Few of us understand what on Earth is being waffled about. And those who do, prefer not to waste their time by joining in. The reason I'm making this seemingly outrageous statement is that NOBODY HAS EVER QUANTIFIED, not even once, what they are waffling about. This demonstrates a great ignorance of the subject. (Remember what Lord Kelvin said about the ability to measure and quantify what it is you are gabbing about and how that ability is directly related to what you really know about it.) Perhaps somebody might be prepared to state the power actually radiated from feedlines in watts. At least it may create the impression you know what you are talking about. It might possibly be at such a low level that, in the great majority of cases, it's not worth all the megabytes of bandwidth which are wasted on it. As an unbiased World Citizen, I now find myself half-way down a bottle of Merlo, 2000, a produce of France. But I have in reserve some decent Californian stuff. May I say how saddened I am to learn about the terrible disastrous storm which has befallen some of the Southern states. ---- Reg, G4FGQ |
Wes Stewart wrote:
SNIPPED Do you suppose Britain or any of the other developed nations will be sending any relief aid? Only AFTER the middle east countries set the example by their donations!! |
dansawyeror wrote:
Below is a link to a site that claims to model coax radiation from a dipole. http://www.smeter.net/feeding/feedpowr.php Please note that the third wire to ground creates the unbalance that causes feedline radiation. You seem to be confusing cause and effect. The cause of the feedline radiation is the existence of that third wire, not SWR. All it proves is that feedline radiation is caused by that third wire path which unbalances the source currents. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
If it is true it proves that a driving a "bad" load can cause the coax feedline
to radiate a significant portion of the feed energy. At this point I am not sure what word to use other then 'bad', but I due intend to perform experiments to measure and verify what is happening and the model. Dan Cecil Moore wrote: dansawyeror wrote: Below is a link to a site that claims to model coax radiation from a dipole. http://www.smeter.net/feeding/feedpowr.php Please note that the third wire to ground creates the unbalance that causes feedline radiation. You seem to be confusing cause and effect. The cause of the feedline radiation is the existence of that third wire, not SWR. All it proves is that feedline radiation is caused by that third wire path which unbalances the source currents. |
On Tue, 30 Aug 2005 20:05:05 -0700, dansawyeror
wrote: I have tried several parameters and have gotten results from a very low level with a close match to over 90% of power radiated in the feedline when the dipole resonance is far from the transmit frequency. We may use this model for a couple of things. It is available and it predicts radiation. That gives the opportunity to create a test. I for one are willing to experiment. Dan Will you please describe the details of your experiment where you claim 90% of the power is radiated from the feedline? How did you make this measurement? Walt, W2DU |
I have to call BS on this one Dan!
On Tue, 30 Aug 2005 20:05:05 -0700, dansawyeror wrote: I have tried several parameters and have gotten results from a very low level with a close match to over 90% of power radiated in the feedline when the dipole resonance is far from the transmit frequency. We may use this model for a couple of things. It is available and it predicts radiation. That gives the opportunity to create a test. I for one are willing to experiment. Dan |
Reg, G4FGQ wrote:
"Perhaps somebody might be prepared to state the power actually radiated from feedlines in watts." Someone has. A feedline is a way to get energy from here to there. Usually we want to keep all the energy on or close to the line as it travels and lose as little as possible to radiation and conversion to heat. Terman gives an approximate formula credited to Sterba and Feldman for radiation from a 2-wire nonresonant line, provided that the lengrh is at least 20X the spacing and the spacing is less than 0.1 wavelength: Radiated power/Isquared=160(pi D/lambda)squared. D/lambda is the spacing in wavelengths I is the rms line current. Terman notes that the parallel line radiates 4X the power that a doublet of length equal to the line spacing would, providing that the line and doublet currents are equal. Terman provides a figure to be used to adjust the estimated radiation upwards for longer feedlines (up to 5 wavelengths), and for greater heights (up to 0.5 wavelength above the earth). See 1943 "Radio Engineers` Handbook", page 194. Best regards, Richard Harrison, KB5WZI |
Dan wrote:
"We may use this model for a couple of things." Dan attached a posting from Reg, G4FGQ. Reg`s description of the "Centre-Fed Dipole - Radiation from Coaxial Feedline" tells it as it is. About 45 years ago, I (Richard Harrison) worked for a company exploring for oil in the Chaco Jungle of Bolivia. Communications were by HF radio at all sites in the jungle and at offices in Cochabamba and La Paz. Radios were Collins 30K-5 transmitters and 51-J receivers at all locations. All antennas were center-fed 1/2-wave dipoles up only about 1/4-wave due to economics and generally were broadside to some favored direction. The feedline was also about 1/4-wavelength for convenience, but the antennas loaded, took a lot of power and the 30-K finals dipped OK. Soil at the radio sites was mostly sand and didn`t really ground anything very much. The length of the coax presented a high impedance to the outside shield of the coax cable at the dipole, whatever it might have been. At 1/4-wave antenna height, directionality was small and whatever the coax radiated, it likely filled-in any nulls. All stations could clearly hear all others all day, so the operators were satisfied. Much of the radiation was straight up so we likely bebefitted from near vertical incidence propagation. The transmitters shook the aether with their power, but the 51-J has a nice dial but not much else. Nevertheless, everything worked. My previous employer had given away all its 51-J`s, they were so sorry, and replaced them with Hammarland SP-600`s. I didn`t take a 51-J, I was using one of the Super Pro`s. My experience is anecdotal. Best regards, Richard Harrison, KB5WZI |
"Cecil Moore" wrote in message ... dansawyeror wrote: Below is a link to a site that claims to model coax radiation from a dipole. http://www.smeter.net/feeding/feedpowr.php Please note that the third wire to ground creates the unbalance that causes feedline radiation. You seem to be confusing cause and effect. The cause of the feedline radiation is the existence of that third wire, not SWR. All it proves is that feedline radiation is caused by that third wire path which unbalances the source currents. -- 73, Cecil http://www.qsl.net/w5dxp Cecil How wrong would it be to say that the *coax* part of the line doesnt radiate at all? I see this as a situation where the *outer shield* of a transmission line is conducting current that radiates. It seems that a "balanced" antenna that is comprised of a single conductor and a L shaped conductor that includes the outer conductor of the coax, could be fed with a balanced line for modeling. Jerry |
Jerry Martes wrote:
"Cecil Moore" wrote in message ... dansawyeror wrote: Below is a link to a site that claims to model coax radiation from a dipole. http://www.smeter.net/feeding/feedpowr.php Please note that the third wire to ground creates the unbalance that causes feedline radiation. You seem to be confusing cause and effect. The cause of the feedline radiation is the existence of that third wire, not SWR. All it proves is that feedline radiation is caused by that third wire path which unbalances the source currents. -- 73, Cecil http://www.qsl.net/w5dxp Cecil How wrong would it be to say that the *coax* part of the line doesnt radiate at all? I see this as a situation where the *outer shield* of a transmission line is conducting current that radiates. It seems that a "balanced" antenna that is comprised of a single conductor and a L shaped conductor that includes the outer conductor of the coax, could be fed with a balanced line for modeling. Jerry What I was taught is that in a properly installed antenna system the coax will not radiate. If the antenna is not properly matched to the coax you get current flow along the outside of the coax shield. Dave WD9BDZ |
dansawyeror wrote:
If it is true it proves that a driving a "bad" load can cause the coax feedline to radiate a significant portion of the feed energy. You need to define "bad load". A "bad load" for unbalanced line is a balanced load, no matter what the impedance. A "bad load" for balanced line is an unbalanced load, no matter what the impedance. The third wire used in the aforementioned software is designed to unbalance the system, no matter what the impedance. To illustrate a balanced system, a fourth wire needs to be added in parallel with and about four inches away from the third wire. Then compare the currents in the third and fourth wires under conditions of changing loads. I due intend to perform experiments to measure and verify what is happening and the model. Please feel free to experiment but at least a dozen participants of this newsgroup already know what is happening and have been trying to tell you. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
On Wed, 31 Aug 2005 16:17:42 -0500, "David G. Nagel"
wrote: What I was taught is that in a properly installed antenna system the coax will not radiate. If the antenna is not properly matched to the coax you get current flow along the outside of the coax shield. Dave WD9BDZ Please read: http://www.w2du.com/r2ch21.pdf |
Jerry Martes wrote:
How wrong would it be to say that the *coax* part of the line doesnt radiate at all? The physical construction of the coax (ideal version) ensures that the inside of the coax doesn't radiate because, for ideal coax, the internal currents are perfectly balanced. Any unbalance in the currents is forced to the outside shield by the laws of physics. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
David G. Nagel wrote:
What I was taught is that in a properly installed antenna system the coax will not radiate. If the antenna is not properly matched to the coax you get current flow along the outside of the coax shield. Is "matched" the correct word to use there? A 50 ohm balanced dipole is perfectly "matched" to 50 ohm coax but the feedline will likely radiate. A 50 ohm balanced dipole is not "matched" to 600 ohm balanced line but with proper attention to details the feedline will like not radiate much. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
"David G. Nagel" wrote in message ... Jerry Martes wrote: "Cecil Moore" wrote in message ... dansawyeror wrote: Below is a link to a site that claims to model coax radiation from a dipole. http://www.smeter.net/feeding/feedpowr.php Please note that the third wire to ground creates the unbalance that causes feedline radiation. You seem to be confusing cause and effect. The cause of the feedline radiation is the existence of that third wire, not SWR. All it proves is that feedline radiation is caused by that third wire path which unbalances the source currents. -- 73, Cecil http://www.qsl.net/w5dxp Cecil How wrong would it be to say that the *coax* part of the line doesnt radiate at all? I see this as a situation where the *outer shield* of a transmission line is conducting current that radiates. It seems that a "balanced" antenna that is comprised of a single conductor and a L shaped conductor that includes the outer conductor of the coax, could be fed with a balanced line for modeling. Jerry What I was taught is that in a properly installed antenna system the coax will not radiate. If the antenna is not properly matched to the coax you get current flow along the outside of the coax shield. Dave WD9BDZ Dave I'd agree with your statement if "matched" doesnt refer to *impedance* match. Jerry |
"Cecil Moore" wrote in message ... Jerry Martes wrote: How wrong would it be to say that the *coax* part of the line doesnt radiate at all? The physical construction of the coax (ideal version) ensures that the inside of the coax doesn't radiate because, for ideal coax, the internal currents are perfectly balanced. Any unbalance in the currents is forced to the outside shield by the laws of physics. -- 73, Cecil http://www.qsl.net/w5dxp Cecil I failed to emphisze that any current on the outside of a coaxial transmission line are out of the bounds of the defination of Coax. Jerry |
Reg wrote, most lucidly,
"Found a corkscrew and I've just opened the Californian. I'd like to try some of your Oregon stuff - do you have any. ;o) " Why, yes, I do. Picked up a decent stash over the Indepencence Day (that's US independence from England...) holiday a couple months ago, at a nice wine and arts fair in Eugene. Do _you_ have any? Are there any good English wines I should try when I'm next in the neighborhood?? (Did Monty Python ever do a skit about English wines to match the one they did about "Fine Australian Table Wines"?) Beats babbling on about old wives any day. Cheers, Tom |
David G. Nagel wrote:
What I was taught is that in a properly installed antenna system the coax will not radiate. If the antenna is not properly matched to the coax you get current flow along the outside of the coax shield. Dave WD9BDZ It's too bad people are being taught this. As a number of others have correctly said, impedance match has nothing to do with whether or not a coax (or symmetrical twinlead line) radiates. Radiation is due to other, unrelated causes. Roy Lewallen, W7EL |
David G. Nagel wrote:
SNIPPED What I was taught is that in a properly installed antenna system the coax will not radiate. If the antenna is not properly matched to the coax you get current flow along the outside of the coax shield. Dave WD9BDZ A balanced antenna fed with balanced line, and the line goes away from the antenna perpendicular [90 degrees angle] for a minimum of 1/4 wavelength is a properly installed antenna system: [line has minimum or no radiation] A balanced antenna fed with balanced line, and the line goes away from the antenna non-perpendicular [other than 90 degrees angle] for a minimum of 1/4 wavelength is an improperly installed antenna system: [line will radiate] A balanced antenna fed with un-balanced line, and the line goes away from the antenna perpendicular [90 degrees angle] for a minimum of 1/4 wavelength is an improperly installed antenna system: [line will radiate] A balanced antenna fed with un-balanced line, and the line goes away from the antenna non-perpendicular [other than 90 degrees angle] for a minimum of 1/4 wavelength is an improperly installed antenna system: [line will radiate] A balanced antenna fed with un-balanced line and a properly installed balun, and the line goes away from the antenna perpendicular [90 degrees angle] for a minimum of 1/4 wavelength is a properly installed antenna system: [line has minimum or no radiation]. An un-balanced antenna fed with balanced line is an improperly installed antenna and the line will radiate. NEED I CONTINUE?? |
Ham op wrote:
A balanced antenna fed with balanced line, and the line goes away from the antenna perpendicular [90 degrees angle] for a minimum of 1/4 wavelength is a properly installed antenna system: [line has minimum or no radiation] The line will have minimum or no radiation only if fed at the transmitter end with a balanced feed. An unbalanced feed will create unequal currents in the conductors, resulting in radiation. A symmetrical (e.g., twinlead) line doesn't assure balance (equal and opposite currents in the two conductors), and an unbalanced line will radiate, regardless of its physical construction. A balanced antenna fed with balanced line, and the line goes away from the antenna non-perpendicular [other than 90 degrees angle] for a minimum of 1/4 wavelength is an improperly installed antenna system: [line will radiate] True for both symmetrical line and coax. Radiation is due to common mode current induced on the line. It can be reduced by inserting "current baluns" (common mode chokes) in the line. A couple spaced about a quarter wavelength apart are usually adequate. A balanced antenna fed with un-balanced line, and the line goes away from the antenna perpendicular [90 degrees angle] for a minimum of 1/4 wavelength is an improperly installed antenna system: [line will radiate] The amount of common mode current and therefore radiation depends on the length of the path along the outside of the coax to ground. A current balun (common mode choke) at the feedpoint will reduce the current and therefore radiation. A balanced antenna fed with un-balanced line, and the line goes away from the antenna non-perpendicular [other than 90 degrees angle] for a minimum of 1/4 wavelength is an improperly installed antenna system: [line will radiate] This is due to induced common mode current. The mechanism is identical to that when a symmetrical line is used. A balanced antenna fed with un-balanced line and a properly installed balun, and the line goes away from the antenna perpendicular [90 degrees angle] for a minimum of 1/4 wavelength is a properly installed antenna system: [line has minimum or no radiation]. Correct. A current balun at the feedpoint reduces the conducted common mode current. Induced common mode current is prevented by symmetrical feedline placement. An un-balanced antenna fed with balanced line is an improperly installed antenna and the line will radiate. Coax feeding an unbalanced antenna will radiate just as much as twinlead feeding an unbalanced antenna. NEED I CONTINUE?? A lot of people have learned a list of handy rules like this without having much of an understanding of where they came from or under what circumstances they do and don't apply. For anyone interested in learning more about common mode current, feedline radiation, types of feedlines, and feedline radiation, I suggest taking a look at http://eznec.com/Amateur/Articles/Baluns.pdf. It deals only with conducted common mode current and doesn't discuss induced common mode current -- I'll try to get together a supplement covering that topic when I have time. There was some discussion on this newsgroup not long ago about induced common mode current, but I can't locate the topic right off. It should be possible to find it with a google search. Be sure to also look at the work by Walt Maxwell, W2DU referenced at the end of the balun article. Roy Lewallen, W7EL |
Walt,
I reported a model at the following link: http://www.smeter.net/feeding/feedpowr.php At resonance it reports a 'normal' pattern. Off resonance it reports up to 90% feedline radiation. I intend to mimic the results this weekend. I will build a 10 meter dipole, misfeed it, an measure the results. I will let you know. Dan Walter Maxwell wrote: On Tue, 30 Aug 2005 20:05:05 -0700, dansawyeror wrote: I have tried several parameters and have gotten results from a very low level with a close match to over 90% of power radiated in the feedline when the dipole resonance is far from the transmit frequency. We may use this model for a couple of things. It is available and it predicts radiation. That gives the opportunity to create a test. I for one are willing to experiment. Dan Will you please describe the details of your experiment where you claim 90% of the power is radiated from the feedline? How did you make this measurement? Walt, W2DU |
Reg Edwards wrote:
I am truly disappointed with the long-running discourse on balanced and unbalanced feedlines and the power radiated therefrom. It's been going on for years. Nobody, especially poor novices, has ever learned anything from it. 99% of it is bafflegab. Few of us understand what on Earth is being waffled about. And those who do, prefer not to waste their time by joining in. The reason I'm making this seemingly outrageous statement is that NOBODY HAS EVER QUANTIFIED, not even once, what they are waffling about. This demonstrates a great ignorance of the subject. (Remember what Lord Kelvin said about the ability to measure and quantify what it is you are gabbing about and how that ability is directly related to what you really know about it.) Perhaps somebody might be prepared to state the power actually radiated from feedlines in watts. At least it may create the impression you know what you are talking about. It might possibly be at such a low level that, in the great majority of cases, it's not worth all the megabytes of bandwidth which are wasted on it. As an unbiased World Citizen, I now find myself half-way down a bottle of Merlo, 2000, a produce of France. But I have in reserve some decent Californian stuff. May I say how saddened I am to learn about the terrible disastrous storm which has befallen some of the Southern states. ---- Reg, G4FGQ Instead of sitting around for years reading the "bafflegab" and then complaining about it, why didn't you QUANTIFY it yourself and report it so as to make yourself useful? John |
dansawyeror wrote:
At resonance it reports a 'normal' pattern. Off resonance it reports up to 90% feedline radiation. I intend to mimic the results this weekend. I will build a 10 meter dipole, misfeed it, an measure the results. I will let you know. Try "misfeeding" a 10m dipole with N*1/2WL 450 ohm ladder-line through a good choke and get back to us. (N is a natural number) -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
"Walter Maxwell" wrote Will you please describe the details of your experiment where you claim 90% of the power is radiated from the feedline? How did you make this measurement? ================================== Walt, If you erect a 10 meter 1/2-wave dipole and center-feed it with a 50 feet length of coax, and then transmit on 1.9 MHz, 99.9 percent of available power will be radiated from the feedline and only 0.1 percent from the antenna itself. SWR on the line will be about 1400 and the amount of power available will not be very great. ---- Reg. |
If anyone besides me is having trouble keeping track of what Reg is
trying to say, maybe the following recent quotes will help. That is, they'll help you understand why you're having trouble keeping track. For what it's worth, I agree with the first quotation of 8/31. But apparently Reg doesn't, even though he said it. Roy Lewallen, W7EL 8/30: Perhaps somebody might be prepared to state the power actually radiated from feedlines in watts. At least it may create the impression you know what you are talking about. 8/30: Roy, do everybody a favour, by stating, numerically, how much power is radiated from feedlines. Then somebody might have some confidence in what you are bafflegabbing about. 8/31: Any current which finds its way on to (3), which ought to flow in the antenna, will result in the coax participating in the radiation pattern of the whole antenna structure. BUT IT IS A SILLY QUESTION TO ASK HOW MANY WATTS ARE RADIATED FROM THE FEEDLINE. OR, CONVERSELY, HOW MANY MICROWATTS ARE PICKED UP BY THE FEEDLINE ON RECEIVE. The feedline cannot be treated in isolation as if it behaves independently of the antenna. For starters, the radiation resistances of the feedline and antenna do not add arithmetically. They interact with each other. 8/31: If you erect a 10 meter 1/2-wave dipole and center-feed it with a 50 feet length of coax, and then transmit on 1.9 MHz, 99.9 percent of available power will be radiated from the feedline and only 0.1 percent from the antenna itself. http://www.smeter.net/feeding/feedpowr.php: Centre-Fed Dipole - Radiation from Coaxial Feedline Author: R.J.Edwards G4FGQ © 25th March 2003 Program Notes .. . . With no loss in accuracy, to simplify the model, the transmitter is located at the dipole centre and the coaxial line is replaced by a single conductor of the same diameter as the coaxial braid. There are 3 radiating elements. Radiation resistance, input impedance, and input current of each element is calculated. Finally, the percent of total power radiated by each of the three elements is calculated. |
Reg Edwards wrote:
If you erect a 10 meter 1/2-wave dipole and center-feed it with a 50 feet length of coax, and then transmit on 1.9 MHz, 99.9 percent of available power will be radiated from the feedline and only 0.1 percent from the antenna itself. I think most RF people would consider radiant heat from the feedline to be heat dissipation (as opposed to RF radiation). -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Reg, put up the corkscrew, your arguing with yourself. Here, we put people
in rubber rooms that exhibit these propensities... "Reg Edwards" wrote in message ... "Walter Maxwell" wrote Will you please describe the details of your experiment where you claim 90% of the power is radiated from the feedline? How did you make this measurement? ================================== Walt, If you erect a 10 meter 1/2-wave dipole and center-feed it with a 50 feet length of coax, and then transmit on 1.9 MHz, 99.9 percent of available power will be radiated from the feedline and only 0.1 percent from the antenna itself. SWR on the line will be about 1400 and the amount of power available will not be very great. ---- Reg. |
Roy, W7EL wrote:
"The calculations you qupte assume equal and opposite (i.e. purely differential mode) currents in the feedline conductors. The topic under discussion is radiation due to common mode current." Radiation from current on the outside of a coax feedline can be calculated too by using formulas appropriate to the actual current distribution. Refer to the infamous "ARRL Antenna Book" figure (Fig 24 on page 26-16 of my 19th edition), "showing various current paths at feed point of a balanced dipole fed eith unbalanced coaxial cable." From the text: "The antenna current in arm 2 is thus equal to the difference between I2 and I3. The magnitude of I3 (on coax exterior) is proportional to the relative impedance in each current path beyond the split." Best regards, Richard Harrison, KB5WZI |
Richard Harrison wrote:
"The antenna current in arm 2 is thus equal to the difference between I2 and I3. The magnitude of I3 (on coax exterior) is proportional to the relative impedance in each current path beyond the split." Could be Roy or Walt wrote that. :-) -- 73, Cecil, http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
"Walter Maxwell" wrote in message ... On Thu, 1 Sep 2005 07:35:35 +0000 (UTC), "Reg Edwards" wrote: "Walter Maxwell" wrote Will you please describe the details of your experiment where you claim 90% of the power is radiated from the feedline? How did you make this measurement? ================================== Walt, If you erect a 10 meter 1/2-wave dipole and center-feed it with a 50 feet length of coax, and then transmit on 1.9 MHz, 99.9 percent of available power will be radiated from the feedline and only 0.1 percent from the antenna itself. SWR on the line will be about 1400 and the amount of power available will not be very great. ---- Reg. Sorry to disagree, Reg, But if a balun at the feedline-antenna junction is a perfect balun at 1.9 MHz, NO power will be radiated from the feedline with any amount of SWR, even 1400. It is true that only 0.1% of the power will be radiated, but the 99.9% will be reflected back to the input of the line, not radiated,, making a 50-ohm transmitter see a 1400:1 mismatch. In the steady state the transmitter will only deliver 0.1% of its available power into the line due to the mismatch. The 1400 SWR inside the coax will not cause it to radiate, unless the outer conductor is full of holes. ============================ Walt, Back along the thread you asked for details of the experiment which demonstrated that a feedline could radiate as much as 90 percent (of the available power). But you received no reply. So I provided an example in which nearly all the power is radiated by the feedline simply because the short antenna is unable to accept very much and is inefficient. The power available at the antenna input HAS to go somewhere if it cannot be accepted by the antenna. Walt, my statement is correct. There is NO choke balun involved. The unbalanced coax goes straight into the balanced antenna which inevitable injects power into the outside of the coax. The choke is omitted just to acheive that purpose. I made the statement because you seemed to doubt it was possible for 90 percent of the available power ever to be radiated from an unbalanced feedline. I may possibly have mis-interpretted you. Admitted, there may not be very much power available at the antenna due to high SWR and loss within the coax. But nevertheless most of the power will be radiated by the outside of the feedline. The feedline makes an excellent, efficient antenna wire compared with the dipole. An approximate analysis can possibly be done by using Eznec and placing the transmitter directly at the center of the dipole. The outer conductor of the coax can be represented by a thick wire down to ground, the transmission loss inside the coax being ignored without having the slightest effect on the experiment. Incidentally, the length of the feedline in wavelengths and the resistance of the bottom end ground connection will have a considerable effect on results. I mention Eznec because of the number of people who swear by it. For Roy's benefit, the experiment will not tell him how many watts are radiated from the feedline because it is impossible to separate the three radiation resistances involved and there will be only one very peculiar radiation pattern from their combination. I entered this extremely long-standing discussion only to say that, in general, the amount of "Power Radiated from a Feedline" is not worth the effort which has been expended in waffling about it. I use the words "Power Radiated from a Feedline" only because, to make myself understood, I am obliged to use the same confusing language as used by old-wives for the last 50 years. I try to be helpful. ---- Reg, G4FGQ |
Reg Edwards wrote:
So I provided an example in which nearly all the power is radiated by the feedline simply because the short antenna is unable to accept very much and is inefficient. But how much of that power is heat and how much is RF? With an SWR of 1000:1, the I^2*R losses are bound to be pretty high. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
On Thu, 1 Sep 2005 07:35:35 +0000 (UTC), "Reg Edwards"
wrote: "Walter Maxwell" wrote Will you please describe the details of your experiment where you claim 90% of the power is radiated from the feedline? How did you make this measurement? ================================== Walt, If you erect a 10 meter 1/2-wave dipole and center-feed it with a 50 feet length of coax, and then transmit on 1.9 MHz, 99.9 percent of available power will be radiated from the feedline and only 0.1 percent from the antenna itself. SWR on the line will be about 1400 and the amount of power available will not be very great. ---- Reg. Sorry to disagree, Reg, But if a balun at the feedline-antenna junction is a perfect balun at 1.9 MHz, NO power will be radiated from the feedline with any amount of SWR, even 1400. It is true that only 0.1% of the power will be radiated, but the 99.9% will be reflected back to the input of the line, not radiated,, making a 50-ohm transmitter see a 1400:1 mismatch. In the steady state the transmitter will only deliver 0.1% of its available power into the line due to the mismatch. The 1400 SWR inside the coax will not cause it to radiate, unless the outer conductor is full of holes. Walt |
But how much of that power is heat and how much is RF?
With an SWR of 1000:1, the I^2*R losses are bound to be pretty high. ================================= Your question is not relevant. We are concerned only with the possible high ratio of feedline radiation to radiation from the antenna. If you are worried about transmission loss in the coax then that can be reduced by transmitting at a higher frequency than 1.9 MHz where the 10m dipole provides a better terminating impedance and SWR will be lower. With an appropriate line-length in wavelengths the coax can still be forced into radiating better than the short 10m dipole. But the conditions under which this occurs are NEVER those under which the system would normally be operated. Under normal conditions, eg., transmitting on 29 MHz, radiation from the feedline is low enough never to cause interference to the neighbor's television. If such an unwelcome event should occur then it will be due to legitimate high-power radiation from the dipole and your conscience will be clear. Now Cecil, you can stop trying to pull my leg. ;o) ---- Reg. |
On Thu, 1 Sep 2005 23:57:37 +0000 (UTC), "Reg Edwards"
wrote: "Walter Maxwell" wrote in message .. . On Thu, 1 Sep 2005 07:35:35 +0000 (UTC), "Reg Edwards" wrote: "Walter Maxwell" wrote Will you please describe the details of your experiment where you claim 90% of the power is radiated from the feedline? How did you make this measurement? ================================== Walt, If you erect a 10 meter 1/2-wave dipole and center-feed it with a 50 feet length of coax, and then transmit on 1.9 MHz, 99.9 percent of available power will be radiated from the feedline and only 0.1 percent from the antenna itself. SWR on the line will be about 1400 and the amount of power available will not be very great. ---- Reg. Sorry to disagree, Reg, But if a balun at the feedline-antenna junction is a perfect balun at 1.9 MHz, NO power will be radiated from the feedline with any amount of SWR, even 1400. It is true that only 0.1% of the power will be radiated, but the 99.9% will be reflected back to the input of the line, not radiated,, making a 50-ohm transmitter see a 1400:1 mismatch. In the steady state the transmitter will only deliver 0.1% of its available power into the line due to the mismatch. The 1400 SWR inside the coax will not cause it to radiate, unless the outer conductor is full of holes. ============================ Walt, Back along the thread you asked for details of the experiment which demonstrated that a feedline could radiate as much as 90 percent (of the available power). But you received no reply. So I provided an example in which nearly all the power is radiated by the feedline simply because the short antenna is unable to accept very much and is inefficient. The power available at the antenna input HAS to go somewhere if it cannot be accepted by the antenna. Walt, my statement is correct. There is NO choke balun involved. The unbalanced coax goes straight into the balanced antenna which inevitable injects power into the outside of the coax. The choke is omitted just to acheive that purpose. snip I use the words "Power Radiated from a Feedline" only because, to make myself understood, I am obliged to use the same confusing language as used by old-wives for the last 50 years. I try to be helpful. ---- Reg, G4FGQ Thank you for the detailed explanation, Reg, I'm sorry I began transmission while off frequency. I jumped on too soon without considering you were talking specifically a condition without a balun. You are correct about the feedline radiation without the balun. However, with the 1400 SWR in the case you described, because even though the SWR at the input of the feedline will be substantially less that at the load, the mismatch seen by the source will allow only an insignificant amount of the available power to be delivered to the feedline. Walt |
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