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As an alternative to using a choke on the feedline, what sort of
results might one get with a standard J-pole by using a half-wave coaxial balun and tapping up a bit further on the elements? Still don't think that would clean up spillover currents... If I were to use a J pole, and wanted decoupling, I would add a 1/4 wave coax section below the feed, and then have a set of radials at that point. They can be grounded to the mast, or left free, as long as they are connected to the shield. The 1/4 wave of coax is physical length, not electrical. But I've found in testing, it's not all that critical. Of course, he wouldn't be able to claim "no radials" at that point, but it would be pretty well decoupled. This is the same basic design cushcraft used with the ringo ranger. In that case, the coax length was 50 inches long, to a set of 20 inch radials. They used the longer length due to the dual 5/8 design. They actually seem to claim that 50 inches of coax as a 3rd radiating element, but I don't quite see it that way. If that were truly the case, it would beat the isopole. The comparison between the isopole and the ringo ranger 2 show how important decoupling is. They are both appx dual 5/8 designs. No real difference in element length. The isopoles improved decoupling is what make it the winner when you compare the two head to head. A sleeve would also be easy to use with a J pole... Actually, I sort of prefer the center fed 1/2 wave "sleeve" dipole vertical, with a 2nd lower decoupling sleeve, over the usual J pole design. MK |
Ian, not wishing to be classed amongst the old wives you have very carefully avoided talking about "power radiated from feedlines". You have shifted to using clamp-on ammeters. But people who DO discuss things in such terms are unable to justify the use of chokes by quantifying the power actually radiated and setting limits on what power level is acceptable or is not acceptable. If they can't measure or calculate the power level then they know nothing about what they are are talking. Refer to what Lord Kelvin said about measurements. Can you suggest an acceptable level of amps as measured on a clamp-on ammeter? ---- Reg, G4FGQ ---------------------------------------------------------------------- --------- "Ian White GM3SEK" wrote in message ... Reg Edwards wrote: They who discuss "power radiated from the feedline", yet are unable to measure (in watts) or calculate (in watts) the MAGNITUDE of the effect, belong to a set of waffling old wives. Those who take no precautions to prevent their feedline from becoming part of the antenna, belong to a set of people who don't even know what their antenna IS. There's a part you call "the antenna", and another part you call "the feedline". Wishful thinking will not stop RF current from flowing directly from one to the other. How is it possible to decide whether or not a choke or balun is needed, and where to locate it, unless the magnitude of what one MIGHT wish to prevent is known. Try a clamp-on RF current meter, a little modeling... or even a little common sense. There's a place called "the feedpoint" where the antenna and the feedline are connected directly together. Might that be a good location for a choke to keep them separate? Yes, it almost certainly would. Chokes may also be needed at other locations, but it's hard to justify anywhere else as your *first* choice. (The exception is the Carolina Windom and similar antennas where part of the feedline is intended to radiate. But even there, they put a good choke at the point where they want RF currents on the feedline to stop.) Queen Elizabeth 1 of England had the good sense to take a bath every six months "whether I need it or not". If you don't know whether your feedline smells of RF, then follow her excellent advice and use a choke. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Dave Platt wrote:
In article , Ian White GM3SEK wrote: The version of the "non-standard J-pole" which Cebik models is fed at bottom center, in a dimensionally-symmetrical way. His current plot seems to show equal currents at this feedpoint, and this would seem to make the use of a choke or balun on the feedline somewhat less significant than with a standard J-pole. An antenna model without a feedline will *force* equal and opposite currents at the feedpoint - it is always fed through the perfect balun! Cebik's plot of currents in the "standard" J-pole shows unequal currents all the way down to the bottom, and he notes this in his text and asserts the need for a choke. He doesn't say whether his models do or do not include a feedline. If they did, the configuration of the feedline would become another important variable which he couldn't fail to mention. Also the drawings show no feedline. Add an un-choked feedline to the model (another thick wire, representing the coax shield) at either side of the feedpoint, and see where the current goes now. If you don't use a choke on a real-life antenna, there's nothing to stop the current going wherever it likes. Agreed. The results are likely to be quite variable depending on the feedline distance to the nearest ground. Seems to me that the worst case would result from a small integral multiple of 1/2 wavelength, no? That's right. At the point where the coax shield connects to the antenna, the current will divide three ways, between the antenna, the inside of the shield, and the outside of the shield. The split will depend on the ratio of the impedances in each of those three directions. If the impedance for current flowing down the outside of the shield is low (which any multiple of 1/2 wavelength grounded at the bottom will achieve) then away the current will go - there's nothing to stop it. A feedline choke creates a high impedance against current flow down the outside, so the current from the inside of the shield flows almost exclusively into the antenna. Even if you choke a J-pole at the feedpoint, there will also be induced currents further down the feedline because the antenna and the feedline are usually installed in a straight line. But that doesn't override the need to choke the feedline at the most obvious place. My guess is that in most simple J-pole installations, the feedline radiation and the resulting disturbance of the antenna's omni pattern are probably not going to be worth worrying about too much. Nearby buildings, trees, etc. are likely to result in larger differences in the far-field pattern than any quirks in the antenna's own pattern. Most people using J-poles won't worry, that is true... but that's mostly because J-poles are used in relatively undemanding applications where you either hit the repeater or packet node, or you don't. In defence of Al, if the J-pole is mounted directly on a car roof, then there's no point in attempting to choke the feedpoint. But if it's mounted on a mast, the mast and feedline will radiate. How much will depend on the exact installation, and is pretty well unknown unless you can measure the actual RF currents. As an alternative to using a choke on the feedline, what sort of results might one get with a standard J-pole by using a half-wave coaxial balun and tapping up a bit further on the elements? That is a workable feed method, but the half-wave balun is a voltage balun. The antenna is asymmetrical, so the balun is acting as center-tapped voltage source which is trying to push equal currents into the unequal impedances on either side of the feedpoint. That will never quite succeed, so there will always be some out-of-balance current left over. Like all voltage baluns, the half-wave coax type doesn't do anything directly to *prevent* the out-of-balance current from flowing away on the outside of the feedline. On the contrary, there is a hard-wired connection that will *allow* such currents to flow. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Reg Edwards wrote:
Ian, not wishing to be classed amongst the old wives you have very carefully avoided talking about "power radiated from feedlines". You have shifted to using clamp-on ammeters. But people who DO discuss things in such terms are unable to justify the use of chokes by quantifying the power actually radiated and setting limits on what power level is acceptable or is not acceptable. If they can't measure or calculate the power level then they know nothing about what they are are talking. That's not really quite true. Can you or anyone else tell how many watts is radiated from each element of a Yagi? How? Then how do you know which elements are important an which are not? Roy Lewallen, W7EL |
Reg Edwards wrote:
Ian, not wishing to be classed amongst the old wives you have very carefully avoided talking about "power radiated from feedlines". You have shifted to using clamp-on ammeters. But people who DO discuss things in such terms are unable to justify the use of chokes by quantifying the power actually radiated and setting limits on what power level is acceptable or is not acceptable. If they can't measure or calculate the power level then they know nothing about what they are are talking. Refer to what Lord Kelvin said about measurements. Can you suggest an acceptable level of amps as measured on a clamp-on ammeter? No. Reg knows perfectly well that the RF current is only one part of a much bigger picture. An acceptable level is one that: 1. Does NOT make the microphone bite your lips (or does not leave lasting scars :-) 2. Does NOT cause your transmitter to act up because there's too much RF current flowing through your station, trying to find "ground" 3. Does NOT cause RFI to your family and neighbours 4. Does NOT cause unpredictable changes in transmitter loading 5. Does NOT lead to unacceptable pickup of interference when you're trying to receive. So that "acceptable level" depends entirely on each individual's particular station layout, how they operate, where they live, what kinds of consumer electronics the family and neighbours use, how they are installed... and how much that individual ham cares about getting along with the family and the neighbours. Every case is totally individual. That is why every individual needs to do his own thinking and make his own decisions. The only "old wives' tale" is that somebody else can do it for you, or tell you from 5000 miles away what does or doesn't matter. You don't actually need to measure amps in order to make those decisions. Basically it's all about simple practical things like the list above. Where the RF current meter really helps is if you decide you do have an RFI problem. Then it lets you *see* how well you're managing to fix it. Too much of RFI investigation work is like groping in the dark. The RF current meter is like taking a blindfold off. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
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That's not really quite true. Can you or anyone else tell how many
watts is radiated from each element of a Yagi? How? Then how do you know which elements are important an which are not? Roy Lewallen, W7EL ================================ Neither you, I or anybody else knows. But, as usual, you have put your finger on the source of the trouble. It's a descriptive language problem. To speak in terms which most people understand, when Cecil acquired his burn the injury was just as likely to come from the antenna as it was from the feedline. Actually, it came from the PA via the tuner. It is not entirely unrelated to the confusion about standing waves caused by referring to the so-called SWR meter as an SWR meter, when it doesn't do anything of the sort and, in any case, there is no line in a position on which SWR can be measured. ---- Reg, G4FGQ |
Ian White GM3SEK wrote:
Where the RF current meter really helps is if you decide you do have an RFI problem. Then it lets you *see* how well you're managing to fix it. Too much of RFI investigation work is like groping in the dark. The RF current meter is like taking a blindfold off. And the "RF current meter" doesn't have to be expensive or complicated. A before/after relative current value may be all one needs. I use a toroid with ten turns feeding a 1N34A curcuit and a DC voltmeter with very short leads. It was "calibrated" using forward current into a 50 ohm load. Simple measurements like this are covered in, "Baluns, What They Do And How They Do It", by Roy, W7EL, in the ARRL Antenna Compendium, Vol 1, article available at: http://www.eznec.com/Amateur/Articles/ -- 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 =--- |
.... my gawd, you go into far too much verbiage for far too simple of
explanation(s) of your personal, views, beliefs and offerings... I do choose to ignore them and toss 'me into "what nm5k likes" bucket--which frankly, "charlotte--I don't give a damn." .... if the only time you hear these complaints are from me, no need to worry, if others point out your nature is less than desirable (and new members--not the good ole boys here who are into the "mutual admiration societies", "legends of their own mind(s) clubs) then you may want to use it to your advantage--or not... John wrote in message oups.com... what is never GOOD is having another tell one WHAT IS GOOD for them... if you do that in normal life, expect to get told off as often as here... What is good is talking to people that don't have their heads up their rearend. All I'm doing is stating fairly simple facts. If you choose to ignore them, fine with me... But don't start with your goofball behavior pattern just because I'm trying to clarify that *no* type of feed on an elevated vertical is capable of fully decoupling the antenna on it's own. I quote from Al's first post.... "One think you may have missed, the original post was about a Arrow Antenna J-Pole. This is an Open Stub type J-Pole, Not a Closed Stub type like the copper pipe ones. The OSJ does not need a choke, it does not have a problem with feedline radiation or a problem with Common Mode Currents. " That statement is *false* if you want the best performance. That type of antenna is most certainly able to suffer from feedline radiation. Sure, it may be better decoupled from the line using his method vs another, but that does not deal with spillover currents. There must be a 2nd decoupling device to deal with those currents. The antenna does include that as sold. If you think I really give a hoot about Arrow antennas, or Al, or whoever, you are sadly mistaken. He would have never heard from me if he hadn't said that his J pole can't suffer from common mode problems. I'm a dumbass compared to many of these people, and even I know thats a silly statement to make, unless you like holes in your toes. Oh...And I can't ever recall ever being "told off" on any post, on any NG...I don't have the behavior patterns, hyperactivity, etc, that you seem to exhibit. So I don't attract the attention that people like you do. The only people that ever get me going are smartasses. I have run across 4-5 realŪ smartasses on my journey through the internet. They tried to tell me off, but I quickly showed them that that was an effort in futility. I'm rated a black belt when it comes to smartasses. You wanna be #6 on the list? Go ahead...Make my day.... At this point, I don't consider you a full fledged smartass... You are like a 9 year old that is hyperactive from gorging on chocolate, that turned out to actually be coated preluden pills... So I cut you a bit of slack...So far.....Don't push your luck... You would have never heard from me, except that your post struck me as overall BS...As usual.... Note... This room is less about antennas than it is about egos, hero worship, and the "good ole boys club." Only people that are defensive, or are trying to cover something up, or trying to claim some silly nonsense would feel this way. All people have ego's to varying degrees. Best to ignore.... I would never worship a man. I don't belong to any clubs, but I reckon if I did, I would prefer a bunch of good ole boys, over a bunch of anal retentive "girly men" who constantly feel victimized by the world in general... MK |
Cecil Moore wrote:
Ian White GM3SEK wrote: Where the RF current meter really helps is if you decide you do have an RFI problem. Then it lets you *see* how well you're managing to fix it. Too much of RFI investigation work is like groping in the dark. The RF current meter is like taking a blindfold off. And the "RF current meter" doesn't have to be expensive or complicated. A before/after relative current value may be all one needs. I use a toroid with ten turns feeding a 1N34A curcuit and a DC voltmeter with very short leads. It was "calibrated" using forward current into a 50 ohm load. Simple measurements like this are covered in, "Baluns, What They Do And How They Do It", by Roy, W7EL, in the ARRL Antenna Compendium, Vol 1, article available at: http://www.eznec.com/Amateur/Articles/ There are also full constructional details and examples on my site, at: http://www.ifwtech.co.uk/g3sek/clip-on/clip-on.htm -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
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