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#11
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"Quarter wave ground mounted radials are a waste of wire."
On Wed, 26 Sep 2007 16:56:27 GMT, "Wayne"
wrote: It would be interesting to follow the change in VSWR as you add individual radials. Some years ago, there was a discussion of adding radials until the VSWR stopped increasing. This was based on an expected feedpoint resistance of 36 ohms, and an assumption that the ground losses were 15 ohms or less. So at the worst case, the VSWR was near 1:1. The best case, with a ground loss approaching 0 ohms, would have a VSWR of 50/36= 1.39:1. Hi Wayne, This strategy may also have you stop adding radials too early when your antenna presented 36 Ohms looking into a ground loss of 33 Ohms (same SWR of 1.39). 73's Richard Clark, KB7QHC |
#12
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"Quarter wave ground mounted radials are a waste of wire."
"Richard Clark" wrote in message
The best case, with a ground loss approaching 0 ohms, would have a VSWR of 50/36= 1.39:1. ___________ A set of 120 buried radials each at least 1/4-wave long (free space value) will produce an r-f ground resistance of around 2 ohms, maybe less in soil with very good conductivity. But in any case the impedance existing between the base of a series-fed vertical monopole and the common point of the buried radials will depend on the electrical height of the monopole which includes the ratio of its height to its width, as well as the r-f resistance in the ground system itself. Broadcast stations will install the tower and radials, measure the Z from the tower base to the radials, and transform whatever that value is to 50 +j0 ohms using a network at the base of the tower. Fewer radials will mean that the ground loss will increase, and system radiation efficiency will decrease (even if the VSWR is 1:1) RF |
#13
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"Quarter wave ground mounted radials are a waste of wire."
"Richard Clark" wrote in message ... On Wed, 26 Sep 2007 16:56:27 GMT, "Wayne" wrote: It would be interesting to follow the change in VSWR as you add individual radials. Some years ago, there was a discussion of adding radials until the VSWR stopped increasing. This was based on an expected feedpoint resistance of 36 ohms, and an assumption that the ground losses were 15 ohms or less. So at the worst case, the VSWR was near 1:1. The best case, with a ground loss approaching 0 ohms, would have a VSWR of 50/36= 1.39:1. Hi Wayne, This strategy may also have you stop adding radials too early when your antenna presented 36 Ohms looking into a ground loss of 33 Ohms (same SWR of 1.39). 73's Richard Clark, KB7QHC Right. I guess the better way to state it is to stop adding radials when the r component stops dropping. I've always wanted to try that experiment, but real estate considerations have prevented it. |
#14
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"Quarter wave ground mounted radials are a waste of wire."
On Sun, 23 Sep 2007 12:46:30 -0400, "Rick (W-A-one-R-K-T)"
wrote: I just read the following on one of the mailing lists I subscribe to: "Quarter wave ground mounted radials are a waste of wire and a hold over from the olden days. Check the antenna handbook, the new philosophy is more and shorter. The thing is that the bulk of the energy from the vertical antenna is near the base of the antenna and this is what you are trying to capture. A quarter wave radial sounds logical but the planet will detune it so a quarter wave means nothing to the current." What say you all? Read this and see if you really want to know more. http://www.bencher.com/pdfs/00361ZZV.pdf A challenge to all you experts out the Can you find anything you disagree with in this document? John Ferrell W8CCW "Life is easier if you learn to plow around the stumps" |
#15
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"Quarter wave ground mounted radials are a waste of wire."
"John Ferrell" wrote in message ... On Sun, 23 Sep 2007 12:46:30 -0400, "Rick (W-A-one-R-K-T)" wrote: I just read the following on one of the mailing lists I subscribe to: "Quarter wave ground mounted radials are a waste of wire and a hold over from the olden days. Check the antenna handbook, the new philosophy is more and shorter. The thing is that the bulk of the energy from the vertical antenna is near the base of the antenna and this is what you are trying to capture. A quarter wave radial sounds logical but the planet will detune it so a quarter wave means nothing to the current." What say you all? Read this and see if you really want to know more. http://www.bencher.com/pdfs/00361ZZV.pdf A challenge to all you experts out the Can you find anything you disagree with in this document? John Ferrell W8CCW "Life is easier if you learn to plow around the stumps" An excellent reference John. All nicely explained and with pictures. :-) Mike G0ULI |
#16
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"Quarter wave ground mounted radials are a waste of wire."
On Sep 26, 9:56 am, "Wayne" wrote:
"Nate Bargmann" wrote in message news On Sun, 23 Sep 2007 12:59:49 -0500, Richard Fry wrote: "Rick wrote ... What say you all? Read "Ground Systems as a Factor in Antenna Efficiency" by Brown, Lewis & Epstein of RCA Labs, published in the June, 1937 issue of The Proceedings of the I.R.E. It proves otherwise. Honest question, does it matter whether the objective is a strong ground wave as in AM broadcast or skywave as in amateur operation? I am installing a Hustler 4BTV this fall and have put down (so far) 16 radials that range from about 18 to 35 feet in length due to my servere space restrictions. I may add 16 more later this fall as an experiment. 73, de Nate It would be interesting to follow the change in VSWR as you add individual radials. Some years ago, there was a discussion of adding radials until the VSWR stopped increasing. This was based on an expected feedpoint resistance of 36 ohms, and an assumption that the ground losses were 15 ohms or less. So at the worst case, the VSWR was near 1:1. The best case, with a ground loss approaching 0 ohms, would have a VSWR of 50/36= 1.39:1. I want to attribute the original discussion to W2DU, but I stop short of that, in case I don't remember correctly. I think it would be a lot MORE interesting to follow the change in _impedance_ as you add wires. I can see a lot that can go wrong with "adding radials till the SWR stops increasing." I don't think that the case of 33 ohms ground resistance is one to seriously consider*, but if you're operating over really poor ground and/or your "quarter wave" antenna isn't really a quarter electrical wave, you may see significant reactive component that you'll never resolve with an SWR meter. You'll likely get very confused by it instead. *What's the "ground resistance" with as few as two radials, even if there's no earth ground nearby? What if there are no radials but the ground is really awful? Some "what-ifs" with EZNEC or the like can give you an appreciation for what happens... Cheers, Tom |
#17
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"Quarter wave ground mounted radials are a waste of wire."
An excellent reference John. All nicely explained and with pictures. :-) Mike G0ULI I am continually amazed at the amount and quality of information on the Internet. I am also dismayed how hard it is to separate out the good from the worthless. John Ferrell W8CCW "Life is easier if you learn to plow around the stumps" |
#18
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"Quarter wave ground mounted radials are a waste of wire."
On Sep 27, 7:45 am, John Ferrell wrote:
On Sun, 23 Sep 2007 12:46:30 -0400, "Rick (W-A-one-R-K-T)" wrote: I just read the following on one of the mailing lists I subscribe to: "Quarter wave ground mounted radials are a waste of wire and a hold over from the olden days. Check the antenna handbook, the new philosophy is more and shorter. The thing is that the bulk of the energy from the vertical antenna is near the base of the antenna and this is what you are trying to capture. A quarter wave radial sounds logical but the planet will detune it so a quarter wave means nothing to the current." What say you all? Read this and see if you really want to know more. http://www.bencher.com/pdfs/00361ZZV.pdf A challenge to all you experts out the Can you find anything you disagree with in this document? John Ferrell W8CCW "Life is easier if you learn to plow around the stumps" That seems a very practical ap note. But since you issued the challenge, I'll say I disagree with the wording of an early sentence, where it says that ground return currents are "greatly attenuated" if they come through lossy earth. Clearly, the current is not attenuated; the current is what it is. However, the current through lossy ground causes power dissipation (and loss of radiated power) in the ground. I think the meaning is clear, but the wording would not pass muster with a good technical editor. Cheers, Tom |
#19
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"Quarter wave ground mounted radials are a waste of wire."
On Fri, 28 Sep 2007 20:21:23 -0700, K7ITM wrote:
On Sep 27, 7:45 am, John Ferrell wrote: On Sun, 23 Sep 2007 12:46:30 -0400, "Rick (W-A-one-R-K-T)" wrote: I just read the following on one of the mailing lists I subscribe to: "Quarter wave ground mounted radials are a waste of wire and a hold over from the olden days. Check the antenna handbook, the new philosophy is more and shorter. The thing is that the bulk of the energy from the vertical antenna is near the base of the antenna and this is what you are trying to capture. A quarter wave radial sounds logical but the planet will detune it so a quarter wave means nothing to the current." That seems a very practical ap note. But since you issued the challenge, I'll say I disagree with the wording of an early sentence, where it says that ground return currents are "greatly attenuated" if they come through lossy earth. Clearly, the current is not attenuated; the current is what it is. However, the current through lossy ground causes power dissipation (and loss of radiated power) in the ground. I think the meaning is clear, but the wording would not pass muster with a good technical editor. Cheers, Tom Good catch. It could have been more precisely stated. In defense of the the point, Dictionary.com offers this definition of "attenuated"- to weaken or reduce in force, intensity, effect, quantity, or value. OTH, lossy earth is a limiting factor to the current component of the equation. I think it unlikely to be misinterpretd so I would be inclined to leave it alone. Of course, I am not a Technical Editor. John Ferrell W8CCW "Life is easier if you learn to plow around the stumps" |
#20
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"Quarter wave ground mounted radials are a waste of wire."
"K7ITM" wrote
... I'll say I disagree with the wording of an early sentence, where it says that ground return currents are "greatly attenuated" of they come through lossy earth. Clearly, the current is not attenuated; the current is what it is. However, the current through lossy ground causes power dissipation (and loss of radiated power) in the ground. ___________ But without a low-loss r-f ground for a monopole such as provided by a good buried radial system, those returning r-f currents ARE greatly attenuated before they can enter into the ground terminal of the antenna system. That ground resistance is in series with the radiation resistance of the monopole, and so will reduce the current that will flow on the monopole -- hence the field it will radiate. RF |
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