|
Quarterwave vertical with radials
"Roy Lewallen" wrote in message ... snip A normal ground plane is a large sheet of metal that reflects the radio wave emitted by the radiating element. "Normal"? Where have you seen an antenna mounted over a metal ground plane many wavelengths in diameter? Perhaps a UHF antenna in the middle of the top of a car, but that's about it. snip Which prompts me to ask a question: If a quarterwave vertical antenna has many radials only a few feet above the ground, and these radials could be made progressively longer and longer, does the antenna eventually fail to "know" where the ground is? How long is "very long" to bring this about (if it happens)? I kicked some numbers around. By the formula two times antenna_length-squared divided by wavelength [2D^2/lambda], I make the far-field distance for a 14 MHz quarterwave vertical be only 2.5 meters [less than a quarter wavelength] ... but typical radials are already longer than that, aren't they? So this isn't a near-field/far-field boundary issue, is it? |
Quarterwave vertical with radials
Tom Ring wrote: wrote: Maybe reading one of your own posts will jog your memory a bit. In it we see you VERY CLEARLY stated two radials would cancel each other's radiation. 73 Tom Won't matter, he'll have an explanation for it, and it will be anyone's fault but his. tom K0TAR You were right Tom. He came up with one and it wasn't his fault. |
Quarterwave vertical with radials
wrote:
Tom Ring wrote: Won't matter, he'll have an explanation for it, and it will be anyone's fault but his. You were right Tom. He came up with one and it wasn't his fault. On the contrary, I confessed it was my fault for missing four days of postings and thus missing the "two and only two radial" context. -- 73, Cecil http://www.qsl.net/w5dxp |
Quarterwave vertical with radials
All antennas consist of conductors which have current conducted to them
from sources and induced in them by coupling to fields from other conductors or other parts of the same conductor. These currents create fields. Ground plane antennas work exactly the same as all others. In that way they're simple to understand. Yes, you can view it this way or that, with various degrees of accuracy and inaccuracy. The problem is that people begin to believe that the alternate views are really what happens, rather than attempts at simplifying and understanding things. Before you know it, you've got mirrors, "ground" high above the Earth, impossible reflections, and other dubious concepts which end up leading people farther and farther from really understanding the basic principles involved. Roy Lewallen, W7EL David wrote: One of the earlier postings suggested that the quarterwave vertical antenna with radials was elementary and easy to understand. I have never found this antenna easy to understand. RF experts on this newsgroup cannot agree on whether i) the radials reflect the wave or ii) the field from the radials cancels out. The standard academic books show that the principle behind the vertical ground plane antenna is that the vertical radiating element emits the wave, and is reflected by the ground plane. You can view a conductor as having current pushed through it by a RF source, or the current can be induced in the conductor by the wave. This is a boundary condition in Maxwell's equations, referred to in theory of transmission lines and guided waves. You can view the radials as reflecting the wave and having current induced in them, or they can have current pushed through them by the RF source. This is probably the same thing, due to the arrangement of all antenna parts forming the antenna impedance. In image theory, the impedance comes from both the self impedance and the mutual impedance. . . . |
Quarterwave vertical with radials
"Not everything that can be counted counts, and not everything that
counts can be counted." - Albert Einstein (1879-1955) Zen Roy Lewallen wrote: All antennas consist of conductors which have current conducted to them from sources and induced in them by coupling to fields from other conductors or other parts of the same conductor. These currents create fields. Ground plane antennas work exactly the same as all others. In that way they're simple to understand. Yes, you can view it this way or that, with various degrees of accuracy and inaccuracy. The problem is that people begin to believe that the alternate views are really what happens, rather than attempts at simplifying and understanding things. Before you know it, you've got mirrors, "ground" high above the Earth, impossible reflections, and other dubious concepts which end up leading people farther and farther from really understanding the basic principles involved. Roy Lewallen, W7EL David wrote: One of the earlier postings suggested that the quarterwave vertical antenna with radials was elementary and easy to understand. I have never found this antenna easy to understand. RF experts on this newsgroup cannot agree on whether i) the radials reflect the wave or ii) the field from the radials cancels out. The standard academic books show that the principle behind the vertical ground plane antenna is that the vertical radiating element emits the wave, and is reflected by the ground plane. You can view a conductor as having current pushed through it by a RF source, or the current can be induced in the conductor by the wave. This is a boundary condition in Maxwell's equations, referred to in theory of transmission lines and guided waves. You can view the radials as reflecting the wave and having current induced in them, or they can have current pushed through them by the RF source. This is probably the same thing, due to the arrangement of all antenna parts forming the antenna impedance. In image theory, the impedance comes from both the self impedance and the mutual impedance. . . . |
Quarterwave vertical with radials
The vertical element is connected to the centre conductor (RF live). The
radials are connected to 0V on the transceiver. If only the vertical is connected, the antenna still radiates although not as well. If only the radials are connected, the antenna does not radiate because the radials are connected to 0V and not a varying voltage. With both vertical and radials connected, the vertical element radiates the wave. The wave is reflected by the radials as boundary condition of Maxwell's equations. The reflection induces a current in the radials. This current has a standing wave on it. Do you think the above is correct? All parts of the antenna form the impedance. Without radials, the impedance is poor and the vertical element does not radiate well. Other explanations say that displacement currents go through the air and terminate on the radials. The displacement currents then becomes conduction current in the radials. Displacement current is another anomaly with electromagnetic theory. I notice that two people have simulated the vertical antenna with radials using EZNEC, and obtained different results. One simulation shows that the radials radiate, the other shows that they do not. |
Quarterwave vertical with radials
The vertical element is connected to the centre conductor (RF live). The
radials are connected to 0V on the transceiver. No. They're connected to the shield/braid of the feedline. There's no assurance that this point will be at "0V" with respect to anything in particular except itself, and in particular it usually won't be at 0 volts with respect to the transceiver's chassis / output jack (except perhaps momentarily, twice per RF cycle). If only the vertical is connected, the antenna still radiates although not as well. .... because the outside of the feedline will tend to act as a poorly-tuned radial/counterpoise. If only the radials are connected, the antenna does not radiate because the radials are connected to 0V and not a varying voltage. With both vertical and radials connected, the vertical element radiates the wave. The wave is reflected by the radials as boundary condition of Maxwell's equations. The reflection induces a current in the radials. This current has a standing wave on it. Do you think the above is correct? Not really, no. It's a mistake to think that the radials "are connected to 0V and not a varying voltage". You're falling into the trap of thinking that "ground" is some sort of magical "zero volt" reference which is the same everywhere. That isn't true even at DC, and it's certainly not true at RF! All parts of the antenna form the impedance. Without radials, the impedance is poor and the vertical element does not radiate well. The vertical element radiates very well indeed... it'll radiate all of the power which is fed into it, except for a small amount of loss. The problem isn't that it doesn't radiate. The problem is that it's difficult to feed power into it, much of the time. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Quarterwave vertical with radials
On Tue, 18 Jul 2006 20:34:32 +0100, "David" nospam@nospam wrote:
If only the vertical is connected, the antenna still radiates although not as well. If only the radials are connected, the antenna does not radiate because the radials are connected to 0V and not a varying voltage. snip I notice that two people have simulated the vertical antenna with radials using EZNEC, and obtained different results. One simulation shows that the radials radiate, the other shows that they do not. Hi Dave, Your statements above show a serious problem with understanding the operation of antennas. The radials are not potted plants merely arranged along the ground (or in the air) to give a sense of symmetry and balance. You would go further to engage more in dialogue rather than simply posting statements. Much of the utility of radials has been discussed, revisited, and rehashed to no apparent effect against what you offer above. The last sentence is outrageously wrong for any of a number of reasons (or proof of some pretty stupid simulation). 73's Richard Clark, KB7QHC |
Quarterwave vertical with radials
Agreed, the centre junction of the radials is not always at 0V. Current
flows along the coax braid on the inside, meaning that the inside part of the coax braid and radials junction can be any voltage. The radials have a voltage gradient along them because of the standing wave. Because the return current flows on the inside of the coax braid, it is normally safe to touch or go near the outside of the braid. For permanent low installations in a public area, coax should be used instead of twin feeder. The fact that the return current flows on the inside of the braid gives coax its shielding properties. |
Quarterwave vertical with radials
On Tue, 18 Jul 2006 21:32:13 +0100, "David" nospam@nospam wrote:
Agreed, the centre junction of the radials is not always at 0V. Hi David, In isolation, this statement offers nothing at all, unless, of course, you are talking about an unexcited system. The notion that 0V inhabits some greater portion of the antenna, or its radials, or its junction is a strange concept to its normal operation. David, AE6EO, has similar concerns that you are presenting what I would call a naive representation of radiators. Current flows along the coax braid on the inside, meaning that the inside part of the coax braid and radials junction can be any voltage. The radials have a voltage gradient along them because of the standing wave. In that sense, 0V does reside at some favored points, but this is not an explanation of anything. Because the return current flows on the inside of the coax braid, it is normally safe to touch or go near the outside of the braid. For permanent low installations in a public area, coax should be used instead of twin feeder. The fact that the return current flows on the inside of the braid gives coax its shielding properties. Is this germane to a particular point? Again, these are simply statements, and they appear unconnected to any kind of dialog. 73's Richard Clark, KB7QHC |
Quarterwave vertical with radials
|
Quarterwave vertical with radials
However, I still think that with only the vertical element connected to the
centre conductor, the vertical element radiates although not very well. With only the radials connected to the outer braid, the radials do not radiate. The RF live of the transceiver is normally connected to centre conductor, and the RF ground side (the 0V side) is normally connected to the braid. The outer of the connector on the transceiver is possibly connected internally to the case and mains Earth. |
Quarterwave vertical with radials
On Wed, 19 Jul 2006 20:33:29 +0100, "David" nospam@nospam wrote:
However, I still think that with only the vertical element connected to the centre conductor, the vertical element radiates although not very well. With only the radials connected to the outer braid, the radials do not radiate. The RF live of the transceiver is normally connected to centre conductor, and the RF ground side (the 0V side) is normally connected to the braid. The outer of the connector on the transceiver is possibly connected internally to the case and mains Earth. Hi David, Only the slimmest margin of this comes close, and not enough to explain anything. 0V, ground, live, dead, hot, cold, or whatever you want to call it has long since disappeared from the scene at the other end of a cable or twin lead. Radials do not mimic these terms even if the illusion of continuity suggests otherwise. There is no "rf side" nor is there a "ground side" to appeal to. Such distinctions are reserved for very untechnical allusions. 73's Richard Clark, KB7QHC |
Quarterwave vertical with radials
I agree that in the coax side, there is no real 0V because the current has a
standing wave. But the transceiver has a DC power supply (normally 12V) that is controlled or modulated. One side of the power supply is regarded as 0V and connected to 0V rail of circuits. The modulated side is the RF live that is connected to the centre contact of transceiver output connector. In theory, while transceiver is transmitting, if I touched the outer of the output connector, I would not expect to get a shock or RF burn. If I touched the centre contact of output connector, then I would expect to get a shock or RF burn. Admittedly, I could touch the outer and find that I am close enough to the centre contact for RF to capacitively couple into me. In real life, I would never touch the output connector while the rig is transmitting. Does RF live and ground not exist on the output connector of the transceiver? |
Quarterwave vertical with radials
I agree that in the coax side, there is no real 0V because the current has a
standing wave. But the transceiver has a DC power supply (normally 12V) that is controlled or modulated. One side of the power supply is regarded as 0V and connected to 0V rail of circuits. The modulated side is the RF live that is connected to the centre contact of transceiver output connector. In theory, while transceiver is transmitting, if I touched the outer of the output connector, I would not expect to get a shock or RF burn. If I touched the centre contact of output connector, then I would expect to get a shock or RF burn. Admittedly, I could touch the outer and find that I am close enough to the centre contact for RF to capacitively couple into me. In real life, I would never touch the output connector while the rig is transmitting. Does RF live and ground not exist on the output connector of the transceiver? "Live" and "ground" aren't absolute, universal things. These terms exist only in relation to a specific reference point, which you must choose. Yes, it's true that the outer rim of the coaxial connector is usually tied to the chassis of the transceiver, and that this chassis is also tied to your DC ground. So, the potential voltage between the connector and your body is usually low and it's not all that likely to shock you. However, this doesn't mean that this same thing is true at the other end of the coaxial cable (i.e. up at the antenna)! Although the far end of the coax braid is at (or very close to) DC ground, it's far enough away in RF terms (that is, in terms of wavelengths) that its voltage is going to be very different much of the time. Consider also the case in which your transceiver isn't "grounded" at all... it's sitting on a wooden table, powered by a battery, and the coax cable runs directly from it to the antenna. There's no "third wire" in the power cord, or other grounding wire connecting the transceiver to a water pipe or electric outlet or a grounding rod or anything like that. Let's further assume that your antenna is a vertical, with radials, elevated up on an insulated mast. Now, when you transmit - what's the voltage present at the base of each radial? If it's "0V", then why, and with respect to what? -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Quarterwave vertical with radials
On Wed, 19 Jul 2006 23:14:33 +0100, "David" nospam@nospam wrote:
I agree that in the coax side, there is no real 0V because the current has a standing wave. Hi Dave, This doesn't really make much sense. Even discounting that and trying to extrapolate your response, it isn't even 0V then and not true for SWR in general. But the transceiver has a DC power supply (normally 12V) that is controlled or modulated. A modulated power supply? Even if you mean regulated, it bears no resemblence to the topic. One side of the power supply is regarded as 0V and connected to 0V rail of circuits. The modulated side is the RF live that is connected to the centre contact of transceiver output connector. It will, then, come as a surprise to you that the output of the transmitter comes from an AC coupled link. You may choose either wire in that link to go to any part of a ground system, or conversely, to what you consider to be the radiator. It makes no difference because there is no Ground Hot Cold Dead Live or any other distinction until you plant that wire. You can paint that wire turquoise in the belief that it is safe, and grab it at a later time to fry you. In theory, while transceiver is transmitting, if I touched the outer of the output connector, I would not expect to get a shock or RF burn. Expectations here are a belief system, not a proof. I suggest you stay away from situations that encourage such lethal presumptions. If I touched the centre contact of output connector, then I would expect to get a shock or RF burn. Admittedly, I could touch the outer and find that I am close enough to the centre contact for RF to capacitively couple into me. In real life, I would never touch the output connector while the rig is transmitting. Does RF live and ground not exist on the output connector of the transceiver? There are no absolutes as you might expect. Plenty of correspondents here complain daily of problems stemming from what you might deny. If you worked your rig on 10M or 20M from the second story window, you are so far from ground, electrically, that its apparent proximity is only an illusion. 73's Richard Clark, KB7QHC |
Quarterwave vertical with radials
David wrote:
I agree that in the coax side, there is no real 0V because the current has a standing wave. But the transceiver has a DC power supply (normally 12V) that is controlled or modulated. One side of the power supply is regarded as 0V and connected to 0V rail of circuits. The modulated side is the RF live that is connected to the centre contact of transceiver output connector. In theory, while transceiver is transmitting, if I touched the outer of the output connector, I would not expect to get a shock or RF burn. If I touched the centre contact of output connector, then I would expect to get a shock or RF burn. Admittedly, I could touch the outer and find that I am close enough to the centre contact for RF to capacitively couple into me. In real life, I would never touch the output connector while the rig is transmitting. Does RF live and ground not exist on the output connector of the transceiver? Begin with a self-contained, battery-powered transmitter. Put a large metal plate on the ground, stand on the plate, and set the radio on the plate. Touch the radio. No burn. Touch the center conductor. Burn. Just like you said. But now put the radio on top of an insulator, and connect the center conductor of the antenna connector to the metal plate. Touch the center conductor. No burn. Touch the radio. Burn. Whoa -- you got burned from "0V" -- the "cold" side! No fair! Finally, insulate the radio and disconnect the center conductor from the plate. Hold on to the radio and touch the center conductor. Burn. Hold on to a wire going to the center conductor. No burn. Touch the radio while you're holding the wire. Burn. You don't prevent a shock by declaring or "regarding" something to be ground or "0V". You do it by connecting it to the same potential as your body. And declaring it to be "live" doesn't cause the burn -- what causes it is that you're touching something that's at a different potential than your body. Roy Lewallen, W7EL |
| All times are GMT +1. The time now is 10:05 PM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com