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80m mobile antenna question
I have been mobilling for years but never on 80/75m. Getting ready to
build something to mount on my Toyota 4Runner. Given the following choices I would appreciate some advice. Assume that both my roof mount and rear trailer mount are "perfect" and will not be the determining factor. Choice #1: Mount a 1" or so mast starting at the trailer hitch going vertical for 4.5' and then have the loading coil (which clears the roof line) and finally a 6.5' whip. Base height is about 2' off the ground and the top is 13+' off the ground. Choice #2: Mount a 1" mast 4' high from the top of the roof, then the loading coil, and then a 5' whip which is vertical for 2' and then horizontal for 3'. Base height about 6' and top height about 12.5'. Choice #2 will have a lower ground loss (good) than choice #1 but choice #1 being taller will have a higher radiation resistance (good) than choice #1. For an 80m antenna which of the tradeoffs generally is more important for an antenna this size? Thanks, Larry Benko, W0QE |
80m mobile antenna question
Larry Benko wrote:
Slight ERROR! Should be: Choice #2 will have a lower ground loss (good) than choice #1 but choice #1 being taller will have a higher radiation resistance (good) than choice #2. Choice #2 will have a lower ground loss (good) than choice #1 but choice #1 being taller will have a higher radiation resistance (good) than choice #1. For an 80m antenna which of the tradeoffs generally is more important for an antenna this size? Thanks, Larry Benko, W0QE |
80m mobile antenna question
I can't prove this but I suspect there will be very little difference.
The wavelength on 80/75 meters is so much longer than the car body that going from a low mount to a high one will be almost unnoticeable. Ground loss will be about the same because the capacitance between the car body and ground is the important factor and does not depend on where the whip is mounted. What will matter greatly is the Q of the coil. Make it inherently as high as you can and keep it away from metal parts of the car body. Resonate it and match it and you will have lots of fun. 80/75 is a great band for mobile and much underused. Bill, W6WRT |
80m mobile antenna question
Bill Turner wrote:
I can't prove this but I suspect there will be very little difference. The wavelength on 80/75 meters is so much longer than the car body that going from a low mount to a high one will be almost unnoticeable. Ground loss will be about the same because the capacitance between the car body and ground is the important factor and does not depend on where the whip is mounted. What we found at the CA shootouts is that when the bottom section runs closely parallel to the vehicle body, as it does with a trailer hitch mount on an SUV, the field strength is much lower than if that bottom section is in the clear, e.g. mounted on the roof of the SUV. Of course, roof mounting creates a different set of problems. -- 73, Cecil http://www.qsl.net/w5dxp |
80m mobile antenna question
What we found at the CA shootouts is that when the bottom
section runs closely parallel to the vehicle body, as it does with a trailer hitch mount on an SUV, the field strength is much lower than if that bottom section is in the clear, e.g. mounted on the roof of the SUV..... I often wonder about this myself, but never get around to trying a bumper mount. In the past, I've always preferred to have the lower mast and coil as clear of the body as possible. But on the other hand, if I mounted the base on the bumper, I could have a longer mast below the coil. It's hard to decide which would be better on paper. But...On my "play" truck, I decided to go whole hog. I mounted the base of the antenna on the rear pillar of my cab, back behind my head. The base of the antenna is appx 64 inches off the ground. Yes, it kicks butt... But I sometimes wonder how it would do with the bumper mount, and longer lower mast. The problem is I have campers on both of my trucks, and have always been afraid to have the lower mast right up against the back tailgate, and camper. It's hard to decide of the longer antenna would outweigh the higher mount, and shorter antenna. I think really the only way to know for sure is to actually try and compare both. But in the past, and present, I'm a "high mounter" as far as mobile whips. BTW, I was out camping in Utopia TX about 2 months ago, and had the chance to really give that truck and antenna a good workout. It was browning the food. I was S9 plus to all TX stations, and even S 9 to a Salt Lake City puter receiver listened to on the internet. That was 80m...On 40m, it's even better. Course, that antenna when parked is 14 ft tall, and has the coil at 8 ft from the base. It's 11 ft tall in the driving mode. Even the short version is tall, when mounted on the cab of that truck. The radio was the 706 barefoot. I use no amp when mobile. I do know it's really bad news to have the coil near body metal. But I've never had that problem yet on my various vehicles. MK |
80m mobile antenna question
In article ,
Cecil Moore wrote: What we found at the CA shootouts is that when the bottom section runs closely parallel to the vehicle body, as it does with a trailer hitch mount on an SUV, the field strength is much lower than if that bottom section is in the clear, e.g. mounted on the roof of the SUV. That seems to match up with various peoples' experiences that I've heard. Close spacing in this way makes the signal weaker, and also seems to make the antenna more difficult to tune/match properly. This also makes sense from an engineering point of view. The closely-parallel spacing of the bottom section and the metal vehicle body would form a transmission line of sorts. This transmission-line section would not radiate much (or efficiently) - its radiation resistance would be quite low. As a result, the antenna's feedpoint impedance would be lower than otherwise (requiring a more aggressive impedance step-up of some sort to match a 50-ohm line). The coil and whip would be above the body, and would still be able to radiate, but you'd be left with something akin to a bottom-loaded whip with no high-current radiating section, rather than a center-loaded radiator with a low-loss high-current radiating section below the coil. In effect, a close/parallel mounting of this sort would seem to sacrifice much of the radiating power of this type of antenna. -- 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! |
80m mobile antenna question
"Cecil Moore" wrote in message et... wrote: What we found at the CA shootouts is that when the bottom section runs closely parallel to the vehicle body, as it does with a trailer hitch mount on an SUV, the field strength is much lower than if that bottom section is in the clear, e.g. mounted on the roof of the SUV..... I often wonder about this myself, but never get around to trying a bumper mount. In the past, I've always preferred to have the lower mast and coil as clear of the body as possible. But on the other hand, if I mounted the base on the bumper, I could have a longer mast below the coil. What worked like a charm for me was using the trailer hitch hole on my GMC pickup and removing the tailgate. I looked for a fiberglass aftermarket tailgate but couldn't find one. -- 73, Cecil http://www.qsl.net/w5dxp With that in mind, I have a friend who has a Ford Exploder--I mean, EXPLORER :) --- with his DK3 mounted on a homebrew mount level with the rear bumper. The bad part of it (IMHO) is the loading coil is level with the body about where the rear window is and about 8 inches from the body. I mentioned to him that it would be better to get the coil up in the clear above the truck, but he is says he can't get in his carport. Well, what about this: move the coil UP to clear the body and use a shorter whip? IOW, faced with the lesser of two evils, which would be better. Left as is with longer whip and putting up with the loss caused by proximity to body metal, or coil clearing the top of the truck and a shorter whip--even it it has to be 5 feet instead of 6 1/2? I voted for the higher coil and shorter whip. What say ye? :) 73 Jerry K4KWH |
80m mobile antenna question
Jerry wrote:
I voted for the higher coil and shorter whip. What say ye? :) Within reason, the higher the coil, the better. I only had one foot of antenna above my coil, a one foot section upon which was mounted a large horizontal top hat. -- 73, Cecil http://www.qsl.net/w5dxp |
80m mobile antenna question
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80m mobile antenna question
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80m mobile antenna question
Bill Turner wrote:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ No doubt that is correct. So how about this: I have a '95 Thunderbird which I dearly love and don't want to cut holes in. I've been think of going to a welding shop and having a metal piece made which I could bolt to the frame in the back and which would stick out about six inches or so behind the rear bumper, and installing a ball mount on it. This will keep the lower part of the antenna about a foot away from the body and allow a nice, long whip overall. The loading coil would be in the center, homebrew of course. :-) And not a hole in sight. Comments? Bill, W6WRT I'm not sure why, but most amateurs don't seem to realize that the whip isn't an "antenna" and the car "ground", but each is half of a dipole-like antenna. The car part is often much more important with regard to radiation characteristics and efficiency than the whip part. With the arrangement you suggest, the antenna consists of a vertical wire -- the whip -- and a fat, horizontal "wire" -- the car. Whatever current flows into the whip, an equal current flows over the outside of the car, originating at the base of the whip. Any antenna with a low horizontal wire will be quite lossy, because the wire's current will induce a heavy current in the lossy ground beneath the wire, or car. The best arrangement, as others have pointed out, is to mount the antenna right at the center of the top of the car. This makes the car "wire" vertical, a much more efficient arrangement, which the "shootouts" consistently show. You'll also find that larger trucks, which effectively form a longer vertical "wire" for the car part, outdo smaller ones for the same whip. Of course, sometimes you don't have any choice, and you just have to do the best you can. I once had a bumper mounted antenna consisting of a CB whip base loaded with an inductor wound on a powdered iron core to resonate on 40 meters. The car was a VW Squareback, so the antenna had the increased disadvantage of proximity between the square back and the antenna. As others have pointed out, this can reduce efficiency farther. Yet I had a successful QSO with JA while driving down Highway 101, running 8 watts, CW. So you can still communicate and have lots of fun even with a very sub-optimal system. But anyone wanting to improve his system has a much better chance of doing it if he has a basic understanding of how the antenna really works. Roy Lewallen, W7EL |
80m mobile antenna question
Roy Lewallen wrote:
The best arrangement, as others have pointed out, is to mount the antenna right at the center of the top of the car. This makes the car "wire" vertical, a much more efficient arrangement, which the "shootouts" consistently show. You'll also find that larger trucks, which effectively form a longer vertical "wire" for the car part, outdo smaller ones for the same whip. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~ Well, maybe. The problem is, the higher the mounting point, the shorter the whip has to be to be legal to drive down the road. If your car was 13 feet five inches tall, your whip could only be one inch long. How efficient would that be on 80 meters? The point being, everything is a tradeoff of one thing for another. If the shootouts say a rooftop is best, ok, but I have to say I'm surprised. At a relatively low frequency like 80 meters, the car body is more of a coupler to the earth rather than a real "ground" of it's own. Given that, then the longer the whip part, the better. HF mobile antennas are a fascinating subject and one of these days I will set up a "shootout" range on my 2.5 acres here in the desert and do some shooting of my own. Bill, W6WRT |
80m mobile antenna question
Bill Turner wrote:
So how about this: I have a '95 Thunderbird which I dearly love and don't want to cut holes in. I've been think of going to a welding shop and having a metal piece made which I could bolt to the frame in the back and which would stick out about six inches or so behind the rear bumper, and installing a ball mount on it. This will keep the lower part of the antenna about a foot away from the body and allow a nice, long whip overall. The loading coil would be in the center, homebrew of course. :-) The only way to improve on that on 75m would be to mount a piece of sheet metal on fiberglass poles connected at the ends of both bumpers. The piece of horizontal sheet metal, located 13.5 feet from the ground, would have the same footprint as the T-bird and would be used as the top hat. You do want optimum performance don't you? :-) -- 73, Cecil http://www.qsl.net/w5dxp |
80m mobile antenna question
Roy Lewallen wrote:
I'm not sure why, but most amateurs don't seem to realize that the whip isn't an "antenna" and the car "ground", but each is half of a dipole-like antenna. My S10 trailer hitch mounted configuration exhibited considerable directivity toward the front of the pickup on 17m. -- 73, Cecil http://www.qsl.net/w5dxp |
80m mobile antenna question
Cecil Moore wrote:
The only way to improve on that on 75m would be to mount a piece of sheet metal on fiberglass poles connected at the ends of both bumpers. The piece of horizontal sheet metal, located 13.5 feet from the ground, would have the same footprint as the T-bird and would be used as the top hat. You do want optimum performance don't you? :-) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~ I think my T-Bird might actually fly. :-) 73, Bill W6WRT |
80m mobile antenna question
Roy Lewallen, W7EL wrote:
"I`m not sure why, but most amateurs don`t seem to realize that a whip isn`t an "antenna" and the car "ground", but each is half of a dipole-like antenna." Not exactly.. In a common balanced dipole, each half has the same current quantity and direction, though in one half the current flows toward the feedpoint while it flows away in the other half. From such a dipole, both its halves contribute equally to its radiation. Action of a common ground plane is different. When its balanced radials are perpendicular to its whip, radiation from its radials zeros out leaving the whip to do all the radiation. Ideally, a whip mounted on a vehicle or directly on the earth behaves the same. It is the whip which radiates. An antenna is also called an aerial. It is defined as that part of a radio station which radiates or receives radio waves into or from space. An antenna ground system is defined as that portion of an antenna system closely associated with the earth and including an extensive conducting surface which may be the earth itself. Most radio amateurs have it right. Best regards, Richard Harrison, KB5WZI |
80m mobile antenna question
Richard Harrison wrote:
Roy Lewallen, W7EL wrote: "I`m not sure why, but most amateurs don`t seem to realize that a whip isn`t an "antenna" and the car "ground", but each is half of a dipole-like antenna." Not exactly.. In a common balanced dipole, each half has the same current quantity and direction, though in one half the current flows toward the feedpoint while it flows away in the other half. From such a dipole, both its halves contribute equally to its radiation. Action of a common ground plane is different. When its balanced radials are perpendicular to its whip, radiation from its radials zeros out leaving the whip to do all the radiation. Ideally, a whip mounted on a vehicle or directly on the earth behaves the same. It is the whip which radiates. A ground plane is a poor model of how currents flow along a car body. Consider an antenna mounted on top of a car. From the base of the antenna, the current flows equally in all directions away from the base of the antenna, like a ground plane. This current doesn't contribute much radiation, for the reasons you state. But then it reaches the edge of the top of the car and flows downward. All the portions of the current are now flowing the same direction, and their fields don't cancel but add in phase. The net result is the same as if it were just flowing down a fat wire the height of the car. If the car is eight feet high, the field from the car will equal the field from an eight foot whip. In fact, unless the whip is top loaded to make the current uniform, the car will radiate more than the whip, because the capacitance of the car to ground will tend to give the car a uniform current distribution, like a top hat does to a whip. This will increase the radiated field strength from the car. Now consider a bumper mounted antenna. The current will spread from the base and proceed around the car. More will probably flow on the bottom than the top and sides due to coupling with the ground, but all portions will be flowing in the same direction and all will radiate. There is no place on the car where the current distribution or flow pattern resembles current on a ground plane. An antenna is also called an aerial. It is defined as that part of a radio station which radiates or receives radio waves into or from space. An antenna ground system is defined as that portion of an antenna system closely associated with the earth and including an extensive conducting surface which may be the earth itself. The problem here is that the currents don't care how you define things. They flow where the physical laws dictate. Defining "ground" doesn't make them behave differently. Most radio amateurs have it right. If your view represents that of most amateurs, they don't. Roy Lewallen, W7EL |
80m mobile antenna question
On Thu, 2 Mar 2006 10:56:15 -0600, (Richard
Harrison) wrote: Action of a common ground plane is different. When its balanced radials are perpendicular to its whip, radiation from its radials zeros out leaving the whip to do all the radiation. Ideally, a whip mounted on a vehicle or directly on the earth behaves the same. It is the whip which radiates. Boy Richard, you sure missed the boat on that one! Roy is quite correct in stating that a vehicle's body behaves as one side of a dipole. A lopsided dipole to be sure, but one half the antenna just the same. A number of years ago I did a study using NEC modeling comparing VHF mobile whips (1/4, 1/2 and 5/8-wavelength) mounted on different vehicles. I created wire frame vehicles models for a full and mid-size passenger cars, a small pickup truck and an SUV. The results for the same antenna mounted top-dead-center on the different vehicles was quite noticeable sometimes substantial. An article I wrote on the subject can be viewed at: http://k6mhe.com/files/mobile_vhf_ant.pdf 73, Danny, k6MHE email: k6mheatarrldotnet http://www.k6mhe.com/ |
80m mobile antenna question
Quite interesting reading. Have you received peer comments?
Dan Richardson wrote: On Thu, 2 Mar 2006 10:56:15 -0600, (Richard Harrison) wrote: Action of a common ground plane is different. When its balanced radials are perpendicular to its whip, radiation from its radials zeros out leaving the whip to do all the radiation. Ideally, a whip mounted on a vehicle or directly on the earth behaves the same. It is the whip which radiates. Boy Richard, you sure missed the boat on that one! Roy is quite correct in stating that a vehicle's body behaves as one side of a dipole. A lopsided dipole to be sure, but one half the antenna just the same. A number of years ago I did a study using NEC modeling comparing VHF mobile whips (1/4, 1/2 and 5/8-wavelength) mounted on different vehicles. I created wire frame vehicles models for a full and mid-size passenger cars, a small pickup truck and an SUV. The results for the same antenna mounted top-dead-center on the different vehicles was quite noticeable sometimes substantial. An article I wrote on the subject can be viewed at: http://k6mhe.com/files/mobile_vhf_ant.pdf 73, Danny, k6MHE email: k6mheatarrldotnet http://www.k6mhe.com/ |
80m mobile antenna question
Roy Lewallen wrote:
A ground plane is a poor model of how currents flow along a car body. If the car body was 1/2WL in the air, would the antenna be more efficient? -- 73, Cecil http://www.qsl.net/w5dxp |
80m mobile antenna question
Dan Richardson wrote:
Roy is quite correct in stating that a vehicle's body behaves as one side of a dipole. A lopsided dipole to be sure, but one half the antenna just the same. Seems the truth might lie somewhere in between. If the ground plane of a vertical antenna is near the ground, there are losses. If the ground plane of a vertical antenna is located 1/2WL above ground, the losses are a lot less. I'll bet that if the vehicle were located 1/2WL in the air, the efficiency would increase. -- 73, Cecil http://www.qsl.net/w5dxp |
80m mobile antenna question
Cecil Moore wrote:
Dan Richardson wrote: Roy is quite correct in stating that a vehicle's body behaves as one side of a dipole. A lopsided dipole to be sure, but one half the antenna just the same. Seems the truth might lie somewhere in between. If the ground plane of a vertical antenna is near the ground, there are losses. If the ground plane of a vertical antenna is located 1/2WL above ground, the losses are a lot less. I'll bet that if the vehicle were located 1/2WL in the air, the efficiency would increase. Kind of tough though going under power lines, bridges and overpasses :-) |
80m mobile antenna question
Amos Keag wrote:
Cecil Moore wrote: I'll bet that if the vehicle were located 1/2WL in the air, the efficiency would increase. Kind of tough though going under power lines, bridges and overpasses :-) What if the vehicle is a helicopter? :-) -- 73, Cecil http://www.qsl.net/w5dxp |
80m mobile antenna question
On Thu, 02 Mar 2006 17:46:19 -0500, Amos Keag
wrote: Quite interesting reading. Have you received peer comments? Naw. However, those nec models are available at my web site and you can run you own analysis if you like. Danny email: k6mheatarrldotnet http://www.k6mhe.com/ |
80m mobile antenna question
Danny K6MHE wrote:
"Boy Richard you sure missed on that one!" A broadcast tower over a perfect ground system is the source of radiated energy even though its image in the ground system produces a pattern which behaves as if there were a dipole, the lower half of which is buried. The earth is not radiating. It is conducting. The tower above the earth is the source of radiation. Every ground radial in the broadcast system (usually all 120 of them), has a twin running in the opposite direction. All radials are tied together at the base of the tower. So the current in the radials all starts out in the same phase and stays roughly in the same phase as it progresses outward. It declines in magnitude away from the feedpoint. That`s the reason ground radials don`t need to be unlimited in length. You don`t need radials after the current plays out. As current travels in opposite directions in the groind radials. the fields they prodoce add to zero. The two halves of a dipole are fed with opposite polarities at their feedpoint. this puts the two halves running in opposite directions in-phase. Their fields thus reinforce. A 1/4-wave ground plane in free space has the same power gain as a center-fed 1/2-wave dipole. The matched power radiated by either ground plane or dipole is the same, but the resistance at the feedpoint of the ground plane is only 50% that of the dipole. Radiation resistance is defined as the resistance at the high current point of the antenna unless otherwise specified. Radiated power is (I) squared times the radiation resistance. Danny did not specify where he thought I erred in my previous posting. I said that a whip mounted on a vehicle is not exactly like a dipole. I meant that the whip did most of the radiating because it carried a concentration of current in the same direction while in the car body the current is dispersed in various directions, some of which canncel in their effects. I still insist that is the case. Best regards, Richard Harrison, KB5WZI |
80m mobile antenna question
ORIGINAL MESSAGE:
Dan Richardson wrote: That may have some validity in the VHF and higher ranges, but on HF - particularly on 80 meters - a car body's size is a small fraction of a wavelength (as is the whip portion). Consequently the vehicle body acts like the one half of a dipole antenna. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Q. How can a car body which is a "small fraction" of a wavelength act like one half of a dipole? A. It can't. Q. Well, what does it do then? A. It acts like a short piece of wire leading from the bottom of the whip to the actual ground plane, namely the earth itself. Q. Does that help any? A. Probably a little, but remember the piece of wire (the car body) is only a few feet long. Not very much on 80 meters. Q. Thanks, I get it now. A. You're welcome. 73, Bill W6WRT |
80m mobile antenna question
Bill Turner wrote:
ORIGINAL MESSAGE: Dan Richardson wrote: That may have some validity in the VHF and higher ranges, but on HF - particularly on 80 meters - a car body's size is a small fraction of a wavelength (as is the whip portion). Consequently the vehicle body acts like the one half of a dipole antenna. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Q. How can a car body which is a "small fraction" of a wavelength act like one half of a dipole? A. It can't. Q. Well, what does it do then? A. It acts like a short piece of wire leading from the bottom of the whip to the actual ground plane, namely the earth itself. Q. Does that help any? A. Probably a little, but remember the piece of wire (the car body) is only a few feet long. Not very much on 80 meters. Q. Thanks, I get it now. A. You're welcome. 73, Bill W6WRT Actually it is acting as one half of a dipole. It is just a non-resonant half of a dipole. Remember "di" means two. Dave WD9BDZ |
80m mobile antenna question
I'm afraid people are getting too hung up by trying to squeeze
everything into various pigeon holes like "dipole" and "ground". You'll have to think beyond those narrow and poorly defined and understood categories and look at the basics of antenna operation in order to understand what's happening. The field radiated from a conductor is determined by two things: the amount of current on it, and the length of the path the current takes. Theorists have known this for well over a century. The most sophisticated antenna analysis programs break the current paths into very short pieces ("segments"), calculate the current on each piece, and then calculate the resulting field from the product of the current and the segment length. Fields from various parts of the conductors can cancel or reinforce to any degree. (Mathematically, they add vectorally.) If you don't or can't believe this, you needn't bother continuing. For those still reading, let's imagine a 16 foot vertical wire with a tiny 3.5 MHz signal generator at the center. This is known in textbooks as a "dipole", but how things behave aren't dictated by what we call them, so feel free to insist it's a "seagull", "pizza", "xfppftm", or whatever makes you comfortable. The signal generator has two terminals, and any generator must have equal currents in and out of its two terminals. If you don't or can't believe that, brush up on Kirchoff's current law. If that doesn't do it, there's no need to continue further. Let's suppose the generator is producing one amp RMS of RF current. If, say, 0.2 amp is flowing upward out of the top terminal at a given instant, 0.2 amp is flowing upward into the bottom terminal at the same instant. By inspection, one amp RMS is flowing upward in the vertical wire immediately above and below the generator. By a number of techniques, we can show that the current decreases nearly linearly from the center to the ends. That is, four feet from the center, either above or below the source, the current is 1/2 amp. At the antenna tips, the current is zero, which we should expect: there's nowhere for it to go. It should be obvious that the wire above the source is radiating the same field strength as the wire below the source -- for each little piece of the wire above the source there's a piece below the source carrying exactly the same current. And as it turns out, the fields from all parts of both wires add completely in phase directly broadside to the wire, and only partially in phase in other directions. So at least directly broadside, we can say that the contribution from each wire is equal and proportional to its total field strength. Ok, now let's make one of the wires "ground" and the other a "whip", because we like to do that, right? Let's call the top wire a "whip", and bottom load it. We add an inductor (very small, physically, to avoid adding another dimension to this analysis) between the signal generator's top terminal and its connection with the upper wire. We can make the inductor the proper value to make the upper wire/inductor self resonant if it were grounded, or we can make the inductor about twice as large to make the whole dipole resonant. It doesn't matter. Now let's see what happened to the radiation from the "whip" and "ground" wires. There's no change whatsoever! The currents are exactly the same as they were before, on both wires. They still taper from the center to the tips as before. They both radiate equally. All we've done is change the impedance seen by the generator. If you don't believe this, perhaps you can explain why they won't. Next, let's replace the lower wire with a cylinder like a tank, say 10 feet in diameter but still 8 feet high. What happens then? Surely it must now be "ground", and "ground" doesn't radiate, does it? Well, it does radiate. The one amp flowing into the bottom generator terminal spreads out radially over the top of the cylinder. Although the current density decreases as we move out from the center, the total current also decreases. If only the cylinder top was present and the rest of the cylinder missing, the current would drop to nearly zero at the edge. But because of the presence of the rest of the cylinder, the current at the edge drops to about half the value at the center. The half which remains flows down the cylinder sides. This would result in the field from the cylinder being about half the field from the "whip" if the current decreased to zero at the bottom of the cylinder as it does at the top of the whip. But the current along the sides of the cylinder doesn't drop to zero at the bottom of the walls because it can flow onto the bottom of the cylinder. The average current on the whip is 0.5 amp, and on the cylinder (from a model) about 0.35 amp, so the cylinder's field is about 3 dB less than that of the "whip". Not quite what most of envision when we think of a "ground". If we top load the whip with a 10 foot diameter top hat, its average current increases to about 0.9 amp. But its presence also reduces the amount of current drop from the center to the edge of the cylinder top due to mutual coupling. The end result is larger current along the cylinder sides and very nearly the same field strength ratio between the "whip" and cylinder. So far this analysis has taken place in free space. What happens if we put the cylinder bottom just above the ground, say six inches? Now, surely, the cylinder is "ground"! But the current still flows down the sides and radiates just like the old original vertical lower wire did. And putting the bottom close to ground increases the current along the sides! The coupling between the cylinder bottom and ground acts somewhat like a top hat does to a whip, and increases the average current. Instead of 0.35 amp, it increases to about 0.42. Now the cylinder's field is only about 1.5 dB less than that of the "whip". I hope this has encouraged at least a few people to think a little before declaring every conductor to be either an "antenna" or a "ground plane" and assuming that by doing so they'll somehow cause it to behave in some predetermined and only vaguely understood fashion. Roy Lewallen, W7EL Bill Turner wrote: ORIGINAL MESSAGE: Dan Richardson wrote: That may have some validity in the VHF and higher ranges, but on HF - particularly on 80 meters - a car body's size is a small fraction of a wavelength (as is the whip portion). Consequently the vehicle body acts like the one half of a dipole antenna. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Q. How can a car body which is a "small fraction" of a wavelength act like one half of a dipole? A. It can't. Q. Well, what does it do then? A. It acts like a short piece of wire leading from the bottom of the whip to the actual ground plane, namely the earth itself. Q. Does that help any? A. Probably a little, but remember the piece of wire (the car body) is only a few feet long. Not very much on 80 meters. Q. Thanks, I get it now. A. You're welcome. 73, Bill W6WRT |
80m mobile antenna question
David G. Nagel wrote:
Actually it is acting as one half of a dipole. It is just a non-resonant half of a dipole. Remember "di" means two. Dave WD9BDZ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~ In a strict sense you are correct, but in the context here where one half of the dipole is an eight-foot whip and the other half is four feet of car body, we don't have much of an 80 meter antenna without the coupling from car body to earth ground. Bill, W6WRT |
80m mobile antenna question
Roy Lewallen wrote:
I hope this has encouraged at least a few people to think a little before declaring every conductor to be either an "antenna" or a "ground plane" and assuming that by doing so they'll somehow cause it to behave in some predetermined and only vaguely understood fashion. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~ A very good explanation, thank you Roy. However... in your example of the giant tin can in free space, the top of the tin can is acting like a ground plane, the side is acting like an antenna and the bottom is again acting like a ground plane, just as we have been saying. When this model is transferred to a car body, the bottom of the car, in addition to the above, is also acting like one plate of a capacitor coupling the signal to the earth below it, commonly known as "ground". If someone disagrees with this I believe we have a problem with semantics more than physics. In other words, we are arguing over nothing. Bill, W6WRT |
80m mobile antenna question & "Skeleton Sleeve Antenna"
Howdy Dan:
Thanks for the info on "Mobile Vhf Ant.pdf", very interesting stuff. I was checking out the rest of your site, and found an interesting pdf on the "Skeleton Sleeve Antenna", and it's similarity of the J-pole, and you state that the distance from the 1/4 wave elements, and the 1/2 wave element is not that critical. I have heard this before about the elements on the J-pole too, but what I don't understand is why this dimension is not critical, it is an electrical path that should be subtracted from both elements, right? Is it correct to shorten both elements by 1/2 of the dimension between the elements? -- SeeYaa:) Harbin Osteen KG6URO !sdohtem noitpyrcne devorppa-tnemnrevog troppus I "Dan Richardson" wrote in message ... On Thu, 2 Mar 2006 10:56:15 -0600, (Richard Harrison) wrote: Action of a common ground plane is different. When its balanced radials are perpendicular to its whip, radiation from its radials zeros out leaving the whip to do all the radiation. Ideally, a whip mounted on a vehicle or directly on the earth behaves the same. It is the whip which radiates. Boy Richard, you sure missed the boat on that one! Roy is quite correct in stating that a vehicle's body behaves as one side of a dipole. A lopsided dipole to be sure, but one half the antenna just the same. A number of years ago I did a study using NEC modeling comparing VHF mobile whips (1/4, 1/2 and 5/8-wavelength) mounted on different vehicles. I created wire frame vehicles models for a full and mid-size passenger cars, a small pickup truck and an SUV. The results for the same antenna mounted top-dead-center on the different vehicles was quite noticeable sometimes substantial. An article I wrote on the subject can be viewed at: http://k6mhe.com/files/mobile_vhf_ant.pdf 73, Danny, k6MHE email: k6mheatarrldotnet http://www.k6mhe.com/ |
80m mobile antenna question
If I remember correctly, the higher the coil goes, doesn't its value
have to increase as well? If so, might the coil dimensions become a bit too big to handle? Scott N0EDV Jerry wrote: "Cecil Moore" wrote in message et... wrote: What we found at the CA shootouts is that when the bottom section runs closely parallel to the vehicle body, as it does with a trailer hitch mount on an SUV, the field strength is much lower than if that bottom section is in the clear, e.g. mounted on the roof of the SUV..... I often wonder about this myself, but never get around to trying a bumper mount. In the past, I've always preferred to have the lower mast and coil as clear of the body as possible. But on the other hand, if I mounted the base on the bumper, I could have a longer mast below the coil. What worked like a charm for me was using the trailer hitch hole on my GMC pickup and removing the tailgate. I looked for a fiberglass aftermarket tailgate but couldn't find one. -- 73, Cecil http://www.qsl.net/w5dxp With that in mind, I have a friend who has a Ford Exploder--I mean, EXPLORER :) --- with his DK3 mounted on a homebrew mount level with the rear bumper. The bad part of it (IMHO) is the loading coil is level with the body about where the rear window is and about 8 inches from the body. I mentioned to him that it would be better to get the coil up in the clear above the truck, but he is says he can't get in his carport. Well, what about this: move the coil UP to clear the body and use a shorter whip? IOW, faced with the lesser of two evils, which would be better. Left as is with longer whip and putting up with the loss caused by proximity to body metal, or coil clearing the top of the truck and a shorter whip--even it it has to be 5 feet instead of 6 1/2? I voted for the higher coil and shorter whip. What say ye? :) 73 Jerry K4KWH |
80m mobile antenna question
Cecil Moore wrote:
Amos Keag wrote: Cecil Moore wrote: I'll bet that if the vehicle were located 1/2WL in the air, the efficiency would increase. Kind of tough though going under power lines, bridges and overpasses :-) What if the vehicle is a helicopter? :-) Ahah!! No we have to consider the whop whop effect!! :-) |
80m mobile antenna question
Scott wrote:
If I remember correctly, the higher the coil goes, doesn't its value have to increase as well? If so, might the coil dimensions become a bit too big to handle? The bigger the top hat, the smaller the required reactance. -- 73, Cecil http://www.qsl.net/w5dxp |
80m mobile antenna question
Bill Turner wrote:
David G. Nagel wrote: Actually it is acting as one half of a dipole. It is just a non-resonant half of a dipole. Remember "di" means two. Dave WD9BDZ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~ In a strict sense you are correct, but in the context here where one half of the dipole is an eight-foot whip and the other half is four feet of car body, we don't have much of an 80 meter antenna without the coupling from car body to earth ground. Bill, W6WRT No argument here Bill. The point I guess I was trying to make is that a dipole antenna system is two elements no matter what you make them of. I use a Hi Sierra screwdriver antenna on my Honda Element. Even though is is an impressive construct I don't harbor any illusions that it is an efficient radiator. The body of the car is longer than the length of the screwdriver, coil and whip. I have also used an Outbacker. Some say that is a good antenna for it type, I have not had that good of a result with it. Of course I am using my mobile for Civil Air Patrol and the Outbacker does not fit that frequency very well on the precut tuning jacks. I have found this thread to be interesting but I think that is had passed that point. I do not consider myself to be anything other than an interested amateur and always consider your comments with great interest. Thank you for your personal comments. Dave WD9BDZ |
80m mobile antenna question
Cecil, W5DXP wrote:
"The only way to improve on that on 75m would be to mount a piece of sheet metal on fiberglas poles connected at ythe ends of both bumpers." Kraus gives some support to that idea. Cecil has the 3rd edition of "Antennas" In that edition, there is a "Disc antenna" on page 720 with some similarity to cecil`s suggestion. The "flush-disk" antenna, (d) in Figure 21-11 is said to be comparable to a 1/4-wave vertical in performance, but has no projection. It could be covered with a dielectric sheet, make no noise in the wind, and break out no fluorescent tubes in parking garages. But, at 75m, the 0.3 lambda dia. depression to contain it would measure 22.5 meters. That woud require a vehicle that was very large indeed. At VHF and UHF it could be very practical. Best regards, Richard Harrison, KB5WZI |
80m mobile antenna question
Bill Turner wrote:
Roy Lewallen wrote: I hope this has encouraged at least a few people to think a little before declaring every conductor to be either an "antenna" or a "ground plane" and assuming that by doing so they'll somehow cause it to behave in some predetermined and only vaguely understood fashion. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~ A very good explanation, thank you Roy. However... in your example of the giant tin can in free space, the top of the tin can is acting like a ground plane, the side is acting like an antenna and the bottom is again acting like a ground plane, just as we have been saying. When this model is transferred to a car body, the bottom of the car, in addition to the above, is also acting like one plate of a capacitor coupling the signal to the earth below it, commonly known as "ground". If someone disagrees with this I believe we have a problem with semantics more than physics. In other words, we are arguing over nothing. Bill, W6WRT I interpreted your comments and those by some others as claiming that radiation from the car is insignificant, and that it therefore isn't effectively part of the antenna. I attempted to show that this isn't generally true. I also showed that coupling to the ground actually increases radiation from the car. So either I've convinced you by my illustration, or I misinterpreted your earlier remarks. Roy Lewallen, W7EL |
80m mobile antenna question
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Question for Roy (was 80m mobile antenna question)
Roy, your analogy of the car body as a tin can really got me to
thinking. With the whip mounted dead center on the top of the car, I can see how the roof acts like a ground plane (a very short one) but I'm puzzled about the radiation from the lower part of the car body. If one visualizes RF flowing through the sides, hood and trunk of the car, the currents will all be in phase with each other (roughly, of course) but the currents are displaced in space by several feet. How does this affect the net radiation from the car body as a whole? Is there some addition or subtraction due to having the same current, same phase but at a different location in space, and arranged in a more or less 360 degree pattern? An interesting thought. 73, Bill W6WRT |
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