Ok, just trying to wrap my brain around some of the concepts discussed here in the forums on the topic of antennas and transmitters. Some of the current talk about antennas seems to indicate that if they're mounted on a tower, then the tower itself radiates enough to need to be counted as a "ground wire" rather than a ground. MORE
But extending that concept, say an antenna/transmitter is on the top of a 20 story steelframe building and grounded to the frame of that building. It seems rather questionable to me that a 100 millwatt transmitter would actually be able to put enough EMF into the building to use it effectively as a radiator? That seems a lot of metal for a very small amount of power to do anything with. Or if the antenna was ground mounted with the xmitter at its base and grounded directly into the ground at the top of a mountain, surely we aren't saying that the mountain's elevation above sea level is being used to radiate with in a fashion that would require it to be counted in the 3M total for antenna, feedline and ground wire (if used) model? On the other end of the spectrum, if the antenna and xmitter are 300 ft in the air and the only path to ground is a thin wire 300 ft long, ok, I can see where it'd be a concern that the ground wire is acting more as a radiator than a ground for at least some of it's length there.. But somewhere between those two extremes there must be a point where at least most theories would agree that it acts as a ground as opposed to just a ground wire.. and another point where it acts as a functionally radiating antenna more than it does a ground. I don't think that there's much debate that a good ground is an effective part of at least most antenna systems. But considering the low resistance that a typical ground cable and rod present (since it being a very low resistance is the whole purpose to using heavy guage wires for such things and/or pipe driven into the ground), it's hard to see where it's going to produce enough radiation to be of much use as a ground. We're talking about a magnetic field for RF, and the voltage drop over the length of any actual practical ground conductor or even a grounded tower would be very small. The current would also not be much, since a 100 milliwatt system can't actually put much into a short radiator at the frequencies we're talking and even 20' of tower would still be very tiny. If we're talking about any sort of ground conductor that is at all large, I would think any "power" the tower might develop as a radiator would be eaten up by the eddy that currents the little bit of stray RF we're talking about would be trying to excite in it. Next dumb grounding question.. How do the buried radials do much if the antenna *is* elevated? I'd think that they'd be less effective, since the actual radiator will be at a greater physical distance and as such the coupling between them would be less? Ok, the advantage of the height might more than make up for that in actual practice since it would get the antenna more clear of obstructions like buildings and trees and power lines and closer to a theoretical view of the physical horizon.. But wouldn't it cancel some of the advantages of the buried ground radials? Ok, I can see where it might possibly couple effectively if the radiator was a significant wavelength fraction obove the physical earth, but for 1Mhz that'd be about a 250 foot tall tower, which I don't think many people *could* build or maintain (and which would need markers to keep planes from hitting it). So with people mounting xmitter/antennas on a tower or a 20' piece of water pipe, how does it couple with those buried radials at all, as opposed to just seeing it as "poor ground conductivity" like if there were no radials? Daniel
First off, I'm not an engineer but I do have a little knowledge on how things should work. The key is to find that magical reasonance point as good as possible to get the most from the xmtr..
I look at it as a mobile antenna situation. The radiator may be designed by formula to be near reasonance per feedpoint impedance but the ground "Plane" or area is an unknown factor. Tuning is provided in most cases to appease the 50 ohm coax in electrical length of th radiator but the ground is set to what is available in the vehicle which is never the same in different installs or locations of the antenna. But, There is one thing in common with all of them.. Near field ground.. And it's rarely ever at any magical wavelength of the used frequency..
The Rangemaster, the SStran and others provide adjustment to electrically shorten/lengthen the radiator via different settings at the output section and the reference is ground for the "Target" voltage readings at set up. The SStran give a voltage level to shoot for and the Rangemaster provides a chart to work from to assure that the R.F. output is within the legal levels. The SStran is easy. If you can find the "Sweet Spot" that gives you the voltage reading that they want you to shoot for, You know that you are near the prefered designed output impeadance. The Rangemaster on the other hand is much more complex. If you can find the "Sweet Spot" near the top of the chart, You know that you have enough ground return to achieve the designed feedpoint impeadance. The farther down the chart you go to get a desired I/V balance, The more mismatch you have to deal with but the R.F. output is adjustable to compensate for the losses in the system.. (To a Point) The farther down the chart, The less bandwidth you have..
From my experiences, I've found that a near field ground gives the best results. There must be enough current flow back to the xmtr to give it something to work with. I really don't believe that there's any magical length/area involved but the ground would be better if it was somewhat in resonance per frequency which would require a coil or something. This would make it illegal in the F.C.C.'s eyes. I will say the more "Close Range" collection area, The better..
.. say an antenna/transmitter is on the top of a 20 story steelframe building and grounded to the frame of that building. It seems rather questionable to me that a 100 millwatt transmitter would actually be able to put enough EMF into the building to use it effectively as a radiator? That seems a lot of metal for a very small amount of power to do anything with.[
Maybe a good way to understand this is to consider a 1 kW commercial AM broadcast station using a single, 1/4-wave tower (about 240 feet high for a 1 MHz carrier frequency). The groundwave field strength 1 km from that station will be about 300 mV/m. Now if the station reduces their tx power to 100 mW, that amounts to a 40 dB reduction in radiated power. The field strength at 1 km at that low power also will drop 40 dB, which value will then be 3 mV/m. It is a linear process, and does not "break down" just because the power applied to that ~240 foot tall radiator is small.
Or if the antenna was ground mounted with the xmitter at its base and grounded directly into the ground at the top of a mountain, surely we aren't saying that the mountain's elevation above sea level is being used to radiate with in a fashion that would require it to be counted in the 3M total for antenna, feedline and ground wire (if used) model?
Correct. The earth is the electrical reference plane for the radiating conductors of a MW monopole.
On the other end of the spectrum, if the antenna and xmitter are 300 ft in the air and the only path to ground is a thin wire 300 ft long, ok, I can see where it'd be a concern that the ground wire is acting more as a radiator than a ground for at least some of it's length there. But somewhere between those two extremes there must be a point where at least most theories would agree that it acts as a ground as opposed to just a ground wire.. and another point where it acts as a functionally radiating antenna more than it does a ground.
Physics shows that the height at which the ground wire begins functioning as an antenna occurs at the point where it exits the earth. As an example, a "shunt-fed" radiator is nothing more than an uninsulated vertical conductor whose base is attached to an r-f ground. The fields it generates are identical to those of a conductor insulated from ground and fed in series across a base insulator (other things equal).
I don't think that there's much debate that a good ground is an effective part of at least most antenna systems. But considering the low resistance that a typical ground cable and rod present (since it being a very low resistance is the whole purpose to using heavy guage wires for such things and/or pipe driven into the ground), it's hard to see where it's going to produce enough radiation to be of much use as a ground.
The ground wire may have very low resistance to the flow of direct current (maybe in the milliohm range), but it will not have that low resistance to the flow of r-f energy. Going back to the shunt-fed tower, it has extremely low DC resistance, but still has significant impedance to r-f energy. That is why both of them can/will radiate.
How do the buried radials do much if the antenna *is* elevated? I'd think that they'd be less effective, since the actual radiator will be at a greater physical distance and as such the coupling between them would be less?
Buried radials can reduce the losses that the r-f ground currents incur as they are returned to the transmitter via that radial ground system, and any "ground wire" that connects the radials to the tx. The flow of that r-f current through the exposed ground wire sets up radiation, just it does on a shunt fed tower. The better the radial ground system, the higher the current that will be returned to the tx, and the greater the net field radiated by the antenna system (other things equal).
Ok, the advantage of the height might more than make up for that in actual practice since it would get the antenna more clear of obstructions like buildings and trees and power lines and closer to a theoretical view of the physical horizon.. But wouldn't it cancel some of the advantages of the buried ground radials?
No, because most of the radiation from an elevated 3-m antenna system occurs from the long ground wire below it, which radiation starts where it exits the earth.
Good questions, Daniel.
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Great stuff there, 12vman, it really helps with understanding the methods folks are talking about here, especially as they apply to two of the most popular transmitters on this forum.
Excellent answers Rich, that does clear a good bit of it up for me.
I would like to return to one part of your reply to explore it a little further..
Maybe a good way to understand this is to consider a 1 kW commercial AM broadcast station using a single, 1/4-wave tower (about 240 feet high for a 1 MHz carrier frequency). The groundwave field strength 1 km from that station will be about 300 mV/m. Now if the station reduces their tx power to 100 mW, that amounts to a 40 dB reduction in radiated power. The field strength at 1 km at that low power also will drop 40 dB, which value will then be 3 mV/m. It is a linear process, and does not "break down" just because the power applied to that ~240 foot tall radiator is small.
Or if the antenna was ground mounted with the xmitter at its base and grounded directly into the ground at the top of a mountain, surely we aren't saying that the mountain's elevation above sea level is being used to radiate with in a fashion that would require it to be counted in the 3M total for antenna, feedline and ground wire (if used) model?
Correct. The earth is the electrical reference plane for the radiating conductors of a MW monopole.
Excellent way of explaining that, by the way. But wouldn't it be assuming that the tx at 100 mw would be able to use the quarter wave tower as effeciently as it did with 1 KW? What about the losses due to eddy currents and the hysteresis threshold? Particularly with the hysteresis, it takes power to do the work of overcoming the "inertia" of the object (tower in this case) becoming magnetized in the first place for the magnetic lines of force to be emitted. Until that "inertia" (using that word instead of resistance to avoid confusing the issue) is overcome, then to my understanding the object will not become electromagnetically excited in such a way that it could radiate more effectively than the mountain would with a transmitter mounted at ground level atop a mountain.
So it's not exactly linear. It might be when you're above the hysteresis threshold and if the power being consumed by eddy currents doesn't consume what would be radiated as RF, but not all the way down to "zero millwatts" can it be linear. Now, those losses might be very negligible at 1KW, but at some point in a mass of iron/steel they are going to be at least a considerable percentage of the power the smaller xmitter has to offer that tower.
Agreed that the mountain would be acting as "the electrical reference plane for the radiating conductors of a MW monopole", but we wouldn't say that it is radiating like a "ground wire" might, but as a ground.
My thought is that at some point in mass, a tower, building or other structure would offer no more "advantage" to the signal of a 100 mw xmitter than an extension of the earth like a mountain would.
Which is not to discount that most antennas do benefit from the presence of a functional ground to allow the earth current which returns to the tx.
At least at higher frequencies, the RF frequency could result in the "skin effect" applying more and as such the losses could be less, but at the MW frequencies around 1 MHZ I'd think we're still dealing with the basic rules which apply for AC electromagnetic fields in an iron/steel mass like they do in a transformer. So wouldn't hysterisis threshold especially still apply in trying to get a 240 ft actual broadcast tower for a 1 kw xtmitter to effectively radiate with the small percentage of 100 mw that a part15 xmitter would be able to offer to what it "sees" as it's ground?
Sorry if I'm a bit dense on this sort of thing or not understanding correctly, but my background is more working with AF and sometimes tinkering with high voltage induction coils over the years than it has been in actual practical experiments with RF.
Daniel
But wouldn't it be assuming that the tx at 100 mw would be able to use the quarter wave tower as effeciently as it did with 1 KW? What about the losses due to eddy currents and the hysteresis threshold?.... So it's not exactly linear. It might be when you're above the hysteresis threshold and if the power being consumed by eddy currents doesn't consume what would be radiated as RF, but not all the way down to "zero millwatts" can it be linear.
Sorry, but it isn't possible to extrapolate the electrical characteristics of an audio or power transformer to apply to antennas. The equations for the radiation resistance and therefore the ultimate efficiency of an antenna system have no terms relating them to the amount of power applied to that antenna system.
So at a power level of 1 femtowatt, 1 terawatt, or any other value, a given antenna system will radiate them all with equal efficiency (other things the same).
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Well, transformers are an example, but hysteresis is basic physics of magnetic electromagnetic effects, not something strictly applying to ac transformers. Particularly in ferromagnetic materials like iron, steel, etc it will take a certain amount of work to drive the magnetic field to a given state, and further it will then have to be driven back the other way to zero and beyond to create an electromagnetic wave that can escape the material to be radiated.
I don't recall the math on the hysteresis threshold (or hysteresis loop in the case of an oscillating waveform), but I do recall the chart of it is a curve both ways, not linear. Now, you may well be correct that above that threshold it may be linear, but what I was saying is it is not linear all the way down to zero because of the hysteresis.
Hysteresis in something like a steel tower is physics, and is due to the magnetic domains existing in that body of ferromagnetic material. Now, if we were making a tower of aluminum or copper, the situation would be a bit different and I'm not sure what rules would apply. But a mass of iron/steel has it's properties, and since an iron slug is a material that can be used to tune a coil at the frequencies we're discussing, it seems logical to me that it is apparently sufficiently reactive over this frequency range that its effect will not be zero. As such the power that the tower itself could radiate (discounting whatever the vertical antenna alement is radiating for the moment) logically couldn't be a direct linear proportion if the power available is small and the tower is large. The elecromagnetic wave has to work both ways against the magnetic properties of the steel tower, and there has to be some power used for that which is simply "loss".
Now, if it was a wooden tower of the same size and we were running a wire up it to act as a "ground", I can see and quite agree with your point that of how that "ground wire" would be radiating as well, and that would be likely why the "ground wire (if used)" phrase is included in the 3 meter limit. But for this example you were talking about a 250 ft commercial broadcast tower, and the ones I have seen have been steel.
Sorry if I seem overly argumentative on what may seem a small point of physics. Your answers have been excellent and thought provoking, and I compliment you on them.
Daniel
In commercial AM antennas, the tower is the antenna. It's live don't touch it. They are "grounded" at the base (feed point), as well as some sort of lightning protection device to jump a lightning strike across an air gap to ground.
Well, transformers are an example, but hysteresis is basic physics of magnetic electromagnetic effects, not something strictly applying to ac transformers. ... Hysteresis in something like a steel tower is physics, and is due to the magnetic domains existing in that body of ferromagnetic material. Now, if we were making a tower of aluminum or copper, the situation would be a bit different and I'm not sure what rules would apply... The elec(t)romagnetic wave has to work both ways against the magnetic properties of the steel tower, and there has to be some power used for that which is simply "loss".
The skin effect of conductors carrying r-f currents produces different behavior than described in your quote above. Skin effect results in the redistribution of the current over the cross-section of the conductor in such a way as to cause those parts of the conductor having the highest reactance (the parts nearest the center) to carry the least current. The magnitude of the effect on r-f resistance and inductance increases with frequency, conductivity, magnetic permeability, and conductor size.
This reality is used in the construction of some types of r-f transmission lines and antennas where conductors are hollow tubes, yet their r-f characteristics are the same as if the tubes were solid. The depth of r-f current penetration from the perimeter of a round conductor is only a few hundred millionths of a meter at medium wave frequencies.
A steel conductor used as a medium wave radiator can have higher r-f loss than one of the same length made of copper, because the conductivity of steel is poorer than copper. Steel also will have different radiation resistance, because the velocity of propagation along a steel conductor is slower than along a copper conductor.
But it remains that the equations to calculate these values have no relationship to the amount of r-f power applied to such radiators.
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Thanks Rich, that clears the matter up for me a lot!
Really great answers and I appreciate you taking the time to explain these matters.
Daniel