- December 14, 2007 at 2:29 pm #16175RichGuest
Total posts : 45366
markk wrote I followed a ham forum where some guy built one, it didn’t work, and someone nearby came over, helped him fix his lc network, and then it worked and worked well for him.
Probably such forums would be a good place to look for the help you are asking for here. No doubt quite a few hams will have experimented with the EH concept for the 160-meter ham band, which is just above the AM broadcast band.
Good luck to you. Let us know what you learn, please.
//December 15, 2007 at 1:18 am #16176markkGuest
Total posts : 45366
What can you do? I don’t know. I can follow the design program to create the physical antenna itself… The radiating elements, made to size, waterproof it, etc.
what I need is for someone who understands how, to build (or just design, using off the shelf components so I can just assemble something) a very low power version of the phase shift/matching network that I wouild put in the box with the transmitter, rather than outside as part of the antenna.
Even better, if someone has the stuff to analyze this, and do the matching… well, we can get all the parts to you and have you align it.
I have a nice WIDE OPEN SPACE for testing. I also want to test it against a more conventional antenna.
MarkDecember 29, 2007 at 10:48 am #16201Ermi RoosGuest
Total posts : 45366
Markk should be congratulated for his enthusiasm for antenna testing. Experimentation is the best way to learn just about anything. His purpose for initiating this thread was to ask for help in designing the coils he needs for building the antenna he wants to test, but he was not able to get the help. The reason is that designing networks that have RF coils is not easy. Usually, design formulas are not enough, and a lot testing is needed.
To illustrate this problem with making RF coils, I will point out that each Part 15 AM station require a loading coil that has several hundred microhenries for resonating a short antenna. A really good loading coil for Part 15 AM has not yet been developed. All loading coils are very lossy, and there is no way to get the losses very low. The loss resistance in loading coils is usually underestimated. It is typical to assume that the loading coil loss resistance is only two ohms. This is an impossibly low coil loss. The inductive reactance needed to resonate a Part 15 antenna is often in the vicinity of 3000 ohms. To get a coil loss of only two ohms requires a Q of about 3000/2 = 1500. A coil with a Q that high is nearly impossible to make. The HP (i.e. Agilent) Q Meter has a maximum reading of 1000. I have tested thousands of coils using this meter, and I never came close to the maximum measurement limit of Q. 200 is usually easy to get. 300 is not too tough, either. In rare cases, I have approached 700. It was mentioned in this thread that a loading coil can be shielded in a metal box to avoid the wire length being counted toward the three meter limit of Section 15.219(b). Unfortunately, shielding significantly increases the capacitance of the coil, and the increased coil capacitance reduces Q.
So, even though RF coils have been around as long as radio itself, they are not easy to design. Also, there is no point in asking help from someone who doesn’t think that the CFA and related antennas work as advertised. Such a person would not think that any coil design can work.
Then there is the problem of getting meaningful test results. Accurate field strength meters are too expensive for hobbyists. About the best the amateur can do is make range tests and get reception reports. Such data is very subjective. Even a wet strand of spaghetti can be caused to radiate, and the signal can be heard on a radio receiver. But, if getting reception reports is the best you can do, then get reception reports. This is still worthwhile experimentation.
I have a suggestion for getting around the phase shifting and matching network design problem: Just don’t use any coils or matching networks at all in your antenna. I’m completely serious about this. This is not any kind of joke. Don’t even bother too much about the shape of the antenna. Select any sizeable piece of metal and call it your “antenna.” A metal garbage can lid would be perfect. Mount your antenna as high as you can. A length of wire (your “transmission line”) needs to be run between your antenna and your transmitter. You can connect the transmitter circuit ground to some sort of external ground, or simply assume that the transmitter is connected to ground somehow. Now you’re in business. You can tune up your transmitter and you will probably get good results. The higher you get the antenna above ground, the better it will work. This arrangement, however, may not comply with FCC regulations.
If you want more specific instructions, I have invented what I call the “No-Field Antenna,” which, despite it’s name, can produce a strong radio signal. The antenna itself hardly radiates at all, but the total antenna system can radiate a lot, and has a gain that can be nearly (within a fraction of a dB) as high as as that of a quarter wave tower. I don’t claim that my system can exceed the gain of a quarter wave tower, however.
The No-Field Antenna has four radials, each two meters long, connected together at right angles at one point. The radial length is not critical. I selected two meters only because that is the radius of a CFA tested at Goiania, Brazil. Each radial has equal RF current flowing through it, but the currents flow in four different directions at right angles, causing the RF fields produced by the currents to nearly cancel each other out. That is the reason for the name “No-Field Antenna.” This cross-shaped antenna is mounted in a horizontal plane. A wire from the transmitter is the transmission line, and connects to the center of the cross. I assumed that the cross is mounted 62 feet above the ground. I analyzed this system at 1600 kHz using the free demo NEC program downloaded from http://www.eznec.com. The input impedance to the transmission line is 8.95 – j472.9 ohms. The reactance is a lot lower than for a typical Part 15 AM antenna. If there were no ground resistance and loading coil resistance at all, the efficiency would be 100%. Of course, there has to be at least some ground resistance and loading coil resistance. Let’s suppose that there is a really low ground resistance of 2 ohms. Let’s also suppose that the loading coil has a very high Q of 600. This gives a loading coil loss resistance of 472.9/600 = .79 ohms. The total loss resistance is about 2.8 ohms. The efficiency of this No-Field Antenna system is 8.95/(8.95+2.8) = 76 %. This is more than twice the efficiency actually measured for the CFA at Goiania, Brazil. This may be mostly because the ground and loading coil resistances at Goiania were greater than I postulated here for my antenna. I claim that my No-Field Antenna (NFA) can be made to work as well as a CFA (or its supposed equivalent, the EH). I don’t have to resort to any theories not yet recognized by science. It can be analyzed by the NEC program, which is based on conventional electromagnetic theory. Best of all, the NFA is much easier to construct than the others.
The NFA can have good efficiency even if it is mounted much lower above the ground than 62 feet. The trick is to mount the NFA near a quarter wave monopole. Of course nobody would actually have an AM broadcast tower in the back yard, but it is possible, with some difficulty, to construct a quarter wave monopole using pipe and tubing. Guy wires or ropes would be needed for stability. The monopole should be well grounded. By mounting the NFA near the quarter wave monopole, the NFA would be capacitively coupled to the quarter wave monopole, causing the quarter wave antenna to radiate with high efficiency.January 5, 2008 at 7:39 am #16244Ermi RoosGuest
Total posts : 45366
Q. What do you call someone who can’t tell the difference between a dougnut and a teacup?
A. A topologist.
Q. Why didn’t Gauss invent group theory?
A. Because he wasn’t Abel.
Math majors hear jokes like this in college. They are inside jokes. Most people don’t understand them, because they are intended for people who are (or will become) mathematical professionals. Here are the explanations: A topologist is someone who studies topology, which is a branch of geometry that determines the properties of figures without regard to any deformation except cutting and joining. Some consider Gauss to be the greatest of all of the mathematicians in history. (Some of the other possible candidates for this honor are Newton, Archimedes, and Euler.) Abel was another outstanding mathematician. He died in this twenties, but he had established a great reputation during his short lifetime. Abel invented group theory.
I think I may have told something like an inside joke in the previous post in this thread. While everything I said is true, some readers may not have understood my point. This would not be fair, so I will explain what I meant.
I did not invent anything about the “No Field Antenna” I described, except the name. This antenna is better known as as the “X” antenna. It has been around for a very long time. The best-known relative of the “X” antenna is the “inverted L,” which has a vertical element attached to one end of a horizontal element. This antenna is very popular for the 160 meter ham band because it is at resonance when the total length of the vertical and horizontal elements is about a quarter wavelength. The inverted L is not as difficult to construct as a quarter wave vertical, and the horizontal element can fit in some confined spaces. The radiation is both horizontally and vertically polarized.
Another related antenna is the “T” antenna, in which a vertical element is attached to the center of a horizontal element. This antenna also produces vertically and horizontally polarized radiation, but the horizontally polarized radiation is weak because the RF currents in the two horizontal halves go in opposite directions, and the fields partially cancel each other out.
The “X” antenna has two horizontal elements that cross each other at right angles at their centers, and they are attached to a vertical element where they cross. The currents in the horizontal elements of the “X”antenna go in four different directions, and the fields nearly exactly cancel each other out. So the “X” antenna has vertically polarized radiation, but practically no horizontally polarized radiation. It is possible to cross more than two horizontal elements together to form a “star” antenna. The “star” antenna works about the same as an “X” antenna. It is possible to replace the horizontal “star” with a horizontal disk. In that case, we have a top-loaded antenna with a circular capacitive hat. This is also about the same as an”X” antenna with the same radius as the disk. Only the vertical elements of these last three antennas radiate. But, the horizontal elements still provide a very useful function. They increase the electical height of the vertical element and increase the radiation resistance. In these antennas, the vertical elements are true transmission lines, but they are transmission lines that radiate. It is not necessary for the top hat to be a disk. Any piece of metal that is comparable in horizontal dimensions to the disk will work as well as a disk.
The point I was trying to illustrate is that it is not necessary for a piece of metal you call an “antenna” to radiate at all if it is attached to a vertical “transmission line.” The antenna can be a poor radiator, or even a non-radiator, because practically all of the radiation is done by the vertical transmission line. This is the essence of the CFA, EH, and all the rest of the antennas of this type.
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