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.