Total posts : 45366
It happens that two threads dealing with Part 15 HF have emerged simultaneously. On the other thread, Rich made the good point that, using the field strength limits of Part 15.225, the field strength at a distant receiver picking up the skywave signal would be very low. However, in the system I mentioned, the peak field strength would be directed upward toward the ionosphere, and the field strength in the horizontal direction, where the field strength measurement to determine compliance to Section 15.225 would be made, would be much lower.
A simple high-angle radiator would be a horizontal half wave dipole a half wavelength above the ground. The peak radiation would be directed upward about 30 degres. The horizontal component of the radiation would be very low. This applies to an open area. It would be hard to predict what the radiation pattern would be if the antenna were mounted in a back yard, with surrounding trees and structures.
An omnidirectional high angle radiator can be a verical monopole slightly less than one wavelength high (between 85 % to 95 % of the wavelength). This antenna would be quite tall, and it would not be resonant. An antenna tuner would be needed to match the transmitter to the antenna. The angle of the peak radiation is about 45 degrees. My comments about alteration of the radiation pattern by nearby trees and structures apply here, also.
As for finding a circuit for the transmitter, any number of Part 15 MF designs can be adapted for HF. The frequency multiplication factor would be less than 10. A good example is the MF transmitter schematic pictured in the recent “Loading coil” thread. This one already has an “L” network at the output, which may be adapted for matching the transmitter to a coax cable. Small-signal transistors can be easily used with a half watt of input power. It would not take a lot of additional effort to make a transmitter with one watt of output power. It is my guess, however, that, in practical setups, the field strenth limits of Section 15.225 will not be met if one watt of radiated power is used. But the ideal theoretical horizontal component can be very small. So, it is hard to say what the maximum power can be.
A good free NEC program for antenna analysis can be downloaded from http://www.eznec.com. The free demo program is limited in its capabilities mostly by the fact that only 20 segments are allowed for analysis. The low number of segments available limits the accuracy of the results. However, I have found that the demo program is good enough for the simple radiators used with Part 15 transmitters. Rich has repeatedly demonstrated how useful the NEC program can be. The tutorial (the “test drive” section) in the downloadable instruction manual gets you started right away. It is easy to find the input impedances and radiation patterns of various simple.antennas. Rich has indicated that, if you are going to pay for an NEC program, EZNEC may not be the optimum choice. However, I am very pleased with the free demo program. I had previously done antenna analysis using pencil, paper, and a calculator (originally the HP desktop scientific calculator of the late Sixties). I can do much more now, even with the free version of the EZNEC program.