| I have been a practicing Ham Radio Operator for nearly 30 years, (FCC call sign KC 8 UXZ -General Class operator). My prinicpal area of interest throughout that time has been the design and construction of wire antennas. While almost any random lenght of wire may be used as a receiver antenna, for optimum receive the wire should be cut to 1/4 or 1/2 resonant lenth at the desired frequency. In many instances a resonant antenna will exhibit as much as 20db gain over a random length while also offering good rejection of unwanted signals. The resonant lenght is the physical distance a radio wave travels through free space on one cycle. All Radio Frequency energy travels at the speed of light which is 300,000,000 meters per second. By dividing 300 by the frequency expressed in MegaHertz (Mhz) we can then determine the distance the signal travels on one inpulse. When designing for a broad band of freqquencies such as found on the AM broadcast band we must first find the frequency at band center. The AM band is 550Khz through 1600 khz therefore we subtract 550 from 1600 to find 1050khz or 1.05mhz is band center. Dividing the the distance radio travels in one second 300 million meters by the frequency in Mhz we find the distance the signal travels on one impulse at frequency- 300/ 1.05 = 285.71 meters. (round to 285) 1/4wave is now 285/4= 71.25 meters allowing 39" per meter it is then 71.25 x 39" = 2778.25inches / 12 = 231.56ft. If you should ever have opportunity to see an AM broadcast station antenna up close you would note that the tower is mounted on insulators at the ground and the entire vertical structure of the tower is the antenna. Approximately 231ft vertical. To get some idea of the relationship of operating frequency to resonant length, the antennas on cell phones are 1/4 wave verticals at their operating frequency. In the early days of AM braodcast when home receivers were much large than what they are today, it was common to take a 1/4 wave lenght of wire and form an expanding loop on the inside of the back cover of the receivers. They worked exceptionally well for fringe area reception, althoght they were very directional and one had to physically rotate the radio receiver for maximum reception. (They are directional in a plane at right angles to the loop) As the physical size of radio recievers began to decrease they then began making antennas by winding a resonant length of insulated wire around a tubular form. Again, highly directional in a plane at right angles to the form. Prenently portable AM receiver antennas are a 1/4 wave lenght of resonant wire wrapped around a ferrite bar or rod form. The ferrite core deminishes the directivity somewhat so that portable radios will work in all postitions, although they are still highly directional for weak signals from distant stations. To compensate for weak signals most receivers now have a receive RF amplifier circuit with an Automatic Gain Control (AGC circuit) between the antenna and the tuner section. Capture area is another prime consideration. For maximum performance a wire antenna should be one long straight lenth of wire at the desired frequency, but obviously a 231.5ft length of wire could present a problem. It is for that reason that AM antennas are commonly wound on a form to make a loop or rod antenna. A simple method of making an AM receive antenna would be to use a 231.5ft lengh of fine diameter insulated were such as awg 20 to awg 30 transformer wire wound around a lenght of PVC plumbing pipe. Be careful to try to keep the spacings of the wire even, then coat it whole antenna with a layer of polyurethane to seal it and keep the wire in place. For even greater performance you could construct a Di-pole antenna in the same manner. Begin at the center of the pipe and wind one 231.5 ft length toward one end of the pipe. Using a second wire the same lenght wind a second coil from the center to the opposite end of the pipe. A coax is then attached at the center with the center conductor attached to one element and the outer braid attached to the other element. For FM broadcast reception the band is 88 to 108 mhz, therefore band center is 98 mhz. 300 / 98mhz = 3.06 meters 3.06 x 39" = 119 inches for a full wave antenna or 29.75" for a 1/4 wave. | 
