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Amazingly, no one has expressed what units these constants 300 and 984, etc.
represent or the fundamental formula: wavelength = velocity of propagation/frequency = velocity (units of length per second)/frequency (cycles per second or Hertz) = length per cycle If the velocity is expressed in meters per second, the wavelength is expressed in meters, if feet per second then in feet. In free space, velocity of propagation of radio waves is 300 million metres per second or 984 million feet per second. When divided by frequency in millions of cycles per second (MegaHertz), we get the length of one cycle or wavelength in meters or feet, respectively. In air, the velocity slows down slightly so a wavelength is slightly shorter. The coefficient of velocity (or velocity factor) expresses the fraction the speed is relative to that in free space. In air, it is 99.7%. In denser media, it is slowed more. Typically, we don't consider velocity of propagation as the reason for the approx 95% factor for antenna calculations below 30 MHz but as a convenient way of accommodating the real-world effects of conductor diameter and capacitance at the ends on the resonant frequency. It is a rule-of-thumb factor as a starting point for trimming an antenna for resonance at a certain frequency - not critical for receiving. Relative velocity of propagation is important in transmission lines, when used to set the relative phase at each element of an array of driven antenna elements, and can be as low as 66% that of free space in coaxial cables using polyethylene insulation between the inner conductor and the shield. Thus, at 10 MHz, a wavelength is: - 30 metres, in free space - 29.91 metres, in air - ~ 28.5 metres for a 'full wavelength' resonant antenna (varies with end effects and conductor diameter) - 19.79 metres in Belden 8241 (RG-59) coaxial cable. Regards, Tom .. "Michael A. Terrell" wrote in message ... David wrote: On Mon, 11 Apr 2005 17:09:32 GMT, "Michael A. Terrell" wrote: RHF wrote: DF, . And the 'original' Question Is (Was) : " formula for calculating the length of a full-wave antenna wire " . "How Do I" ? Calculate the Length of Wire I need to build a Wire Antenna ? http://groups.yahoo.com/group/Shortw...a/message/2884 . This webpage does a very good job of providing an answer. - - - How Do I Find the WaveLength of a Frequency ? - - - . GoTo= http://www.radiomods.co.nz/radiomath.html . IN THEORY - The Numbers Are : Meters = 300 Divided by Frequency in MHz Feet = 984 Divided by Frequency in MHz Inches = 11,811 Divided by Frequency in MHz. Not "In theory" but in free space. IN PRACTICE - {The-Real-World} - The Numbers Are : Meters = 285 Divided by Frequency in MHz Feet = 936 Divided by Frequency in MHz Inches = 11,235 Divided by Frequency in MHz. This is caused by the propagation delay in the conductor the antenna is made of. In other words, the wire is measurably slower that free space. This is speced as the "Propagation Delay" and is stated as a percentage. Look at the data on Coaxial cable for examples. Other Questions - Asked-and-Answered : * How do I find the frequency of a wave length ? * How do I Calculate the Length of Wire I need to build a Wire Antenna. ? [ You must use the following Math to Correctly Cut an Antenna. ] - One {Full} Wave Length (WL) - Three-Quarter Wave Length (3/4 WL) - Five-Eighths Wave Length (5/8 WL) - One-Half Wave Length (1/2 WL) - One-Quarter Wave Length (1/4 WL) - One-Eighth Wave Length (1/8 WL) . FULL WAVE LENGTH WIRE (WL) ANTENNA IN FEET = 936 DIVIDED BY FREQUENCY . Plus the Age Old - How Do I Convert : * Meters-to-Feet ? * Feet-to-Meters ? . . iane ~ RHF The wave doesn't travel as fast in a solid conductor and the voltage reverses sooner (i.e. closer to where it started.) That's another way to describe propagation delay. -- Former professional electron wrangler. Michael A. Terrell Central Florida |
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