Dipole coverage distances

 



The above chart may need some explanation. I have marked the chart up showing coverage of a horizontal antenna at 3/8 wavelength high. On 40 meters this would correspond to a height of close to 50 feet. At that height the maximum radiation is centered on roughly 55 degrees. Zero degrees being horizontal and 90 degrees being straight up. At angles just above 70 and just below 40 the radiation decreases by 3 dB. Normally these 3 dB points are used to define the limits of the beam width of an antenna. There is not much difference when operating within theses “3 dB points”. The chart shows the upper and lower limits of a 3/8 wave high dipole. The fact that is says dipole is not significant as this applies to all horizontal antennas and not just dipoles. Since the ionosphere height varies from about 250 to 400 KM the corresponding distance for any radiation angle will vary depending on this height. For the 3/8 high horizontal antenna we can predict the minimum and maximum distances that can best be covered by an antenna at that height. If the ionosphere is at its maximum height of 400 KM we can expect to best cover a distance from 300 miles to 560 miles. If the ionosphere is lower, such as 250 KM, we can expect the best range of distances to be from 125 miles to 375 miles. Another way to look at this is that a horizontal antenna at this height will be best from a minimum distance of  about 125 miles out to a maximum of 560 miles. This is not to say that good communication can not be had at other distances, because if signals are out of the “3 dB beamwidth” they will only be not as strong as they could be with an antenna at the optimal height for that distance at that time. Most of us are stuck with antennas at only one height with out the ability to raise and lower then a will! We also are usually limited in how high we can get them up. Charts such as this do give us insight in how height affects our transmission range and maybe help us decide how high we would like our antennas. The above example is for single hop communication. And my explanation is for communication using the F Layer of the ionosphere. The chart does include a curve for a 100 KM high E Layer and the coverage distance is significantly less if signals are refracted by that layer. In general communication will be by the F layer at night on all bands and on the higher bands both at night and during the day. However skip via the E Layer can occur on any band at any time, that is called sporadic E skip. It can produce extremely strong signals on any band and is especially noticeable on 10 meters when the band is otherwise thought to be dead! 

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