Some basic antenna information

 Antenna information.


This is some basic fundamental antenna information that may help understand a little about how antennas work.

First of all antennas need to be different sizes for different frequencies. For example a short antenna ( in Terms of wavelength) will not radiate as well or as much energy as a longer antenna. The basic radiation is proportional to the product of amps on the antenna times the length. The term is meter-amperes or feet-amperes. Given the same current on a wire a longer wire radiates more than a short antenna. Along the same lines, for a given length of antenna increasing the current will radiate a stronger signal. As long as the product of meters times amperes is the same the radiation will be the same. It must be noted that longer antennas (over 1/2 wavelength) will have different radiation patterns. For such long antennas the directive pattern will also depend on the location of the feed point. For example a one wave long center fed antenna will have a broadside pattern when fed in the center and more of an X pattern if fed at the end. This is due to phase of the current along the antenna. Pure radiation ( regardless of directivity) is proportional to length times current, while directivity depends on geometry of the antenna and its feed point location. 

Another factor is efficiency of the antenna. The short antenna has a low radiation resistance while a long antenna usually has a higher radiation resistance. While the  actual resistance of the antenna or wire in real ohms may be almost the same, the efficiency depends on the ratio of radiation resistance to the radiation resistance plus the actual ohmic resistance of the antenna. If ohmic loss is zero then the efficiency is 100%. As the ohmic loses increase the efficiency goes down. This situation is made much worse with the low radiation resistance of short antennas. 


One thing I always do is try and determine the current profile on an antenna. It will usually be somewhat sinusoidal. That means there will be places of high current periodically along the length of a antenna. In most cases it is not too hard to estimate where they high current points are. These are the locations of the maximum radiation from the antenna. As an example this is at the center of a simple half-wave dipole. At the ends of the dipole the current is low. Based on this it is easy to see that the center 1/3 of the half-wave dipole radiates the most energy while the ends much smaller amounts. The obvious conclusion is that it is best to get the center of the dipole high and in the clear but bending the ends down or using inductive loading on that part of the dipole does not reduce radiation significantly. 


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