Topband: Antenna Impedance vs efficiency

[David Gilbert]

Ford Peterson wrote:

Is there any intrinsic loss attributable to coupling a relatively low impedance system to the high impedance of free space?  E.g. is there any benefit to using a higher impedance system?


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Hi, Ford.

A wave traveling through space has both an electric field and a magnetic 
field.  The orientations of the two fields are perpendicular to each 
other and both look like waves that vary with time and distance, just 
like voltage and current do on a transmission line.  The physical 
orientation of the electric field is determined by polarization of the 
antenna.  The electric field of a wave traveling through space is the 
analogy of the voltage on a transmission line and the magnetic field is 
the analogy of current, and the ratio of the electric field to the 
magentic field is 377 ohms in free space. 

Since the antenna launches the signal into space, by definition it 
essentially provides the transformation to the impedance of free space.  
At least that's how my college field theory professor described it 
almost 40 years ago.  Maybe he was only making an analogy.  Anyway, the 
junction of the feedline to the antenna is a different node in the total 
system and its impedance has no direct relationship to the impedance of 
free space.

On the other hand, it would seem that certain antennas must have an 
inherently better ability to couple a signal into free space.   I don't 
know this for sure, but I suspect that a reactive feedpoint represents 
an antenna that couples improperly to free space.  We are able to adjust 
that condition by the use of matching networks of various sorts to 
override the antenna and put voltage and current (and therefore electric 
field and magnetic field) back into proper phase.  Please consider this 
last paragraph as speculation on my part, and I'd appreciate someone 
correcting me if it is erroneous.

Dave
AB7E



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