Topband: Antenna Impedance vs efficiency

[Ford Peterson]

Topbanders!  The repository of all (ok most) knowledge about RF...

Some time ago, I gave a seminar to the local radio club about SWR.  To provide an illustration, I used a visual explanation including a garden hose--actually a series of various diameter garden hoses tied end-to-end.  I explained that any transition from one diameter to another caused an impedance 'bump' that had a negative impact on flow.  In my analogy, going from 5/8" hose to 1/2" hose induced a higher pressure and reduced flow due to back pressure.  Going from the 5/8" to 1/2" to 3/4" caused a loss of pressure after the 1/2" hose and likewise an additional reduction in flow.  The analogy went over pretty good with those members that, until that point, had no explanation for using 50 ohm coax vs 75 ohm coax or why there was a need for antenna matching network--basic stuff.

Lately I've been pondering the notion of the impedance of free space.  I'm told that the characteristic impedance of free space is 377 ohms.  I have no idea where that comes from, or how it is measured.  But I do know that the characteristic impedance of a 1/4wL monopole, properly constructed, is about 38 ohms.  That's a ~10:1 impedance ratio.  What am I missing here?  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?

Ford-N0FP
ford at cmgate.com