[Elecraft] EARC End Fed 6-40 question (Was Re: cobweb antenna)

[Ron D'Eau Claire]

The "grounding" issue with end feeding a half wave radiator is in keeping
the rig at a low RF voltage to avoid "RF in the Shack". 

Unlike a short monopole (1/4 wavelength long or less), very little RF
current flows into the end of a half wave radiator so the "ground" has very
little impact on its efficiency. Short monopoles have relatively high RF
currents at the feed point so the "ground" has a huge impact on their
efficiency. 

The RF current at the feed point is divided between the radiator and the
ground connection according to their relative impedances. Where only a tiny
current is flowing into the radiator, only a very tiny current flows into
the ground connection.

With a relatively bad ground with an impedance of, say 300 ohms, connecting
an end half wave antenna with a typical impedance of 3000 ohms or so means
that about 90% of the RF power is delivered to the antenna and only 10% is
lost in the ground impedance. 

Using the same ground with a 1/4 wave monopole with, say, 35 ohms impedance
means that about 90% of the RF power is absorbed in the ground resistance
and only 10% is delivered to the antenna. With shorter monopoles the
impedance drops very fast in a non-linear fashion. It's not unusual to see
feed point impedances of 2 or 3 ohms for short "loaded" antennas. In that
case 98% or more of the RF power is consumed in the "ground" and only 1% or
2% is radiated. 


73 Ron AC7AC