Tom Donaly wrote:
Actually, no one really knows whether it's really a sine curve
or not because no one has ever been able to solve the integral
equation that would give an exact answer.

It would only be a sine curve if the reflected current was equal
to the forward current, i.e. the antenna was lossless (no I^2*R losses
and no radiation). We know that the reflected current is roughly about
90% of the value of the forward current at the feedpoint of a dipole.
So the total current distribution approximates a cosine wave. In the
textbooks you will find general assumption statements like Kraus':

"IT IS GENERALLY ASSUMED that the current distribution of an infinitesimally
thin antenna is sinusoidal, and that the phase is constant over a 1/2WL
interval, changing abruptly by 180 degrees between intervals."

For all real-world current waves, there is an attenuation factor. The
reflected current arriving back at the feedpoint is always less than
the forward current. That's why the feedpoint impedance,
(Vfor+Vref)/(Ifor+Iref) is low but never zero for a dipole. The net
current cosine function is a ballpark assumption, not actual reality.

This is interesting because (Vfor+Vref)/(Ifor+Iref) is 75 ohms for a
1/2WL dipole but is about 12 ohms for a 75m bugcatcher. That means
the reflected waves are closer in magnitude to the forward waves
in the 75m bugcatcher than they are for a 1/2WL dipole. That makes
sense since the tip of the antenna is closer to the feedpoint for
the 75m bugcatcher than for the 1/2WL dipole.
--
73, Cecil http://www.qsl.net/w5dxp


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