[Elecraft] Home made Sigma-GT5 & KRC2 or SGC?

[George, W5YR]

Earl, I am confused over your statement. that the radiation resistance Rrad
of a dipole is 72 ohms regardless of the feedpoint.

The driving point impedance of an antenna depends upon where you elect to
feed it. Feed a horizontal dipole in the center and its driving point
impedance is in the order of 72+j0 if it is resonant and the appropriate
distance above the ground. Feed that same antenna at the end, and its
driving point impedance may be 3000 ohms.

In either case the power radiated is determined by the I^2 Rrad involved.
Thus, at the center where a relatively high current exists, the 72 ohm
radiation resistance accounts for the radiated power.

But at the end of the antenna, the current is quite small, yet the I^2Rrad
still must account for the radiated power.

The only explanation that fits both cases is that the radiation resistance
is a function of driving point location and thus varies as the location
varies. With endfeed, the radiation resistance must be much greater than
with center feed. In fact, if the antenna is lossless, that 3000 ohm driving
point impedance will *be* Rrad.

If you could explain your statement further, I would appreciate seeing what
point(s) I am missing.

Thanks!

73, George W5YR
Fairview, TX
w5yr@att.net
http://www.w5yr.com


----- Original Message ----- 
From: "Earl W Cunningham" <k6se@juno.com>
To: <mikemccoy@austin.rr.com>
Cc: <elecraft@mailman.qth.net>
Sent: Sunday, March 21, 2004 1:37 PM
Subject: Re: [Elecraft] Home made Sigma-GT5 & KRC2 or SGC?


> Mike, K5PU wrote:
>
> "I'm sure you're right but I'm referring to a center fed vertical
> *dipole* (ala Sigma GT5) and not an end fed vertical."
> ==========
> It doesn't matter whether a half-wave vertical is center-fed or end-fed.
> It's radiation resistance is still 72 ohms and it is subject to ground
> losses.
>
> 73, de Earl, K6SE