"Wayne" wrote in
news:7L%Gh.506$Ih.268@trnddc02:


Wayne wrote:
When the subject of antenna efficiency comes up, it often involves a
discussion of ground losses on verticals. What about, for example,
a dipole? Could one calculate "power out/power in" by measuring the
VSWR and declaring that everything not reflected was transmitted?
It would seem more accurate to actually measure power out and power
in, but that introduces inaccuracies by having to calibrate the
setup. Thoughts?

"Roy Lewallen" wrote in message
...
There's no direct way to measure the total power being radiated other
than sampling the field at many points in all directions and
integrating. "Reflected" power is not power that isn't transmitted.
You can find the power being applied to the antenna by subtracting
the "reverse" or "reflected" power from the "forward" power, but that
tells you nothing about what fraction is radiated and what fraction
lost as heat.

Roy Lewallen, W7EL

Thanks for the reply. My dipole example is intended to avoid
transmission line issues by not having one, and the elements are
assumed to be reasonably low loss. If I do some quick
back-of-the-envelope calculations, for a VSWR of 1.3:1, I get an
efficiency of about 98.3% (using the equation 1-gamma^2). Assuming a

You probably mean 1 less the magnitude of the reflection coefficient
squared, which I would write as 1-|Gamma|^2 or 1-rho^2. Your implication
is that "reflected power" is (necessarily) lost, that is wrong.

If you do not have a transmission line, why are you trying to use
transmission line concepts to solve the problem? The power delivered to
the antenna is the real part of V^2/Z where V is the feedpoint voltage
and Z is the feedpoint impedance.

resistance of 1 ohm in the dipole conductors the efficiency I
calculate is about 98.6% (72/73).

The number you guess could be reasonable, and if it is truly the
equivalent R at the feedpoint (rather than R/unit length * length), it
does impact the antenna efficiency in the way you calculate.

Owen