On Jun 4, 9:53*am, Roy Lewallen wrote:
Richard Fry wrote:
*To determine efficiency you'd have to make some field strength measurements
(usually performed with a calibrated field strength meter) in order to determine
how much of the power going into the antenna terminals is being radiated
into free space.

The radiation resistance present at the base of an electrically short,
linear, monopole (whip) antenna of various ODs can be calculated
rather accurately using equations found in various antenna engineering
textbooks . . .

This is true only if you don't confuse the idealized textbook models
with real antennas. ...

For the sake of discussion, below are two pastes from the same NEC
model using the demo version of EZNEC v. 5.0 -- which rather well
support my earlier post that the radiation resistance (NOT the
impedance) of an electrically short monopole is a function of its
electrical length, and not the loss resistance of the r-f ground and/
or the loading coil.

CASE 1 = Zero loss resistance and reactance in the r-f ground, and
zero loss resistance in the loading coil:

EZNEC Demo ver. 5.0

1650 kHz 3 meter monopole 6/4/2010 10:50:57 AM

--------------- SOURCE DATA ---------------

Frequency = 1.65 MHz

Source 1 Voltage = 0.08578 V at 35.09 deg.
Current = 0.4986 A at 0.0 deg.
Impedance = 0.1408 + J 0.09888 ohms
Power = 0.035 watts
SWR (50 ohm system) 100 (25.17 ohm system) 100


CASE 2 = Same model as above, except with a total of 25 ohms loss in a
loading coil and r-f ground, and no reactance in the r-f ground:

EZNEC Demo ver. 5.0

1650 kHz 3 meter monopole 6/4/2010 10:49:40 AM

--------------- SOURCE DATA ---------------

Frequency = 1.65 MHz

Source 1 Voltage = 0.9386 V at 0.22 deg.
Current = 0.03729 A at 0.0 deg.
Impedance = 25.17 + J 0.09579 ohms
Power = 0.035 watts
SWR (50 ohm system) = 1.987 (25.17 ohm system) =
1.004

EZNEC calculated the radiation resistances of these two cases to be
0.14 ohms and 0.17 ohms, respectively -- fairly close, but not exact.
Perhaps Roy could comment on the reason why their agreement using NEC/
EZNEC is not better.

Those wanting a good resource for the measured results for monopoles
of less than 1/8 electrical wavelength might try to locate the paper
by Carl E. Smith and Earl M. Johnson titled PERFORMANCE OF SHORT
ANTENNAS, published in the October, 1947 edition of the Proceedings of
the I.R.E.

The equation for the radiation resistance of short antennas given in
that paper is independent of the resistive losses in any loading coil
or r-f ground system.

RF