On Aug 30, 5:12 am, Richard Fry wrote:
On Aug 29, 6:34 pm, Cecil Moore wrote:
However an assumption might be taken from some posts here that a short
vertical radiator loaded to resonance is the full electrical
equivalent of an unloaded, resonant vertical of about 1/4-wavelength,
while it is not. That is my point.

The short vertical radiator loaded to resonance *IS* the full
*electrical* length of an unloaded, resonant vertical of about 1/4WL,
which is related to the feedpoint impedance. It is NOT the full
*physical* length which is related to radiation resistance and
efficiency.

The feedpoint impedance of any electrically long 90 degree standing-
wave antenna, including resonant loaded mobile antennas, is:

Zfp = (Vfor-Vref)/(Ifor+Iref) on the antenna, not on the feedline.

The reflected voltage has undergone a 180 degree phase shift. The
reflected current has undergone a 360 degree phase shift. Part of the
phase shift occurs in the loading coil. A typical resonant mobile
antenna is *electrically* 90 degrees long. If it was less than 90
degrees long *electrically* it would exhibit capacitive reactance at
the feedpoint.

Let's discuss a base-loaded configuration which is less complicated
than a center-loaded configuration. (1) The delay through the loading
coil is part of that 90 degrees. (2) The delay through the stinger is
part of that 90 degrees. (3) The phase shift at the coil to stinger
junction is part of the 90 degrees. Tom, W8JI, assumes a lumped
inductor for calculating the phase shift at the coil to stinger
junction but a 75m bugcatcher loading coil is NOT a lumped inductor -
it is a distributed network existing in the real world with an
associated real-world delay through the coil.

The other rail of the argument assumes all of the "missing degrees"
come from the coil and none from the coil to stinger junction. Both
sides are wrong. All three phase shift components listed above exist
in a base-loaded mobile antenna.

(There are four phase shift components in a center-loaded mobile
antenna. Degrees of electrical length are actually lost at the low Z0
base section to high Z0 loading coil junction. That's why the
inductance (coil delay) has to increase for center-loaded
configurations.)

Interestingly enough, a base-loaded mobile antenna functions like the
dual-Z0 stubs covered on my web page and can be analyzed in the same
manner:

http://www.w5dxp.com/shrtstub.htm

Here is a simplified approximate representation of what a base-loaded
mobile antenna looks like electrically:

FP------Z01=5000 ohms------+------Z02=500 ohms------

The Z01 portion is the base loading coil and the Z02 portion is the
stinger. The Z0 of the loading coil can be obtained from the
inductance calculator at:

http://hamwaves.com/antennas/inductance.html
--
73, Cecil, w5dxp.com