RadioBanter - View Single Post

March 9th 06, 06:20 PM posted to rec.radio.amateur.antenna

David Shrader wrote:
wrote:

SNIPPED

If I have a 10 degree tall base loaded antenna it is a ten degree tall
antenna. It is NOT 90 degree resonant antenna with "80 degrees of
missing length" in the inductor, nor with that 80 degree long inductor
behave like 80 degrees of antenna length would.

I beg to differ.

If I have a 15 degree long physical antenna, center loaded at 10
degrees, with 5 degrees above the coil I do have a 15 degree physical
antenna. That does not mean the antenna is NOT 90 degrees elctrically
long! Resonance requires that the reactive components cancel both in
amplitude and phase! Each reactive component introduces phase shift into
the system. The antenna, without a loading coil, is composed of three
terms: resistance [radiation and loss], self capacitance, and self
inductance. In a shortened antenna the self capacitance dominates and
the resultant phase shift is NOT zero. It is required to add inductance
to achieve resonance [phase shift = 0]. If an antenna is electrically 15
degrees long and the self inducance does not reduce the reactive phase
shift to zero PHASE SHIFT MUST BE ADDED TO THE ANTENNA for resonance.
This phase shift is accomplished by the loading coil.

Now, when that antenna is fed with 1 ampere [Imax] at the base of the
antenna and the feed current follows a cosine distribution to the base
of the coil [I = Imax*cos(theta)][theta=10], you claim that the current
exiting the coil is also Imax*cos(theta), or 98.5% of max value.

However, if we start with zero current at the tip, a valid initial
condition, and let current increase by a sine function then I =
Imax*sin(theta1][theta1 = 5 degrees] The result is simply 9% of max
value. There seems to be a disconnect here. 98.5% = 9% ?????????

If you claim Imax @ 98.5% exits the coil and has a value of 96.6% [I2]
at the tip then boundary conditions require total reflection. That
requires a 180 degree phase reversal at the I2 amplitude to satisfy the
boundary condition. Now the reflected current into the top of the coil
is -I3 = 99.6% of -I2. The reflected current exiting the bottom of the
coil is, by your reasoning -I3. The reflected current at the base is
96.6% of -I2, or 93% of Imax.

If I understand you correctly, then the measured value at the base of
the 15 degree antenna is NOT 1 ampere but only 0.07 amperes.

Obviously, the coil is acting as something more than a simple L. It is
adding and inductive phase shift. The vertical has capacitance to the
local ground. The vertical also has a self inductance!! That self
inducatance is insufficient to complete the 90 phase shift required for
resonance. Therefore, I offer that the loading coil provides the
required additional inductance for resonance.

You can replace the antenna with a box containing a series resistor and
capacitor, and except for the field there's no steady state way to tell
it from an antenna. A physically small inductor such as a toroid will
function exactly the same in both cases. (I limited it to being
physically small, since a larger inductor will interact with the
antenna's field.) So your explanation should work just as well when the
inductor is in series with a simple RC as when it's in series with an
antenna. I can easily write the equations describing the voltage and
current at every part of the resulting RLC circuit, using circuit
analysis techniques which have been around for over a century. You're
saying either that they're wrong, or that you can tell by looking at the
terminals of a black box whether it contains an antenna or a simple RC.
Can you describe the method you'd use, restricting yourself to steady
state measurements, how you'd tell the difference?

Roy Lewallen, W7EL