"AI4QJ" wrote in
:
"Owen Duffy" wrote in message
...
....
That didn't come out very clearly. As I understand it, Cecil argues
that there is substantial phase change in the forward and reflected
wave components when considering the helix as a transmission line.
I don't know if this is too simplistic but here goes:
The forward wave maintains it's phase as it encounters a new purely
resitive medium (like a straight copper wire or PCB trace). The
reflected wave that goes back into the helix transmission line refects
as a mirror image of the forward wave, just as light would reflect
from a partial mirror. This indeed would be a phase change relative to
the incident wave, approx. 90 degrees TIME or about 62.5 nsec at 4MHz
in the example under consideration. From here we can go on to discuss
the amplitude of the reflected wave and what it takes to cause
resonance and a 1:1 standing wave to appear. The standing wave will
not have a time phase shift so it cannot be used to measure delay, but
voltage and current will of course have a 90 degrees distance phase
shift across the transmission line.
I am open to correction on this.
The seems to me a bit confused, and I can't really understand it.
Looking at the coil Tom used:
If you use the inductance calculator at
http://hamwaves.com/antennas/inductance.html to determine Beta, you get
2.008 rad/m. The coil is .254m long, so that suggests a one-way phase
delay of 0.51 rad, or 29.2°. Some suggest that this coil will replace the
equivalent electrical length in a quarter wave monopole to shorten it
with inductive loading, and irrespective of the position of the loading
coil... which is not consistent with experience that a larger loading
coil is needed the further it is from the base.
One of the proponents posted on eham, the following solution to a loading
coil for 160m: "The VF of a 6" dia., 4 TPI coil on 160m would be about
0.02. Whatever number of degrees you want the coil to occupy, wind it
accordingly.", note the independence of coil size and location on the
monopole.
Owen