The Mobile antenna websites practically tell you to keep the coax at 18 feet, or
else. I thought that was true, until numerous people at this group and several
websites said that is nonsense.

For a NGP antenna, the shield is the counterpoise, and if you change the
length of that, you'll detune the antenna. If you can alter things by
moving the coax around, you've got a problem. Think of coax as a
"signal hose". The RF should stay inside of the coax, not run along
the outside and affect things. If things change, start by fixing your
ground. That's one reason I took my antenna off the magnet. The other
was so that it won't get knocked over by a low branch on the trail.

I immediately noticed the antenna worked better with a real ground.

For a NGP antenna, the shield is the counterpoise, and if you change the
length of that, you'll detune the antenna. If you can alter things by
moving the coax around, you've got a problem. Think of coax as a
"signal hose". The RF should stay inside of the coax, not run along
the outside and affect things. If things change, start by fixing your
ground. That's one reason I took my antenna off the magnet. The other
was so that it won't get knocked over by a low branch on the trail.


And one last thing,

The speed of the signal INSIDE the coax (the velocity factor) is slower
then the speed of the signal OUTSIDE (on the shield). While 17.21 feet
is a quarter wave on the outside of the shield, the inside 1/4 wave is
shorter. If you want to see the actual SWR at the feedpoint, then use
a 1/2 wave electrical length of coax. This will shift the phase of the
mismatch back into it's original position at the other end of the feedline.

(I learned all this stuff while I was still a single bander, and still
laugh at all the ham's that still believe the coax length BS.)

I don't dislike CB. It's another band to use. I dislike all the noise on it now!
If I can't find someone to talk to on one band, I've got others to use now.

BTW, repeaters suck! I've only got to abide by Part 97, not what some control op
thinks his interpretation of the rules are. I'm simplex only these days.

(Sorry, no letters or warnings have been recieved by me! I just said **** it.)
And I didn't say that on the air!!!!!!!!!!!!!

On Tue, 28 Jun 2005 14:36:23 -0400, Scott in Baltimore
wrote:

The speed of the signal INSIDE the coax (the velocity factor) is slower
then the speed of the signal OUTSIDE (on the shield). While 17.21 feet
is a quarter wave on the outside of the shield, the inside 1/4 wave is
shorter. If you want to see the actual SWR at the feedpoint, then use
a 1/2 wave electrical length of coax. This will shift the phase of the
mismatch back into it's original position at the other end of the feedline.

(I learned all this stuff while I was still a single bander, and still
laugh at all the ham's that still believe the coax length BS.)
*****

And I have the biggest laugh because most CBers as well as Hams have a
peanuts view of what a transmission line is or how signals act on and
in them.

First off, while the coax can be inside the field of radiation, the
signal from the transmitter to the antenna travels solely inside the
transmission line. That is between the center conductor and the
shield. The energy transmitted travels in the dielectric and it is the
dielectric that slows the wave down and casue loses. Even the worst
coax, RG-58 has sufficient shield as to not cause leakage through the
shield at 27 MHz. Maybe a 10 GHz. but not 27 MHz.

Common mode currents occur on the shield and are just that currents.
They can come from poor ground connection at the antenna feed point or
can be induced currents due to the coax being within the fear feild
energy of the antenna. Often common mode currents are also rich in
harmonic energy and that is what reradiates and cause TVI and
interference.


james

On Tue, 28 Jun 2005 19:36:53 GMT, james wrote
in :

On Tue, 28 Jun 2005 14:36:23 -0400, Scott in Baltimore
wrote:

The speed of the signal INSIDE the coax (the velocity factor) is slower
then the speed of the signal OUTSIDE (on the shield). While 17.21 feet
is a quarter wave on the outside of the shield, the inside 1/4 wave is
shorter. If you want to see the actual SWR at the feedpoint, then use
a 1/2 wave electrical length of coax. This will shift the phase of the
mismatch back into it's original position at the other end of the feedline.

(I learned all this stuff while I was still a single bander, and still
laugh at all the ham's that still believe the coax length BS.)
*****

And I have the biggest laugh because most CBers as well as Hams have a
peanuts view of what a transmission line is or how signals act on and
in them.


You can say -that- again.....


First off, while the coax can be inside the field of radiation, the
signal from the transmitter to the antenna travels solely inside the
transmission line. That is between the center conductor and the
shield. The energy transmitted travels in the dielectric and it is the
dielectric that slows the wave down and casue loses.


The energy in a coax travels on the conductors -and- in the dielectric
-and- within the magnetic fields. The propogation delay of a line is
the combined phase delays of distributed capacitance -and- distributed
inductance in the line. The dielectric constant only -seems- to be the
determining factor of coax propogation delay because the conductors
are straight. IOW, if you replace the center conductor with a coil you
will introduce an additional propogation delay into the coax which is
-independent- of the dielectric constant (and will have constructed a
device known to us old farts as a 'helical resonantor'). Regardless,
it has no relevance to this discussion.


Even the worst
coax, RG-58 has sufficient shield as to not cause leakage through the
shield at 27 MHz. Maybe a 10 GHz. but not 27 MHz.

Common mode currents occur on the shield and are just that currents.
They can come from poor ground connection at the antenna feed point or
can be induced currents due to the coax being within the fear feild
energy of the antenna.


One of the most misunderstood terms in radio is "common-mode current".
It simply means that current is moving in the same direction, and in
phase, on two or more conductors. It occurs in a coax when current on
the -inside- of the shield is in phase with the current on the center
conductor. Any RF current on the -outside- of a coax has -nothing- to
do with common-mode currents -- it's simply the result of RF spilling
out of the coax or being induced onto it from an external field.


Often common mode currents are also rich in
harmonic energy and that is what reradiates and cause TVI and
interference.


Hogwash. Harmonics don't just appear because of common-mode currents.
They must come from a source -- i.e, the transmitter. And conductors
of common-mode currents don't have any magical properties that let
them conduct or radiate harmonics any better than the fundamental
frequency. That's RF voodoo.







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On Tue, 28 Jun 2005 15:39:57 -0700, Frank Gilliland
wrote:

The energy in a coax travels on the conductors -and- in the dielectric
-and- within the magnetic fields. The propogation delay of a line is
the combined phase delays of distributed capacitance -and- distributed
inductance in the line. The dielectric constant only -seems- to be the
determining factor of coax propogation delay because the conductors
are straight. IOW, if you replace the center conductor with a coil you
will introduce an additional propogation delay into the coax which is
-independent- of the dielectric constant (and will have constructed a
device known to us old farts as a 'helical resonantor'). Regardless,
it has no relevance to this discussion.
*****

Well the dielectric constant does have a direct effect on the
capacitance as well as the spacing between the two conductors. Still
the TEM wave propogates through the dielectric and induces currents in
the center and outer conductor. Propogation of a TEM wave can be
mathematically describe by the Pyonting Vector. The TEM wave is an
alternating E and H field.

The currents induced into the conductors have depth only to that of
sigma or the skin depth. I am not sure a coiled center conductor would
introduce anymore delays than a solid or even stranded center
conductor. On face evidence it would seem that it might but only if
the coil's turns per inch were suffieciently low enough as to not
appear to the traveling wave as a solid conductor.

james

On Wed, 29 Jun 2005 16:17:33 GMT, james wrote
in :

On Tue, 28 Jun 2005 15:39:57 -0700, Frank Gilliland
wrote:

The energy in a coax travels on the conductors -and- in the dielectric
-and- within the magnetic fields. The propogation delay of a line is
the combined phase delays of distributed capacitance -and- distributed
inductance in the line. The dielectric constant only -seems- to be the
determining factor of coax propogation delay because the conductors
are straight. IOW, if you replace the center conductor with a coil you
will introduce an additional propogation delay into the coax which is
-independent- of the dielectric constant (and will have constructed a
device known to us old farts as a 'helical resonantor'). Regardless,
it has no relevance to this discussion.
*****

Well the dielectric constant does have a direct effect on the
capacitance as well as the spacing between the two conductors. Still
the TEM wave propogates through the dielectric and induces currents in
the center and outer conductor. Propogation of a TEM wave can be
mathematically describe by the Pyonting Vector. The TEM wave is an
alternating E and H field.


Well, let's put it this way: the radio and antenna don't connect to
the dielectric of a coax.


The currents induced into the conductors have depth only to that of
sigma or the skin depth. I am not sure a coiled center conductor would
introduce anymore delays than a solid or even stranded center
conductor. On face evidence it would seem that it might but only if
the coil's turns per inch were suffieciently low enough as to not
appear to the traveling wave as a solid conductor.


I'm sure you have studied the lumped-constant equivalent of a
transmission line.....






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On Wed, 29 Jun 2005 13:56:53 -0700, Frank Gilliland
wrote:

I'm sure you have studied the lumped-constant equivalent of a
transmission line.....
****

And Maxwell's equations

james

On Wed, 29 Jun 2005 13:56:53 -0700, Frank Gilliland
wrote:

Well, let's put it this way: the radio and antenna don't connect to
the dielectric of a coax.
*****

No it does not directly.

I know this concept is not easy to see but at the begining of the
coax, one can then consider the energy that travels down the coax as a
TEM wave. It is inside the dielectric where the E and H fields of the
traveling wave can be measured and found.

In transmission lines it is by far easier to think of E and H fields
within the the transmission line. Once that concept is mastered then
the rest is rather easy.

When the wave reaches the end, you have the final induced currents.
You can take a dipole and look at it as if the legs were an extension
of the transmission line. This can better be seen if you consider a
dipole and it is fed with open twin lead. The leads of the dipole then
are an extention of the twin lead except they are now at 90 degrees to
the transmission line.

Current is high when the magnetic field is high. This is so because
the induced current is controlled by the density of the magnetic
field. The E field is high when magnetic field is low. The E field
does not require current but voltage. On a center fed dipole the
impedance is low and the corresponding currents are high. The E field
off teh antenna is also low. As you progress a quarter wave from the
feed point in either direction the H field increases and the E field
decreases. With increasing H field the RF currents induced in the
antenna are high. Thus Ohm's law hald true. Z = I^2*R. Where R is the
radiation resistance of the antenna. The ends of a center fed dipole
are high impedance.

james

On Tue, 28 Jun 2005 15:39:57 -0700, Frank Gilliland
wrote:

One of the most misunderstood terms in radio is "common-mode current".
It simply means that current is moving in the same direction, and in
phase, on two or more conductors. It occurs in a coax when current on
the -inside- of the shield is in phase with the current on the center
conductor. Any RF current on the -outside- of a coax has -nothing- to
do with common-mode currents -- it's simply the result of RF spilling
out of the coax or being induced onto it from an external field.
*******

Yes misunderstood.

I have yet to really see any coax of decent quality that has
suffiecient gaps in the shield to allow a 27 Mhz wave to have
appreciable leakage. Even with 80% coverage the holes in the shield
are so little of a wavelength that I would dare say less than 1/10,000
of the energy of the TEM wave propogating down the coax can "leak"
out.

As for common mode currents the coax itself can have induced currents
in the chield from fields radiated from the antenna. Depending on
where the coax is located to a conducting surface, you can develope
various intensity of currents. Yes you need two conductors minimum to
have comnmon mode. Earth can be one of those conductors.

On Tue, 28 Jun 2005 15:39:57 -0700, Frank Gilliland
wrote:

Often common mode currents are also rich in
harmonic energy and that is what reradiates and cause TVI and
interference.


Hogwash. Harmonics don't just appear because of common-mode currents.
They must come from a source -- i.e, the transmitter. And conductors
of common-mode currents don't have any magical properties that let
them conduct or radiate harmonics any better than the fundamental
frequency. That's RF voodoo.
*****

Harmonics are not there due to just common mode. My mistake there.

What I was thinking and what I wrote were not well alligned.

To generate harmonics off a the shield of the coax from an external
induced current, one needs a means of rectification. That can come
from two dissimilar metals that are not properly electrically
connected. Then the shield can become a radiator of externally induced
currents. It is the diode effect of two dissimilar metals that is the
source of harmonics.

james