On Tue, 28 Jun 2005 22:29:55 GMT, Lancer wrote:

Thanks James;
I don't think my post was a smart ass post. Nor do I think I have
ever made a smart ass post directed at you. I posted what I learned
from the guys in the antenna group. It seems that you know all the
answers, no point in me continuing this conversation with you...

Lance
*****

No I don't know all the answers. Never claimed to.

Nor did I intend the post to be such or a smart ass post. If you took
it that way I apologize for it was not intended to be that.

Just that current in a inner conductor and shield of the coax is
induced by the TEM (Transverse ElectroMagnetic) wave that travels from
the source to the load in the space between the two conductors. That
is a hard concept for many to understand. But once that is understood
then many of the mysteries of transmission lines becomes rather
simple. All to often many think that the signal travels down the
inside and outside conductors. It does not.

james

On Tue, 28 Jun 2005 15:40:40 -0700, Frank Gilliland
wrote:

I think you have that a little misconstrued..... reflection of the
load to the transmitter by a half-wavelength coax is equal to the
-load- regardless of the characteristic impedance of the -coax-.

****

You may be right there. Don't have all my library restored here at my
new place and having to remember.

And Lancer was right, RF on the shield at the feedpoint -will- change
the input impedance of the coax because the shield is no longer
grounded, which is a necessary condition for proper operation of the
coax.
********

1) The impeadance of the coax never changes unless its dielectrtic, or
ratio of outer diameter to inner diamter changes.

2) the reflection coefficient of the load does not change unless there
is a physical change in the load.

3) currents on the shield from other sources will not change the
impedance of the load reflected towards the source.

4) currents on the shield may alter what the source sees as a load
impedance. It may well be possible that the source could see the
antenna and some other load in paralell and then vectorally add the
two impedances.

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.....






----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

On Tue, 28 Jun 2005 23:17:15 -0500, Cecil Moore
wrote in :

Frank Gilliland wrote:
Impedance matching of an SWR meter is generally unimportant since most
SWR meters used for HF have a directional coupler that is much shorter
than the operating wavelength.

Point is that they are usually calibrated for Z0=50 ohms
and are in error when used in Z0 environments differing
from Z0=50 ohms, e.g. Z0=75 ohms.


The point is that the error is insignificant when the directional
coupler is much shorter than the wavelength. The error is even more
insignificant when there are a host of variables and confounds between
the SWR meter and the transmitted field that can (and frequently do)
affect the objective -- field strength. It's much simpler (and just
plain logical) to measure the field strength directly instead of
measuring an abstract value halfway towards the objective and relying
on nothing more than speculation that the rest is working according as
expected.





----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

On Wed, 29 Jun 2005 20:53:42 GMT, james wrote
in :

On Tue, 28 Jun 2005 15:40:40 -0700, Frank Gilliland
wrote:

I think you have that a little misconstrued..... reflection of the
load to the transmitter by a half-wavelength coax is equal to the
-load- regardless of the characteristic impedance of the -coax-.

****

You may be right there. Don't have all my library restored here at my
new place and having to remember.

And Lancer was right, RF on the shield at the feedpoint -will- change
the input impedance of the coax because the shield is no longer
grounded, which is a necessary condition for proper operation of the
coax.
********

1) The impeadance of the coax never changes unless its dielectrtic, or
ratio of outer diameter to inner diamter changes.

2) the reflection coefficient of the load does not change unless there
is a physical change in the load.

3) currents on the shield from other sources will not change the
impedance of the load reflected towards the source.

4) currents on the shield may alter what the source sees as a load
impedance. It may well be possible that the source could see the
antenna and some other load in paralell and then vectorally add the
two impedances.


And #4 is exactly why #1 is incorrect: the 'characteristic' impedance
of a coax is constant, but it's 'input' impedance varies according to
load mismatch at the other end. If it wasn't for this fact, a tuner at
the radio end would be useless. But the point here is that if the SWR
meter is left floating with the coax shield (both of which should be
RF grounded) then the measurement can be darn near anything.






----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

"Frank Gilliland" wrote in message
...
On Tue, 28 Jun 2005 23:17:15 -0500, Cecil Moore
wrote in :

Frank Gilliland wrote:
Impedance matching of an SWR meter is generally unimportant since most
SWR meters used for HF have a directional coupler that is much shorter
than the operating wavelength.

Point is that they are usually calibrated for Z0=50 ohms
and are in error when used in Z0 environments differing
from Z0=50 ohms, e.g. Z0=75 ohms.


The point is that the error is insignificant when the directional
coupler is much shorter than the wavelength.

It is the directional coupler that is balanced for a particular value of Z0.

Tam/WB2TT


The error is even more
insignificant when there are a host of variables and confounds between
the SWR meter and the transmitted field that can (and frequently do)
affect the objective -- field strength. It's much simpler (and just
plain logical) to measure the field strength directly instead of
measuring an abstract value halfway towards the objective and relying
on nothing more than speculation that the rest is working according as
expected.





----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet
News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption
=----

Frank Gilliland wrote:
Point is that they are usually calibrated for Z0=50 ohms
and are in error when used in Z0 environments differing
from Z0=50 ohms, e.g. Z0=75 ohms.

The point is that the error is insignificant when the directional
coupler is much shorter than the wavelength.

Nope, that's not the point at all. It is true that a 50 ohm
SWR meter designed for HF may not work on 70 cm but the error
I'm talking about is the calibration error in a 50 ohm SWR meter
designed for HF and used on HF in, for instance, a Z0 = 450 ohm
environment instead of its calibrated-for 50 ohm environment. It
works perfectly in a 50 ohm environment at the HF frequency of
operation. Here's the proof using a 50 ohm SWR meter:

XMTR--1/2WL 450 ohm line--SWR meter--1/2WL 450 ohm line--50 ohm load

The 50 ohm SWR meter will read 1:1, nowhere near the actual SWR

XMTR--1/4WL 450 ohm line--SWR meter--1/4WL 450 ohm line--50 ohm load

The 50 ohm SWR meter will read 81:1, nowhere near the actual SWR

An SWR meter calibrated for 450 ohms will correctly read 9:1
in both cases.
--
73, Cecil http://www.qsl.net/w5dxp


----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups
---= East/West-Coast Server Farms - Total Privacy via Encryption =---

james wrote:
If the transmitter has a refelction coefficient of zero and the load
say .3, then that reflected power from the load is dissapated as heat
in the output circuits and any final transistors or tubes.

Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.
--
73, Cecil http://www.qsl.net/w5dxp


----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups
---= East/West-Coast Server Farms - Total Privacy via Encryption =---

On Tue, 28 Jun 2005 16:18:56 -0700, Frank Gilliland
wrote:

Yep. And I should add that 18' of coax is recommended not because of
it's propogation characteristics -inside- the coax, but because of
it's velocity factor on the -outside- of the shield which is nearly 1.
IOW, when the shield of an 18' length of coax is grounded only at one
end, that ground will be reflected at the other end of the coax. At
least that's the theory. In practical use it's not perfect, but it's
still better than a fully ungrounded radio or antenna
*****

your reasoning does not pass the common sense rule.

if 18 feet is special because the velocity factor outside the shield
is nearly 1, then no other length has a velocity factor of nearly 1?
What would cause the outside velocity factor to change?

Beisdes I really don't care to rehash this topic to much more. Been
there, done that and don't care to review it right now.

james

On Wed, 29 Jun 2005 16:42:49 -0500, Cecil Moore
wrote:

Sometimes yes, sometimes no. If the reflected current arrives out
of phase with the forward current, then the final dissipation can
actually be *reduced* by the mismatch.
*****

Power is power. Phase is not a problem. Take the mafnitude of the
transmitted power and teh magnitude of the reflected power. The
results are phaseless. The magnitudes add linearly.

QED

james