Current across the antenna loading coil - from scratch
 

Gene, W4SZ wrote:
"However, the physical entities do not have two values at once in the
same time and place."

Richard Harrison wrote:
You can measure each of the two simultaneous constituents with the right
equipment. A Bird Thruline wattmeter uses a directional coupler to
separate forward direction power from reverse direction power. These are
obbtainable at the same time and place anywhere in a 50-ohm coax line.
Individual volts and amps in each direction are easily calcuable from
the powers indicated in each direction.

That's not true.

The directional coupler in a Bird meter samples the across vector
(voltage) from a capacitive divider and adds it to a sample voltage of
the through vector (current) from a current transformer in a
predetermined ratio. After that sum, the output is rectified.

I can place it in a system with NO standing waves and it will show
standing waves. I can place it in a system with standing waves and have
it show NO standing waves. It does not measure standing waves, it
simply measures the ratio and phase of voltage and current at one point
in the transmission line.

There can NEVER be current flowing at that point in two directions at
the same instant of time, and the Bird does not even contain a system
that samples standing waves.

Now I can build a piece of test gear that does directly read standing
waves, but it requires a line sampling lwength of at least 1/4 wl.
Such a device would be totally independent of the actual operating
impedance, and could read either current or voltage.

The Bird meter is NOT that type of unit.

73 Tom

Current across the antenna loading coil - from scratch
 

Apparently the sentiment

On Mon, 10 Apr 2006 23:05:48 -0400, "Yuri Blanarovich" wrote:
You don't want to explain W5DXP models and answer his questions.

bears no correlation to

On Tue, 11 Apr 2006 04:08:34 GMT, Cecil Moore wrote:
BOTTOM LINE: Until you can prove that a mobile antenna is 90
degrees long, your argument is just another straw man.

that is of any significance to "Current across the antenna loading
coil - from scratch"

Current across the antenna loading coil - from scratch
 

On Mon, 10 Apr 2006 16:04:56 -0400, Michael Coslo
wrote:

Not that I could fan the flames any more anyhow, but just what was the
original discussion about anyhow?

Hmmm, Mike, I bet you didn't find a pony in that pile of "responses"
did you?

73's
Richard Clark, KB7QHC

Current across the antenna loading coil - from scratch
 

Cecil Moore wrote:
Gene Fuller wrote:
(snip)
C'mon, you know as well as anybody that inductance of a coil tends to
increase as n-squared. Yes, there are all kinds of special cases and
correction factors.


Increasing the length of a coil or transmission line doesn't
change its velocity factor at a fixed frequency.
(snip)

That is an interesting hypothesis.

How would you go about testing its validity?

(Have you heard of end effects?)

Current across the antenna loading coil - from scratch
 

wrote:
I can place it in a system with NO standing waves and it will show
standing waves. I can place it in a system with standing waves and have
it show NO standing waves. It does not measure standing waves, it
simply measures the ratio and phase of voltage and current at one point
in the transmission line.

Yes, but if it is calibrated for 50 ohms and installed in a 50 ohm
environment, it is indeed accurately reporting forward and reflected
power. It is accurately separating the forward wave energy from the
reflected wave energy. If it weren't, Bird would be guilty of false
advertising.
--
73, Cecil http://www.qsl.net/w5dxp

Current across the antenna loading coil - from scratch
 

John Popelish wrote:

Cecil Moore wrote:
Increasing the length of a coil or transmission line doesn't
change its velocity factor at a fixed frequency.

That is an interesting hypothesis.

Since I know you are going to nit-pick that statement, I
probably should add "appreciably" in front of "change". :-)

How would you go about testing its validity?

The velocity factor of a piece of transmission line doesn't
change appreciably with length. The velocity factor of a
straight wire doesn't change appreciably with length.

I would think that a two wavelength coil would be approximately
twice as long as a one wavelength coil which would be
approximately twice as long as a 1/2 wavelength coil.

The equation for the velocity factor of a coil depends upon:
1. The diameter of the coil
2. The number of turns per unit length
3. The frequency

None of those factors are dependent upon the length of the
coil.

(Have you heard of end effects?)

Of course, it's the 5% difference between 468/f and
492/f. I'm not talking super accuracy here - just better
accuracy than anyone has yet measured.

It is akin to your suggestion that a coil be installed
between two current nodes and its number of degrees
calculated from that. I will try to take that same
coil that I have been talking about and use your
suggestion to see how close the results are.

However, I am preparing for a 6 state Harley road trip over
the Easter holidays and will not be back until Monday.
--
73, Cecil http://www.qsl.net/w5dxp

Current across the antenna loading coil - from scratch
 

Cecil Moore wrote:
(snip)
However, I am preparing for a 6 state Harley road trip over
the Easter holidays and will not be back until Monday.

I envy you. I haven't been on a decent motorcycle ride since my
12,000 mile loop from Virginia to Alaska, and back, in May, 2002.

Kill a few bugs for me.

Current across the antenna loading coil - from scratch
 

Tom, W8JI wrote:
"The directional coupler in a Bird meter samples the across vector
(voltage) from a capacitive divider and adds it to a sample voltage of
the through vector (current) from a current transformer in a
predetermined ratio.

That`s close. In the cartridges is a loop terminated in a diode.
Capacitive coupling of the loop to the center conductor of the precision
ccoax supplies the voltage sample. Inductive coupling of the loop
supplies the current sample.

I`ve described operation several times here and once in this thread, so
I won`t repeat it.

SWR is easy to get from the forward and reflected indications of the
wattmeter.

VSWR = 1 + sq.rt. (ref. PWR / for. PWR)
Divided by 1 - sq.rt. (ref. PWR / for. PWR)

Bird supplies a family of VSWR lines on a graph of forward power vs.
reflected power for those who would avoid the calculation. They can also
supply a slide-rule to do rhe same.

Best regards, Richard Harrison, KB5WZI

Current across the antenna loading coil - from scratch
 

Reg Edwards wrote:
"K7ITM" wrote


W8JI-Tom, Tom
Donaly, Ian White, Roy Lewallen, Gene Fuller, Reg Edwards, I, and
others I can think of are NOT, repeat NOT, absolutely NOT, most
definitely NOT, talking about a lumped-circuit model.

==========================================

I do wish you would't take my name in vain about what I might have
said or not said. I have in fact said little or nothing about lumped
circuits or anything else in this stupid, ridiculous argument. Please
don't drag me down to your level.
----
Reg Edwards.



If the argument is that ridiculous, why do you continue
to read it?
73,
Tom Donaly, KA6RUH

Current across the antenna loading coil - from scratch
 

Responding to no one in particular.

This is starting to make me miss the shorter Fractenna threads.

Phil, comeonback good buddy.