Hi,

This is a hypothetical question, if one needs to send more power down
a line than its capacity can 2 or more lines be paralleled? Is the
equivalent characteristic impedance of the combined lines the same as
that of a single individual line?

A*s*i*m*o*v

.... The bird of paradise alights only upon the hand that does not grasp.

On Thu, 02 Jun 2005 21:00:00 -0400, Asimov
wrote:

Hi,

This is a hypothetical question, if one needs to send more power down
a line than its capacity can 2 or more lines be paralleled? Is the
equivalent characteristic impedance of the combined lines the same as
that of a single individual line?

A*s*i*m*o*v

... The bird of paradise alights only upon the hand that does not grasp.


The short answer is no to the question of the impedence being the same.
though the current would be halfed in each line. I'm not sure that I have
see a four wire open line in the past but if my memory is correct it fed
two dipoles not one.
73 Dave kc1di




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Asimov wrote:
This is a hypothetical question, if one needs to send more power down
a line than its capacity can 2 or more lines be paralleled? Is the
equivalent characteristic impedance of the combined lines the same as
that of a single individual line?

Let's say we parallel two pieces of lossless 50 ohm coax such
that the voltages at the load (V1 and V2) are equal magnitude
and phase. The currents at the load (I1 and I2) will be equal
magnitude and phase.

For matched line operation, V1/I1 = V2/I2 = 50 ohms.
Pload = (V1*I1)+(V2*I2) = 2*(V1*I1)
Vload = V1 = V2
Iload = I1 + I2
Rload *for matched line operation* = V1/(I1+I2) = V1/(2*I1).
Rload = 1/2(V1/I1) = 50/2 = 25 ohms. So 25 ohms is the
equivalent Z0 of two pieces of 50 ohm coax in *parallel*.

Conversely, if you use one piece of 50 ohm coax going to the
load and one piece of 50 ohm coax coming back from the load
to achieve a shielded balanced feedline, the Z0 of that
feedline is 2*Z0 = 100 ohms and the two lines are in *series*.
--
73, Cecil http://www.qsl.net/w5dxp

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KC1DI wrote:
The short answer is no to the question of the impedence being the
same. though the current would be halfed in each line. I'm not sure
that I have see a four wire open line in the past but if my memory is
correct it fed two dipoles not one.

An interesting article on such is "Quadro-Line: The Ideal
UHF Transmission Line", ARRL Antenna Compendium, Vol. 3.
It's four box-spaced wires in a parallel type transmission
line. Two adjacent wires have a Z0 of 200 ohms. Paralleling
the four wires yields a Z0 of 100 ohms. Choosing two diagonal
wires yields a Z0 of 250 ohms.
--
73, Cecil http://www.qsl.net/w5dxp

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For 60hz power lines there's no problem, and impedence isn't usually an
issue,
but since we're on an antenna group, we assume you're referring to RF
transmission line.
If you parallel 2 lines - the overall impedence halves
If you run 2 lines in series (which looks like parallel) its like having
twinlead made from coax - and the impedence is doubled.
I've used combinations of both for making matching sections.

The hand of paradise lights the birds only.
A*b*a*c*a*d*a*b*a
..-- ....- .--. -- .---
dadadidahdidah

"Asimov" wrote in message
...
Hi,

This is a hypothetical question, if one needs to send more power down
a line than its capacity can 2 or more lines be paralleled? Is the
equivalent characteristic impedance of the combined lines the same as
that of a single individual line?

A*s*i*m*o*v

... The bird of paradise alights only upon the hand that does not grasp.

As others have said, yes, you can run two parallel lines. An easy way
to think about the impedance: assume the lines are independent, and
each has a matched load equal to Zo. Parallel them, and the load is
Zo/2. Since a (uniform) line with matched load presents a Zo load to
the source, the source with two such parallel lines will see a Zo/2
load.

BUT...BEWARE! What if the two lines are not equal electrical lengths?
The results may surprise you. Details are left as an exercise for the
reader. You can use standard transmission line equations, or use a
program like RFSim99, to see what happens. Try, for example, a couple
of lengths of lossless 100 ohm line with v.f. = .66, one of length =
100 feet and one of length = 101 feet. (I'm curious what others get
for results...)

Cheers,
Tom

On Tue, 31 May 2005 23:00:00 -0500, Asimov wrote:

Hi,

This is a hypothetical question, if one needs to send more power down a
line than its capacity can 2 or more lines be paralleled?

The 4 wire transmission line is described in the
ARRL Electronics Data Book. And the ARRL
Antenna Handbook.

73, Ed. EI9GQ.


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On 1 Jun 2005 19:19:39 -0700, "K7ITM" wrote:

As others have said, yes, you can run two parallel lines. An easy way
to think about the impedance: assume the lines are independent, and
each has a matched load equal to Zo. Parallel them, and the load is
Zo/2. Since a (uniform) line with matched load presents a Zo load to
the source, the source with two such parallel lines will see a Zo/2
load.

BUT...BEWARE! What if the two lines are not equal electrical lengths?
The results may surprise you. Details are left as an exercise for the
reader. You can use standard transmission line equations, or use a
program like RFSim99, to see what happens. Try, for example, a couple
of lengths of lossless 100 ohm line with v.f. = .66, one of length =
100 feet and one of length = 101 feet. (I'm curious what others get
for results...)

I guess this was too much work for anyone else [g].

There is a repeating pattern, but at the first frequency of ~3.2296
MHz the input Z -- infinity and the phase changes sign. It looks
like a parallel resonant circuit.

Thanks, Wes. And in case anyone missed it, since the input impedance
goes to infinity, you're delivering no power to the input, so there is
no voltage across the load. The transmission is a comb of notches,
spaced in frequency by (speed of light)*(v.f.)/(total length of line)
-- in this case, 3.2296MHz as Wes says. In other words, the first
notch and the notch spacing is equal to the frequency at which the
total line length is one wavelength. If the difference between the two
line sections is small and they have loss, the notch is not very deep.
If you could actually get lossless line, it's a way to make very
narrow, very deep notches, but then if you could get lossless line, you
could probably also get lossless capacitors and inductors and make
deep, narrow notches with them.

On the other hand, if you have a wavelength of line connected in a loop
and put a load 1/4 wave from a source (which is 3/4 wave around the
other direction), you get quite a good notch even if the line is
slightly lossy. Look up "hybrid junctions" for more info.

Cheers,
Tom

Good explanation.

You might add that this applies to coaxial libes whose fields do not
interact. For open wire line the picture is considerably different
because the eleccctric and magnetic fields overlap.

Jim

On Wed, 01 Jun 2005 07:50:06 -0500, Cecil Moore
wrote:

Asimov wrote:
This is a hypothetical question, if one needs to send more power down
a line than its capacity can 2 or more lines be paralleled? Is the
equivalent characteristic impedance of the combined lines the same as
that of a single individual line?

Let's say we parallel two pieces of lossless 50 ohm coax such
that the voltages at the load (V1 and V2) are equal magnitude
and phase. The currents at the load (I1 and I2) will be equal
magnitude and phase.

For matched line operation, V1/I1 = V2/I2 = 50 ohms.
Pload = (V1*I1)+(V2*I2) = 2*(V1*I1)
Vload = V1 = V2
Iload = I1 + I2
Rload *for matched line operation* = V1/(I1+I2) = V1/(2*I1).
Rload = 1/2(V1/I1) = 50/2 = 25 ohms. So 25 ohms is the
equivalent Z0 of two pieces of 50 ohm coax in *parallel*.

Conversely, if you use one piece of 50 ohm coax going to the
load and one piece of 50 ohm coax coming back from the load
to achieve a shielded balanced feedline, the Z0 of that
feedline is 2*Z0 = 100 ohms and the two lines are in *series*.