shorted 1/8 wave transmission line
 

Hello Group,

If I have an RG-58 coax and it is shorted at the load end. At the electrical
1/8 wave lenght what would be the impedance seen at the other end?

I understand that a shorted 1/4 wave length reflects an open, but was
interested in what happens at the 1/8 wave frequency.

Tnx de KJ4UO

The impedance would be +j50 ohms, that is, 50 ohms of inductive
reactance. If you open the far end, you'll see -j50 ohms, that is, 50
ohms of capacitive reactance, at the input.

Roy Lewallen, W7EL

PDRUNEN wrote:

Hello Group,

If I have an RG-58 coax and it is shorted at the load end. At the electrical
1/8 wave lenght what would be the impedance seen at the other end?

I understand that a shorted 1/4 wave length reflects an open, but was
interested in what happens at the 1/8 wave frequency.

Tnx de KJ4UO

PDRUNEN wrote:
If I have an RG-58 coax and it is shorted at the load end. At the electrical
1/8 wave lenght what would be the impedance seen at the other end?

I understand that a shorted 1/4 wave length reflects an open, but was
interested in what happens at the 1/8 wave frequency.

For lossless coax, it would be purely reactive, close to +jZ0. Questions
like that are easy when one understands the Smith Chart.
--
73, Cecil http://www.qsl.net/w5dxp



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Roy,

Many years ago an article was published showing nomographs for using 1/8
wave lines for universal impedance matching. Do you recall where and when
that appeared, by any remote chance? I know you can do all this with
computer programs now, but I can't easily carry a computer or software into
many of the places I go.

--
Crazy George
W5VPQ
Remove N O and S P A M imbedded in return address

"Roy Lewallen" wrote in message
...
The impedance would be +j50 ohms, that is, 50 ohms of inductive
reactance. If you open the far end, you'll see -j50 ohms, that is, 50
ohms of capacitive reactance, at the input.

Roy Lewallen, W7EL

PDRUNEN wrote:

Hello Group,

If I have an RG-58 coax and it is shorted at the load end. At the
electrical
1/8 wave lenght what would be the impedance seen at the other end?

I understand that a shorted 1/4 wave length reflects an open, but was
interested in what happens at the 1/8 wave frequency.

Tnx de KJ4UO

Crazy George wrote:
... I can't easily carry a computer or software into
many of the places I go.

Everyone else does.



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The input reactance of a shorted length of line is given by -

Inductive Xin = j * Zo * Tangent(Theta)

where Theta is line length in degrees.

For a 1/8-wavelength line Theta = 360/8 = 45 degrees, Tangent(45) = 1.0,
and for Zo = 50 ohms the input inductive reactance is also 50 ohms.

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

For an open circuit length of line -

Capacitative Xin = -j * Zo / Tangent(Theta)

So for open-circuit 1/8-wavelength line and Zo = 50 ohms, input reactance is
a capacitative -j50 ohms.

The resistive component of input impedance is very small because line loss
is very small for 1/8 wavelength.
----
Reg, G4FGQ

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

"PDRUNEN" wrote in message
...
Hello Group,

If I have an RG-58 coax and it is shorted at the load end. At the
electrical
1/8 wave lenght what would be the impedance seen at the other end?

I understand that a shorted 1/4 wave length reflects an open, but was
interested in what happens at the 1/8 wave frequency.

Tnx de KJ4UO

Sorry, I don't recall having seen that.

Roy Lewallen, W7EL

Crazy George wrote:

Roy,

Many years ago an article was published showing nomographs for using 1/8
wave lines for universal impedance matching. Do you recall where and when
that appeared, by any remote chance? I know you can do all this with
computer programs now, but I can't easily carry a computer or software into
many of the places I go.

--
Crazy George
W5VPQ
Remove N O and S P A M imbedded in return address

Reg, let me guess: you dug that Smith Chart out after all huh? he he he

Btw I had a Power Quality engineer that I was discussing ground line
impedances with remind me that the same 1/4 wave phenomenon can happen in
runs of ground and bonding too. The same radial or parallel or "web" of
connections alleviates that risk with lightning grounding as it does with RF
grounding.

Jack

"Reg Edwards" wrote in message
...
The input reactance of a shorted length of line is given by -

Inductive Xin = j * Zo * Tangent(Theta)

where Theta is line length in degrees.

For a 1/8-wavelength line Theta = 360/8 = 45 degrees, Tangent(45) = 1.0,
and for Zo = 50 ohms the input inductive reactance is also 50 ohms.

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

For an open circuit length of line -

Capacitative Xin = -j * Zo / Tangent(Theta)

So for open-circuit 1/8-wavelength line and Zo = 50 ohms, input reactance
is
a capacitative -j50 ohms.

The resistive component of input impedance is very small because line loss
is very small for 1/8 wavelength.
----
Reg, G4FGQ

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

"PDRUNEN" wrote in message
...
Hello Group,

If I have an RG-58 coax and it is shorted at the load end. At the
electrical
1/8 wave lenght what would be the impedance seen at the other end?

I understand that a shorted 1/4 wave length reflects an open, but was
interested in what happens at the 1/8 wave frequency.

Tnx de KJ4UO

For lossless coax, it would be purely reactive, close to +jZ0. Questions
like that are easy when one understands the Smith Chart.
--
73, Cecil

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

They are even easier WITHOUT the unessessary over-complication of the chart.

A student's valuable time is much better spent learning about transmissiom
lines instead of how to use an antique chart. The answer can be worked out
in the head in less than a second. Tan(45) = 1.0000000 and therefore X =
Zo.

Sorry to hear about your pH problem. Isn't there an ant-acid preparation
available in this modern day and age?

----
Reg, G4FGQ

"Jack Painter"
snip
Btw I had a Power Quality engineer that I was discussing ground line
impedances with remind me that the same 1/4 wave phenomenon can happen in
runs of ground and bonding too. The same radial or parallel or "web" of
connections alleviates that risk with lightning grounding as it does with
RF
grounding.

Jack

Jack:

Alleviates? NO! Reduces somewhat, maybe, and that is one of the most
difficult things to get across to people with little theoretical knowledge.
Even if it were the perfectly conducting sphere so loved in textbooks,
impedance still exists, and the instantaneous voltage at point A will be
different from point B. All you are doing is increasing the current
carrying capability, so it is less likely to blow up due to a direct strike.
Instantaneous voltage difference with respect to a remote reference can
still rise to a gazillion volts, no matter how much copper you put in there,
or how you configure it.

--
Crazy George
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