Jim Kelley wrote:

Cecil Moore wrote:
If there is 75 ohm coax on the input of the Bird, the reflected
power reported by the Bird on the coax will be off by an infinite
percent. That's pretty inaccurate.

Why wouldn't the meter correctly indicate the reflection resulting from
the mismatch between the 50 ohm wattmeter and the 75 ohm transmission line?

The question implies that the mismatch would cause appreciable
reflections. I have not witnessed that happening at HF but
perhaps others have. On MFJ meters, for instance, the one inch
wire through the ferrite toroid probably wouldn't cause an
appreciable mismatch.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:

Jim Kelley wrote:


Cecil Moore wrote:

If there is 75 ohm coax on the input of the Bird, the reflected
power reported by the Bird on the coax will be off by an infinite
percent. That's pretty inaccurate.


Why wouldn't the meter correctly indicate the reflection resulting
from the mismatch between the 50 ohm wattmeter and the 75 ohm
transmission line?


The question implies that the mismatch would cause appreciable
reflections. I have not witnessed that happening at HF but
perhaps others have. On MFJ meters, for instance, the one inch
wire through the ferrite toroid probably wouldn't cause an
appreciable mismatch.

The Bird manual shows a section of 50 ohm transmission line. But even
the N connectors are 50 ohms. I assume even one of those would show up
on a TDR.

ac6xg

Owen Duffy wrote:
No Cecil, the thread subject and the quote of your text in the first
message of the post was the main point of discussion:

Yes, the thread subject was the main point. Z0's other than 50
ohms do not force the V/I ratio to 50 ohms (for matched lines)
and 2% of a 10 MHz WL of RG-213 is of a *sufficient* length to
force the V/I ratio to 50 ohms.

The transmission line length must only be long enough such that
the V/I ratio is forced to the Z0 value. According to some pretty
smart guys I asked, that's about 2% of a wavelength.

I've already apologized for my memory being faulty on that one.
If what you want are additional apologies, you've got it. I
hereby triple dog apologize for my faulty memory. The above
statement was incomplete and was therefore wrong. What more
do you want me to say about it? It was wrong, so of course,
it "is not born out in practice".

It seems that the statement you have quoted is not born out in
practice, though I note that you "seriously doubt that is true".

What I seriously doubted being true is that a 50 ohm Bird wattmeter
has a 0.2 reflection coefficient when placed in a 75 ohm feedline.
I just don't think that 40mm (1.5") is enough length to establish a
50 ohm environment inside the Bird. But I might be wrong on that one.
I know my Autek WM-1 doesn't cause appreciable reflections when
placed in a 75 ohm feedline.

I think, from the beginning, you have confused what Reg said with
what I said. I am basically on your side in the argument.

***Additional Context***
The question of a year ago was not what is the minimum length
required to force V/I to Z0 (although that can be had from Kevin's
posting quoted from s.p.e.) I was arguing with Reg over whether
the lengths of coax into and out of *my* SWR meter forces V/I to
be 50 ohms. I use two foot lengths of RG-400. (That's ~2% of a
wavelength at 10 MHz.) Two feet is a *sufficient* length to cause
V/I to equal Z0 (in a matched line). Two feet is not the *absolute
minimum* length but is instead a *sufficient* length and the length
I use in my shack.
--
73, Cecil http://www.qsl.net/w5dxp

Jim Kelley wrote:
The Bird manual shows a section of 50 ohm transmission line. But even
the N connectors are 50 ohms. I assume even one of those would show up
on a TDR.

Probably would, but would it cause appreciable reflections?
I don't think it would have |rho|=(75-50)/(75+50)=0.2
when used with 75 ohm coax. Owen said something about 40mm.
I just don't think 40mm (1.5") is long enough to establish
a 50 ohm environment but I could be wrong. I've never owned
a Bird. I suspect |rho| would calculate out to be much less
than 0.2. And of course there would be two discontinuities,
one from 75 ohms to 50 ohms and one from 50 ohms back to 75
ohms. Given the short length between those two discontinuities,
one probably interferes with and tends to cancel the effects
of the other as would happen with a zero length of feedline.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
Jim Kelley wrote:

The Bird manual shows a section of 50 ohm transmission line. But even
the N connectors are 50 ohms. I assume even one of those would show
up on a TDR.


Probably would, but would it cause appreciable reflections?
I don't think it would have |rho|=(75-50)/(75+50)=0.2
when used with 75 ohm coax. Owen said something about 40mm.
I just don't think 40mm (1.5") is long enough to establish
a 50 ohm environment but I could be wrong. I've never owned
a Bird. I suspect |rho| would calculate out to be much less
than 0.2. And of course there would be two discontinuities,
one from 75 ohms to 50 ohms and one from 50 ohms back to 75
ohms. Given the short length between those two discontinuities,
one probably interferes with and tends to cancel the effects
of the other as would happen with a zero length of feedline.

Ummm, I don't know, Cecil......sounds a little fishy to me.

ac6xg

Jim Kelley wrote:
Ummm, I don't know, Cecil......sounds a little fishy to me.

Let's say you had a one inch piece of 50 ohm coax in the
middle of a longer piece of 75 ohm coax. I'll bet the
reflections from one discontinuity would virtually
cancel the reflections from the other discontinuity.
After all, the math should work if the length of the
50 ohm piece is zero, or 1/2WL, or 1WL, or ...

In fact, a one inch piece of 50 ohm coax should cause
the same steady-state interference conditions as a (one
inch plus one wavelength) piece of 50 ohm coax. Don't
you agree?
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:

Jim Kelley wrote:

Ummm, I don't know, Cecil......sounds a little fishy to me.


Let's say you had a one inch piece of 50 ohm coax in the
middle of a longer piece of 75 ohm coax. I'll bet the
reflections from one discontinuity would virtually
cancel the reflections from the other discontinuity.
After all, the math should work if the length of the
50 ohm piece is zero, or 1/2WL, or 1WL, or ...

In fact, a one inch piece of 50 ohm coax should cause
the same steady-state interference conditions as a (one
inch plus one wavelength) piece of 50 ohm coax. Don't
you agree?

So basically you want me to agree that it's ok to use 75 ohm connectors
in a 50 ohm system - as long as you use two of them separated by 0, 1/2,
or 1 (etc) wavelength. Could be true, I suppose. Still, I'd rather you
tried it out on your radio first. :-)

ac6xg

Jim Kelley wrote:
So basically you want me to agree that it's ok to use 75 ohm connectors
in a 50 ohm system ...

Heck, you probably already use PL-239's. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:

Jim Kelley wrote:

So basically you want me to agree that it's ok to use 75 ohm
connectors in a 50 ohm system ...


Heck, you probably already use PL-239's. :-)

When I use them, they're effectively 50 ohm connectors. What are they
when you use them, Cecil? :-)

ac6xg

Jim Kelley wrote:
When I use them, they're effectively 50 ohm connectors. What are they
when you use them, Cecil? :-)

PL-259's are notorious for not being 50 ohms.
--
73, Cecil http://www.qsl.net/w5dxp