"Cecil Moore" wrote
Reg, I dug up some calculations from sci.physics.electromag
from about a year ago that indicate one foot of 50 ohm coax
on each side of the Bird is enough to make the line real,
i.e. not imaginary, and that's a conservative estimate.

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

Cec, you forgot to say sci.physics.electromag were working at 500 MHz
and above. The one and only so-called SWR meter I have stops at 30
MHz. ;o)
----
Reg.

Cecil Moore wrote:
Ian White G/GM3SEK wrote:
All agreed. Along with the math that Cecil has retrieved and quoted
again, everything points towards the distance in question being a
function of coax diameter only; and not wavelength.

Please forgive my previous senior moment.
It was ~2% of a wavelength at 10 MHz for RG-213.
It appears that one foot of coax on each side of
a Bird wattmeter is enough to establish Z0 at
50 ohms which forces Vfor/Ifor=Vref/Iref=50,
the necessary Bird boundary conditions.

The Bird doesn't require any upstream and downstream boundary
conditions. You can insert the instrument between any source impedance
and any load impedance, and what it reports is entirely about the load
impedance, unaffected by the source impedance.

However, it was scaled and calibrated assuming a 50 ohm system reference
impedance, so in order to read correctly, it requires you to agree that
your system reference impedance is 50 ohms too.

The element is trying to sample the voltage and current at a single
point along the instrument's internal line. Because that line is
physically quite long, it is built as an accurate 50-ohm line so that
the instrument will cause minimal disturbance when inserted somewhere
along a 50-ohm cable.


--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On Tue, 11 Oct 2005 16:57:51 GMT, Cecil Moore wrote:


****Quote****
Newsgroups: sci.physics.electromag
From: "Kevin G. Rhoads"
Date: Tue, 07 Oct 2003 12:49:14 -0400
Subject: Transmission Line Question
....
So unless almost all the power diverts into an undesireable
mode (by a factor of more than a million to one), one foot
of cable should see pure TEM at the end.
***End Quote***

But Cecil, nowhere in the analysis you quoted does it estimate how
much power is diverted from the dominant mode at the discontinuity.

If the explanation of the discontinuity is that some power is
converted from dominant propagation mode to another mode, and that
those other modes are evanescent, it seems that this analysis of the
impact of the discontinuity considers only estimating the decay rate
of the evanescent modes without estimation of the relative magnitude
of the power diverted to those modes in the first place.

Owen
--

Reg Edwards wrote:
Let this little anecdote be a friendly warning to they who use meters
with a 0 to infinity SWR scale, or scaled in terms of forward and
reverse power.

If the scale were linearly calibrated for |rho| = 0 to 1,
would you still be objecting?

SWR = (1+|rho|)/(1-|rho|) for 0 = |rho| = 1

When SWR = infinity, it doesn't mean infinite current
through the meter. It just means that Vref = Vfor
where |Vref|/|Vfor| = |rho|. My Heathkit HM-15 allows
for full scale to equal Vfor. Then Vref is applied.
The scale is a linear indication for |rho|. The
corresponding SWR scale just follows the above equation.
--
73, Cecil http://www.qsl.net/w5dxp

Reg Edwards wrote:
Cec, you forgot to say sci.physics.electromag were working at 500 MHz
and above.

Not true, Reg. My question was specified using RG-213 at 10 MHz.
--
73, Cecil http://www.qsl.net/w5dxp

Ian White G/GM3SEK wrote:
The Bird doesn't require any upstream and downstream boundary
conditions.

When Bird requires a 50 ohm environment, they are requiring
50 ohm boundary conditions for the reading to be valid. If
you install the Bird in a 450 ohm environment on both sides
of the wattmeter, for instance, it will NOT read a valid forward
power and reflected power. In a matched-line 450 ohm environment
with absolutely zero reflected power, the Bird will indicate an
SWR of 9:1, a |rho| of 0.8 and a ratio of reflected power to
forward power of 0.64 even when the reflected power is zero.
--
73, Cecil http://www.qsl.net/w5dxp

Owen Duffy wrote:
If the explanation of the discontinuity is that some power is
converted from dominant propagation mode to another mode, and that
those other modes are evanescent, it seems that this analysis of the
impact of the discontinuity considers only estimating the decay rate
of the evanescent modes without estimation of the relative magnitude
of the power diverted to those modes in the first place.

I've always had a rule of thumb that 100 times the diameter of
the coax was enough length to 99% establish the TEM mode so
Kevin's explaination made sense to me.

Apparently, you are not satisfied with Kevin's explaination. Kevin
Rhodes email address is available on Google. The reason not to
publish it here is to avoid spaming his email account.
--
73, Cecil http://www.qsl.net/w5dxp

On Tue, 11 Oct 2005 21:15:22 GMT, Cecil Moore wrote:


Not true, Reg. My question was specified using RG-213 at 10 MHz.

True enough, but in the context of the question as to whether the Bird
43 reads sufficiently accurately, the transmission line on which one
is interested in the decay of the evanescent modes is the Bird
Thruline coupler section, not Rg-213 or any other cable that might be
attached to the Bird.

Owen
--

On Tue, 11 Oct 2005 21:25:53 GMT, Cecil Moore wrote:

Ian White G/GM3SEK wrote:
The Bird doesn't require any upstream and downstream boundary
conditions.

When Bird requires a 50 ohm environment, they are requiring
50 ohm boundary conditions for the reading to be valid. If
you install the Bird in a 450 ohm environment on both sides
of the wattmeter, for instance, it will NOT read a valid forward
power and reflected power. In a matched-line 450 ohm environment
with absolutely zero reflected power, the Bird will indicate an
SWR of 9:1, a |rho| of 0.8 and a ratio of reflected power to
forward power of 0.64 even when the reflected power is zero.

(I am assuming your 450 ohm line to be an unbalanced line, impractical
as that is, but the issues of balance to unbalanced transition are
just noise to the discussion.)

Is this about whether the Bird readings are correct for the conditions
on the Bird Thruline, or whether the meter readings are extensible to
the adjacent transmission line without further interpretation /
modelling?

The Bird readings should be correct for the conditions on the Bird
Thruline. You can safely extend those measurements literally to the
adjacent line where the adjacent line is the same as the Bird Thruline
and of negligible loss. In other cases, knowing the line parameters,
you may be able to use the measurements to some extent to calculating
some conditions on the other line.

Though the Bird readings in your example for Forward and Reflect Power
cannot be assumed valid for the adjacent line, the net power should be
correct.

I don't think anyone is suggesting that the Bird could be used in a
general sense to estimate the VSWR on your 450 ohm line.

Owen
--

On Tue, 11 Oct 2005 14:09:03 -0400, "Fred W4JLE"
wrote:

I would then assume you disregard anything written in books as it falls in
the same category.

Hi Fred,

Certainly anything that is third hand and name dropping - but you
already knew that from my previous posting.

73's
Richard Clark, KB7QHC