I would sure like to get some information from you.
You don't sound like the CIA so I'll do my best.
You are correct. A home-made slotted line can be any value Zo provided you
know what it is. But it is prudent to be close to the standard 50 or 75
ohms because of the Zo of any other cables and connectors to be used in
conjunction with it. Otherwise you will forever be calculating corrections
for impedance mismatches.
And how do you know what Zo is? You CALCULATE it from measured dimensions
of the line's cross-section. After all, that's why you constructed it in
the first place. Uncertainty in Zo depends on cross-section measuring
accuracy and, of course, on the accuracy of the calculating formula.
The most simple cross-section is circular coaxial for which Zo = 60*Ln(D/d)
exactly. But there's difficulty in producing a narrow slot without
mechanical distortion. If you can think of a way of doing it that's fine.
It depends on what measuring accuracy is required. Fortunately, Zo is
relatively insensitive to absolute measurements on the cross section. Quite
a big fat, strong, easily-measured line can be made.
Note that absolute measurements of the cross-section are not required -
only the RATIO D/d is needed. This freedom also applies to all other shapes
of cross section. There is no requirement for a length of line to be
absolutely straight unless you intend to make an international measurement
standard out of it. ;o)
The next most simple cross section is a square or rectangular aluminium
trough or channel with the open top capped with 2 parallel straight strips
spaced apart by the width of the slot which should be no wider than
absolutely necessary. The separate parts can be screwed together.
A problem is providing support for the inner conductor of a long length of
line. Ideally it should be rigid enough to be self-supporting without
significant sag. Hard aluminium (Duralumin) alloy will be OK. A single
support, half-way along the line, could be made from a THIN rigid plastic
plate about an inch long.
A square or rectanglar cross section will allow the probe carraige to slide
smoothly along the line. The formula for calculating line Zo will be fairly
simple. I'm sure it will be available from some reliable source. Just don't
even dream of deriving a formula from first principles. Accuracy of
determination of Zo should be within a few percent even using a wooden
ruler.
Departure of the inner conductor itself from a circular cross section, eg.,
square or moderately rectangular, will not detract from measurement accuracy
for the degree of accuracy you say you require.
Below 200 or 300 MHz there's no need to be very particular about the coaxial
connectors at the ends of the line. You should be more concerned about
uncertainty of the impedance of the coaxial cables used in conjunction with
it.
Can't think of anything else at present.
How do you propose to use a slotted line to measure unknown impedances?
----
Reg, G4FGQ
I think you are the
kind of guy who would know about how this line might be made to work. I
have some 'notions' that may well be wrong, and I'm too easily confused to
work this out alone. It seems to me that a "home made" slotted line
wouldnt
have to be 50 ohms, yet it could accurately measure load impedances.
For my purpose, I'd want to try to get the line impedance as close to 50
as practical. But, if the slotted line's impedance was 48 ohms or 52
ohms,
the line would give quite good information on load impedance. That would
require alot of testing to actually determine the Zo of the line. But, I
would like only to get real close. I dont need a lab standard.
Since I've gotten so involved with this news group discussion, I'll
start
building a slotted line useing that 5 foot long piece of copper tube at
Home
Depot. Still, it would be more satisfying to know more about what I'm
attempting. It occurred to me that, if building a slotted line for
impedance measurement at 137 MHz was easy, someone would have done it
already. So, I'm not real confidant that I'll be successful.
I'd thought I might be able to use my Oscilloscope with a sensitive
probe
for the "voltage measurement". But, since I'm hearing nothing from this
group about how slotted lines are home made for VHF impedance measurement,
I'm not confidant I'll be successful.
Of course it is not you who is out of date. It is me. I've been away
from electronics and antennas for the last 35 years.
Jerry
"Reg Edwards" wrote in message
...
A slotted line is a fundamental Standard of Impedance. It relates
impedance
measurements to the absolute standard of length, the International
Metre!
Its weakest feature is the probe mechanism itself. Its electrical
presence
is unpredictable. Incalculable. Uncertainty can only be minimised.
Or am I 60 years out of date?
Does anyone know what is the uncertainty in measuring an impedance of,
say,
around 100 ohms at 10 MHz, as may be claimed by a National Measurement
Standards Laboratory.
What may be claimed by an instrument manufacturer in the sales blurb is
another matter.
----
Reg, G4FGQ
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