On Mon, 10 May 2004 03:12:05 GMT, "Jerry Martes"
wrote:

I'm condidering building a slotted line for measuring impedance at 137
MHz. I find no referances to home made lines thru my quick Google search.
Does anyone know of any publication that show how someone has already worked
out the problems of contructing one?

Jerry

Hi Jerry,

It is not all that hard to do in fact. HP solved that problem long
ago by thinking outside of the box by thinking box. As with other
test equipment issues, it is simply a matter of planning and testing
with very simple methods. For those whose greatest physical effort in
Ham radio is sliding a credit card across the showroom display
counter, this is called "work."

Basically you construct your coaxial line with the usual interior
line, but unlike the conventional expectations, you do not try to
emulate the outer portion as a cylinder. You construct the outer
portion as two parallel conductive planes (appropriately shorted to
the connector shells at each end (hence the allusion to box):

view HP 805C Slotted Line picture on ebay at:
http://web.ask.com/redir?bpg=http%3a...html&qte=0&o=0

Where the two planes stand apart, you insert a probe to measure the
potential along the line. The depth of the line within and in between
the two parallel surfaces insures the line isolation (no leakage) as
well as preserving the line characteristic Z.

However, anyway that you look at it (even the lecher line suffers from
this) you run the risk of over coupling and throwing the measurement
into confusion (very simple to make errors). The problem is the probe
will introduce its own SWR and gum up the works if it lacks
sensitivity. I won't bother too much with dimensions here, but
instead offer a formula for such a structu
Zc = (138/sqrt(e))·log(4h/pi·d)
where
e: dielectric constant (= 1 for air)
d: interior line diameter
h: wall separation

You will want to build it long enough to be more than a wavelength of
course.

You will also need to calibrate it to determine the residual SWR it
presents to the system (this will reveal construction errors). Off
hand, I would suggest that the walls be roughly a 2 to 4 cm apart and
at least a 20 cm wide (larger wouldn't hurt).

Build one quick and dirty to get your gross mistakes out of the way
without spending too much time on them. I can guarantee no one here
could build it right the first time (including yours truly). Once
you've got the first pass attempt on the bench, then we can talk about
how to use it right. ;-)

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Mon, 10 May 2004 03:12:05 GMT, "Jerry Martes"
wrote:

I'm condidering building a slotted line for measuring impedance at 137
MHz. I find no referances to home made lines thru my quick Google
search.
Does anyone know of any publication that show how someone has already
worked
out the problems of contructing one?

Jerry

Hi Jerry,

It is not all that hard to do in fact. HP solved that problem long
ago by thinking outside of the box by thinking box. As with other
test equipment issues, it is simply a matter of planning and testing
with very simple methods. For those whose greatest physical effort in
Ham radio is sliding a credit card across the showroom display
counter, this is called "work."

Basically you construct your coaxial line with the usual interior
line, but unlike the conventional expectations, you do not try to
emulate the outer portion as a cylinder. You construct the outer
portion as two parallel conductive planes (appropriately shorted to
the connector shells at each end (hence the allusion to box):

view HP 805C Slotted Line picture on ebay at:

http://web.ask.com/redir?bpg=http%3a...html&qte=0&o=0

Where the two planes stand apart, you insert a probe to measure the
potential along the line. The depth of the line within and in between
the two parallel surfaces insures the line isolation (no leakage) as
well as preserving the line characteristic Z.

However, anyway that you look at it (even the lecher line suffers from
this) you run the risk of over coupling and throwing the measurement
into confusion (very simple to make errors). The problem is the probe
will introduce its own SWR and gum up the works if it lacks
sensitivity. I won't bother too much with dimensions here, but
instead offer a formula for such a structu
Zc = (138/sqrt(e))·log(4h/pi·d)
where
e: dielectric constant (= 1 for air)
d: interior line diameter
h: wall separation

You will want to build it long enough to be more than a wavelength of
course.

You will also need to calibrate it to determine the residual SWR it
presents to the system (this will reveal construction errors). Off
hand, I would suggest that the walls be roughly a 2 to 4 cm apart and
at least a 20 cm wide (larger wouldn't hurt).

Build one quick and dirty to get your gross mistakes out of the way
without spending too much time on them. I can guarantee no one here
could build it right the first time (including yours truly). Once
you've got the first pass attempt on the bench, then we can talk about
how to use it right. ;-)

73's
Richard Clark, KB7QHC

Richard

I'm pretty sure I'll try to make it coax instead of "slab" like HP does
it. I saw some 2 inch copper pipe at Home Depot today. They sell it in 5
foot lengths for about $22.00. I'm OK with using only 1/2 wavelength for
the slotted coax. I thought I might be able to 'get away with' using my
scope for detecting the line voltages. I am wide open for learning that my
approach has extremely difficult to solve problems. It all seems fairly
easy right now.
I was hoping to get a response from someone who either had built a line or
knew of a publication on the pitfalls associated with making a slotted line.

Jerry

On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes"
wrote:

[Richard's good stuff snipped]

You can lead 'em to water, but you can't make 'em drink.

|
| I'm pretty sure I'll try to make it coax instead of "slab" like HP does
|it. I saw some 2 inch copper pipe at Home Depot today. They sell it in 5
|foot lengths for about $22.00. I'm OK with using only 1/2 wavelength for
|the slotted coax. I thought I might be able to 'get away with' using my
|scope for detecting the line voltages. I am wide open for learning that my
|approach has extremely difficult to solve problems. It all seems fairly
|easy right now.
| I was hoping to get a response from someone who either had built a line or
|knew of a publication on the pitfalls associated with making a slotted line.

Wes

Maybe I missed your point. I am trying to learn how to build a slotted
line for impedance measurements at 137 MHz. It would be real helpfull to me
to see how someone has built one at home. I'd sure appreciate hearing about
how you did it or what you know about how I could build one here in my
garage.
In fact, I'd like to know more about why a person (HAM type) would use the
slab line configuration in preferance to coaxial.

Jerry

"Wes Stewart" wrote in message
...
On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes"
wrote:

[Richard's good stuff snipped]

You can lead 'em to water, but you can't make 'em drink.

|
| I'm pretty sure I'll try to make it coax instead of "slab" like HP does
|it. I saw some 2 inch copper pipe at Home Depot today. They sell it in
5
|foot lengths for about $22.00. I'm OK with using only 1/2 wavelength for
|the slotted coax. I thought I might be able to 'get away with' using my
|scope for detecting the line voltages. I am wide open for learning that
my
|approach has extremely difficult to solve problems. It all seems fairly
|easy right now.
| I was hoping to get a response from someone who either had built a line
or
|knew of a publication on the pitfalls associated with making a slotted
line.

Jerry Martes wrote:
In fact, I'd like to know more about why a
person (HAM type) would use the slab line
configuration in preference to coaxial....

One advantage of slab line is that when you
drop something like a set screw into the
line, it falls right on through. (Been there,
done that).

I've used the HP slab-line instrument and
obtained very good, repeatable results with
it.

Others have commented that the big advantage
is ease of fabrication, and I have to agree.
Cutting a slot in a water pipe seems like a
lot of trouble for minimum gain.

Another alternative might be a trough line.
This is a slab line with bottom side closed
off. The advantage this would provide is a
means of supporting the center conductor
by means of a longitudinal, dielectric strip
between it and the bottom plate of the line.
A long piece of Teflon fiberglass would be
ideal for this. The line dimensions would
have to be adjusted to maintain a 50-ohm Zo.

Anyway, it's an interesting project. Good
luck with it.

Jim, K7JEB
Glendale, AZ

On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes"
wrote:
I was hoping to get a response from someone who either had built a line or
knew of a publication on the pitfalls associated with making a slotted line.

Hi Jerry,

In response to this, and your email, your wishes are most likely to
evince a deafening silence. If you are trying to survey those who
have built one from a population of those who used them, I think
you've already hit a saturation of 2 possible users. Your own
construction effort would qualify you to expand the population by 50%.

As for why a slab style over a cylinder style. That for me is taken
on faith that greater minds, in the person of HP Metrologists, figured
that one out a long time ago. Myself, I can infer their rationale
that perhaps demands more precision and accuracy than you care to
pursue; but as any path demands a handcrafted solution, why put in 80%
of the effort for a 20% design when 90% effort would double or triple
your return?

Harkening back to my days at Metrology school, I can imagine that the
slab method was chosen because of the inevitable inner line sag that
would inject residual SWR into the system. Sagging between two plates
would seem to me to be a non-issue. Sagging within a cylinder may not
bring enough residual SWR to cause you grief either, but you have to
build and test one to discover the error of your presumption if it
disappoints you.

As for using your scope to eke out the voltage measurements. That is
a tantalizing thought, but the big boys accomplish more with less.
Simplicity is the keyword, with thinking outside of the box. You are
focusing on the literal, absolute voltage measurement when SWR is all
a matter of relativity that affords orders of magnitude more
sensitivity and resolution (and hence accuracy).

Research the Agilent archives for the Metrology papers of the 1960s.
The discussion is very accessible (with only the math necessary to
perform a real measurement) and focused to the concept and the theory.
The writing of that era is a hallmark of clarity.

73's
Richard Clark, KB7QHC

On Mon, 10 May 2004 18:13:00 GMT, Richard Clark
wrote:
Sagging between two plates would seem to me to be a non-issue.

On further reflection (no pun), I would retract this because line sag
would necessarily induce a voltage variation between the probe and the
line that did not actually exist (probe tracking is very important, of
course). Sag may not introduce residual SWR, but it may appear to.

Also, yes, you could use a less than one wavelength long slotted line
- provided you had a sliding load to make up for the remaining length.
In other words, instead of moving the probe to measure the
crests/troughs, you move the load (or both very carefully). When you
introduce limitations into design and wish to maintain precision, it
necessarily follows that you need more standards to compare against.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Mon, 10 May 2004 18:13:00 GMT, Richard Clark
wrote:
Sagging between two plates would seem to me to be a non-issue.

On further reflection (no pun), I would retract this because line sag
would necessarily induce a voltage variation between the probe and the
line that did not actually exist (probe tracking is very important, of
course). Sag may not introduce residual SWR, but it may appear to.

Also, yes, you could use a less than one wavelength long slotted line
- provided you had a sliding load to make up for the remaining length.
In other words, instead of moving the probe to measure the
crests/troughs, you move the load (or both very carefully). When you
introduce limitations into design and wish to maintain precision, it
necessarily follows that you need more standards to compare against.

73's
Richard Clark, KB7QHC

Richard

This is a good example of how out of touch I am. I thought I could get
all the impedance measuring data from a line only slightly over 1/4 wave
long. Even after thinking about it for several minutes, I cant convince
myself that the slotted line has to be more than 1/2 wave long.

Jerry

"Richard Clark" wrote in message
...
On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes"
wrote:
I was hoping to get a response from someone who either had built a line
or
knew of a publication on the pitfalls associated with making a slotted
line.

Hi Jerry,

In response to this, and your email, your wishes are most likely to
evince a deafening silence. If you are trying to survey those who
have built one from a population of those who used them, I think
you've already hit a saturation of 2 possible users. Your own
construction effort would qualify you to expand the population by 50%.

As for why a slab style over a cylinder style. That for me is taken
on faith that greater minds, in the person of HP Metrologists, figured
that one out a long time ago. Myself, I can infer their rationale
that perhaps demands more precision and accuracy than you care to
pursue; but as any path demands a handcrafted solution, why put in 80%
of the effort for a 20% design when 90% effort would double or triple
your return?

Harkening back to my days at Metrology school, I can imagine that the
slab method was chosen because of the inevitable inner line sag that
would inject residual SWR into the system. Sagging between two plates
would seem to me to be a non-issue. Sagging within a cylinder may not
bring enough residual SWR to cause you grief either, but you have to
build and test one to discover the error of your presumption if it
disappoints you.

As for using your scope to eke out the voltage measurements. That is
a tantalizing thought, but the big boys accomplish more with less.
Simplicity is the keyword, with thinking outside of the box. You are
focusing on the literal, absolute voltage measurement when SWR is all
a matter of relativity that affords orders of magnitude more
sensitivity and resolution (and hence accuracy).

Research the Agilent archives for the Metrology papers of the 1960s.
The discussion is very accessible (with only the math necessary to
perform a real measurement) and focused to the concept and the theory.
The writing of that era is a hallmark of clarity.

73's
Richard Clark, KB7QHC

Richard

I used to think the HP slab configuration was choosen so as to put the
probe in a place of minimum fiels within the line. That might be needed so
that the probe introduces a minimum disturbance, or influence. The truth
is; I dont really know why slab is prefered. I thpought I could attach some
light dielectric supports along the bottom of the center conductor to
eliminate the serious sag. I think the diam. ratio for the condoctors is
close to 2.3 to 1 for 50 ohms without dielectric loading. Thats something I
could evaluate after I decide how to make a slotted line.

All thoughts from 'readers' will be appreciated, I'm way out of touch with
antenna design these days.

Jerry

How 'bout this...'

"Jerry Martes" wrote in message
news
"Richard Clark" wrote in message
...
On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes"
wrote:
making a slotted line.

Hi Jerry,
[...] As for why a slab style over a cylinder style. That for me is
taken
on faith that greater minds, in the person of HP Metrologists, figured
that one out a long time ago. ...
73's
Richard Clark, KB7QHC

Richard

I used to think the HP slab configuration was choosen so as to put the
probe in a place of minimum fiels within the line.
Jerry


Gents.
What you are calling "slab line" is more properly called "Stripline".
It is a very well defined type of transmission line...that is, it has exact
equations (unlike the one-sided Micro-strip). I suspect that Jerry's
conclusion is right-on. At least theoretically, there is zero field along a
line through the center conductor, parallel to the ground planes. So it
appears that you would indeed disturb things the least...as long as your
'probe' exited parallel to the ground planes.
Ignore everything but the lower right figure on page 3 of the stripline E
field:
http://www.altera.com/literature/wp/lvdsboardwp.pdf

It is all sort of 'sucked' into the ground planes...
The best pix I could find... I think as long as the probe has a small cross
section when viewed from the line you will disturb the field the least.
Seems to me in the old days, the probe consisted of a 1N21 diode...
--
Steve N, K,9;d, c. i My email has no u's.