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.

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.


This month's QEX has an article with some information about slotted lines
(used at 2.4 GHz in that article if I remember).

Torsten Clay
N4OGW

Jerry Martes wrote:

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

Though you don't usually see it for sale, "coax" can be square as well
as round.

Why build a square line? Because it is a hell of a lot easier to mount
and hold on to while you are doing the metal work and easier to build
a centering probe carriage on a flat surface.

--
Jim Pennino

Remove -spam-sux to reply.

FWIW here, using network analyzers may be preferable to slotted lines for RF
system measurements. The paper linked below tells why...

http://broadcastengineering.com/ar/b...nts_revisited/

RF

Visit http://rfry.org for FM broadcast RF system papers.

"Jerry Martes" wrote in message .. .
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?

Seems like the postings I've seen here are off on the wrong foot.
Yep, there will be problems, but I've learned a lot more by building
and experimenting with things that didn't work, than listening to or
arguing with people telling me that they won't work. And in fact, a
lot of the time they DO work well enough for me to accomplish what I
wanted.

So I'd like to encourage Jerry to have a go at the copper-pipe slotted
line, keeping in mind the pitfalls that have been mentioned and
thinking of ways around them. I know I would have no trouble at all
making a uniform narrow slot in such a pipe, maintaining a radial
angle straightness well under a degree, without using a milling
machine. I know I'd have no trouble supporting the 7/8" diameter
center conductor to maintain the impedance within a fraction of a
percent over the line length. With proper geometry, sag would not be
an issue for coupling. (Keeping the line vertical is one way, but not
the only way.) Perhaps more important is the initial straightness of
the pipes. 3/4"nom pipe is stiff enough that I can see doing this
with support only at the ends, so impedance AND velocity factor
variation caused by dielectric supports are no longer issues.

Will everything go smoothly? Heck no! Will Jerry learn from doing
it? You bet! Go for it, Jerry, and please see if you can write up
something about your results, to share with others. Listen to the
advice you get, but make up your own mind about what will work best
for you. You should be able to make very decent estimates of the
effects from the problem areas folk have mentioned, through either
formulas or measurements or both. You should be able to make tests on
your line to see if it's performing like you think it is, by applying
known loads and testing at various frequencies, using assorted
detectors and probes.

Cheers,
Tom

Jerry Martes wrote:
"I`m considering building a slotted line for measuring impedance at 137
MHz."

The 2nd edition of the RSGB "VHF-UHF Manual" has a home-built "trough
line" on page 10.26. Dimensions are given for 50 ohms and the line is
expected to be usable down to 145 MHz at its length of 36 to 40 inches.
It needs to be at least 1/2-wavelength.

Best regards, Richard Harrison, KB5WZI