I still remember a demonstration of transmission line phenomena given by
Dick Silberstein, W0YBF, at the Denver Radio Club more than 30 years
ago. He had made a "slotted line" (more accurately a trough line) from a
wire tightly suspended in the "V" of a piece of angle aluminum or steel.
It worked very well. It was pretty large -- I believe the frequency he
used was around 50 MHz. You could use a smaller piece of angle material
or suspend it deeper in the valley for a higher frequency.

Roy Lewallen, W7EL

Jerry Martes wrote:

Cecil

I've always ignored the use of twin lead for impedance measurement. But,
it makes sense. I may try building something like that, just for fun.

I certainly havent thought this "twin lead" line thru but it seems that
one could be easily built from a length of PVC pipe with wires attached to
its sides.
This seems too easy and relatively accurate for "home in the garage" use.
I'd bet I can find some info on the Internet for FM and TV frequency use.
Thanks for jogging my mind.

Jerry

"Richard Clark" wrote in message
...
On Wed, 30 Mar 2005 14:45:12 GMT, "Jerry Martes"
wrote:

Your thoughts on sweep frequency for impedance measurement is very
interesting. If I could figure out a way to build the "sensors" I'd
probably build a sweep system for 2 meters.

The ARRL handbooks of yore have several designs. Google "monimatch".


Although I could handle
building a sweep frequency generator, I wouldnt know how to convert the
"incident and reflected" to an impedance.

Software would do that.

No it wouldn't, unless you could figure out some way to get vector magnitude
and phase from the two channels.


I suspect that, if sweep frequency impedance measurements was simple
enough for Me to do, someone would already have done it and published in
one
of the HAM magazines.

Perhaps not the ham magazines; I know I've written at least three articles
in Kitplanes magazine as to how to do it for aircraft antenna VSWR.

Jim

On Wed, 30 Mar 2005 08:42:14 -0800, "RST Engineering"
wrote:

Although I could handle
building a sweep frequency generator, I wouldnt know how to convert the
"incident and reflected" to an impedance.

Software would do that.

No it wouldn't, unless you could figure out some way to get vector magnitude
and phase from the two channels.

Hi All,

I have since researched the archives to find my reference:
http://users.adelphia.net/~n2pk/

What can be done, what has been done, and what might be inferred is
revealed at this page.

73's
Richard Clark, KB7QHC

Roy

That trough configuration is probably more practical than the copper tube
coax thing I made. I dont have any current interest in 50 MHz impedance
measurement. But, I'll sure consider that "angle trough" method for any low
cost tools for impedance measurements. I realize now that a slotted line
even for 137 MHz is too large to have around in my garage. It requires too
much room to set up the equipment. It is kinda interesting to consider
something made from PVC so it is "disposable". I dont know how I'd handle
the transitions to and from the coax. But, it seems like a guy could use a
couple 4:1 baluns to get from 50 ohm coax to 200 ohm twin lead. I'm
pretty sure I could find a couple long lengths of copper wire or tube that I
could attach along the sides of some 1 inch PVC to make a 200 ohm line.
That would be about 20 feet long for 6 meters. Wow, thats big.

Maybe I should just get back to learning EZNEC. You guys actually get
good impedance predictions for almost any antenna.

Jerry




"Roy Lewallen" wrote in message
...
I still remember a demonstration of transmission line phenomena given by
Dick Silberstein, W0YBF, at the Denver Radio Club more than 30 years ago.
He had made a "slotted line" (more accurately a trough line) from a wire
tightly suspended in the "V" of a piece of angle aluminum or steel. It
worked very well. It was pretty large -- I believe the frequency he used
was around 50 MHz. You could use a smaller piece of angle material or
suspend it deeper in the valley for a higher frequency.

Roy Lewallen, W7EL

Jerry Martes wrote:

Cecil

I've always ignored the use of twin lead for impedance measurement.
But, it makes sense. I may try building something like that, just for
fun.

I certainly havent thought this "twin lead" line thru but it seems that
one could be easily built from a length of PVC pipe with wires attached
to its sides.
This seems too easy and relatively accurate for "home in the garage"
use. I'd bet I can find some info on the Internet for FM and TV frequency
use. Thanks for jogging my mind.

Jerry

Richard

I'm way behind on the learning curve. I had read about that Vector
Network Analyzer last year. it looked too complicated for me to build.
And, in my ignorance, I thought that system was what the MFJ used.

The Smith Chart plots from his VNA data really impresses me. I could
possible get interested in building something like that for VHF, if I
understood it.

Jerry



"Richard Clark" wrote in message
...
On Wed, 30 Mar 2005 08:42:14 -0800, "RST Engineering"
wrote:

Although I could handle
building a sweep frequency generator, I wouldnt know how to convert the
"incident and reflected" to an impedance.

Software would do that.

No it wouldn't, unless you could figure out some way to get vector
magnitude
and phase from the two channels.

Hi All,

I have since researched the archives to find my reference:
http://users.adelphia.net/~n2pk/

What can be done, what has been done, and what might be inferred is
revealed at this page.

73's
Richard Clark, KB7QHC

On Wed, 30 Mar 2005 17:44:45 GMT, "Jerry Martes"
wrote:

The Smith Chart plots from his VNA data really impresses me. I could
possible get interested in building something like that for VHF, if I
understood it.

Hi Jerry,

Take the plunge, that is the best way to learn. You already exhibit
positive motivation and that's the hard part of learning.

73's
Richard Clark, KB7QHC

On Wed, 30 Mar 2005 09:19:40 -0800, Richard Clark
wrote:

[snip]

Hi All,

I have since researched the archives to find my reference:
http://users.adelphia.net/~n2pk/

What can be done, what has been done, and what might be inferred is
revealed at this page.

This is the greatest thing since sliced bread.

I've built one for ~$250.00. Here's a photo taken during construction
and checkout:

http://www.qsl.net/n7ws/ANA_1.jpg (350K)

Currently it's limited to 60 MHz although variants are in the works to
increase this. Another minor limitation is that the software is
DOS-based although Windows software is under development by other
users. Xp users, like me, have a couple of hoops to jump through to
access the parallel port but it's no big deal.

The accuracy, however, is lab quality and is comparable to commercial
network analyzers costing tens to hundreds of $K like I used to use,
although it's achieved at the expense of measurement speed.

Also see:

http://groups.yahoo.com/group/N2PK-VNA/

A higher cost alternative with much reduced dynamic range is he

http://radio.tentec.com/Amateur/vna

On Wed, 30 Mar 2005 11:15:41 -0700, Wes Stewart
wrote:

This is the greatest thing since sliced bread.

Hi Wes,

I can think of no greater commendation from an authority. Thanx for
the insights offered.

73's
Richard Clark, KB7QHC

On 3/30/05 2:52 AM, in article ,
"Richard Clark" wrote:



You will need a stable source capable of 1 to 10mW power. You will
need a well shielded communications receiver to act as a detector.
You will also need a steady hand to balance the bridge (which may be
part of your problem with the noise bridge). Nulls are often more
than 100 dB deep. If you lack sufficient shielding for the detector,
this will degrade the nulls considerably. If you lack stability, you
will never find the null.

Richard, what do you mean by "If you lack sufficient shielding for the
detector, this will degrade the nulls considerably."

The source is the built in noise generator in the bridge. My detector is my
Icom communications receiver, connected via coax to the bridge.
What additional shielding would be needed??

UWE

On Thu, 31 Mar 2005 12:28:46 GMT, Jaggy Taggy
wrote:
Richard, what do you mean by "If you lack sufficient shielding for the
detector, this will degrade the nulls considerably."

The source is the built in noise generator in the bridge. My detector is my
Icom communications receiver, connected via coax to the bridge.
What additional shielding would be needed??

Hi Uwe,

This is entirely revealed by practice. Simply answer how wide is the
null? If you don't know how wide it should be, then chances are you
don't know how well your system is connected. As I pointed out
elsewhere, I can obtain nulls that are 100dB deep. This is one simple
measure of width; although not in terms of width, it is very
distinctive and correlates with the same consideration. Far more
frequently my nulls are 120+dB deep (down into the receiver noise from
a signal level of 1mW).

The problem with a noise bridge is power. To observe that same depth
of null requires a noise source of several Watts (not sure of the
actual figure now, but retail sources don't come close). If you
balance your bridge and you run out of signal before you run out of
balance, then your settings are at the wrong Z. What appeared to be a
balanced null was actually the lack of source power to indicate you
were off-set. For a noise bridge, you have to keep adjusting even
when the noise seems to have been nulled. You keep going until you
find the noise signal climbing again, and then split the difference
between the two edges of the silent span of readings. If that is too
wide, you could have quite a time of this.

Now, consider if your shielding is not tight and you could hear your
source along a path that was not through the bridge = you never hear a
null, or you get a very very broad null that still contains
noise/signal. A receiver's shielding that is good to -100 to -120dB
is exceptional. In our hobby it is not exactly commonplace, but it is
found in good equipment. However, how good your equipment is can only
be determined through actual testing as shielding is also a function
of grounding, and there everyone is an expert but few are well
grounded.

73's
Richard Clark, KB7QHC