On Mon, 4 Oct 2004 15:19:41 -0500, "John Smith"
wrote:

|Okay, then, I will present data measured this day for this antenna:
|
|http://www.sophisticatedsolutions.us...d%20Dipole.jpg
|
|This is shown in "Antennas for All Applications" on page 820, figure 23-17
|(a).
|
|I built the antenna wholly from RG58. The center conductor of the right half
|is not connected at either end. It is 14.375 inches wide and averages a
|little less than .5 inches between the centers of the top and bottom
|conductors. Where the coax is shown exiting the antenna, is a female,
|flangeless, chassis mount, BNC connector so that I can replace the antenna
|with a short.
|
|My test set up is a VHF oscillator, a vector voltmeter, and a Narda dual
|directional coupler. I use a 66 inch piece of RG58 from the output of the
|directional coupler to go to the antenna. The short circuits I use are the
|best I could make from BNC connectors. The 50 Ohm load I used for
|comparative measurements is one of those used for network terminators. Yes,
|I am aware they are not instrumentation quality, but it's what I have.
|
|For a given frequency, I replace the antenna with the short and adjust the
|amplitude of the oscillator and the controls of the vector voltmeter so that
|the reference channel (A) is 10 mV and the phase is 180 degrees. I record
|channel B's amplitude. I then remove the short and connect the antenna. I
|then read and record channels A, channel B, and the phase. From these data I
|calculate the impedance (per HP's AN 77-3, thanks to Wes Stewart).
|
|The first item measured is the 50 Ohm terminator. I also measured it at the
|conclusion of the tests to see if there were any differences and there were
|none.
|
|Here are the results computed from the data:
|
|Freq (MHz) Impedance (Ohms)
|
|410 46.4 + 6.0i (50 Ohm terminator)
|
|380 9.7 - 12.5
|390 3.5 - 5.7
|400 5.1 + 1.3i
|410 5.1 + 6.5i
|415 4.0 + 10.0i
|425 2.5 + 15.7i
|
|This is surprising to me and doesn't make a whole lot of sense. For one
|thing, I would have expected the impedance to vary wildly over the range
|shown. For another, the low impedance seems, well, really low.
|
|Is the trend of the data as shown to be expected? Well, maybe the reactive
|part? The real part seems to make a little sense except at the extremes.
|
|Can I trust this data to be even approximately right? I mean, can I now say
|that the antenna in question is maybe nice due to the natural balun but I
|might as well forget it as a simple antenna because to the low impedance?
|
|Or, should I say this is utter nonsense, the antenna is probably okay, it's
|just my equipment, setup, or lack of knowledge giving erroneous data?
|
|Your opinions are welcome.

John,

First of all you are to be commended for running these experiments.
Without a bit more study of your situation I can't comment too much
but I wanted to throw out a couple of quick ideas.

1) Do you have the VVM probes terminated in 50 ohm?

2) If you don't have the line stretcher as in Fig 7 of the note, are
you recalibrating at each test frequency?

3) How well is your signal source terminated, in other words do you
know its source match?

Each of these things can affect the outcome.

More later,


Wes

Hi, Wes -


"Wes Stewart" wrote in message
...
On Mon, 4 Oct 2004 15:19:41 -0500, "John Smith"
wrote:

John,

First of all you are to be commended for running these experiments.
Without a bit more study of your situation I can't comment too much
but I wanted to throw out a couple of quick ideas.

1) Do you have the VVM probes terminated in 50 ohm?


Yes. They came witht the kit and are called terminators. They are 50 Ohms
each.


2) If you don't have the line stretcher as in Fig 7 of the note, are
you recalibrating at each test frequency?


I do not have the stretcher. I recalibrate at every frequency change.


3) How well is your signal source terminated, in other words do you
know its source match?


I only know that the signal source is an HP 3200B. It directly feeds the
Narda dual directional coupler through a few feet of RG58.


Each of these things can affect the outcome.

More later,


Wes

Thanks, Wes.

John

On Mon, 4 Oct 2004 16:45:31 -0500, "John Smith"
wrote:

|Hi, Wes -
|
|
|"Wes Stewart" wrote in message
.. .
| On Mon, 4 Oct 2004 15:19:41 -0500, "John Smith"
| wrote:
|
| John,
|
| First of all you are to be commended for running these experiments.
| Without a bit more study of your situation I can't comment too much
| but I wanted to throw out a couple of quick ideas.
|
| 1) Do you have the VVM probes terminated in 50 ohm?
|
|
|Yes. They came witht the kit and are called terminators. They are 50 Ohms
|each.

Okay.
|
|
| 2) If you don't have the line stretcher as in Fig 7 of the note, are
| you recalibrating at each test frequency?
|
|
|I do not have the stretcher. I recalibrate at every frequency change.
|
Understood.
|
| 3) How well is your signal source terminated, in other words do you
| know its source match?
|
|
|I only know that the signal source is an HP 3200B. It directly feeds the
|Narda dual directional coupler through a few feet of RG58.

If I remember that correctly the '3200 is nothing but a p-p oscillator
and a waveguide-below-cutoff probe. If your VVM reference probe
readings are changing much between frequencies and/or
calibration/measurement, try a 6 or 10 dB pad right on the generator
output and see what happens.

When you're calibrating using a short, the source Z has really got to
be nailed down.

Wes

"Wes Stewart" wrote in message
...
On Mon, 4 Oct 2004 16:45:31 -0500, "John Smith"

If I remember that correctly the '3200 is nothing but a p-p oscillator
and a waveguide-below-cutoff probe. If your VVM reference probe
readings are changing much between frequencies and/or
calibration/measurement, try a 6 or 10 dB pad right on the generator
output and see what happens.


The 3200B is the oscillator they use in the HP AN 77-3 you sent to me. I'm
using it the same way they did except for the stretcher and a Narda (rather
than HP) coupler. However, I'll try to run a test and determine how much
difference there is using a pad.


When you're calibrating using a short, the source Z has really got to
be nailed down.

Wes


Thanks.

John

"Wes Stewart" wrote in message
...
On Mon, 4 Oct 2004 16:45:31 -0500, "John Smith"
wrote:

Freq (MHz) Impedance (Ohms)

410 46.4 + 6.0i (50 Ohm terminator)

380 9.7 - 12.5
390 3.5 - 5.7
400 5.1 + 1.3i
410 5.1 + 6.5i
415 4.0 + 10.0i
425 2.5 + 15.7i


| 3) How well is your signal source terminated, in other words do you
| know its source match?
|
|
|I only know that the signal source is an HP 3200B. It directly feeds the
|Narda dual directional coupler through a few feet of RG58.

If I remember that correctly the '3200 is nothing but a p-p oscillator
and a waveguide-below-cutoff probe. If your VVM reference probe
readings are changing much between frequencies and/or
calibration/measurement, try a 6 or 10 dB pad right on the generator
output and see what happens.

When you're calibrating using a short, the source Z has really got to
be nailed down.

Wes


Okay. I repeated the test using an HP 355C attenuator set for 10 dB and at
400 MHz got 4 + 3i on the antenna. I also checked my 50 Ohm network
terminator with this setup and it measured 44 + 4i. The data are different,
but they're not an order of magnitude different, at least.

So, although my measurements aren't repeatable, they are sloppily
consistent. That is, although I can't say exactly what the antenna impedance
is with confidence, I am beginning to believe that it really is very low in
impedance. Am I drawing an erroneous conclusion too early? I can wait a
little longer to draw an erroneous conclusion.

John

On Mon, 4 Oct 2004 20:58:07 -0500, "John Smith"
wrote:

|
|"Wes Stewart" wrote in message
.. .
| On Mon, 4 Oct 2004 16:45:31 -0500, "John Smith"
| wrote:
|
|Freq (MHz) Impedance (Ohms)
|
|410 46.4 + 6.0i (50 Ohm terminator)
|
|380 9.7 - 12.5
|390 3.5 - 5.7
|400 5.1 + 1.3i
|410 5.1 + 6.5i
|415 4.0 + 10.0i
|425 2.5 + 15.7i
|
|
| | 3) How well is your signal source terminated, in other words do you
| | know its source match?
| |
| |
| |I only know that the signal source is an HP 3200B. It directly feeds the
| |Narda dual directional coupler through a few feet of RG58.
|
| If I remember that correctly the '3200 is nothing but a p-p oscillator
| and a waveguide-below-cutoff probe. If your VVM reference probe
| readings are changing much between frequencies and/or
| calibration/measurement, try a 6 or 10 dB pad right on the generator
| output and see what happens.
|
| When you're calibrating using a short, the source Z has really got to
| be nailed down.
|
| Wes
|
|
|Okay. I repeated the test using an HP 355C attenuator set for 10 dB and at
|400 MHz got 4 + 3i on the antenna. I also checked my 50 Ohm network
|terminator with this setup and it measured 44 + 4i. The data are different,
|but they're not an order of magnitude different, at least.
|
|So, although my measurements aren't repeatable, they are sloppily
|consistent. That is, although I can't say exactly what the antenna impedance
|is with confidence, I am beginning to believe that it really is very low in
|impedance. Am I drawing an erroneous conclusion too early? I can wait a
|little longer to draw an erroneous conclusion.

First of all, neglecting the feed method, the antenna is a simple
folded dipole. In free space, or an approximation thereof, it should
have a feedpoint Z of about 300 ohm. (See the ARRL Antenna book for a
description of why this is so under "Special Antenna Types", p.2-32 in
the 17th edition) In the presence of other (non-resonant) objects, it
may differ from this but not a whole lot.

In theory, the "natural balun" doesn't change the impedance of the
feedpoint whatsoever. By "feedpoint" I mean the gap between the ends
of the folded element, not the "tee" connection opposite.

At the outside of the tee connection, the voltage is zero thus this
point can be grounded, connected to the boom in a Yagi, etc. without
upsetting anything. Likewise the coax feeder can be introduced here
and run through one side of the element without upsetting anything
either.

But, a nominal 300 ohm load is terminating a 50 ohm line, so the usual
transforming effects are in play. The input Z of an arbitrary length
line is---well, arbitrary. If the line is many wavelengths long, then
when the frequency is changed, the long lines effect kicks in and the
input Z is going to vary rapidly with respect to frequency.

Second. I believe that you need to determine the parameters of your
directional coupler. As Richard pointed out, your B1/A1 numbers are
pretty unstable.

So here's what I recommend. First verify that the "A" and "B" probes
read the same thing when connected to the same source. Then put your
pad right at the input connector of the coupler. Terminate the
reverse port and connect your VVM "A" probe to the ouput connector and
the "B" probe to the forward port.

The ratio reading is the forward coupling factor of the directional
coupler. Vary the frequency and see how this changes and note some
values. Move the "B" probe to the reverse port and terminate the
forward port. Note the readings at the same frequency.

Reverse the input and output ports and repeat the measurements.
Ideally, the data sets will track closely. If they don't then you
have a problem. Serious differences might indicate damage to the
internal terminations. This assumes that this is a true dual coupler
and not single line coupler with the termination applied to the unused
port externally.

If the numbers are consistant, then you can determine the directivity
by computing the ratio between the two readings on a given port when
the coupler is reversed.

I'm going to stop here and assume you understand the consquences of
poor directivity on measurement accuracy. If you don't then I can
expound further later.

Wes

"Wes Stewart" wrote in message
...
On Mon, 4 Oct 2004 20:58:07 -0500, "John Smith"
wrote:

|
|"Wes Stewart" wrote in message
.. .
| On Mon, 4 Oct 2004 16:45:31 -0500, "John Smith"
| wrote:
|
|Freq (MHz) Impedance (Ohms)
|
|410 46.4 + 6.0i (50 Ohm terminator)
|
|380 9.7 - 12.5
|390 3.5 - 5.7
|400 5.1 + 1.3i
|410 5.1 + 6.5i
|415 4.0 + 10.0i
|425 2.5 + 15.7i
|
|
| | 3) How well is your signal source terminated, in other words do you
| | know its source match?
| |
| |
| |I only know that the signal source is an HP 3200B. It directly feeds
the
| |Narda dual directional coupler through a few feet of RG58.
|
| If I remember that correctly the '3200 is nothing but a p-p oscillator
| and a waveguide-below-cutoff probe. If your VVM reference probe
| readings are changing much between frequencies and/or
| calibration/measurement, try a 6 or 10 dB pad right on the generator
| output and see what happens.
|
| When you're calibrating using a short, the source Z has really got to
| be nailed down.
|
| Wes
|
|
|Okay. I repeated the test using an HP 355C attenuator set for 10 dB and
at
|400 MHz got 4 + 3i on the antenna. I also checked my 50 Ohm network
|terminator with this setup and it measured 44 + 4i. The data are
different,
|but they're not an order of magnitude different, at least.
|
|So, although my measurements aren't repeatable, they are sloppily
|consistent. That is, although I can't say exactly what the antenna
impedance
|is with confidence, I am beginning to believe that it really is very low
in
|impedance. Am I drawing an erroneous conclusion too early? I can wait a
|little longer to draw an erroneous conclusion.

First of all, neglecting the feed method, the antenna is a simple
folded dipole. In free space, or an approximation thereof, it should
have a feedpoint Z of about 300 ohm. (See the ARRL Antenna book for a
description of why this is so under "Special Antenna Types", p.2-32 in
the 17th edition) In the presence of other (non-resonant) objects, it
may differ from this but not a whole lot.

In theory, the "natural balun" doesn't change the impedance of the
feedpoint whatsoever. By "feedpoint" I mean the gap between the ends
of the folded element, not the "tee" connection opposite.

At the outside of the tee connection, the voltage is zero thus this
point can be grounded, connected to the boom in a Yagi, etc. without
upsetting anything. Likewise the coax feeder can be introduced here
and run through one side of the element without upsetting anything
either.

But, a nominal 300 ohm load is terminating a 50 ohm line, so the usual
transforming effects are in play. The input Z of an arbitrary length
line is---well, arbitrary. If the line is many wavelengths long, then
when the frequency is changed, the long lines effect kicks in and the
input Z is going to vary rapidly with respect to frequency.

Second. I believe that you need to determine the parameters of your
directional coupler. As Richard pointed out, your B1/A1 numbers are
pretty unstable.

So here's what I recommend. First verify that the "A" and "B" probes
read the same thing when connected to the same source. Then put your
pad right at the input connector of the coupler. Terminate the
reverse port and connect your VVM "A" probe to the ouput connector and
the "B" probe to the forward port.

The ratio reading is the forward coupling factor of the directional
coupler. Vary the frequency and see how this changes and note some
values. Move the "B" probe to the reverse port and terminate the
forward port. Note the readings at the same frequency.

Reverse the input and output ports and repeat the measurements.
Ideally, the data sets will track closely. If they don't then you
have a problem. Serious differences might indicate damage to the
internal terminations. This assumes that this is a true dual coupler
and not single line coupler with the termination applied to the unused
port externally.

If the numbers are consistant, then you can determine the directivity
by computing the ratio between the two readings on a given port when
the coupler is reversed.

I'm going to stop here and assume you understand the consquences of
poor directivity on measurement accuracy. If you don't then I can
expound further later.

Wes

Thanks, Wes. I'm going to have to stop the experiments for a few days, but
I'll try to get back to you.

John

"Wes Stewart" wrote in message
...
On Mon, 4 Oct 2004 20:58:07 -0500, "John Smith"
wrote:

Second. I believe that you need to determine the parameters of your
directional coupler. As Richard pointed out, your B1/A1 numbers are
pretty unstable.

So here's what I recommend. First verify that the "A" and "B" probes
read the same thing when connected to the same source.

I put the oscillator to the center of the HP Power Splitter. I then put a
probe tee on each of the splitter outputs followed by a 50 Ohm terminator.
The difference between the A and B channels was maybe a needle's width.
Reversing the splitter made no difference. Swapping the terminators made no
difference.


Then put your
pad right at the input connector of the coupler. Terminate the
reverse port and connect your VVM "A" probe to the ouput connector and
the "B" probe to the forward port.

The ratio reading is the forward coupling factor of the directional
coupler. Vary the frequency and see how this changes and note some
values. Move the "B" probe to the reverse port and terminate the
forward port. Note the readings at the same frequency.

From 350 MHz to 450 MHz the forward port coupling was -25.9 dB to -26.2 dB.

Reverse the input and output ports and repeat the measurements.
Ideally, the data sets will track closely.


I got the same here within about a tenth of a dB.


If they don't then you
have a problem. Serious differences might indicate damage to the
internal terminations. This assumes that this is a true dual coupler
and not single line coupler with the termination applied to the unused
port externally.

If the numbers are consistant, then you can determine the directivity
by computing the ratio between the two readings on a given port when
the coupler is reversed.

I'm going to stop here and assume you understand the consquences of
poor directivity on measurement accuracy. If you don't then I can
expound further later.

Wes


I guess it looks okay. Thanks, Wes.

John (KD5YI)

On Tue, 5 Oct 2004 15:59:32 -0500, "John Smith"
wrote:

|
|"Wes Stewart" wrote in message
.. .
| On Mon, 4 Oct 2004 20:58:07 -0500, "John Smith"
| wrote:
|
| Second. I believe that you need to determine the parameters of your
| directional coupler. As Richard pointed out, your B1/A1 numbers are
| pretty unstable.
|
| So here's what I recommend. First verify that the "A" and "B" probes
| read the same thing when connected to the same source.
|
|I put the oscillator to the center of the HP Power Splitter. I then put a
|probe tee on each of the splitter outputs followed by a 50 Ohm terminator.
|The difference between the A and B channels was maybe a needle's width.
|Reversing the splitter made no difference. Swapping the terminators made no
|difference.

Excellent.
|
|
| Then put your
| pad right at the input connector of the coupler. Terminate the
| reverse port and connect your VVM "A" probe to the ouput connector and
| the "B" probe to the forward port.
|
| The ratio reading is the forward coupling factor of the directional
| coupler. Vary the frequency and see how this changes and note some
| values. Move the "B" probe to the reverse port and terminate the
| forward port. Note the readings at the same frequency.
|
|From 350 MHz to 450 MHz the forward port coupling was -25.9 dB to -26.2 dB.

Okay. Not per nameplate, but now you know.
|
| Reverse the input and output ports and repeat the measurements.
| Ideally, the data sets will track closely.
|
|
|I got the same here within about a tenth of a dB.

Great.
|
|
| If they don't then you
| have a problem. Serious differences might indicate damage to the
| internal terminations. This assumes that this is a true dual coupler
| and not single line coupler with the termination applied to the unused
| port externally.
|
| If the numbers are consistant, then you can determine the directivity
| by computing the ratio between the two readings on a given port when
| the coupler is reversed.

You still need to do this.

|
| I'm going to stop here and assume you understand the consquences of
| poor directivity on measurement accuracy. If you don't then I can
| expound further later.
|
| Wes
|
|
|I guess it looks okay. Thanks, Wes.

You're welcome.

Wes