Coaxial folded dipole (was: Natural balun/Antenna on 9/26/2004)
 

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 (KD5YI)

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 15:19:41 -0500, "John Smith"
wrote:

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

Hi John,

How about the raw data? How about a detailed description of the NARDA
coupler? It should have a calibration plate affixed to the side of it
with freq vs. coupling marks (generally pretty close).

I would note you have a considerable SWR, but this was expected going
into the test (however, in an inverse proportion, which may be
meaningful here).

73's
Richard Clark, KB7QHC

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

"John Smith" wrote in message
...
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
........................................
John,
Discounting the absolute values, the numbers seem to make sense, except for
the 9.7. Might it have been 2.7? There seems to be resonance at around 400.
The impedance goes more reactive in both directions from there, and the real
part goes down monotonically, except for the 9.7

I looked at the picture, and it is not 100% obvious to me what gets
connected at the balanced point. Just for kicks, I am going to try that,
somewhere within the range of an MFJ269.

Tam/WB2TT

"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

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

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

Hi John,

How about the raw data? How about a detailed description of the NARDA
coupler? It should have a calibration plate affixed to the side of it
with freq vs. coupling marks (generally pretty close).

The Narda coupler has no calibration plate. It says model 30611, serial no.
4235. It also appears to have been made for Motorola, as their part number
appears on it. One end says "BTS" and the opposite end says "ANT". At the
BTS end, on the side, there is a connector which says -30 dB. Similarly on
the antenna end.


Raw data:

Reference Measurement
Freq A1 B1 ?1 A2 B2 ?2
415 1 0.76 180 0.82 0.535 158
410 1 0.77 180 0.905 0.57 165
400 1 0.79 180 1.08 0.695 177
390 1 0.81 180 1.02 0.72 -167
425 1 0.743 180 1.06 0.72 145
380 1 0.815 180 0.86 0.485 -151
410 1 0.749 180 0.535 0.029 118
425 1 0.695 180 1.04 0.695 143


I would note you have a considerable SWR, but this was expected going
into the test (however, in an inverse proportion, which may be
meaningful here).

73's
Richard Clark, KB7QHC

"Tam/WB2TT" wrote in message
...

"John Smith" wrote in message
...
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
........................................
John,
Discounting the absolute values, the numbers seem to make sense, except
for the 9.7. Might it have been 2.7? There seems to be resonance at around
400. The impedance goes more reactive in both directions from there, and
the real part goes down monotonically, except for the 9.7

I looked at the picture, and it is not 100% obvious to me what gets
connected at the balanced point. Just for kicks, I am going to try that,
somewhere within the range of an MFJ269.

Tam/WB2TT


Hi, Tam -

I will try to repeat the test at that frequency.

By the balanced point, I assume you mean at the bottom center. It is a
female BNC connector, facing downward. A halfwave length of RG58 goes off
the left side and folds. The coax is soldered in normal fashion to the
connector. Another halfwave piece of RG58 goes off the right side and folds.
The center conductor of the right side piece is not connected on either end.
The shield of the right side coax is soldered to the shell of the BNC and
the two pices of coax is joined as shown in the figure.

If by the balanced point you meant at the top center of the figure, the
center conductor only of the left side coax is soldered to the shield only
of the right side coax.

If this description is not clear, let me know and I'll try again. I would
take a picture and make it available, but I'm afraid it would only confuse
due to lack of detail.

Thanks,
John

"Tam/WB2TT" wrote in message
...

"John Smith" wrote in message
...
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
........................................
John,
Discounting the absolute values, the numbers seem to make sense, except
for the 9.7. Might it have been 2.7? There seems to be resonance at around
400. The impedance goes more reactive in both directions from there, and
the real part goes down monotonically, except for the 9.7

Well, measured again, at 380 MHz I get 0.9 - 4.4i.

I looked at the picture, and it is not 100% obvious to me what gets
connected at the balanced point. Just for kicks, I am going to try that,
somewhere within the range of an MFJ269.

Tam/WB2TT


This is getting discouraging.

John