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 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

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

"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

On Mon, 4 Oct 2004 19:30:19 -0500, "John Smith"
wrote:

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.

Hi John,

This is not a NARDA model number according to their own catalogue,
although I see it described as NARDA in more than one ebay auction. I
would say this is a special run for Motorola (which is probably their
contract number with NARDA).

However, all ebay auctions list this as an 960 MHz device.

Typically, directional couplers are within their nominal ratings only
over a octave range and some of those octaves from within their
catalogue are 500 MHz to 1 GHz. Others are 450 MHz to 900 MHz. Some
are listed as 0.05 GHz to 1 GHz, but the coupling is VERY MUCH
different than nominal outside the octave range (by as much as 10 - 15
dB). Generally, you don't suffer this much variation near the octave
band edges, but they do track off from their otherwise flat response.

It looks like you need to measure the coupling directly (at both
ports) across your frequencies of interest. The coupling factor is
not so important as is the ports tracking. I will discuss the raw and
finished data separately.

73's
Richard Clark, KB7QHC