"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

"John Smith" wrote in message
...

"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

John, This turned out simpler than I thought. I did a quick and dirty test
with a folded dipole made up of two 3 foot pieces of RG58, and a feedline
1/2 WL at 160 MHz. Unfortunately, this puts me too close to the upper limit
of the MFJ, but you can see what happens. BTW, I meant the top for the
balanced point. Turns out it makes no difference whether the center
conductor of the left side is connected to the center conductor, or the
shield, of the right side.


My numbers:

F R X
145 30 89
150 10 60
155 ? 7? 14? This looks like 165
160 8 25
165 7 13
170.25 6 0
175 5 14

I would say that if anything, my numbers make less sense than yours. I also
got a reading of 1+j0 at about 65 MHz. Don't know what that means, other
than a dead short. I will hook it up as a regular folded dipole, and see if
I get anything like 300 Ohms. Probably tomorrow.

Was this claimed to be a 50 Ohm antenna?

Tam/WB2TT

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

On Mon, 4 Oct 2004 19:30:19 -0500, "John Smith"
wrote:
400 5.1 + 1.3i
Reference Measurement
Freq A1 B1 ?1 A2 B2 ?2
400 1 0.79 180 1.08 0.695 177

Hi John,

Well, from the two results above, and referencing my copy of Appl.
Note 77-3, Page 7, section "Measuring Rho 100 to 1000 MHz," there are
a number of issues here.

Your B1/A1 is quite off the mark (but certainly correctable, afterall,
that is the purpose of its measurement).

|Rho| = B2·A1/A2·B1 = 0.695·1/1.08·0.79 = 0.815

Casting the magnitude and angle onto a Smith Chart would suggest your
5.1 + 1.3i Ohms is close enough given your data.

The only kicker is port tracking, but I have a hunch that probably is
not an issue.

This bears further consideration. Too bad Kraus did not choose to
elaborate.

73's
Richard Clark, KB7QHC

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).

Just an off the cuff remark and a possible clue.

It's not clear from the picture or explanation if the center
conductor is continious from the right half of the antenna to the
left. I understand the center conductor does not connect to the
right half at the T.

For the case that it does continue:
Looking that the picture, if the right side were parallel to the left
I'd call that a 1/4 wave 1:1 balun with a shorted load connected
rather than an antenna. That would result in a significant reactance
or a short depending on frequency. The sorted load would be the
continued centor conductor in the right half.

For the case that it does Not continue:
I'd call that a 1/4 wave 1:1 balun with an open load connected rather
than an antenna. That would result in a significant reactance again.

There is a third case:
A connect dot is missing at the junction of the center conductor where
it meets the shield of the right loop (top center). If the lengths
were ~1/4 wave for each side then the impedence at the center would
be high for the left center conductor and the right shield and that
would likely be a tuneable folded dipole.

Did I miss something about the antenna design?

As drawn it looks like an attempt to take a parallel line balun (coax
with a 1/4wave 1:1 balun) and make it serve as a radiator. There is
detail missing one possible dimensions and other connections.

FYI: if you used one of those cheap eithernet Tees to create the
junction, I've found them to be very poor at UHF. Use a good quality
one with TFE insulation and test it seperately first.

Allison

wrote in message
...
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).

Just an off the cuff remark and a possible clue.

It's not clear from the picture or explanation if the center
conductor is continious from the right half of the antenna to the
left. I understand the center conductor does not connect to the
right half at the T.

The center conductor in the right half is not used at all. The right half
should actually be a piece of coax with the center conductor removed.

For the case that it does continue:
Looking that the picture, if the right side were parallel to the left
I'd call that a 1/4 wave 1:1 balun with a shorted load connected
rather than an antenna. That would result in a significant reactance
or a short depending on frequency. The sorted load would be the
continued centor conductor in the right half.

For the case that it does Not continue:
I'd call that a 1/4 wave 1:1 balun with an open load connected rather
than an antenna. That would result in a significant reactance again.

There is a third case:
A connect dot is missing at the junction of the center conductor where
it meets the shield of the right loop (top center). If the lengths
were ~1/4 wave for each side then the impedence at the center would
be high for the left center conductor and the right shield and that
would likely be a tuneable folded dipole.

Did I miss something about the antenna design?


The center conductor of the left side is connected to the shield of the
right side. The shield of the left side is not connected at the top center.


As drawn it looks like an attempt to take a parallel line balun (coax
with a 1/4wave 1:1 balun) and make it serve as a radiator. There is
detail missing one possible dimensions and other connections.

FYI: if you used one of those cheap eithernet Tees to create the
junction, I've found them to be very poor at UHF. Use a good quality
one with TFE insulation and test it seperately first.

Allison

I wound up not using a T. I simply soldered all connections to a BNC
connector at the bottom center of the figure.

John

"Tam/WB2TT" wrote in message
...

"John Smith" wrote in message
...

"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

John, This turned out simpler than I thought. I did a quick and dirty test
with a folded dipole made up of two 3 foot pieces of RG58, and a feedline
1/2 WL at 160 MHz. Unfortunately, this puts me too close to the upper
limit of the MFJ, but you can see what happens. BTW, I meant the top for
the balanced point. Turns out it makes no difference whether the center
conductor of the left side is connected to the center conductor, or the
shield, of the right side.


My numbers:

F R X
145 30 89
150 10 60
155 ? 7? 14? This looks like 165
160 8 25
165 7 13
170.25 6 0
175 5 14

I would say that if anything, my numbers make less sense than yours. I
also got a reading of 1+j0 at about 65 MHz. Don't know what that means,
other than a dead short. I will hook it up as a regular folded dipole, and
see if I get anything like 300 Ohms. Probably tomorrow.

Was this claimed to be a 50 Ohm antenna?

Tam/WB2TT


Hey, Tam!

This is great information! According to the dimensions, it should be
resonant at 173 MHz. (Close enough... I subtracted 1 inch on each side for
the loop). Your 6 Ohms is close enough to my 5 Ohms to satisfy me that I'm
not getting useless measurements.

I wouldn't worry about the 65 MHz reading, as that's so far away from
resonance that it is probably acting like a shorted half wave transmission
line or some other kind of crazy network.

No, there was no statement given about the antenna impedance. That's why I
started this thread. I was interested in the antenna because:

1. It is at DC ground.
2. It is a half wave, giving a nice omnidirectional pattern if mounted
vertically.
3. No balun is needed as it is inherent.

However, it appears to be not worthwhile after all, owing to its low
feedpoint impedance.

I really appreciate you taking the time to perform your experiment. I'm now
convinced that the antenna is next to worthless.

Thanks,
John
(KD5YI)

"Richard Clark" wrote in message
...
On Mon, 4 Oct 2004 19:30:19 -0500, "John Smith"
wrote:
400 5.1 + 1.3i
Reference Measurement
Freq A1 B1 ?1 A2 B2 ?2
400 1 0.79 180 1.08 0.695 177

Hi John,

Well, from the two results above, and referencing my copy of Appl.
Note 77-3, Page 7, section "Measuring Rho 100 to 1000 MHz," there are
a number of issues here.

Your B1/A1 is quite off the mark (but certainly correctable, afterall,
that is the purpose of its measurement).

|Rho| = B2·A1/A2·B1 = 0.695·1/1.08·0.79 = 0.815

Casting the magnitude and angle onto a Smith Chart would suggest your
5.1 + 1.3i Ohms is close enough given your data.

The only kicker is port tracking, but I have a hunch that probably is
not an issue.

This bears further consideration. Too bad Kraus did not choose to
elaborate.

73's
Richard Clark, KB7QHC


I don't understand. Please enumerate the issues. What do you mean B1/A1 is
off the mark? If they are off the mark, how can the Z be close enough?

Are you saying that I have calculated the Z correctly from the data and you
think port tracking is not at fault? What further consideration?

You're right. Too bad Kraus didn't tell the feedpoint impedance. I suspect I
would not have embarked on this folly if he had. The antenna may have its
uses elsewhere, but I don't need the headaches of matching 5 Ohms at 440
MHz.

John

On Tue, 5 Oct 2004 09:10:19 -0500, "John Smith"
wrote:

I don't understand. Please enumerate the issues. What do you mean B1/A1 is
off the mark?

Hi John,

You should get 1.0 @ 180° (the definition of a short).

If they are off the mark, how can the Z be close enough?

Because what you DID measure, was used as a correction factor per:
|Rho| = B2·A1/A2·B1 = 0.695·1/1.08·0.79 = 0.815
(or you skipped that step)

Are you saying that I have calculated the Z correctly from the data and you
think port tracking is not at fault?

Well, that is really your job to confirm or deny. There is very
little I can accomplish short of that.

What further consideration?

I suppose I could visit my Engineering Library. I will be on campus
for my Nanotechnology seminar today anyway.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Tue, 5 Oct 2004 09:10:19 -0500, "John Smith"
wrote:

I don't understand. Please enumerate the issues. What do you mean B1/A1 is
off the mark?

Hi John,

You should get 1.0 @ 180° (the definition of a short).

Well, that's not possible when feeding a length of RG58 at 400 MHz, is it?
Remember, I said that there was about a 5 foot length of RG58 between the
directional coupler and the load. How can one get 1.0 reflected to the
coupler when the load is a short? That requires zero loss coax.


If they are off the mark, how can the Z be close enough?

Because what you DID measure, was used as a correction factor per:
|Rho| = B2·A1/A2·B1 = 0.695·1/1.08·0.79 = 0.815
(or you skipped that step)

I did that. As far as I can determine, I did it like the HP application note
said to do it.

Thanks for your comments.

John