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dansawyeror
January 1st 06, 03:14 AM
This is the next chapter in the antenna measuring saga. Today's adventure is
trying to measure SWR with an HP 8405A Vector Voltmeter.

The measuring setup is a standard signal generator feeding a tee. One branch
feeds the A input of the 8405A and the other a 20 db directional configured in
reverse, that is feeding the out port. The assumption is the reflected signal
will be read correctly on the in port. The in port is then terminated or
connected to an antenna.

An open termination reads 180 degrees and a 50 Ohm termination reads 0 degrees
and 55 db down from the input. (I assume 20 of that is the coupler so I am
subtracting 20 from that reading - is that a correct assumption.)

Using the method to read the SWR on an antenna produces values very close to
those from several SWR meters used as controls. When the output is terminated in
25 Ohms (2 x 50 Ohm terminators on a tee) the reflected signal reads about 24.5
db. (The B channel won't calibrate to within 1/2 a db so readings are estimates)
Subtracting the 20 db bias yields 4.5 db. Within the error of the instrument
this is pretty close to an SWR of 2:1.

Is there an error in this logic? If the instrument is working well enough to
perform this calculation then it should support more complicated measurements.

Thanks - Dan

Owen Duffy
January 1st 06, 04:25 AM
On Sat, 31 Dec 2005 18:14:04 -0800, dansawyeror
> wrote:

>This is the next chapter in the antenna measuring saga. Today's adventure is
>trying to measure SWR with an HP 8405A Vector Voltmeter.

As I see it, your instrument with accessories (directional coupler,
attenuator, etc) can be used to measure the complex reflection
coefficient (Gamma) at the measurement plane.

Reading the mail, it seems have already worked out how to do that, and
to calculate Z given gamma.

If you want to determine SWR, you don't need the argument of Gamma,
you just need its magnitude rho. SWR=(1+rho)/(1-rho) where
rho=|Gamma|. (You are of course throwing away part of the measurement
data, the part that is the real power of the VVM. You could measure
rho with a directional coupler, calibrated attenuator and xtal
detector or a bolometer power meter... but they won't give you the
phase data that the VVM does.)

The return loss of VSWR=2 is 9.5dB, not 4.5 as you suggest. You need
to review some of these simple relationships. You also seem not to be
properly including the directional coupler in the calibration loop,
why the tee as described, doesn't the 8405A have a tee probe?

Is using a VVM an overkill for determining SWR? Probably, especially
if the complexity results in errors.

Owen
--

Owen Duffy
January 1st 06, 04:34 AM
On Sun, 01 Jan 2006 03:25:23 GMT, Owen Duffy > wrote:


>Reading the mail, it seems have already worked out how to do that, and
>to calculate Z given gamma.

Should read:

Reading the mail, it seems have already worked out how to do that, and
to calculate Z given Gamma (with a captial G).
--

dansawyeror
January 1st 06, 07:12 AM
Where are these equations? I have searched the ARRL handbook for Gamma and did
not find it. This is also the first reference to rho.

I was planning on the Z equation next, yes I remember that one from earlier in
the week.

Owen Duffy wrote:
> On Sun, 01 Jan 2006 03:25:23 GMT, Owen Duffy > wrote:
>
>
>
>>Reading the mail, it seems have already worked out how to do that, and
>>to calculate Z given gamma.
>
>
> Should read:
>
> Reading the mail, it seems have already worked out how to do that, and
> to calculate Z given Gamma (with a captial G).
> --

Owen Duffy
January 1st 06, 07:46 AM
On Sat, 31 Dec 2005 22:12:47 -0800, dansawyeror
> wrote:

>Where are these equations? I have searched the ARRL handbook for Gamma and did
>not find it. This is also the first reference to rho.

I am at my holiday place, and don't have reference manuals here, so I
can't give you specific references. I am confident that the main
formulas you need will be in both the ARRL Handbook and the ARRL
Antenna Handbook (though some versions of the ARRL publications
contain an incorrect formula for Gamma / rho). They will be in any
text on transmission lines.

Because I don't have the Geek font available here, I write Gamma to
mean the uppercase Greek symbol gamma (G). Textbooks should use the
symbol.

"Gamma" (meaning the uppercase Greek Gamma symbol) is the complex
reflection coefficient and Gamma=(Zl-Zo)/(Zl+Zo) where Zl and Zo are
complex quantities. rho (lowercase Greek rho) is the magnitude of
Gamma, rho=|Gamma|. VSWR=(1+rho)/(1-rho). You can rearrange these
expressions to suit your needs. For example,
Zl=Zo*(1+Gamma)/(1-Gamma).

Be careful, there are different notations and meanings in different
texts.

You need a reference!

Owen
--

Owen Duffy
January 1st 06, 08:51 AM
Perhaps this lab document might help you:

http://emclab.concordia.ca/~trueman/elec453/Experiment_2_2005.pdf
--

Cecil Moore
January 1st 06, 04:06 PM
dansawyeror wrote:
> Where are these equations? I have searched the ARRL handbook for Gamma
> and did not find it. This is also the first reference to rho.

Some texts, like ITT's "Reference Data for Radio Engineers"
simply define rho to be complex as does Ramo and Whinnery.

rho = Eref/Efor = -Iref/Ifor = (Z-Z0)/(Z+Z0) =
(Y0-Y)/(Y0+Y) = |rho| /_ 2*psi

where psi is the electrical angle to the nearest voltage
maximum looking back toward the generator from the load.

Walter C. Johnson uses 'k' for the reflection coefficient.
--
73, Cecil http://www.qsl.net/w5dxp

Wes Stewart
January 1st 06, 04:46 PM
On Sat, 31 Dec 2005 18:14:04 -0800, dansawyeror
> wrote:

>This is the next chapter in the antenna measuring saga. Today's adventure is
>trying to measure SWR with an HP 8405A Vector Voltmeter.
>
>The measuring setup is a standard signal generator feeding a tee. One branch
>feeds the A input of the 8405A and the other a 20 db directional configured in
>reverse, that is feeding the out port. The assumption is the reflected signal
>will be read correctly on the in port. The in port is then terminated or
>connected to an antenna.

It would also be nice it you had a 6 - 10 dB pad between the generator
and the directional coupler (DC); located right at the DC. You want
the source match to be set right there and the A probe to sample right
there.

>
>An open termination reads 180 degrees and a 50 Ohm termination reads 0 degrees
>and 55 db down from the input. (I assume 20 of that is the coupler so I am
>subtracting 20 from that reading - is that a correct assumption.)

No and no. Although an open isn't as bad a reference as some folks
think (in coax anyway), a better reference is a short. Then you know
that the reflection is 100% < -180 deg. (Where "<" means "angle of")
An open has a bit of fringing capacitance and a tiny bit of radiation
so it strays (sorry) from 100% < 0 degrees.

The other thing you need to do is normalize all future readings to 0
dB. Because, as Owen notes, you're throwing away phase information in
your quest for SWR only data, then start thinking in terms of return
loss. Return loss is measured (or calculated) against a 0 dB
reference. So from the beginning, when you get a reading with a short
(or open) that ratio becomes your 0 dB reference.

>
>Using the method to read the SWR on an antenna produces values very close to
>those from several SWR meters used as controls. When the output is terminated in
>25 Ohms (2 x 50 Ohm terminators on a tee) the reflected signal reads about 24.5
>db. (The B channel won't calibrate to within 1/2 a db so readings are estimates)
>Subtracting the 20 db bias yields 4.5 db. Within the error of the instrument
>this is pretty close to an SWR of 2:1.

Are you saying that it you step the input power to the "B" channel by
1 dB, it measures a 1/2 dB change, or that with the same signal
applied to A and B they only agree within 1/2 dB?

If it's the former, then stop here and fix the instrument. If it's
the latter, you don't care; you are measuring ratios.
>
>Is there an error in this logic? If the instrument is working well enough to
>perform this calculation then it should support more complicated measurements.

There are errors someplace. A 2:1 SWR is a return loss of 9.55 dB.

Wes Stewart
January 1st 06, 05:13 PM
On Sat, 31 Dec 2005 22:12:47 -0800, dansawyeror
> wrote:

>Where are these equations? I have searched the ARRL handbook for Gamma and did
>not find it. This is also the first reference to rho.

Rho is mentioned on page 5 of AN77-3. Unfortunately, as I have
written a long time ago in other threads, Hewlett-Packard's authors
(and divisions) could not agree among themselves on a standard
notation for complex reflection coefficient.

The same holds true for the rest of the literature. My personal (and
others') choice is to use upper case Gamma for the complex reflection
coefficient and lower case rho for the magnitude of Gamma.

If this was standard then there would be no confusion.

Dave Platt
January 1st 06, 08:15 PM
In article >,
Wes Stewart > wrote:

>>The measuring setup is a standard signal generator feeding a tee. One branch
>>feeds the A input of the 8405A and the other a 20 db directional configured in
>>reverse, that is feeding the out port. The assumption is the reflected signal
>>will be read correctly on the in port. The in port is then terminated or
>>connected to an antenna.
>
>It would also be nice it you had a 6 - 10 dB pad between the generator
>and the directional coupler (DC); located right at the DC. You want
>the source match to be set right there and the A probe to sample right
>there.

The 8405A manual indicates the use of a power divider, and then a pair
of equal-value pads. One side goes to the probe T for the A
(reference) probe and thence to the termination, and the other goes to
the probe T for the B probe and thence to the device-under-test.

The manual is quite clear that the A and B probes need to be connected
to points which are isolated from one another. You really don't want
the oddities of the load connected to the B side to affect the
voltage/phase of the reference signal seen by the "A" probe - it'd
certainly wreck the measurement.

Using a power divider, and resistive pads for isolation is one way to
do this. Using a pad followed by a dual directional coupler (as in
the experiment page to which Owen posted a link - thanks!) is another.

--
Dave Platt > AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Wes Stewart
January 1st 06, 10:01 PM
On Sun, 01 Jan 2006 19:15:12 -0000, (Dave Platt)
wrote:

>In article >,
>Wes Stewart > wrote:
>
>>>The measuring setup is a standard signal generator feeding a tee. One branch
>>>feeds the A input of the 8405A and the other a 20 db directional configured in
>>>reverse, that is feeding the out port. The assumption is the reflected signal
>>>will be read correctly on the in port. The in port is then terminated or
>>>connected to an antenna.
>>
>>It would also be nice it you had a 6 - 10 dB pad between the generator
>>and the directional coupler (DC); located right at the DC. You want
>>the source match to be set right there and the A probe to sample right
>>there.
>
>The 8405A manual indicates the use of a power divider, and then a pair
>of equal-value pads. One side goes to the probe T for the A
>(reference) probe and thence to the termination, and the other goes to
>the probe T for the B probe and thence to the device-under-test.

Actually, my manual does not show this. Although I have the full kit
of a resistive tee, two 50 ohm "N" sampling tees and appropriate
terminations, I don't believe Dan does.
>
>The manual is quite clear that the A and B probes need to be connected
>to points which are isolated from one another.

I've read this someplace, but again my version of the manual (unless
I'm really missing something) doesn't say it. Nevertheless, the
directional coupler provides the isolation between probes.

>You really don't want
>the oddities of the load connected to the B side to affect the
>voltage/phase of the reference signal seen by the "A" probe - it'd
>certainly wreck the measurement.

>Using a power divider, and resistive pads for isolation is one way to
>do this.

Actually, you do want to measure (include) the effects of the load on
the source. Although you ideally want the source to be unchangable
with respect to changes in the load (good source match). That is the
point of my suggestion to pad the source at the input to the coupler.
(Just as does paragraph 2.3 of Owen's reference)

But if the load -does- pull the source you damn sure want to know it
and account for it. The desired ratio is that between the incident
signal and the reflected signal. Sampling somewhere off in isolated
space via a tee and additional padding does -not- yield the incident
signal.

>
> Using a pad followed by a dual directional coupler (as in
>the experiment page to which Owen posted a link - thanks!) is another.

My point exactly. With the dual coupler the forward coupled arm -is-
measuring the incident signal -regardless- of what effects load pull
have on the source output.

Wes Stewart
January 1st 06, 10:35 PM
On Sun, 01 Jan 2006 14:01:18 -0700, Wes Stewart >
wrote:

>On Sun, 01 Jan 2006 19:15:12 -0000, (Dave Platt)
>wrote:
>
>>In article >,
>>Wes Stewart > wrote:
>>
>>>>The measuring setup is a standard signal generator feeding a tee. One branch
>>>>feeds the A input of the 8405A and the other a 20 db directional configured in
>>>>reverse, that is feeding the out port. The assumption is the reflected signal
>>>>will be read correctly on the in port. The in port is then terminated or
>>>>connected to an antenna.
>>>
>>>It would also be nice it you had a 6 - 10 dB pad between the generator
>>>and the directional coupler (DC); located right at the DC. You want
>>>the source match to be set right there and the A probe to sample right
>>>there.
>>
>>The 8405A manual indicates the use of a power divider, and then a pair
>>of equal-value pads. One side goes to the probe T for the A
>>(reference) probe and thence to the termination, and the other goes to
>>the probe T for the B probe and thence to the device-under-test.
>
>Actually, my manual does not show this. Although I have the full kit
>of a resistive tee, two 50 ohm "N" sampling tees and appropriate
>terminations, I don't believe Dan does.
>>
>>The manual is quite clear that the A and B probes need to be connected
>>to points which are isolated from one another.
>
>I've read this someplace, but again my version of the manual (unless
>I'm really missing something) doesn't say it. Nevertheless, the
>directional coupler provides the isolation between probes.
>
>>You really don't want
>>the oddities of the load connected to the B side to affect the
>>voltage/phase of the reference signal seen by the "A" probe - it'd
>>certainly wreck the measurement.
>
>>Using a power divider, and resistive pads for isolation is one way to
>>do this.
>
>Actually, you do want to measure (include) the effects of the load on
>the source. Although you ideally want the source to be unchangable
>with respect to changes in the load (good source match). That is the
>point of my suggestion to pad the source at the input to the coupler.
>(Just as does paragraph 2.3 of Owen's reference)
>
>But if the load -does- pull the source you damn sure want to know it
>and account for it. The desired ratio is that between the incident
>signal and the reflected signal. Sampling somewhere off in isolated
>space via a tee and additional padding does -not- yield the incident
>signal.
>
>>
>> Using a pad followed by a dual directional coupler (as in
>>the experiment page to which Owen posted a link - thanks!) is another.
>
>My point exactly. With the dual coupler the forward coupled arm -is-
>measuring the incident signal -regardless- of what effects load pull
>have on the source output.

I've got to clarify this a bit if I can...

If you have the full set of parts per figure 11 in AN77-3 and you are
using them as shown, then with equal loads on the two ends, the
circuit is essentially a resistive Wheatstone bridge in balance with
the null detected by the difference between probes A and B.

In this case, the "incident" signal -is- measured by the A probe and
the effects to the source by a changing load are incorporated into the
measurement.

In the case at hand, at least as I imagine it, there is no longer an
nice tidy resistive Wheatstone bridge, but some cabling and a
directional coupler in the mix. In this case, the generator is no
longer the "source", the source is the signal at the input to the
coupler. It is my belief (unless I change my mind later) that a
sample derived from a resistive divider remote from the input to the
directional coupler is not a true measure of the incident signal.

Owen Duffy
January 1st 06, 10:45 PM
On Sun, 01 Jan 2006 03:25:23 GMT, Owen Duffy > wrote:

>On Sat, 31 Dec 2005 18:14:04 -0800, dansawyeror
> wrote:
>

>why the tee as described, doesn't the 8405A have a tee probe?

Dan, my reason for this question is that you do not seem to have a
load independent sample of the forward wave (if you are referencing
measurements to the A probe).

Ideally you would use a dual directional coupler, otherwise, you need
to isolate the A sample from load impedance variations and reflections
using a largish attenuator for example.

If you are working the ratios out entirely from the B probe, eg B
probe measurements on s/c and unknown load, then the A measurement
becomes unimportant. Note that while this approach can give you enough
info to measure rho and calculate SWR, it will not permit phase
measurements. This approach dumbs the instrument down to a single
channel RF voltmeter.

Owen
--

dansawyeror
January 1st 06, 11:03 PM
The VVM appears to do a great job of reading 'open circuit' at 180 degrees and
50 Ohms at 0 degrees, however it does not appear to read shorts accurately.
Shorts read at about 150 degrees instead of -180. They also degrade the incoming
signal to the point where is has significant noise on the monitor output line.
Is there a technique for doing this?

Thanks - Dan


>>This is the next chapter in the antenna measuring saga. Today's adventure is
>>trying to measure SWR with an HP 8405A Vector Voltmeter.
>>
>>The measuring setup is a standard signal generator feeding a tee. One branch
>>feeds the A input of the 8405A and the other a 20 db directional configured in
>>reverse, that is feeding the out port. The assumption is the reflected signal
>>will be read correctly on the in port. The in port is then terminated or
>>connected to an antenna.
>
>
> It would also be nice it you had a 6 - 10 dB pad between the generator
> and the directional coupler (DC); located right at the DC. You want
> the source match to be set right there and the A probe to sample right
> there.
>
>
>>An open termination reads 180 degrees and a 50 Ohm termination reads 0 degrees
>>and 55 db down from the input. (I assume 20 of that is the coupler so I am
>>subtracting 20 from that reading - is that a correct assumption.)
>
>
> No and no. Although an open isn't as bad a reference as some folks
> think (in coax anyway), a better reference is a short. Then you know
> that the reflection is 100% < -180 deg. (Where "<" means "angle of")
> An open has a bit of fringing capacitance and a tiny bit of radiation
> so it strays (sorry) from 100% < 0 degrees.
>
> The other thing you need to do is normalize all future readings to 0
> dB. Because, as Owen notes, you're throwing away phase information in
> your quest for SWR only data, then start thinking in terms of return
> loss. Return loss is measured (or calculated) against a 0 dB
> reference. So from the beginning, when you get a reading with a short
> (or open) that ratio becomes your 0 dB reference.
>
>
>>Using the method to read the SWR on an antenna produces values very close to
>>those from several SWR meters used as controls. When the output is terminated in
>>25 Ohms (2 x 50 Ohm terminators on a tee) the reflected signal reads about 24.5
>>db. (The B channel won't calibrate to within 1/2 a db so readings are estimates)
>>Subtracting the 20 db bias yields 4.5 db. Within the error of the instrument
>>this is pretty close to an SWR of 2:1.
>
>
> Are you saying that it you step the input power to the "B" channel by
> 1 dB, it measures a 1/2 dB change, or that with the same signal
> applied to A and B they only agree within 1/2 dB?
>
> If it's the former, then stop here and fix the instrument. If it's
> the latter, you don't care; you are measuring ratios.
>
>>Is there an error in this logic? If the instrument is working well enough to
>>perform this calculation then it should support more complicated measurements.
>
>
> There are errors someplace. A 2:1 SWR is a return loss of 9.55 dB.
>

Dave Platt
January 1st 06, 11:10 PM
In article >,
Wes Stewart > wrote:

>>>The 8405A manual indicates the use of a power divider, and then a pair
>>>of equal-value pads. One side goes to the probe T for the A
>>>(reference) probe and thence to the termination, and the other goes to
>>>the probe T for the B probe and thence to the device-under-test.
>>
>>Actually, my manual does not show this. Although I have the full kit
>>of a resistive tee, two 50 ohm "N" sampling tees and appropriate
>>terminations, I don't believe Dan does.
>>>
>>>The manual is quite clear that the A and B probes need to be connected
>>>to points which are isolated from one another.
>>
>>I've read this someplace, but again my version of the manual (unless
>>I'm really missing something) doesn't say it.

The BAMA copy mentions it in paragraph 3-14. Later text indicates
that attaching the two probes to a single point is an appropriate way
to set phase-zero.

>> Nevertheless, the
>>directional coupler provides the isolation between probes.

I see the issue, and I think I was conflating two different sorts of
measurement regimes.

The splitter/isolator/pad arrangement I was referring to appears on
page 3-3 of the 8405A manual available at BAMA. It's what's
appropriate for doing an in-line test of a transmission line or other
network, where you want to see the effect of the network itself and
can measure (via probe B) at the network's output.

Page 3-4 shows a somewhat similar hookup, which doesn't include the
resistive pads... I presume because the device-under-test (an
amplifier) is assumed to have high isolation as part of its design.

Neither of these hookups wouldn't work for measuring an antenna, since
you can't measure at the antenna's output. Instead, using a
directional coupler provides the necessary isolation, and (as you
point out) lets you determine the incident and reflected signals
accurately.

>I've got to clarify this a bit if I can...
>
>If you have the full set of parts per figure 11 in AN77-3 and you are
>using them as shown, then with equal loads on the two ends, the
>circuit is essentially a resistive Wheatstone bridge in balance with
>the null detected by the difference between probes A and B.
>
>In this case, the "incident" signal -is- measured by the A probe and
>the effects to the source by a changing load are incorporated into the
>measurement.
>
>In the case at hand, at least as I imagine it, there is no longer an
>nice tidy resistive Wheatstone bridge, but some cabling and a
>directional coupler in the mix. In this case, the generator is no
>longer the "source", the source is the signal at the input to the
>coupler. It is my belief (unless I change my mind later) that a
>sample derived from a resistive divider remote from the input to the
>directional coupler is not a true measure of the incident signal.

Hmmm. In the general case, I believe you're correct.

I suspect that the setup shown in the 8405A manual sets up a specific
special case, though. The diagrams and text seem to be defining a
case in which:

- there is a physical and electrical symmetry in the T arrangement -
that is, the power splitter is symmetrical, and the pair of
attenuator pads between the splitter and the (A probe tap) and
(device under test) are matched. The manual makes a point of this
issue.

- The pads being used are matched to the system's transmission line
impedance, so that any reflected signal coming back from the
DUT/coupler sees a proper termination by the source (the pad and
signal generator, in this case) and is not re-reflected.

In this particular situation, I believe that the incident signals
reaching the DUT (the input to the coupler, in this case) and the "A"
probe, would be identical... would they not? The proper termination
of the reflected wave will mean that it won't re-reflect off of the
generator and alter the incident wave. The "A" probe signal (off on
its side of the "T") and a signal read out via the incident-wave tap
on the directional coupler ought to be the same, once the coupling
coefficient is taken into account... no?

--
Dave Platt > AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

dansawyeror
January 1st 06, 11:32 PM
I do not have the 8405a adapter kit or power splitter. I made "probe to bnc"
adapters with 3/4 inch copper plumbing stock. The threads on the back of the
probe are standard thread so it is easy to make an adapter and simply screw the
probes in. This allows the A and B inputs to be connected to the directional
coupler with bnc.

The measuring setup is a signal generator feeding a tee. One side of the tee
connects to the A channel input. The other side feeds the 'output side' of the
coupler. The B port is connected to the coupled signal port. The load is
connected to the coupler input. This should read the reflected signal.

The signal generator is adjusted to read 0 dbm on the A port for all readings.
(This is not possible for the 'shorted' readings). I do not have a pad between
the generator and the tee. However, the above setup appears to support
consistent readings.

Dan


Wes Stewart wrote:
> On Sun, 01 Jan 2006 19:15:12 -0000, (Dave Platt)
> wrote:
>
>
>>In article >,
>>Wes Stewart > wrote:
>>
>>
>>>>The measuring setup is a standard signal generator feeding a tee. One branch
>>>>feeds the A input of the 8405A and the other a 20 db directional configured in
>>>>reverse, that is feeding the out port. The assumption is the reflected signal
>>>>will be read correctly on the in port. The in port is then terminated or
>>>>connected to an antenna.
>>>
>>>It would also be nice it you had a 6 - 10 dB pad between the generator
>>>and the directional coupler (DC); located right at the DC. You want
>>>the source match to be set right there and the A probe to sample right
>>>there.
>>
>>The 8405A manual indicates the use of a power divider, and then a pair
>>of equal-value pads. One side goes to the probe T for the A
>>(reference) probe and thence to the termination, and the other goes to
>>the probe T for the B probe and thence to the device-under-test.
>
>
> Actually, my manual does not show this. Although I have the full kit
> of a resistive tee, two 50 ohm "N" sampling tees and appropriate
> terminations, I don't believe Dan does.
>
>>The manual is quite clear that the A and B probes need to be connected
>>to points which are isolated from one another.
>
>
> I've read this someplace, but again my version of the manual (unless
> I'm really missing something) doesn't say it. Nevertheless, the
> directional coupler provides the isolation between probes.
>
>
>>You really don't want
>>the oddities of the load connected to the B side to affect the
>>voltage/phase of the reference signal seen by the "A" probe - it'd
>>certainly wreck the measurement.
>
>
>>Using a power divider, and resistive pads for isolation is one way to
>>do this.
>
>
> Actually, you do want to measure (include) the effects of the load on
> the source. Although you ideally want the source to be unchangable
> with respect to changes in the load (good source match). That is the
> point of my suggestion to pad the source at the input to the coupler.
> (Just as does paragraph 2.3 of Owen's reference)
>
> But if the load -does- pull the source you damn sure want to know it
> and account for it. The desired ratio is that between the incident
> signal and the reflected signal. Sampling somewhere off in isolated
> space via a tee and additional padding does -not- yield the incident
> signal.
>
>
>> Using a pad followed by a dual directional coupler (as in
>>the experiment page to which Owen posted a link - thanks!) is another.
>
>
> My point exactly. With the dual coupler the forward coupled arm -is-
> measuring the incident signal -regardless- of what effects load pull
> have on the source output.
>

dansawyeror
January 2nd 06, 01:51 AM
Owen,

Yes, you are right. The single coupler doesn't isolate the channels. Putting a
10 dbm attenuator between the tee and the coupler changes the 50 Ohm reading. I
think a dual directional coupler is required. I will have to put this on hold
until that problem is solved.

Thanks - Dan

Owen Duffy wrote:
> On Sun, 01 Jan 2006 03:25:23 GMT, Owen Duffy > wrote:
>
>
>>On Sat, 31 Dec 2005 18:14:04 -0800, dansawyeror
> wrote:
>>
>
>
>>why the tee as described, doesn't the 8405A have a tee probe?
>
>
> Dan, my reason for this question is that you do not seem to have a
> load independent sample of the forward wave (if you are referencing
> measurements to the A probe).
>
> Ideally you would use a dual directional coupler, otherwise, you need
> to isolate the A sample from load impedance variations and reflections
> using a largish attenuator for example.
>
> If you are working the ratios out entirely from the B probe, eg B
> probe measurements on s/c and unknown load, then the A measurement
> becomes unimportant. Note that while this approach can give you enough
> info to measure rho and calculate SWR, it will not permit phase
> measurements. This approach dumbs the instrument down to a single
> channel RF voltmeter.
>
> Owen
> --

Jerry Martes
January 2nd 06, 02:22 AM
"dansawyeror" > wrote in message
. ..
> Owen,
>
> Yes, you are right. The single coupler doesn't isolate the channels.
> Putting a 10 dbm attenuator between the tee and the coupler changes the 50
> Ohm reading. I think a dual directional coupler is required. I will have
> to put this on hold until that problem is solved.
>
> Thanks - Dan

Dan

Are you open to trying to assemble something to measure the reflection
coefficient rather than to put the project on hold?
The HP 41952A Transmission/Reflection Test Set uses only one directional
coupler. It uses a power splitter at the input with a pad to level the
outputs from the "Fwd" and "Rev" ports. I can scan some info from the HP
41952 and E-mail them to you if you have interest in building something.
I would think it would be fairly easy to build devices for HF if you
already have a decent directional coupler.

Jerry

dansawyeror
January 2nd 06, 04:39 AM
Jerry,

Thanks, the antenna handbook has a design for a bi-directional coupler that
looks like it will work. At this point I think the next step is to separate the
input to the A port from the reflected signal. Putting a pad in the circuit to
increase isolation affected the reflected reading. I am also sorting through
those readings.

Dan

Jerry Martes wrote:
> "dansawyeror" > wrote in message
> . ..
>
>>Owen,
>>
>>Yes, you are right. The single coupler doesn't isolate the channels.
>>Putting a 10 dbm attenuator between the tee and the coupler changes the 50
>>Ohm reading. I think a dual directional coupler is required. I will have
>>to put this on hold until that problem is solved.
>>
>>Thanks - Dan
>
>
> Dan
>
> Are you open to trying to assemble something to measure the reflection
> coefficient rather than to put the project on hold?
> The HP 41952A Transmission/Reflection Test Set uses only one directional
> coupler. It uses a power splitter at the input with a pad to level the
> outputs from the "Fwd" and "Rev" ports. I can scan some info from the HP
> 41952 and E-mail them to you if you have interest in building something.
> I would think it would be fairly easy to build devices for HF if you
> already have a decent directional coupler.
>
> Jerry
>
>

Wes Stewart
January 2nd 06, 04:40 AM
On Sun, 01 Jan 2006 22:10:09 -0000, (Dave Platt)
wrote:

>
>In article >,
>Wes Stewart > wrote:
>
>>>>The 8405A manual indicates the use of a power divider, and then a pair
>>>>of equal-value pads. One side goes to the probe T for the A
>>>>(reference) probe and thence to the termination, and the other goes to
>>>>the probe T for the B probe and thence to the device-under-test.
>>>
>>>Actually, my manual does not show this. Although I have the full kit
>>>of a resistive tee, two 50 ohm "N" sampling tees and appropriate
>>>terminations, I don't believe Dan does.
>>>>
>>>>The manual is quite clear that the A and B probes need to be connected
>>>>to points which are isolated from one another.
>>>
>>>I've read this someplace, but again my version of the manual (unless
>>>I'm really missing something) doesn't say it.
>
>The BAMA copy mentions it in paragraph 3-14. Later text indicates
>that attaching the two probes to a single point is an appropriate way
>to set phase-zero.

Bingo. Didn't seem like an "Electrical feature" to me :-)
>
>>> Nevertheless, the
>>>directional coupler provides the isolation between probes.
>
>I see the issue, and I think I was conflating two different sorts of
>measurement regimes.
>
>The splitter/isolator/pad arrangement I was referring to appears on
>page 3-3 of the 8405A manual available at BAMA. It's what's
>appropriate for doing an in-line test of a transmission line or other
>network, where you want to see the effect of the network itself and
>can measure (via probe B) at the network's output.
>
>Page 3-4 shows a somewhat similar hookup, which doesn't include the
>resistive pads... I presume because the device-under-test (an
>amplifier) is assumed to have high isolation as part of its design.

No, that's very similar to figure 11 in AN77-3 that I mentioned below.
Without inserting the device under test per Fig 3-3, but removing the
right hand 50 ohm load and connecting the device there, reflection
measurements can be performed. That's a technique described in
AN77-3. The configuration is that of a Wheatstone bridge as mentioned
earlier.

But I still submit that when you separate the bridge, insert a DC and
some cabling, you lose the symmetry and the signal measured by the A
probe is not necessarily the same as the signal incident at the input
to the DC. Close maybe, but not something I would rely on.

Remember, when doing the calibration there is a 100% reflection. This
can have a huge perturbing effect on the incident signal at the
coupler input if the source is not well matched. That's why I
originally suggested a pad right at the coupler input, especially if
there is some cabling between the generator (or power splitting tee)
and the DC.

>
>Neither of these hookups wouldn't work for measuring an antenna, since
>you can't measure at the antenna's output.

Not so, see above.

>Instead, using a
>directional coupler provides the necessary isolation, and (as you
>point out) lets you determine the incident and reflected signals
>accurately.
>
>>I've got to clarify this a bit if I can...
>>
>>If you have the full set of parts per figure 11 in AN77-3 and you are
>>using them as shown, then with equal loads on the two ends, the
>>circuit is essentially a resistive Wheatstone bridge in balance with
>>the null detected by the difference between probes A and B.
>>
>>In this case, the "incident" signal -is- measured by the A probe and
>>the effects to the source by a changing load are incorporated into the
>>measurement.
>>
>>In the case at hand, at least as I imagine it, there is no longer an
>>nice tidy resistive Wheatstone bridge, but some cabling and a
>>directional coupler in the mix. In this case, the generator is no
>>longer the "source", the source is the signal at the input to the
>>coupler. It is my belief (unless I change my mind later) that a
>>sample derived from a resistive divider remote from the input to the
>>directional coupler is not a true measure of the incident signal.
>
>Hmmm. In the general case, I believe you're correct.
>
>I suspect that the setup shown in the 8405A manual sets up a specific
>special case, though. The diagrams and text seem to be defining a
>case in which:
>
>- there is a physical and electrical symmetry in the T arrangement -
> that is, the power splitter is symmetrical, and the pair of
> attenuator pads between the splitter and the (A probe tap) and
> (device under test) are matched. The manual makes a point of this
> issue.
>
>- The pads being used are matched to the system's transmission line
> impedance, so that any reflected signal coming back from the
> DUT/coupler sees a proper termination by the source (the pad and
> signal generator, in this case) and is not re-reflected.
>
>In this particular situation, I believe that the incident signals
>reaching the DUT (the input to the coupler, in this case) and the "A"
>probe, would be identical... would they not? The proper termination
>of the reflected wave will mean that it won't re-reflect off of the
>generator and alter the incident wave. The "A" probe signal (off on
>its side of the "T") and a signal read out via the incident-wave tap
>on the directional coupler ought to be the same, once the coupling
>coefficient is taken into account... no?

No. The B probe, in the single directional coupler arrangement, is
not measuring -incident-, but reflected signal.

In any event, Dan has stated that he doesn't have all of this stuff
and is stuck using the DC only. My suggestion holds, put a pad at the
DC input, measure the incident at the DC input and of course, the
reflected at the coupled port.

Dave Platt
January 2nd 06, 05:11 AM
In article >,
Wes Stewart > wrote:

>Bingo. Didn't seem like an "Electrical feature" to me :-)

Yeah. I'm not sure just why there would be substantial interaction if
the two points are connected to the same test point, since the rated
impedance is pretty high even without 10:1 isolators.

>But I still submit that when you separate the bridge, insert a DC and
>some cabling, you lose the symmetry and the signal measured by the A
>probe is not necessarily the same as the signal incident at the input
>to the DC. Close maybe, but not something I would rely on.

The signal on the other side of the T-and-attenuator setup wouldn't be
the same as the signal at the input to the DC, certainly, since the
signal at the input of the DC would be affected by the reflected
signal. I don't disagree with you there.

What I suggest, though, is that the signal on the "A" probe (at the
other side of the T from the DC), and a signal as seen at the output
of the DC's "forward" coupler line, ought to be very closely
correlated. They'd differ by the coupler's coupling factor, of
course, and there's be a bit of phase shift from the coupler
(dependent on the coupler line length and the frequency). However,
the loading at the coupler output from the load (or the calibration
short) ought not to affect the signal appearing at the 'forward' tap
on the coupler.

>Remember, when doing the calibration there is a 100% reflection. This
>can have a huge perturbing effect on the incident signal at the
>coupler input if the source is not well matched.

Agreed, and I don't suggest that measuring the incident at the coupler
input is a good idea.


> That's why I
>originally suggested a pad right at the coupler input, especially if
>there is some cabling between the generator (or power splitting tee)
>and the DC.

Agreed.

>No. The B probe, in the single directional coupler arrangement, is
>not measuring -incident-, but reflected signal.

True. I was assuming a double directional coupler, and asserting that
the "forward" output on the coupler will produce a signal equivalent
(except for scaling and perhaps a tad of phase shift) to a signal
taken from the far side of the splitter-and-pads "T".

>In any event, Dan has stated that he doesn't have all of this stuff
>and is stuck using the DC only. My suggestion holds, put a pad at the
>DC input, measure the incident at the DC input and of course, the
>reflected at the coupled port.

Yes, that should work quite well, and I think it'd give results pretty
much equivalent to [1] a dual directional coupler or [2] the
splitter-and-two-pads isolation arrangement.

--
Dave Platt > AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

dansawyeror
January 2nd 06, 05:32 AM
Wes,

I only have copies of a few pages from the app note showing a bi-directional
coupler. Is your reference that the note shows a bridge?

Thanks - Dan

Wes Stewart wrote:
> On Sun, 01 Jan 2006 22:10:09 -0000, (Dave Platt)
> wrote:
>
>
>>In article >,
>>Wes Stewart > wrote:
>>
>>
>>>>>The 8405A manual indicates the use of a power divider, and then a pair
>>>>>of equal-value pads. One side goes to the probe T for the A
>>>>>(reference) probe and thence to the termination, and the other goes to
>>>>>the probe T for the B probe and thence to the device-under-test.
>>>>
>>>>Actually, my manual does not show this. Although I have the full kit
>>>>of a resistive tee, two 50 ohm "N" sampling tees and appropriate
>>>>terminations, I don't believe Dan does.
>>>>
>>>>>The manual is quite clear that the A and B probes need to be connected
>>>>>to points which are isolated from one another.
>>>>
>>>>I've read this someplace, but again my version of the manual (unless
>>>>I'm really missing something) doesn't say it.
>>
>>The BAMA copy mentions it in paragraph 3-14. Later text indicates
>>that attaching the two probes to a single point is an appropriate way
>>to set phase-zero.
>
>
> Bingo. Didn't seem like an "Electrical feature" to me :-)
>
>>>> Nevertheless, the
>>>>directional coupler provides the isolation between probes.
>>
>>I see the issue, and I think I was conflating two different sorts of
>>measurement regimes.
>>
>>The splitter/isolator/pad arrangement I was referring to appears on
>>page 3-3 of the 8405A manual available at BAMA. It's what's
>>appropriate for doing an in-line test of a transmission line or other
>>network, where you want to see the effect of the network itself and
>>can measure (via probe B) at the network's output.
>>
>>Page 3-4 shows a somewhat similar hookup, which doesn't include the
>>resistive pads... I presume because the device-under-test (an
>>amplifier) is assumed to have high isolation as part of its design.
>
>
> No, that's very similar to figure 11 in AN77-3 that I mentioned below.
> Without inserting the device under test per Fig 3-3, but removing the
> right hand 50 ohm load and connecting the device there, reflection
> measurements can be performed. That's a technique described in
> AN77-3. The configuration is that of a Wheatstone bridge as mentioned
> earlier.
>
> But I still submit that when you separate the bridge, insert a DC and
> some cabling, you lose the symmetry and the signal measured by the A
> probe is not necessarily the same as the signal incident at the input
> to the DC. Close maybe, but not something I would rely on.
>
> Remember, when doing the calibration there is a 100% reflection. This
> can have a huge perturbing effect on the incident signal at the
> coupler input if the source is not well matched. That's why I
> originally suggested a pad right at the coupler input, especially if
> there is some cabling between the generator (or power splitting tee)
> and the DC.
>
>
>>Neither of these hookups wouldn't work for measuring an antenna, since
>>you can't measure at the antenna's output.
>
>
> Not so, see above.
>
>
>>Instead, using a
>>directional coupler provides the necessary isolation, and (as you
>>point out) lets you determine the incident and reflected signals
>>accurately.
>>
>>
>>>I've got to clarify this a bit if I can...
>>>
>>>If you have the full set of parts per figure 11 in AN77-3 and you are
>>>using them as shown, then with equal loads on the two ends, the
>>>circuit is essentially a resistive Wheatstone bridge in balance with
>>>the null detected by the difference between probes A and B.
>>>
>>>In this case, the "incident" signal -is- measured by the A probe and
>>>the effects to the source by a changing load are incorporated into the
>>>measurement.
>>>
>>>In the case at hand, at least as I imagine it, there is no longer an
>>>nice tidy resistive Wheatstone bridge, but some cabling and a
>>>directional coupler in the mix. In this case, the generator is no
>>>longer the "source", the source is the signal at the input to the
>>>coupler. It is my belief (unless I change my mind later) that a
>>>sample derived from a resistive divider remote from the input to the
>>>directional coupler is not a true measure of the incident signal.
>>
>>Hmmm. In the general case, I believe you're correct.
>>
>>I suspect that the setup shown in the 8405A manual sets up a specific
>>special case, though. The diagrams and text seem to be defining a
>>case in which:
>>
>>- there is a physical and electrical symmetry in the T arrangement -
>> that is, the power splitter is symmetrical, and the pair of
>> attenuator pads between the splitter and the (A probe tap) and
>> (device under test) are matched. The manual makes a point of this
>> issue.
>>
>>- The pads being used are matched to the system's transmission line
>> impedance, so that any reflected signal coming back from the
>> DUT/coupler sees a proper termination by the source (the pad and
>> signal generator, in this case) and is not re-reflected.
>>
>>In this particular situation, I believe that the incident signals
>>reaching the DUT (the input to the coupler, in this case) and the "A"
>>probe, would be identical... would they not? The proper termination
>>of the reflected wave will mean that it won't re-reflect off of the
>>generator and alter the incident wave. The "A" probe signal (off on
>>its side of the "T") and a signal read out via the incident-wave tap
>>on the directional coupler ought to be the same, once the coupling
>>coefficient is taken into account... no?
>
>
> No. The B probe, in the single directional coupler arrangement, is
> not measuring -incident-, but reflected signal.
>
> In any event, Dan has stated that he doesn't have all of this stuff
> and is stuck using the DC only. My suggestion holds, put a pad at the
> DC input, measure the incident at the DC input and of course, the
> reflected at the coupled port.
>

Owen Duffy
January 2nd 06, 08:15 AM
On Sun, 01 Jan 2006 16:51:37 -0800, dansawyeror
> wrote:

>Owen,
>
>Yes, you are right. The single coupler doesn't isolate the channels. Putting a
>10 dbm attenuator between the tee and the coupler changes the 50 Ohm reading. I
>think a dual directional coupler is required. I will have to put this on hold
>until that problem is solved.
>

Dan, it seems to me that you should be able to make measurements with
a single directional coupler (DC)

You could connect your signal generator to the directional coupler via
a 40dB attenuator, and put the chan A probe T on the sig gen end of
the attenuator. This sample should be fairly independent of the
reflection from the unknown load (to the extent of the 40dB
attenuator), and so approximately proportional to the incident wave
alone.

The sample from the DC "reflected" port (properly terminated) is
fairly independent of the incident wave (depending on the F/B ratio of
the coupler) and so is approximately proportional to the reflected
wave alone.

Calibration of the B channel magnitude with a s/c and o/c taken as
rho=1 provides the basis for measurement of Gamma. The angle of Gamma
should be calibrated to 180 and o deg respectively.

BTW, the angle of Gamma for a 50 ohm termination is unimportant if rho
is very small. The angle of Gamma is real important for s/c and o/c
and ought be almost exactly 180 deg difference (if not, you have a
instrument problem).

Following this procedure, if the magnitude of the B channel on the
unknown load measures for example 9.5dB below the B chan magnitude on
a s/c, then the return loss is 9.5dB and the VSWR is 2:1. rho (the
magnitude of Gamma) is 0.333 and you could measure the phase offset
from the o/c angle to determine the angle of Gamma.

Why won't this work?

Owen
--

dansawyeror
January 2nd 06, 10:34 PM
Owen,

That idea seems to work. I set it up and then looked and the 'interference', the
change in phase based on changing the pad. Zero pad showed several degrees phase
shift from -40 dbm, 10 dbm showed small shift, and 20, 30, and 40 were all about
equal. I decided on 20 dbm as a practical base.


Thanks - Dan


Owen Duffy wrote:
> On Sun, 01 Jan 2006 16:51:37 -0800, dansawyeror
> > wrote:
>
>
>>Owen,
>>
>>Yes, you are right. The single coupler doesn't isolate the channels. Putting a
>>10 dbm attenuator between the tee and the coupler changes the 50 Ohm reading. I
>>think a dual directional coupler is required. I will have to put this on hold
>>until that problem is solved.
>>
>
>
> Dan, it seems to me that you should be able to make measurements with
> a single directional coupler (DC)
>
> You could connect your signal generator to the directional coupler via
> a 40dB attenuator, and put the chan A probe T on the sig gen end of
> the attenuator. This sample should be fairly independent of the
> reflection from the unknown load (to the extent of the 40dB
> attenuator), and so approximately proportional to the incident wave
> alone.
>
> The sample from the DC "reflected" port (properly terminated) is
> fairly independent of the incident wave (depending on the F/B ratio of
> the coupler) and so is approximately proportional to the reflected
> wave alone.
>
> Calibration of the B channel magnitude with a s/c and o/c taken as
> rho=1 provides the basis for measurement of Gamma. The angle of Gamma
> should be calibrated to 180 and o deg respectively.
>
> BTW, the angle of Gamma for a 50 ohm termination is unimportant if rho
> is very small. The angle of Gamma is real important for s/c and o/c
> and ought be almost exactly 180 deg difference (if not, you have a
> instrument problem).
>
> Following this procedure, if the magnitude of the B channel on the
> unknown load measures for example 9.5dB below the B chan magnitude on
> a s/c, then the return loss is 9.5dB and the VSWR is 2:1. rho (the
> magnitude of Gamma) is 0.333 and you could measure the phase offset
> from the o/c angle to determine the angle of Gamma.
>
> Why won't this work?
>
> Owen
> --

dansawyeror
January 2nd 06, 10:36 PM
Now that the setup is reading consistently I will 'test' a loaded 2m monopole
over a 1 m**2 ground plane.

Dan

Owen Duffy wrote:
> Perhaps this lab document might help you:
>
> http://emclab.concordia.ca/~trueman/elec453/Experiment_2_2005.pdf
> --

Owen Duffy
January 2nd 06, 10:56 PM
On Mon, 02 Jan 2006 13:34:54 -0800, dansawyeror
> wrote:

>Owen,
>
>That idea seems to work. I set it up and then looked and the 'interference', the
>change in phase based on changing the pad. Zero pad showed several degrees phase
>shift from -40 dbm, 10 dbm showed small shift, and 20, 30, and 40 were all about
>equal. I decided on 20 dbm as a practical base.

This doesn't make sense... are you using "dbm" to mean decibels of
attenuation, usually written "dB".

The units "dBm" are usually written to qualify a power level with
respect to one milliwatt.

The attenuator on your sig gen might be marked in dBm, but that
applies to the combination of the oscillator, possibly its level
meter, and the attenuator as a system.

Using the wrong terms for things is often a result of a concept gap!

A 20dB attenuator will reduce the effect of the reflected component to
about the same level as you would expect from a practical directional
coupler, more attenuation is better if you have the power from the sig
gen and the VVM probe chan can operate at the higher input level.

Owen
--

Roy Lewallen
January 2nd 06, 11:35 PM
dansawyeror wrote:
> Now that the setup is reading consistently I will 'test' a loaded 2m
> monopole over a 1 m**2 ground plane.

I suggest that you start with an unloaded monopole or some very simple
antenna with a well known impedance. (You will of course have to know
and allow for the effect of the finite ground plane.)

You also need to take measures to prevent coupling between the antenna
and the outside of the feedline. The ground plane you mention will help,
but there can still be substantial coupling. Some high impedance ferrite
beads at the feedpoint and another set about a quarter wavelength down
should reduce the coupling to a small value.

Roy Lewallen, W7EL

dansawyeror
January 3rd 06, 01:10 AM
I proceeded before reading this note. The procedure was to zero the phase meter
on an short and then to test the loaded 2m vertical. The result was +10 dbm
forward (before the 20 dbm pad) and -50 dbm reflected. The coupler measures
about -14 dbm. The total was about -60 dbm, with 34 db of that due to the pad
and coupler. The net is -26 db forward - reflected.

(The phase angle and reflected ware very touchy. It was almost impossible to
adjust by changing frequency. It was easier to 'adjust' it by sitting very still
and moving my arm.)

The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a 1 inch
long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is mounted over a 2
foot square aluminum plate. This antenna should have an input impedance less
then 20 Ohms.

How can it measure very close to 50 Ohms? Is there something wrong with this
analysis?

Thanks - Dan

Roy Lewallen wrote:
> dansawyeror wrote:
>
>> Now that the setup is reading consistently I will 'test' a loaded 2m
>> monopole over a 1 m**2 ground plane.
>
>
> I suggest that you start with an unloaded monopole or some very simple
> antenna with a well known impedance. (You will of course have to know
> and allow for the effect of the finite ground plane.)
>
> You also need to take measures to prevent coupling between the antenna
> and the outside of the feedline. The ground plane you mention will help,
> but there can still be substantial coupling. Some high impedance ferrite
> beads at the feedpoint and another set about a quarter wavelength down
> should reduce the coupling to a small value.
>
> Roy Lewallen, W7EL

Roy Lewallen
January 3rd 06, 01:21 AM
dansawyeror wrote:
> I proceeded before reading this note. The procedure was to zero the
> phase meter on an short and then to test the loaded 2m vertical. The
> result was +10 dbm forward (before the 20 dbm pad) and -50 dbm
> reflected. The coupler measures about -14 dbm. The total was about -60
> dbm, with 34 db of that due to the pad and coupler. The net is -26 db
> forward - reflected.
>
> (The phase angle and reflected ware very touchy. It was almost
> impossible to adjust by changing frequency. It was easier to 'adjust' it
> by sitting very still and moving my arm.)
>
> The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a
> 1 inch long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is
> mounted over a 2 foot square aluminum plate. This antenna should have an
> input impedance less then 20 Ohms.

How did you arrive at this figure? I wouldn't hazard a guess without
modeling it.

> How can it measure very close to 50 Ohms?

1. Inductor loss.
2. Effect of finite size ground plane.
3. Coupling to feedline.
4. Measurement error.

Is there something wrong with
> this analysis?

I don't know. What should the impedance really be?

Roy Lewallen, W7EL

Reg Edwards
January 3rd 06, 04:25 AM
"Owen Duffy" wrote

> Using the wrong terms for things is often a result of a concept gap!
>
===========================================

.. . . . . and using the wrong name for an SWR meter often results in a
concept gap.
----
Reg.

Owen Duffy
January 3rd 06, 07:42 AM
On Mon, 02 Jan 2006 16:10:02 -0800, dansawyeror
> wrote:

> The coupler measures
>about -14 dbm.

What does this mean? Are you trying to tell us that the power on the
coupler port is 14dB less than the through power? What has dBm got to
do with it?

You didn't report the power in the coupler port with a s/c and / or
o/c at the measurement plane. Did you perform this cal?

>(The phase angle and reflected ware very touchy. It was almost impossible to
>adjust by changing frequency. It was easier to 'adjust' it by sitting very still
>and moving my arm.)

That is understandable. How much coax between the A probe and the
load, and the B probe and the load... how many degress does this total
electrical length change for a 1% change in frequency? Does that
explain some of the phase sensitivity?

>
>The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a 1 inch
>long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is mounted over a 2
>foot square aluminum plate. This antenna should have an input impedance less
>then 20 Ohms.

Don't you need to measure some "known" loads. Why not try a 50 ohm
load tee'd to a s/c stub (quarter wave at a known frequency) and see
if you get the predictable results at different frequencies around
resonance. Then try two 50 ohm loads in parallel with the stub. (339mm
of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at
half that frequency it should be 0.85+j50, etc...

>
>How can it measure very close to 50 Ohms? Is there something wrong with this
>analysis?

See if you can trust your measurements on known loads before wondering
why the unknown load isn't what you expect when using unknown
measurement technology... too many unknowns.

Owen
--

Richard Clark
January 3rd 06, 08:06 AM
On Tue, 03 Jan 2006 06:42:53 GMT, Owen Duffy > wrote:

>>(The phase angle and reflected ware very touchy. It was almost impossible to
>>adjust by changing frequency. It was easier to 'adjust' it by sitting very still
>>and moving my arm.)
>
>...Does that
>explain some of the phase sensitivity?

Hi Owen,

Being very touchy, especially to the specifics of sitting very still,
sounds like classic common mode problems.

73's
Richard Clark, KB7QHC

Owen Duffy
January 3rd 06, 09:36 AM
On Mon, 02 Jan 2006 23:06:43 -0800, Richard Clark >
wrote:

>On Tue, 03 Jan 2006 06:42:53 GMT, Owen Duffy > wrote:
>
>>>(The phase angle and reflected ware very touchy. It was almost impossible to
>>>adjust by changing frequency. It was easier to 'adjust' it by sitting very still
>>>and moving my arm.)
>>
>>...Does that
>>explain some of the phase sensitivity?
>
>Hi Owen,
>
>Being very touchy, especially to the specifics of sitting very still,
>sounds like classic common mode problems.

Yes it does Richard. I saw Roy's response regarding isolation of the
feedline, and it is a valid comment. My comment was towards the
reported frequency sensitivity... until the effect of the propagation
delay is removed from the results, the underlying impedance is
obsured.

Owen
--

Wes Stewart
January 3rd 06, 04:25 PM
On Mon, 02 Jan 2006 21:56:37 GMT, Owen Duffy > wrote:

>On Mon, 02 Jan 2006 13:34:54 -0800, dansawyeror
> wrote:
>
>>Owen,
>>
>>That idea seems to work. I set it up and then looked and the 'interference', the
>>change in phase based on changing the pad. Zero pad showed several degrees phase
>>shift from -40 dbm, 10 dbm showed small shift, and 20, 30, and 40 were all about
>>equal. I decided on 20 dbm as a practical base.
>
>This doesn't make sense... are you using "dbm" to mean decibels of
>attenuation, usually written "dB".
>
>The units "dBm" are usually written to qualify a power level with
>respect to one milliwatt.
>
>The attenuator on your sig gen might be marked in dBm, but that
>applies to the combination of the oscillator, possibly its level
>meter, and the attenuator as a system.
>
>Using the wrong terms for things is often a result of a concept gap!
>
>A 20dB attenuator will reduce the effect of the reflected component to
>about the same level as you would expect from a practical directional
>coupler, more attenuation is better if you have the power from the sig
>gen and the VVM probe chan can operate at the higher input level.

Owen

Let me pick a nit or two.

More attenuation is not necessarily better. In theory the improvement
in source match is two times the attenuation, so a 10 dB pad improves
the return loss to no less than 20 dB, even with a zero ohm source,
and with a decent source match of RL = 10 dB or so, is as good as you
need. (I know you know this already)

I say this because it's very likely that the return loss of the
attenuator isn't any better than 25-30 dB, regardless of its
attenuation. For example Narda makes a "precision" Type N attenuator:

http://www.nardamicrowave.com/east/PassiveComponents/pdf/attenuators/FixedPrecision.pdf

Note the VSWR spec, 1.15 at low frequency. That's a 23 dB RL. So
although a 20 dB pad in theory provides a minimum 40 dB RL, the actual
RL can be as little as 23 dB.

Manufacturers have to work really hard and typically use a precision
connector like 3.5mm or 7mm to build a 40 dB RL termination although
Anritsu will sell you a 40 dB RL type N termination for -only- $650
USD.

Also, and this goes back a post or two, where you suggested that if a
pad is used between the generator and the input to the coupler, the
"A" probe (reference/incident) should be between the generator and the
pad.

This is contrary to what I tried to recommend earlier when I said:

"It would also be nice it you had a 6 - 10 dB pad between the
generator and the directional coupler (DC); located right at the DC.
You want the source match to be set right there and the A probe to
sample right there."

Let me offer this thought experiment:

If you had two directional couplers, such that one could be used to
sample the forward signal and the other the reflected, would you place
a pad between them to isolate the generator from the effects of the
load?

Owen Duffy
January 3rd 06, 09:04 PM
On Tue, 03 Jan 2006 08:25:48 -0700, Wes Stewart >
wrote:

....
>Owen
>
>Let me pick a nit or two.
>
>More attenuation is not necessarily better. In theory the improvement
>in source match is two times the attenuation, so a 10 dB pad improves
>the return loss to no less than 20 dB, even with a zero ohm source,
>and with a decent source match of RL = 10 dB or so, is as good as you
>need. (I know you know this already)
>
>I say this because it's very likely that the return loss of the
>attenuator isn't any better than 25-30 dB, regardless of its
>attenuation. For example Narda makes a "precision" Type N attenuator:
>
>http://www.nardamicrowave.com/east/PassiveComponents/pdf/attenuators/FixedPrecision.pdf
>
>Note the VSWR spec, 1.15 at low frequency. That's a 23 dB RL. So
>although a 20 dB pad in theory provides a minimum 40 dB RL, the actual
>RL can be as little as 23 dB.
>
>Manufacturers have to work really hard and typically use a precision
>connector like 3.5mm or 7mm to build a 40 dB RL termination although
>Anritsu will sell you a 40 dB RL type N termination for -only- $650
>USD.
>
>Also, and this goes back a post or two, where you suggested that if a
>pad is used between the generator and the input to the coupler, the
>"A" probe (reference/incident) should be between the generator and the
>pad.
>
>This is contrary to what I tried to recommend earlier when I said:
>
>"It would also be nice it you had a 6 - 10 dB pad between the
>generator and the directional coupler (DC); located right at the DC.
>You want the source match to be set right there and the A probe to
>sample right there."
>
>Let me offer this thought experiment:
>
>If you had two directional couplers, such that one could be used to
>sample the forward signal and the other the reflected, would you place
>a pad between them to isolate the generator from the effects of the
>load?

No, of course not... it just adds another source of error, and
increases the gap between the measurements being made on both probes.

But Dan does not have two directional couplers. To my mind, if the A
probe is sampling the main transmission line, the sample is of the
resultant of the algebraic sum of the forward and reflected waves
rather than a sample of the forward wave alone (well, nearly alone) as
you would get with a directional coupler. My suggestion of placing the
A probe on the source side of the attenuator is to reduce the
contribution of the reflected wave to the A probe measurement. The
attenuator was proposed mainly for isolation of the forward wave
component for measurement, rather than a source matching issue...
which also exists.

Have I got this wrong?

All comments on practical limits of RL from pads / attenuators noted,
and understood.

Owen

>
>
--

dansawyeror
January 4th 06, 06:52 AM
Thanks,

I went back through the calibration procedure.

1. The meter appears to be performing as it is supposed to. The 'static readings
work repeatably and predictably. With a balanced configuration shorts and
opens read correctly.

2. The pad I was using appears to have a frequency shift. With the 'legs
balanced for length' the measurement would change with frequency. This really
confused the measurement process. I put it aside.

3. I have some 23 Ohm couplers. These of course confuse readings when used as
normal couplers. However if they are used in a truly balanced configuration -
that is the legs are configured as close to identical as possible then they
appear to work.

I am going to shelve this until I find or make a 50 Ohm bi-directional coupler.

Thanks again - Dan


Owen Duffy wrote:
> On Mon, 02 Jan 2006 16:10:02 -0800, dansawyeror
> > wrote:
>
>
>>The coupler measures
>>about -14 dbm.
>
>
> What does this mean? Are you trying to tell us that the power on the
> coupler port is 14dB less than the through power? What has dBm got to
> do with it?
>
> You didn't report the power in the coupler port with a s/c and / or
> o/c at the measurement plane. Did you perform this cal?
>
>
>>(The phase angle and reflected ware very touchy. It was almost impossible to
>>adjust by changing frequency. It was easier to 'adjust' it by sitting very still
>>and moving my arm.)
>
>
> That is understandable. How much coax between the A probe and the
> load, and the B probe and the load... how many degress does this total
> electrical length change for a 1% change in frequency? Does that
> explain some of the phase sensitivity?
>
>
>>The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a 1 inch
>>long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is mounted over a 2
>>foot square aluminum plate. This antenna should have an input impedance less
>>then 20 Ohms.
>
>
> Don't you need to measure some "known" loads. Why not try a 50 ohm
> load tee'd to a s/c stub (quarter wave at a known frequency) and see
> if you get the predictable results at different frequencies around
> resonance. Then try two 50 ohm loads in parallel with the stub. (339mm
> of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at
> half that frequency it should be 0.85+j50, etc...
>
>
>>How can it measure very close to 50 Ohms? Is there something wrong with this
>>analysis?
>
>
> See if you can trust your measurements on known loads before wondering
> why the unknown load isn't what you expect when using unknown
> measurement technology... too many unknowns.
>
> Owen
> --

Owen Duffy
January 4th 06, 10:35 PM
On Tue, 03 Jan 2006 08:25:48 -0700, Wes Stewart >
wrote:


>Owen
>
>Let me pick a nit or two.

....

Firstly, thanks for posting AN 77-3, it is a long time since I read
it, and have little recollection of the recommended low frequency test
setup.

The HP 11549A is described as a power splitter, so I am guessing that
it is some kind of hybrid (ie as in hybrid transformer) that in that
role, whilst splitting the power to the "output" ports, will prevent
power flow between "output" ports, so isolating the A probe to some
extent from the reflected wave on the unknown load side.

Additionally, the 8491 attenuator in the load path will improve the
return loss at the B side 11549A port, so that combination seems to be
stabilising the loads presented to the splitter (which if it is a
hybrid, improves is cross port isolation), and improving the RL by the
action of the attenuator and splitter.

Without knowing the loss in the 8491 (I know they were available in
10dB, but I think there were -3, -6, -10 and -20s), or the isolation
across the splitter, it is hard to quantify the total isolation of
reflected wave from the A probe.

It may be that Dan should consider constructing a hybrid or Return
Loss Bridge, whatever you want to call it, it will be cheaper and have
less loss that a dual directional coupler for HF measurements. IIRC,
the ARRL has some simple designs.

Owen
--

Wes Stewart
January 4th 06, 11:21 PM
On Wed, 04 Jan 2006 21:35:40 GMT, Owen Duffy > wrote:

>On Tue, 03 Jan 2006 08:25:48 -0700, Wes Stewart >
>wrote:
>
>
>>Owen
>>
>>Let me pick a nit or two.
>
>...
>
>Firstly, thanks for posting AN 77-3, it is a long time since I read
>it, and have little recollection of the recommended low frequency test
>setup.
>
>The HP 11549A is described as a power splitter, so I am guessing that
>it is some kind of hybrid (ie as in hybrid transformer) that in that
>role, whilst splitting the power to the "output" ports, will prevent
>power flow between "output" ports, so isolating the A probe to some
>extent from the reflected wave on the unknown load side.

Nothing so exotic

www.k6mhe.com/n7ws/HP-11549.pdf

>
>Additionally, the 8491 attenuator in the load path will improve the
>return loss at the B side 11549A port, so that combination seems to be
>stabilising the loads presented to the splitter (which if it is a
>hybrid, improves is cross port isolation), and improving the RL by the
>action of the attenuator and splitter.
>
>Without knowing the loss in the 8491 (I know they were available in
>10dB, but I think there were -3, -6, -10 and -20s), or the isolation
>across the splitter, it is hard to quantify the total isolation of
>reflected wave from the A probe.
>
>It may be that Dan should consider constructing a hybrid or Return
>Loss Bridge, whatever you want to call it, it will be cheaper and have
>less loss that a dual directional coupler for HF measurements. IIRC,
>the ARRL has some simple designs.

Maybe, but I think Dan is trying some VHF measurements and the coupler
approach is really the way to go.


Wes

dansawyeror
January 6th 06, 07:24 AM
I now have use of a b-directional coupler for HF and would like to perform the
following 'tests' however I do not understand them. Can you explain the last a
different way. I don't understand "two 50 Ohm loads in parallel with the stub".

Tests of stubs are now clearly reading correct for various lengths across
frequencies. That is once the 1/4 wave is determined the next odd and even
is very predicable.

Thanks,
Dan

>
> Don't you need to measure some "known" loads. Why not try a 50 ohm
> load tee'd to a s/c stub (quarter wave at a known frequency) and see
> if you get the predictable results at different frequencies around
> resonance. Then try two 50 ohm loads in parallel with the stub. (339mm
> of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at
> half that frequency it should be 0.85+j50, etc...
>
>

Owen Duffy
January 6th 06, 07:48 AM
On Thu, 05 Jan 2006 22:24:54 -0800, dansawyeror
> wrote:

>I now have use of a b-directional coupler for HF and would like to perform the
>following 'tests' however I do not understand them. Can you explain the last a
>different way. I don't understand "two 50 Ohm loads in parallel with the stub".

Parallel two 50 ohm loads with a tee piece, then use another tee to
put the stub in parallel. The tees will be imperfect, but at HF, the
impact should not be major.

>
>Tests of stubs are now clearly reading correct for various lengths across
>frequencies. That is once the 1/4 wave is determined the next odd and even
>is very predicable.

Do you get the correct answers for 1/8, 3/8 wave stubs in parallel
with a 50 ohm load? (1/8 wave s/c stub has an impedance close to
0+jRo, and you need to put it in parallel with the dummy load, so
series equivalent is 25+j25. 25 ohms in parallel with 1.8 wave s/c
stub should be around 20+j10. Put a 25 ohm load at the end of a metre
of coax and check its transformation at different frequencies
(equivalent to quarter wave, eight wave etc) against your Smith Chart
prog.

They are some examples, work some out for what you have at hand.

Prove that your measurement system works on predictable loads.

Owen

>
>Thanks,
>Dan
>
>>
>> Don't you need to measure some "known" loads. Why not try a 50 ohm
>> load tee'd to a s/c stub (quarter wave at a known frequency) and see
>> if you get the predictable results at different frequencies around
>> resonance. Then try two 50 ohm loads in parallel with the stub. (339mm
>> of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at
>> half that frequency it should be 0.85+j50, etc...
>>
>>
--

Walter Maxwell
January 6th 06, 06:21 PM
Reg, I always thought SWR meant Short Wave Radio.

Walt, W2DU

>
>. . . . . and using the wrong name for an SWR meter often results in a
>concept gap.
>----
>Reg.
>

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