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.

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.

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
--

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.

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!

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.

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
--

"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

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

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.