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

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
--

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).
--

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).
--

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
--

Perhaps this lab document might help you:

http://emclab.concordia.ca/~trueman/...ent_2_2005.pdf
--

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

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.

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.

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
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