I am using an HP 8405A vector voltmeter to try and measure the delay
in a transmission line in the 2 meter band.
This is part of a phasing harness for a pair of 2 meter antennas to
put them into circular polarization.

The set up is as follows: A T connector on the signal generator with a
pair of 75 ohm ¼ wave length jumpers from either side of the T. At the
ends I put an adaptor and a bnc T connector of which one side connects
to the probe of the 8405A and the other end of the bnc T is a 50 ohm
load.
The other 75 ohm ¼ wave jumper has the same setup with the bnc T, 50
ohm load and B channel probe of the 8405A.

With this I see a zero degree phase difference on the 8405A as it
should be.
Then I insert a ¼ wave length 50 ohm cable between one of the 75 ohm
cable ends and the bnc T connector / 8405A probe connection point.
There are the same number of adaptors in each side of the lines. The
only addition is one barrel connector and the ¼ wave length of 50 ohm
cable. (this is in the B channel by the way)
The additional barrel connector is part of the ¼ wave length on the 50
ohm line.

The wanted frequency is 145 Mhz.
Rather than seeing 90 degrees delay with the added 50 ohm line section
I only see 60 degrees of delay?? If I run the frequency up to around
180 Mhz I get the 90 degrees delay shown on the 8405A.
What do you suppose I am doing wrong in the measurement setup?

I have measured the delay on the 50 ohm line section by itself (with
the one barrel connector) and I see 90 degrees of delay at 145 Mhz. I
used 20 db pads on each side of the T from the signal generator to
isolate the two sides of the test setup when measuring this cable by
itself.

Thanks
Gary K4FMX

In article ,
Gary Schafer wrote:

I am using an HP 8405A vector voltmeter to try and measure the delay
in a transmission line in the 2 meter band.
This is part of a phasing harness for a pair of 2 meter antennas to
put them into circular polarization.

The set up is as follows: A T connector on the signal generator with a
pair of 75 ohm ¼ wave length jumpers from either side of the T. At the
ends I put an adaptor and a bnc T connector of which one side connects
to the probe of the 8405A and the other end of the bnc T is a 50 ohm
load.
The other 75 ohm ¼ wave jumper has the same setup with the bnc T, 50
ohm load and B channel probe of the 8405A.

With this I see a zero degree phase difference on the 8405A as it
should be.
Then I insert a ¼ wave length 50 ohm cable between one of the 75 ohm
cable ends and the bnc T connector / 8405A probe connection point.
There are the same number of adaptors in each side of the lines. The
only addition is one barrel connector and the ¼ wave length of 50 ohm
cable. (this is in the B channel by the way)
The additional barrel connector is part of the ¼ wave length on the 50
ohm line.

The wanted frequency is 145 Mhz.
Rather than seeing 90 degrees delay with the added 50 ohm line section
I only see 60 degrees of delay?? If I run the frequency up to around
180 Mhz I get the 90 degrees delay shown on the 8405A.
What do you suppose I am doing wrong in the measurement setup?

I have measured the delay on the 50 ohm line section by itself (with
the one barrel connector) and I see 90 degrees of delay at 145 Mhz. I
used 20 db pads on each side of the T from the signal generator to
isolate the two sides of the test setup when measuring this cable by
itself.

Thanks
Gary K4FMX


When you figured the length of the 1/4 wave section, did you account
for the Velocity Factor of the cable?

Me

Gary Schafer wrote:
The wanted frequency is 145 Mhz.
Rather than seeing 90 degrees delay with the added 50 ohm line section
I only see 60 degrees of delay?? If I run the frequency up to around
180 Mhz I get the 90 degrees delay shown on the 8405A.
What do you suppose I am doing wrong in the measurement setup?

I don't understand the description of the test setup so
this may be a stupid question but what would a 50 ohm
SWR meter read if installed in that 50 ohm line during
the test?
--
73, Cecil http://www.qsl.net/w5dxp

Something is not as it seems. I think I understand your setup; the
ends of the 75 ohm lines both see 50 ohms when you're measuring, in
both the zero degree "calibration" phase and with the measurement of
the 50 ohm line inserted...assuming it really is 50 ohm line (though
that shouldn't make a difference anyway). I assume you've tried
swapping probes, and you see the 60 degree phase shift the opposite
direction? The phase shift of 30 degrees going from 145 to 180 doesn't
sound right: it sounds like the phase shift across a resonance, not
the phase shift in a length of line versus frequency. What happens if
you take your phase readings at several points across a range of
frequencies, say 100MHz to 250MHz? What happens if you set up like in
your first "calibration" phase, with the two probes at the outer ends
of the two 75 ohms sections, and then compare between putting the 50
ohm load on (say) the B channel, and then putting the 50 ohm line
section between the BNC T and the load? Things shouldn't change, doing
that. And finally, what is the impdeance looking into the probe?

The stubs represented by the BNC Ts will cause some loading, but I
don't think you could account for 30 degrees that way.

Do you have an independent way to check the phase accuracy of your
meter? Like, a 10 ohm resistor across a probe, and a small
capacitance--4.7pF, 10pF or so--from the generator [monitored by the
other probe] to the 10 ohm. That should give you a known phase shift
close to 90 degrees, if you know the capacitance reasonably accurately.
You should be able to know it quite a bit closer than 30 degrees
anyway.

Someone else mentioned taking velocity factor into account, but if you
had measured off half a meter of line, the velocity factor would give
you MORE phase shift, not less.

Cheers,
Tom

Forgive me for asking questions about seemingly obvious things, but
something isn't as it appears. If it's really like you've described, the
phase difference should be 90 degrees.

Please verify that the added 50 ohm quarter wave section is between the
end of the 75 ohm line and the 50 ohm terminator/probe connector as you
described and not between the signal generator tee and one of the 75 ohm
lines. If the latter, you will get something other than 90 degrees of
phase shift, although I haven't run the numbers to see what it would be.

Are you sure the added quarter wave line is 50 and not 75 ohms?

Also, what's the ratio of voltage magnitudes at the two probes?

Roy Lewallen, W7EL

Gary Schafer wrote:
I am using an HP 8405A vector voltmeter to try and measure the delay
in a transmission line in the 2 meter band.
This is part of a phasing harness for a pair of 2 meter antennas to
put them into circular polarization.

The set up is as follows: A T connector on the signal generator with a
pair of 75 ohm ¼ wave length jumpers from either side of the T. At the
ends I put an adaptor and a bnc T connector of which one side connects
to the probe of the 8405A and the other end of the bnc T is a 50 ohm
load.
The other 75 ohm ¼ wave jumper has the same setup with the bnc T, 50
ohm load and B channel probe of the 8405A.

With this I see a zero degree phase difference on the 8405A as it
should be.
Then I insert a ¼ wave length 50 ohm cable between one of the 75 ohm
cable ends and the bnc T connector / 8405A probe connection point.
There are the same number of adaptors in each side of the lines. The
only addition is one barrel connector and the ¼ wave length of 50 ohm
cable. (this is in the B channel by the way)
The additional barrel connector is part of the ¼ wave length on the 50
ohm line.

The wanted frequency is 145 Mhz.
Rather than seeing 90 degrees delay with the added 50 ohm line section
I only see 60 degrees of delay?? If I run the frequency up to around
180 Mhz I get the 90 degrees delay shown on the 8405A.
What do you suppose I am doing wrong in the measurement setup?

I have measured the delay on the 50 ohm line section by itself (with
the one barrel connector) and I see 90 degrees of delay at 145 Mhz. I
used 20 db pads on each side of the T from the signal generator to
isolate the two sides of the test setup when measuring this cable by
itself.

Thanks
Gary K4FMX

Thanks guys for the replies.

Yes the 50 ohm line is between the 75 ohm line and the bnc/ probe
connection.

I did try the 50 ohm line between the bnc T and the load as K7ITM
suggested. No change in phase.

These lines are factory made, cushcraft, lines and connectors. The 75
ohm sections are RG59 and the 50 ohm section is RG58/U. The connectors
are crimp on PL type. The barrels are PL type.

My additional adaptors are 1 PL type barrel, a PL male to bnc female
adaptor and the bnc T at the ends of each side where the 50 ohm loads
and probe connections are.
The rest of the PL barrel (between the 50 and 75 ohm line) and PL type
T connector at the generator is factory supplied as part of the
network. I added a PL female to bnc male at the PL T to connect to the
generator.

The length of the 50 ohm line is 9 ¾" from shielded area to shielded
area (not counting the center pin exposed when shell is pulled back).
With the barrel installed it makes it 10 ¾" in length that is
shielded.

The two 75 ohm line sections from the center of the T connector to the
shielded part of the connector on the other end are 10 ½" for each.

The following are readings on the 8405A: 50 ohm line is installed in
the B channel side. (with both 75 ohms lines installed as described)

180 Mhz = -90 degrees A channel -5dbm B channel -2dbm.
145 Mhz = -60 degrees A channel -3.5dbm B channel -3dbm

The following sweep was done:
110 Mhz = -55 Degrees.
160 Mhz = -65.
170 Mhz = -80.
180 Mhz = -90.

The following was measured with an MFJ 259B in place of the signal
generator at the input T:
Freq SWR R X.
110 1.0 46 2.
120 1.0 46 0.
130 1.1 44 3.
140 1.2 41 7.
145 1.3 39 8.
175 1.5 33 7.

As far as verifying the 8405 at 90 degrees it seems to give the 90
degree reading with the 50 ohm coax length by itself with the method I
earlier described with the two attenuators and T connector at the
generator.

Reversing the probes gives the same reading but of course the opposite
phase.

My thought is that the extra Barrel, Pl to bnc adaptor and bnc
connector add enough length to change the impedance enough to throw
things off? But the longer length seems like it would give more phase
shift rather than less?

I think th probes are supposed to be around 100K? load. I also have
some isolators for the probes which I tried and still got about the
same readings. They raise the impedance of the probe. Not sure by how
much offhand.

With the 50 ohm line and one barrel connected by itself to the MFJ259
it seems to dip at around 145 Mhz.

Thanks for the help.

Gary K4FMX


On Wed, 03 May 2006 13:29:59 -0700, Roy Lewallen
wrote:

Forgive me for asking questions about seemingly obvious things, but
something isn't as it appears. If it's really like you've described, the
phase difference should be 90 degrees.

Please verify that the added 50 ohm quarter wave section is between the
end of the 75 ohm line and the 50 ohm terminator/probe connector as you
described and not between the signal generator tee and one of the 75 ohm
lines. If the latter, you will get something other than 90 degrees of
phase shift, although I haven't run the numbers to see what it would be.

Are you sure the added quarter wave line is 50 and not 75 ohms?

Also, what's the ratio of voltage magnitudes at the two probes?

Roy Lewallen, W7EL

Gary Schafer wrote:
I am using an HP 8405A vector voltmeter to try and measure the delay
in a transmission line in the 2 meter band.
This is part of a phasing harness for a pair of 2 meter antennas to
put them into circular polarization.

The set up is as follows: A T connector on the signal generator with a
pair of 75 ohm ¼ wave length jumpers from either side of the T. At the
ends I put an adaptor and a bnc T connector of which one side connects
to the probe of the 8405A and the other end of the bnc T is a 50 ohm
load.
The other 75 ohm ¼ wave jumper has the same setup with the bnc T, 50
ohm load and B channel probe of the 8405A.

With this I see a zero degree phase difference on the 8405A as it
should be.
Then I insert a ¼ wave length 50 ohm cable between one of the 75 ohm
cable ends and the bnc T connector / 8405A probe connection point.
There are the same number of adaptors in each side of the lines. The
only addition is one barrel connector and the ¼ wave length of 50 ohm
cable. (this is in the B channel by the way)
The additional barrel connector is part of the ¼ wave length on the 50
ohm line.

The wanted frequency is 145 Mhz.
Rather than seeing 90 degrees delay with the added 50 ohm line section
I only see 60 degrees of delay?? If I run the frequency up to around
180 Mhz I get the 90 degrees delay shown on the 8405A.
What do you suppose I am doing wrong in the measurement setup?

I have measured the delay on the 50 ohm line section by itself (with
the one barrel connector) and I see 90 degrees of delay at 145 Mhz. I
used 20 db pads on each side of the T from the signal generator to
isolate the two sides of the test setup when measuring this cable by
itself.

Thanks
Gary K4FMX

Gary Schafer wrote:
180 Mhz = -90 degrees A channel -5dbm B channel -2dbm.
145 Mhz = -60 degrees A channel -3.5dbm B channel -3dbm

Did anyone else notice that 90/60 = 75/50?
--
73, Cecil http://www.qsl.net/w5dxp

Gary Schafer wrote:
. . .
The length of the 50 ohm line is 9 ¾" from shielded area to shielded
area (not counting the center pin exposed when shell is pulled back).
With the barrel installed it makes it 10 ¾" in length that is
shielded.
. . .

By my reckoning, a free space quarter wavelength at 145 MHz is 20.335
inches. Assuming solid polyethylene dielectric coax, this would be about
13.4 inches of coax. 10-3/4 inches would be about 72 degrees.

The probes have a shunt C of 2.5 pF, resulting in a Z (-X) of about 440
ohms at 145 MHz, not 100k ohms. If you parallel that with 50 ohms and
terminate a 72 degree 50 ohm cable with it, the voltage delay in the
cable is 78 degrees -- that is, the imperfect termination actually
increases the cable delay by 6 degrees. If the cable Z0 is 45 ohms
instead of 50, the delay is a couple of degrees less with that load; if
it's 55 ohms, the delay is a couple of degrees more. Maybe your 50 ohm
termination isn't quite what you think it is -- as you can see, it
doesn't take much to substantially change the cable's delay.

As I've said often, most people don't realize how difficult it is to
make accurate RF measurements.

Roy Lewallen, W7EL

On Wed, 03 May 2006 20:46:06 -0700, Roy Lewallen
wrote:

Gary Schafer wrote:
. . .
The length of the 50 ohm line is 9 ¾" from shielded area to shielded
area (not counting the center pin exposed when shell is pulled back).
With the barrel installed it makes it 10 ¾" in length that is
shielded.
. . .

By my reckoning, a free space quarter wavelength at 145 MHz is 20.335
inches. Assuming solid polyethylene dielectric coax, this would be about
13.4 inches of coax. 10-3/4 inches would be about 72 degrees.

The probes have a shunt C of 2.5 pF, resulting in a Z (-X) of about 440
ohms at 145 MHz, not 100k ohms. If you parallel that with 50 ohms and
terminate a 72 degree 50 ohm cable with it, the voltage delay in the
cable is 78 degrees -- that is, the imperfect termination actually
increases the cable delay by 6 degrees. If the cable Z0 is 45 ohms
instead of 50, the delay is a couple of degrees less with that load; if
it's 55 ohms, the delay is a couple of degrees more. Maybe your 50 ohm
termination isn't quite what you think it is -- as you can see, it
doesn't take much to substantially change the cable's delay.

As I've said often, most people don't realize how difficult it is to
make accurate RF measurements.

Roy Lewallen, W7EL

Measuring RF is difficult! I can't seem to get any two methods to come
out in the same ball park.

If I just have the two 75 ohm lines hooked up with 50 ohm terminations
on the ends and measure with the probes I get 0 degrees phase.

I can hook one of the 75 ohm lines only with a 100 ohm load and one
probe at the generator end and a 50 ohm load at the other end with a
probe and see near 90 degrees.

I can do the same with the 50 ohm line with a 50 ohm load on each end
and see near 90 degrees.

But when I have the two 75 ohm lines and add the 50 ohm line in one
leg I get only around 60 degrees. By pulling the probe of the
connector (B channel) at the end of the 50 ohm line, and keeping the
center pin close but not in contact so there is only light capacitive
coupling the phase shift increases by around 10 degrees or so.

If I add a few elbow adaptors to the 50 ohm line to increase its
length to arund14 ½" inches or so then I see close to 90 degrees.

So it would seem that my 50 ohm line is too short but when measuring
it without the 75 ohm lines it looks to be the right length.

I built a couple of different 90 degree phasing setups several years
ago using two 75 ohm ¼ wave length lines, a spdt relay and a 90 ohm ¼
wave length line all hooked to the relay to be able to reverse
polarization.
I measured that setup in a similar manor only the center of the relay
went to the signal generator and the 50 ohm loads and probes went at
the ends of the 75 ohm lines. I get 60 degrees of phase shift on that
setup too.

I don't remember how I trimmed the 90 ohm line at the time but I do
remember I tried several different methods to try and measure what was
happening and had the same problems I am having now. I didn't have the
8405A then.
It seems I am doing no better with it. :)

I would like to find a repeatable method that will work when adding a
relay and or connectors to be able to know pretty close what the
actual delay is.

Thanks for the help.

Gary K4FMX