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