hi guys,

After an hour or more of pushing and twiddling various buttons and
knobs on my VNA I have at least established two standards: I connect a
short and the (polar) spot flys off to the extreme left-hand-side and
sits stably on the central horizontal axis. I then remove the short and
replace it with an open and the spot jumps across 180 degrees to the
extreme right-hand side on the same plane. All well and good so far.
Next, I replace the open with a lab-grade 50 ohm termination and I wind
up with an arc on the outermost circle of the polar chart at about
two-thirds of the way round the circumference (clockwise) from "12
o'clock" as opposed to a spot in the dead center of the display as I
would expect. What have I done wrong? I have checked this with 3
different 50 ohm loads and they all show an arc at the 'south west'
corner of the chart. What's goin' on here, guys??
confused
Paul

You haven't fully described your setup, so it's hard to tell. What are
you using for a bridge? If I recall correctly, your VNA is an HP8754A,
which won't do S-parameter measurements by itself; it requires an
external bridge (an S-parameter test set, or a reflection-transmission
test set, for example).

The fact that you see an arc on the outermost circle, assuming that you
have it set as a unit circle on the linear reflection coefficient plane
(the Smith Chart outer circle), implies that you have a (nearly) unity
reflection coefficient. Is something almost disconnected so it looks
like a tiny series capacitance?

Okay, thanks. I must admit it is symptomatic of the load's center pin
not quite making contact for some reason, even though the load is fully
screwed home. But the same thing happens with 3 different loads!
I'm using the T/R test set, by the way, although I don't see how that
could explain the problem. I don't have the full S-parameter bridge but
am in the market for one.

Okay, thanks. I must admit it is symptomatic of the load's center pin
not quite making contact for some reason, even though the load is fully
screwed home. But the same thing happens with 3 different loads!
I'm using the T/R test set, by the way, although I don't see how that
could explain the problem. I don't have the full S-parameter bridge but
am in the market for one.

If you're using N connectors, check the center pins of the females.
Make sure that they aren't splayed open.

The local repeater association I'm a part of has had repeated problems
with splayed N connectors. They can be caused by a couple of
problems. A botched install of an N male can cause its center pin to
project too far forwards, out of the shell. We've also seen some N
adapters (Chinese-made I believe) whose center pin was larger than
usual in diameter, or was not tapered in the usual way.

In either case, if you screw an N male connector with a bad center pin
into an N female, the male's pin can force the split segments of the
center female pin outwards. Once this happens, a _good_ N male
connector's pin may not make reliable contact with the female's
damaged center pin. Intermittent nastiness can occur.

It's sometimes possible to bend the female's split pin sections back
together with needle-nose pliers, creating a repair which may or may
not be permanent. A better solution is to replace the damaged female
connectors entirely, and inspect all N male plugs and discard any
whose center pins are too large in diameter, not tapered properly, or
project too far out of the shell.

I suppose that similar problems might occur on BNC or SMA connectors
as well, for related reasons.

--
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!

Okay, thanks for that guys.
One clue that might assist in tracking down this problem is that the
aforementioned arc that appears at about '37 minutes past the hour' on
the outermost polar circle when the 50 ohm termination is fully screwed
in first begins to appear at just before 12 o'clock and gradually
circumambulates around this outer circle anticlockwise to its final
position as it's fully screwed home. I'd have (intuitively) thought
that once the centre pin makes proper contact with its counterpart, the
trace would suddenly flick to close to the centre. But with no
experience of VNAs, that's just a hunch. Any suggestions?

Dave Platt wrote, among other things,
"In either case, if you screw an N male connector with a bad center pin
into an N female, the male's pin can force the split segments of the
center female pin outwards. Once this happens, a _good_ N male
connector's pin may not make reliable contact with the female's
damaged center pin. Intermittent nastiness can occur."

I've seen people suggest that you can mate an N plug with a BNC jack in
a pinch, but what you describe is _exactly_ why you should never do
that. The N plug center pin is a distinctly larger diameter than a BNC
center pin.

Also note that there exist 75-ohm N connectors. Do not try to mate
them with 50 ohm versions. A 75 ohm plug in a 50 ohm jack is likely to
not make good contact, and a 50 ohm plug in a 75 ohm jack may cause
damage to the jack.

Suggestions? You bet! Get out your RFSim99 and build a model of what
you are putting on the RT test set test port, including any connecting
cable. Play with the model to see what will get you the sort of
performance you are seeing.

Note that you haven't really told us what you are plotting. I'm
assuming it's a plot of the complex reflection coefficient on a display
that's scaled to a unit circle. Are you using a standard Smith chart
overlay?

The normal interpretation of the result you describe, however, doesn't
make much sense, since the load seems to go from something like a
nearly pure inductance with reactance equal to the reference resistance
at the center of the plot (50 ohms?), and wanders around to become a
very low impedance (at the left edge, -1+j0 reflection coefficient),
and then around to become capacitive with moderate reactance (35
ohms?). Could you possibly be using some other scaling? What happens
if you are trying to display a value beyond full scale on your display?

I suspect something is not what you think it is in the measuring
instrument, and from past experience, I'd say it could be just about
anything. Careful visual inspections often reveal problems.

Cheers,
Tom

Oh dear. It appears you may be right.
I've just checked with the suppliers and they tell me the plugs I
bought are 75 ohm ones. This really stinks as in their catalogue,
although they don't specifiy the impedance of these plugs, they do
clearly state that they're suitable for RG214 coax. AFAIK, RG214 is
only available in 50 ohm - if anyone knows otherwise please tell me. It
took me a good deal of time and trouble to make up these patch leads so
all in all, I ain't exactly pleased.
Here's the URL for the catalogue page so you guys can tell who's at
fault he

http://cpc.farnell.com/jsp/endecaSea...=CN06870&N=411

Since posting earlier I've been on to the actual manufacturers and they
reckon these plugs are actually 50 ohms. So the supplier's insist
they're 75 and the makers say they're 50! I need to know for sure, as
one or the other is in error. How can one tell, by visual inspection
and or physical measurement, which type is which? There must be
something that's visibly different about the two types.
THanks.

wrote:
Since posting earlier I've been on to the actual manufacturers and they
reckon these plugs are actually 50 ohms. So the supplier's insist
they're 75 and the makers say they're 50! I need to know for sure, as
one or the other is in error. How can one tell, by visual inspection
and or physical measurement, which type is which? There must be
something that's visibly different about the two types.

Yes, look at the thickness of the central pin. A 50 Ohm connector has
a central pin of some 1.6mm thick. The 75 Ohm version is a mere needle,
something like 0.8mm.

It's one of my long-time gripes about coaxial connectors that N and
BNC series of the two impedance levels are sufficiently compatible
to mate together, but not enough to do so without damage. In a lab
with someone ignorant of this fact, it's a source of endless trouble.

Jeroen Belleman