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!