Jeff Carter wrote:
> I've been following this thread with interest, largely because I'm
> ignorant about the utility of these meters. Jim, your description
> fits with what I had concluded about them, that being if you had
> pretty much anything else to use, it would be better. I understand
> how they work but using a GDO always seemed pointless.
> I have seen these things on eBay for years, but I still cannot think
> of a single instance where it would be of more utility than more
> modern antenna tools, so I never bought one.
> My question is, am I wrong? Is there some situation I have not yet
> encountered where only a GDO will do? Jim, you said that for some
> systems it's convenient. I can't picture one for the life of me.
The GDO has two aspects that are convenient:
1) It's inherently a narrow band detection process, so interfering
signals (a real issue if you're trying to dial in a 160m antenna and
there's a pesky AM broadcast station near by)
2) It's not oriented towards a 50 ohm system in particular. You can
adjust the coupling ratio to suit the measurement (granted, more an art
> I usually use a HP network analyzer for general things, and the little
> Autek boxes for spot checks.
You should be able to use the VNA.. Set it up to measure S11, then run
your coax up to a coupling network of some sort. In theory, you'd want
a transformer to transform the 50 ohms to whatever the loop resonant R
would be, but in reality, since you're going to be using loose coupling,
that will set the magnitude. I've used some turns of magnet wire on a
BNC connector (and you can add and remove turns by hand) You'll see the
bump in S11 magnitude at the resonant frequency of the system under test.
(of course, the VNA's probably not handheld. Anritsu has a handheld
(2hands) device that does this sort of thing, though, if you have a few
tens o'kilobucks laying around. It's pretty neat)
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