"Walter Maxwell" wrote in message
news
On Sat, 21 Aug 2004 00:30:48 GMT, "The other John Smith"
wrote:
"Walter Maxwell" wrote in message
.. .
On Fri, 20 Aug 2004 14:48:41 -0500, "Crazy George"
wrote:
What we encountered were dynamic range problems. The instrument
operates
as
HP says, but if a strong enough interfering signal appears at an
input,
it
overloads the first(usually) active device, causing various unwanted
products to appear in band, and sometimes desensitizing the channel.
Urban
problem with many other radio services nearby, and it doesn't help to
be
under the landing path of USAF aircraft with their jammers still
active
either.
Hi George,
I understand the situation you encountered, but as I understand the
'other
John's' situation, he was sufficiently far from strong RF fields such
that
there
would be no problem with dynamic range. My interest is what the meters
showed on
his 8405 that would be recognized as negative resistance.
Walt, W2DU
Hi, Walt -
I'm sorry to say that I did not keep the data. Shame on me. But I will
keep
the data the next time it comes up (if it does) and I will post it here.
As
I said, I have not had negative resistance show up while using the dual
directional coupler, only when using the power splitter and pads.
Different
math relations were used to calculate the results from the two
techniques,
so it might very well be that it will never show up again.
Our country property where the measurements were made is about 10 miles
east
of Sulphur Springs, Texas, and about 80 or so miles from Dallas. There
are
two radio stations there, one on 1230 kHz (1 kW) and one on 95.9 MHz (6
kW).
The nearest microwave tower is about 5 miles west of me and I don't think
I'm in the path. There are no hams closer than 8 miles to me.
The more I think about this, the more convinced I become that I flubbed
the
readings or the calculations. I am learning that you cannot treat 2 meter
and 70 cm signals casually. For example, when placing a short on the load
port of the coupler for calibration purposes, I can see the phase
changing
after the connector has made contact but is still being screwed down.
Thanks for your comments.
John
Hi John,
I believe I said earlier that the distance to any probable source of
interference to your 8405 is sufficient to exclude any interference.
Is it possible to recall what you were measuring, and what the setup was
when
yo;u obtained the negative resistance indications, such that you could
repeat
it just to humor me?
No, I'm afraid not. I made so many measurements that weekend that it sort of
turned my brain to mush. I will set up the apparatus as I did before and
repeat some experiments. I like to be humored, too.
And concerning the phase change when screwing down the short, it's
possible
there's contamination in the screw threads, either on the short or on the
connector on the coupler. On the other hand, at UHF the phase can change
slightly between having the short placed on the coupler before tightening
down
vs being completely tightened down.
The phase change is not a problem, just an observation. After thinking about
it, it makes perfect sense. It was due to the shortening of the path as the
shorted connector was screwed on. The change was not large, maybe a degree
or two. It just surprised me that the setup was that sensitive.
I use an HP 778D coupler with N connectors, but I normally use a BNC short
on an
adapter when establishing a phase and magnitude reference. When using
sufficient
padding between the sig gen and the coupler I find no difference in magnit
ude of
the reflection between the short or open reference.
I use a 11570A kit which includes a power splitter, two probe Ts, two
terminators and a short. I also add two attenuators per the HP application
note. All are type N connectors.
I'm curious concerning what coupler and sig gen you use. My gens are all
HP, the
606A, 608E, and 612.
Recently I have replaced the splitter and attenuators with a Narda dual
directional coupler. It is with this setup that I seem to be moderately
successful. My generator is an HP 3200B VHF Oscillator.
John