On Aug 14, 12:01 pm, Walter Maxwell wrote:
On Tue, 14 Aug 2007 09:53:31 -0700, K7ITM wrote:
Bingo. It's that ratiometric thing that is a big plus for stability.
In a coupler made of all the same metal, or at least metals that have
nearly equal coefficients of expansion, the ratios stay the same, and
it's the dimensional ratios that establish the coupling and
impedances, not the absolute size. Actually, the change in length
does matter, but if you make the assembly a quarter wave long, the
d(coupling)/d(length) is zero at that point anyway. In any event, I
suppose the thermal coefficient of expansion of metals you'd be most
likely to use is small enough that you'd be fine with a shorter
coupler. There doesn't need to be anything terribly complex about the
geometry of the whole thing, either. It's probably safe to say that
changes in the dielectric constant of air due to air pressure and
humidity aren't going to be significant in this case. ;-)

Cheers,
Tom

Tom, I thought this thread concerned measurement of attenuation in transmission lines. On the 11th I posted a
precedure that involves measuring the line input impedances with the line terminated in both a short circuit
and an open circuit, then plugging the measured data into a BASIC program that outputs the attenuation,
complex Zo, and electrical length. My thoughts were that this procedure gives results with more accuracy and
precision than the procedures discussed before my post appeared.

However, I noticed that my post drew zero response. Is my procedure out-of-line, or out dated?

Walt, W2DU

Hi Walt,

Well, yes, the original posting asked if it was reasonable to check
the line attenuation with the other end of the line open and/or
shorted. I think that part of it got hashed out pretty well early on,
before your posting. If I'm not mistaken the OP has a VNA he can use
to do the measurement. By sweeping over a narrow frequency range
(about 200 feet of line; he's interested in the loss at about 1GHz),
he can easily and very quickly see the line impedance and the return
loss. If he's worried about his VNA calibration, I suggested he get a
couple calibrated attenuators that bracket the return loss of his
line, which he has to check occasionally. We just don't ever see much
change in attenuators from reliable vendors, from one check to the
next.

Beyond that, we got into some "basenote drift" along the lines of "how
can you provide reasonably cheaply a way to continuously monitor the
performance?" That's where the stuff about putting something up the
tower to pick off an RF sample came in. Since your posting appears as
a response to one of mine where I was writing about the top-end
monitoring, that may be an additional reason it didn't generate any
responses. On the top-end monitoring, I claim that it's not all that
difficult to make a stable coupled-line hybrid with very low coupling,
and combine that with one of the modern RF power monitoring chips
(esp. the AD8302 which has good temperature stability, and can tell
you the phase relationship and amplitudes of two signals) to look at
either just incident nom. 100 watts of power, or both incident and
reflected. With something like that in place, you'd have added peace
of mind on a continuous basis that everything was behaving as it's
supposed to. It's reasonable to ask if there's much benefit beyond
just monitoring the forward power at both ends of the line, but it
seems like a small incremental effort to add a reflected measurement
if you're doing a forward one. Of course, even continuous monitoring
of the forward power at the top end may be well beyond what the OP has
in mind.

Cheers,
Tom