On Aug 14, 2:22 pm, (J. B. Wood) wrote:
In article , wrote:
J. B. Wood wrote:

The challenge here is, given a transmission line of certain physical
length, to find a measurable value at the operating frequency(s). An RF
signal source with a surplus (but in proper operating order) General Radio
(Genrad) impedance bridge is good for this type of measurement. Keep in
mind that any coupling from the line to nearby structures will affect the
measurement. Sincerely, and 73s from N4GGO,

Even more of a challenge might be getting that impedance bridge to work
at 1 GHz...grin

Hello, and I must have had a senior moment. Forgot what freq the OP was
interested in. Too many years spent making measurements in the 2-30 MHz
band I guess ;-) Of course now we're looking at a vector network analyzer
to make the measurement (not something most Hams have in the shack). I
wonder if MFJ has anything? Sincerely,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

Later on in the thread, the OP said he has an Agilent network
analyzer, presumably a VNA, and an HP power meter. Certainly the VNA,
connected to the feed end with the tower end open or shorted, swept
over a fairly narrow range (since he has 200 feet of line) around
1GHz, should tell him enough to characterize the impedance and the
loss. He indicated that he's happy with that solution, some time back
in the thread.

Cheers,
Tom

"K7ITM" wrote in message
oups.com...
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


Having a power sensor at the top of the tower would mean that it would have
to be calibrated at least annually. This would be deafeating the purpose of
the investigation. The purpose being finding an easier method with
acceptable accuracy of measuring the loss of the cable. Initial test taken
by the enginnering staff show that this may be feasble and possibly even moe
accurate than using the current appoved method.


Jimmie