I am rather fond of the coupled-line hybrid idea: it can be built in
a way that everything stays ratiometric, so the coupling ratio is very
nearly constant over temperature, and of course the directionality
lets you observe things you can't just from monitoring voltage at a
point. It's possible to build one with low coupling without too much
trouble; -60dB coupling isn't out of the question, for sure. I'm
imagining a design I could make reliably with simple machine tools
that would work well for the OP's application: 100 watts at about
1GHz as I recall in the through line, and coupling on the order of
-60dB to get to about -10dBm coupled power and have negligible effect
on the through line. There's a free fields solver software package
that will accurately predict the coupling, and with the right design
and normal machine shop tolerances the coupling and impedance should
be accurate to a fraction of a dB and better than a percent,
respectively. Perhaps I can run some examples to see if I'm off-base
on that, but that's what my mental calculations tell me at the moment.


Actually, the exact coupling ratio probably isn't important in this
application, because it could be "calibrated out". Stability would be a
bigger concern, and it's certainly possible to design a coupler that is
very temperature stable by choosing the right dimensions so that things
change in the right ratios.

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

On Tue, 14 Aug 2007 00:14:31 -0700, K7ITM wrote:

On Aug 13, 10:56 pm, Richard Clark wrote:
On Mon, 13 Aug 2007 13:09:09 -0700, Jim Lux
wrote:

Or, something like a 50k resistor into a 50
ohm load will be about 60 dB down,

Hi Jim,

Unlikely.

With parasitic capacitance at a meager 1pF across the 50K, its Z at
10MHz would compromise the attenuation presenting closer to 50 dB
down. At 1Ghz it would plunge like a rock. This, of course, presumes
a 1/4 watt resistor.

A better solution is to use surface mount resistors where the
parasitics are down at 100aF - but then you will have a frequency
dependant divider unless you can guarantee that the parasitic
capacitance of the 50 Ohm resistor is 100pF (sort of casts us back
into using a 1/4 watt resistor with a padding cap). At 1GHz, it is
not going to look like a trivial 50K load anymore.

100aF??? :-) X(100aF)/X(100pF) = 50k/50 ??? ;-) ;-)

S/B 100fF (trying to watch the Perseids and do math at the same
time).

On Aug 14, 8:45 am, Jim Lux wrote:
I am rather fond of the coupled-line hybrid idea: it can be built in
a way that everything stays ratiometric, so the coupling ratio is very
nearly constant over temperature, and of course the directionality
lets you observe things you can't just from monitoring voltage at a
point. It's possible to build one with low coupling without too much
trouble; -60dB coupling isn't out of the question, for sure. I'm
imagining a design I could make reliably with simple machine tools
that would work well for the OP's application: 100 watts at about
1GHz as I recall in the through line, and coupling on the order of
-60dB to get to about -10dBm coupled power and have negligible effect
on the through line. There's a free fields solver software package
that will accurately predict the coupling, and with the right design
and normal machine shop tolerances the coupling and impedance should
be accurate to a fraction of a dB and better than a percent,
respectively. Perhaps I can run some examples to see if I'm off-base
on that, but that's what my mental calculations tell me at the moment.

Actually, the exact coupling ratio probably isn't important in this
application, because it could be "calibrated out". Stability would be a
bigger concern, and it's certainly possible to design a coupler that is
very temperature stable by choosing the right dimensions so that things
change in the right ratios.

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

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

On Tue, 14 Aug 2007 15:01:32 -0400, Walter Maxwell
wrote:

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?

Hi Walt,

I provided a posting on how to determine the extent of error that was
similarly ignored - don't feel bad. Accuracy isn't all that its
cracked up to be. :-)

73's
Richard Clark, KB7QHC

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

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?
====================================
The MFJ259 antenna analyser can measure coax loss at any frequency
between 1.8 and 170 MHz.


Frank GM0CSZ / KN6WH

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

Jimmie D wrote:

Own, beg, borrow or steal a watt meter. Hook to back of rig, measure
wattage, hookup rig through coax to watt meter, measure wattage--no
conversion math/chart necessary. You'll directly know the loss ...

Regards,
JS