On 9/26/20 8:45 AM, Richard (Rick) Karlquist wrote:
1. There are various ways to measure characteristic impedance,
some are better than others. You didn't say which method you
used. I have the most confidence in terminating the coax with
an accurate 50 ohm load and measuring the input impedance ripple
over a band of frequencies. I learned this from W8JI, who is
nearly always correct. I have not had good luck measuring
inductance of a shorted line and capacitance of an open line
at a spot frequency.
2. Many RG-XX specs don't even claim to be exactly 50 ohm.
In point of fact, there are no currently active RG specifications any
more. What you can say is "built to former RG-XYZ spec" or "RG-XYZ similar"
RG specs haven't been valid for a LONG time (40 years?) so that's not
surprising.
3. The RG specs don't cover foam coax, but you will see
"Foam RG-8U" etc, whatever that means. It is a given
that foam coax is never precision coax.
I don't know whether the MIL-C-17 (which may not even be active any
more.. MIL-C-17G was in 1990) has any foam dielectric coax.
MIL-DTL-17H superseded it in Aug 2006. J came out in 2014, amendment 2
in 2019.
MIL-DTL-17J (W/ AMENDMENT-2), DETAIL SPECIFICATION: CABLES, RADIO
FREQUENCY, FLEXIBLE AND SEMIRIGID, GENERATION SPECIFICATION FOR
(22-FEB-2019)., This specification covers flexible and semirigid cables
with solid and semisolid dielectric cores, with single, dual and twin
inner conductors. Cables covered by this specification are primarily
intended for use as transmission lines to conduct energy in a simple
power transfer continuously or intermittently. In general, these cables
are designed for low-loss, stable operation from the relatively low
frequencies through the higher frequencies in the microwave and radar
regions of the frequency spectrum. Cables may also be used as circuit
elements, delay lines or impedance matching devices. These cables are
supplied under a reliability assurance program as specified in section 3.
semisolid -> foam
I'll disagree on the "precision" aspect of foam - low loss microwave
cable with a variety of foamed dielectric (PTFE, silica) are pretty
standard and used in precision applications (bearing in mind the "teflon
step" phenomenon).
4. Foam is difficult to manufacture with precision, because
the amount of air is hard to control. OTOH, foam has significantly
less loss, so in general, you want to use it.
I don't think this is true with modern manufacturing - foam dielectrics
are more sensitive to crushing and bending too tightly, but they can and
are manufactured with exquisite precision - probably just as tightly
controlled as solids.
Not to say that there isn't "cheap" foam dielectric coax out there, but
I suspect variability in any decent foam coax is comparable to solid
dielectric.
5. Specs on "velocity factor" should be interpreted as "minimum",
especially as stated above where foam is used. Even then, always
check VF if it is important to you. I bought some coax for phasing
lines from a vendor with a good reputation. However, its VF was
less than the spec, and I couldn't shorten it to compensate because
then the length wouldn't reach from point A to point B. Admittedly,
I represented the proverbial corner case, but the vendor made good
on the purchase.
It's interesting, the MIL standards call out the physical dimensions and
construction, the capacitance per unit length (and how to measure it),
but not the inductance (it would fall out of knowing C and Z, of
course), and the loss (and how to measure it).
6. To further complicated the issue: Zo is actually complex and
"dispersive" meaning it varies over frequency. Steve Stearns,
K6OIK, has written some excellent papers on this topic that are
up on the web. He has shown how you can get erroneous results
if you don't do calculations taking into account the complex value
of Zo. It's not sufficient to just model it as a real impedance
cascaded with an ideal resistive attenuator. This actually causes more
trouble in practice than small deviations in the magnitude.
Certainly true.
Rick N6RK
PS: 2x4 lumber consumes 8 square inches out of the original log.
You pay for the sawdust!
Oh there's a fascinating story about the change from 1 5/8x3 5/8 to 1
1/2 x 3 1/2 - trade wars, Canadian vs US, on and on. litigated up and
down about "accurate trade descriptions", differences in "customary in
the industry", hardwoods vs softwoods and the definition there of,
advances in saw technology and tree growing practices. I can't remember
if it went to the Supreme Court or not.
By comparison, coax is easy.
On 9/26/2020 6:53 AM, Artek Manuals wrote:
I recently purchased a nice VNA ( Array Solutions AIM4300 ...good news
/bad news ..contact me off list) . Like a kid with a new chemistry set
at Xmas I set about measuring everything in site with my new "toy".
Much to my naive surprise I found that much of the "50 ohm" coax I had
(DX engineering RG213 and RG8X), actually measured 51.8 ohms .
Although I did have a 75 ft piece of brand-X Hamfest special that
measured 49.7 ohms!
Now I know that in practice there is not enough difference between 50
ohms and 52 ohms in actual use especially at 50MHz and below.
Occasionally though it matters ! Besides the truth in advertising
thing ( 2x4 lumber has always galled me) Does anyone know who makes
legal limit 50 ohm coax that is actually 50 ohms and not 52 ohms?
Dave
NR1DX
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