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.
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.
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.
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.
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.
Rick N6RK
PS: 2x4 lumber consumes 8 square inches out of the original log.
You pay for the sawdust!
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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