Telamon wrote:

The holes in the coax braid are a tiny fraction of the 1/4 wave of the
highest frequency on the chart so I'm doubtful of that.

The coax cable self inductance of the outer braid/shield? Well we
should be able to see an effect of coax diameter size where the larger
diameter should have lower self inductance.

The larger diameter and other wise similar cable has the dip at a
higher frequency. Maybe your buddy is right about the coax outer shield
self inductance.

Many of the coax on the charts are not current but I compared two,
which are 9104 and 9116. Both have the same electrical characteristics
except for the inner and outer DC resistance. 9116 is the larger
diameter cable with lower DC resistance for both conductors. This cable
has its dip on the chart at a higher frequency than the 9104. The 9116
has lower insertion loss (S21) at higher frequencies than the 9104 at
the 6 dB point of about 200 MHz.

If this was a function of Q then it would make sense that the dip would
be at a higher frequency on the larger / lower DC resistance cable but
this is not a function of Q as far as I can see.

I don't know maybe it has something to do with the traveling wave being
T.E.M. in the cable but that should be opposite of the chart result.
Smaller diameter should be better at higher frequency.

I expect that the 9116 is better at higher frequencies because it is
less dispersive at higher frequencies due to the lower ohms per inch of
the conductors. You always have two losses in a transmission line,
which are the conductor and dielectric loss. One or the other can
dominate depending on materials, dimensions and frequency. Here I
expect that the 9116 cable dielectric loss is higher than the 9104 but
the 9116 conductor loss is lower, which in this case dominates.

The greater dispersion of the 9104 coax at higher frequency may cause
the RF energy in the cable to radiate some and not just turn into heat
in the cable or your buddy is right about the outer shield inductance
per foot?

--
Telamon
Ventura, California
------------------------------------
I have textbook somewhere that deal with coax issues and I do remember
doing
a quick review to see if this issue is addressed. From memory, no real
details as
to the "why", just a mention of the effect with increasing frequency.

My friend and I have talked about this several times but never gotten
too serious
about it. He is more concerend then me because he is into weak signal
VHF/UHF
and the decrease in isolation with increasing frequency is a real
bother to him.
While I am a ham who is active on 6M, 2m and 70cm, I must admit that I
haven't
been particularly active in weak signal ops.

He is switching to ~1/2" CATV hardline so any improvement will become
clear
very rapidly. Currently he has some nasty engress in the 137 NOAA WX
Sat
range from a birdie from his laptop. Currently he has the laptop in a
rat wire(1/4"
square holes) for RFI contianment. If the engress remains, it is likely
the issue is inductance.

I forwarded this thread on to anohter friend who designs CATV systems
as I hope
he is much more familiar with the issues.

When pumping near maximum RF through coax I have noticed warm spots but

attributed that more to current nodes then inductance per se. In a
satellite uplink
make over I was brought into I found the reason why the replacement
waveguide
failed. A 90 degree bend was right at a current node and kept melting
the high
temp silver solder, which allowed the preasured air to esacpe tripping
an shut
down alram. I chopped off 1", resoldered the joint and all was well.
Inspite of 4
years of college, an EE degree and 35 years in the business I am still
troubled by
my ignorance on many electronic topics. Too many issues are so esoteric
that
good is too hard to dig up.

I do know that "spiral" wraped shielded cable has "odd" charactoristics
that I
have attributed to shield inductance.


Terry