In an attmept to beat this subject to death, here are a few more links
to usefull
information.

One measure of the degree of isolation bewteen a feedline the outside
is called
"transfer impedance".

From Fluke:
http://www.flukenetworks.com/us/_Promotions/ISV/Glossary.htm

"Transfer Impedance - For a specified cable length, transfer impedance
relates to a
current on one surface of a shield to the voltage drop generated by
this current on
the opposite surface of the shield. Transfer impedance is used to
determine shield effectiveness against both ingress and egress of
interfering signals. Shields with
lower transfer impedance are more effective than shields with higher
transfer
impedance."

Blue Jean Cables has a good simple article with chart.
http://www.bluejeanscable.com/articles/shielding.htm

Beldon has more info at:
http://www.belden-wire.com/Catalog/TechInfo/TechTransfer.htm

One test I made was to use my antenna noise bridge with an additonal
4:1
step down transformer to induce noise in a wire that I ran parallel to
my
coax and triax feedlines. I compared several different types of coax.
The crap
they sell at Radio Shack is marginal. It is better then twisted or zip
cord, but still
sucks. For LF/MW/HF any of the real coax is very close in "leakage". I
am
partial to Belden but there are othre good brands.

When I ran a 6' test "loop" in parallel, ~6" offset, with twisted or
zip cord the
induced/injected RF noise overwelamed evrything except for th e very
strongest
signals. With radio shack coax Ihad to place the test loop against the
coax
for significant ingress. With belden braid and foil coax I could barely
detect the
N, with Triax I couldn't detect it at all.

I tend to think of video as mid level , LF/MW/HF receivers are low
level and transmitters
are high level. While similar, each level has different issues. For
instance I might not
mind lossing 5% of a 100W 40M transmitted signal, but I damn sure don't
want that 5W
to show up in my shack. A 10uV engress in video will not be detectable.
A 10uV
engress in to my LF/MW/SW, or even VHF/UHF will cover all but the
strongest
signals. Coax does not provide 100% isolation, so I take some pains to
keep the
transmitt coax lines seprated from the receive coax lines and to insure
they don't
run parallel for any significant length.

And spiral wraped shielded cable is about the worst choice you can make
for either
video or RF. For very short video or audio runs, up to 1m, spiral might
work IF there
are no RFI sources nearby. Spiral cables are cheap and every VCR/DVD
that I have
seen comes with these cheapies.These should NEVER be used for even
short
connections to a SW!

An example: I have a RF patch bay and one night theAC mains went down.
I was in
mid rebuild so I wanted to patch my DX398 into my patch bay. I had just
installed a
high end DVD and still had the cables in my garbage can. I grabbed the
video (yellow
RCA) a patch to BNC, RCA to BNC, and a RCA to 3.5MM. I was amazed at
how much
RFI from laptop was getting in to the DX398. So I dug out the correct
patch cable, a
1M RG174 BNC to 3.5MM and when I switched cables the RFI vanished. In
disgust
I cut the cables in half and put them back in the trash.

http://www.audioholics.com/techtips/audioprinciples/interconnects/spiral_shield_cables.php

Terry

In article .com,
wrote:

Telamon wrote:

So concerning the "transfer Impedance" link to the Belden site, the
chart of the various cables shows some of the expected (by me that
is) shield performance. The shielding improves at higher
frequencies due to the skin depth of the shield was expected but by
around 100 MHz the shielding starts getting worse again. Can you
explain this apparently consistent result over a number of
different cables? It must be due to a physical characteristic of
coax cable construction. Maybe the frequency response of the cable?


Terry wrote

I have assumed that the inter braid openings, the lower effeciency of
mylar foil, and the basic losses of the cable foil account for this.
A fellow enginer insists it is an effect of self inductance.

Other coax companies charts show very simlar cuirves.

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

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

In article .com,
wrote:

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.

Yeah the spiral wound foil shield sounds like a bad idea. It is sure to
mess up the fields inside the coax.

Please post to the news group if you or your friends turn anything else
up on this subject of higher frequency noise ingress into the coax.

--
Telamon
Ventura, California

Telamon wrote:

Yeah the spiral wound foil shield sounds like a bad idea. It is sure to
mess up the fields inside the coax.

Please post to the news group if you or your friends turn anything else
up on this subject of higher frequency noise ingress into the coax.

--
Telamon
Ventura, California
-------------------------------------------
I have emailed copies of tis thread to several EEs and advanced hams
that I know.
One EE's response wondered when I was going to give up on that 5W
station fron
"lower goatsuckistan".
Funny.

I forwarded this queston through our slaesdroid to Belden.

I am sure there is a very simple answer but darned itf I know what it
is.

Terry