RadioBanter - View Single Post

December 5th 05, 09:19 PM posted to rec.radio.shortwave

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