Charlie wrote:

-ps How many times do you think Davis has tested their 9914 in the past
10+ years?

Before we go much further, is there an engineering data sheet on
Bury-Flex? I couldn't find one on the Davis RF site.

In particular, what is the rated minimum bending radius?

Coaxial cables with braid-over-foil shielding have a generic problem
that if they're bent too sharply, the foil will tear into separated
segments each about an inch long. The cable then relies on the braid for
overall shield continuity. It will still function, especially at low
frequencies, but there isn't much contact pressure to maintain the
continuity between the braid and the foil.

If a cable had been treated in this way, it's not hard to imagine that a
precision measurement at 450MHz would reveal small jumps in the loss and
SWR when the cable is flexed.

But this is NOT something you'd ever notice in a normal amateur station
operation. Even when I was using braid-over-foil coax with 1.5kW at
432MHz, and monitoring the SWR continuously, I never noticed any major
jumps when rotating the antenna. The break-up of the foil only came to
light after the cable had failed for an unrelated reason, and was slit
open for a post-mortem.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:
. . .
Coaxial cables with braid-over-foil shielding have a generic problem
that if they're bent too sharply, the foil will tear into separated
segments each about an inch long. The cable then relies on the braid for
overall shield continuity. It will still function, especially at low
frequencies, but there isn't much contact pressure to maintain the
continuity between the braid and the foil.
. . .

Most interesting! The cable I've been measuring has been kept in a coil
of about 3 feet diameter, but squeezing and handling it has reduced that
to probably about a foot or so at times. But I don't recall the coil
size when I received the cable, and of course I don't know anything
about how it was handled between the manufacturer and delivery to my
home. This might be an explanation for the variablility. I did hear from
someone else a while back that he'd seen variability in a foil-wrapped
cable, but I don't think it was specifically Bury-Flex. I have some
RG-58 size cable with the same general construction which doesn't show
this variability. But it looks like the stress would be worse on the
foil in a larger diameter cable. Also, there seems to be some difference
in how the foil is more-or-less bonded to the PE, and that would also
play a role in the stress.

I want to keep the piece I've measured intact for the time being, but if
I get up the time and interest to do more measurements on another piece
of cable, a post mortem might be revealing.

Roy Lewallen, W7EL

On Sun, 11 Dec 2005 20:35:23 -0800, Roy Lewallen
wrote:


Most interesting! The cable I've been measuring has been kept in a coil
of about 3 feet diameter, but squeezing and handling it has reduced that
to probably about a foot or so at times. But I don't recall the coil
size when I received the cable, and of course I don't know anything
about how it was handled between the manufacturer and delivery to my
home. This might be an explanation for the variablility. I did hear from

Further, you may not know the manufacturers specifications for minimum
bending radius to preserve operating characteristics.

When I went to the Davis site, I found a table of losses for several
cables including BuryFlex, but it did not state the length. I assumed
that the length was 100' from the losses quoted for some other cables.
The VF (82%) was buried in text, and I found no explicit information
on Zo, mechanical properties, bending restrictions, operating
temperatures etc. Some properties may be implied by description as an
RG8 type cable, but min bending radius is likely to be larger than a
solid dielectric / no foil cable.

Perhaps there is spec sheet there somewhere, it didn't leap out at me!

Owen
--