[Elecraft] Balun Questions

[Guy Olinger K2AV]

Robert Nobis wrote:
> After reviewing specs from several manufacturers, the “recommended”
minimum bend radius for RG303 and RG400 is essentially the same at 1.0
inches.

Hi Robert,

I have wrapped RG400 on a two stack of FT240 form factor toroids with never
an issue, without any change in electrical characteristics I could measure.
These were a little less than a half inch radius, something I would never
try with 303 or 142.

A one inch radius or two inch diameter winding, per the listings you have
quoted, would hang loose on most forms. In effect this specifies the
303/142/400 cables useless for winding on toroid cores of any HF suitable
size in use by hams, including even the monumental T500A series toroids.

***However,***

I respectfully suggest that the minimum bending radius that you see
published for RG400 can be ignored for ham purposes at HF and low VHF, and
common sense is better suited to the problem. IMHO the ANSI standard
(ANSI/SCTE 39 2007) uses a crude method better suited to measuring metallic
sheathed cables, and ignores testing the needed characteristics directly,
simply to avoid testing cost and complexity.

I also suggest that everyone carefully study the ANSI standard until it is
clear what they are doing mechanically and see what they are actually
measuring:

http://www.scte.org/documents/pdf/standards/ANSI_SCTE%2039%202007.pdf

The method of measuring is in section 4. They are looking for a limit of 1%
surface deformity when bending.

In the case of RG400 with .195 inch OD, that would be 2/1000 of an inch
(yes, that's three zeros, two one thousandths of an inch) bending deformity
at the surface of the teflon jacket, or half the thickness of an average
human hair.

Anyone who works with teflon knows that the teflon jacket on the outside of
the bend will stretch and the teflon on the inside of the bend will bunch,
due to the difference in the radius, and particularly due to it being a
soft material with no constraint to its outside surface. And there is the
problem of managing to measure the thickness of something soft like teflon
so as not to compress the teflon 2 mils during the measurement of something
with a curved surface.

The teflon dielectric between the inside of the shield and the center
conductor, all we care about, is confined by the double shield, which
opposes the teflon's tendency to deform. Further, the difference in the
radius is smaller inside the shield, dividing down the differential
measured at the surface of the jacket.

The 19 strand center conductor in RG400 will easily follow the teflon in
multiple bendings. The solid center conductor versions (303,142) to a
degree will remember their first bend and will apply that deformity in the
second and later bends, accumulating deformity at that point in the cable
unless the second bend is identical to the first. That is why you see
"once" or "bend once" in some of the listings for RG303 and RG142. I have
no argument with the "bend once" specification in the 303 and 142 listings.
It's relates to the reason I use RG400.

RG400 is the only coax listed for certified aircraft installations in a lot
of aircraft service vendor's web pages. I find 142 mentioned a few times as
being easier to fit with connectors. I have not seen 303 mentioned on an
aircraft service vendor web page.

73, Guy K2AV