[Elecraft] Balun Questions

[Robert Nobis]

Hi Guy,

Thank you.  It is interesting that the ANSI test is based on physical “outer surface" deformity, rather than electrical (rf) performance. Although there is probably a degree of correlation between physical deformity and electrical (rf) performance. I agree the ANSI spec outlines a rather crude test. 

The data I provided came from several manufacturers of coax. I just wanted to point out that the manufacturer’s specifications for both RG303 and RG400 are the same at a 1.0 inch static bending radius. They did not indicate how they came up with these specifications. I believe there is plenty of margin in these specified values that may be required for use in military applications, since both cables are MIL spec rated. However, for ham radio purposes, I believe these specs can be safely ignored, within reason. Although, I probably would not try to bend either RG303 or RG400 any tighter than maybe 0.40 to 0.45 inches. 

I have used RG303/U because that is what I had. If I had RG400/U, I would have used that. In my case, I have seen no measurable performance issues with the tightly wound chokes that I have made with RG303/U.  If care is taken in winding the chokes, I feel either type of coax will do the job. Maybe if I find some RG400/U at the next hamfest I attend, I will buy some and try it. I could then compare results.

It would be interesting to do some actual tests of the electrical impact of bending coax at radii needed for 2.4 inch OD cores, either single or stacked.  Possibly some type of TDR test could be done to see if any deformaty would impact performance of the coax used in chokes, over the HF ham bands.

Again, thanks for sharing your insight and experience.

73,

Bob Nobis - N7RJN
n7rjn at nobis.net


> On Feb 8, 2016, at 18:01, Guy Olinger K2AV <k2av.guy at gmail.com> wrote:
> 
> 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 <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
> 
>