*On Tuesday 29 January 2008 15:07, Richard Gill wrote:*

*A few years ago I bought some Semi-Rigid Coax from out favorite bid site for later use. I am building some Yagi's as described in the Aug 2006 QST article by Jim, W6PQL. The yagi's use a piece of UT-141 Semi Rigid coax as a balun. The coax I have has no ID or number and does not look like the coax in the article. Anyway the coax I have has the follow specs: appears to be made of Copper tube.*

*Has a OD of .141“*

*ID of .120” ~*

*Insulator is a white plastic looking material.*

*Center Conductor is solid wire. 20Gauge (.0365“ Dia) and may be silver wire. Not Copper.*

*Each Piece has a SMA Connector soldered on the end.*

Steve VE3SMA replied:

It looks like the right stuff. UT-141 is one manufacturer's name for what is more generally known as MIL-C-17/130-RG402, which has .141” OD (bare copper), .119“ OD PTFE insulation and .036” diameter inner conductor (silver plated copper-covered steel).

There are many similar variants with different outer conductor materials, etc.

The impedance formula should be

Zo = ( 138/SQRT(e) ) * log(D/d)

where SQRT(e) means “square root of the dielectric constant” and the log is the base 10 type. It comes out to 49.4 ohms using the above dimensions and e = 2.1 which is what my reference gives for PTFE.

73,

Steve VE3SMA