[TowerTalk] Measuring electrical length of coax...

[Jim Lux]

At 08:10 AM 7/14/2005, Dino Darling wrote:
>Hello John, how do you test your lines and what equipment do you use?
>
>I've experimented with the MFJ 259B for stubs and phasing lines, but if it
>is critical, I'll order lines from M2.  I'd sure like a good method that I
>could employ myself.  Also, do you short the end of the coax or leave it
>open.  The MFJ book lists both methods.

In the network analyzer calibration world, it's considered that it's much 
easier to make a good short than a good open. The problem is the fields at 
the end of the open are less well controlled.  For amateur HF use, it's 
probably a wash.  The error in the open is probably in the fractions of an 
inch equivalent.  1 inch is about a half degree on 20m.

You can also use any precision mismatch.  Say you take a couple 50 ohm 
loads and connect them with a T to make a 25 ohm load, or, a 3dB pad 
terminated into a short.



> >...but I even measure the actual phase shift for each delay line of solid
> >dielectric coax, if the application is critical.
> >
> >And with foam coax it is MANDATORY to TUNE (trim) each delay line to the
> >correct electrical length.
> >
> >--John  W0UN
>
>Dino...k6rix at earthlink.net