[TowerTalk] Figuring degrees of a coax line

[donovanf at starpower.net]

Tom,

Unless your 50 ohm phasing lines are terminated in 50 ohms (unlikely in any practical phased array), they will not produce the desired phase delay.  The same applies for 75 ohm lines and 75 ohm terminations.

Transmission lines that are any multiple of 1/4 wavelength are immune from this problem; however, transmission lines of  84 and 71 degrees will produce delays that are highly dependent on VSWR.  Its not worth the effort to accurately calculate the physical length of lines when
VSWR will ruin the calculated phase delay through the lines.

This variable delay effect in mis-terminated transmission lines can be
easily demonstrated on an dual trace oscilloscope, with one trace
displaying the signal generator output and the other trace displaying the signal at the output of the transmission line.  As you vary the transmission line termination impedance, you'll see the delay through the line vary dramatically.

You can calculate the physical length of your line (in inches) using:

11803/fMHz  X  Vf  X  degrees/360

But don't waste your time...    It won't work!

73!
Frank
W3LPL

---- Original message ----
>Date: Tue, 16 Oct 2007 21:33:44 -0700
>From: "Tom Osborne" <w7why at verizon.net>  
>Subject: [TowerTalk] Figuring degrees of a coax line  
>To: "Towertalk" <towertalk at contesting.com>
>
>OK all you math guru's.
>
>I have been trying to figure an easy way to find the length of a couple  of 
>phasing lines.
>
>What I want is an 84 degree and 71 degree phasing line for 7.050.
>
>I found one way, but I thought was kind of a long way around.
>
>If 360 degrees is ~132 feet, is 1 degree .068 feet?  Then can you multiply 
>that by 84 and get the answer (that way it comes out to 30.9 feet).
>
>Is there a simple formula?
>
>I asked a guy at work and he was telling me about signs and cosigns, and I 
>graduated from high school in 1954 :-)  73
>Tom W7WHY