[TowerTalk] Method of calculating phase delay variation

[Jim Thomson]

Date: Tue, 03 Sep 2013 10:54:50 -0700
From: Jim Brown <jim at audiosystemsgroup.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Method of calculating phase delay variation

While the principle at play in Bob's demonstration is important, he used 
the wrong word to describe it. The correct word to describe the reversal 
of a pair of wires carrying signal, and the inversion a a signal in a 
gain stage is "polarity," not "phase."  Polarity is a two-valued 
function -- either positive or reversed -- and it is independent of 
frequency. Phase is a continuously valued function and has the units of 
degrees or radians.

Phase can be described only as the difference between two signals of the 
same frequency, and for any method that we use to shift the phase, 
increases linearly with frequency. For example, the signal from two 
ideal sources that are at different points in space will have a 
different phase relationship with each other at every frequency, and at 
every point in space (because the travel time to every point in space is 
different). When we shift the phase between two signals by means of a 
network, or by adding a length of transmission line to one of them, the 
phase shift will be different at every frequency. But when we reverse 
the polarity of the feed to one of the radiators, the phase does not 
change, but their fields are opposite at all frequencies.

The difference is critically  important in understanding how things 
work. In the field of pro audio, we learned to appreciate and understand 
this principle more than 30 years ago, and to use the right words to 
describe what we're talking about. It's long past time for RF engineers 
to learn them.

73, Jim K9YC

##  I just checked several of my rack mount pro-audio mic pre-amps....
and they all have a switch on em..marked  phase reversal.   They
are just swapping the polarity of the  tip and ring on the balanced line
from the mic.  You are correct of course...it’s a polarity reversal...
but I have yet to see any mic pre-amp made by anybody that is
marked as such. 

Jim   VE7RF