[BC] FM antennas and Velocity of Propagation

[Richard Fry]

Jason Russell wrote about coax that is "electrically longer
than it is physically:"
>I was wondering about some more or less real world scenarios,
>what's happening when it's right, when it's wrong, with and
>end fed antenna vs center fed, and such.
________________

Others already have responded with their reasons about why this 
happens, so I'll comment on the clip above.

The velocity of propagation (v.p.) of a radio wave via a coaxial 
transmission line is important only when the r-f phase of that radio 
wave must have some known value for correct operation of the 
device(s) it is delivering power to.  That pretty much eliminates all 
broadcast applications where a single coax run feeds a single 
radiating element --
such as non-D AM stations..

All bays of FM antennas of two or more bays need be driven with a 
known r-f phase and power, in order (nominally) to produce the 
free-space radiation patterns expected from them.   Commonly the bays 
need to be fed in the same relative phase in order to do that.  FM 
antenna OEMs supply all the inter-bay coax for an array, and of 
course account for its v.p. in the mechanical layout of the antenna 
-- which is frequency-specific.

ERI, for example, spaces the bays of a standard, 1-wave spaced array 
with respect to a v.p. of 0.996 in the interbay lines.  So the bays 
are physically closer to each other than a free-space wavelength (by 
0.4%), but still, all are fed in the same relative phase given the 
v.p. and physical length of the interbay lines.  The elevation gain 
of the array is affected by this change in bay spacing away from 
1-wavelength, but that value can be calculated, and used to arrive at 
a "published" r.m.s. gain for the complete array.

But whether end- or center-fed, an FM antenna array is a controlled 
entity as far as its internal r-f phasing and power distribution is 
concerned.  So the r-f phase at its input connector can be any value, 
meaning that the v.p. and length of the coax connecting the antenna 
to the tx also can be any value.

There are FM antenna designs where the array is split into upper and 
lower halves, each fed by its own run of coax.  In this case the r-f 
phase at the input of each half must be known/controlled, so that the 
radiation from the upper and lower halves combines properly in 
space.  The v.p. of the two coax runs, and their electrical lengths 
then become critical.

Two FM antenna bays (or AM verticals) physically close to each other 
and driven by a single r-f source can produce a wide variety of net 
radiation patterns depending on their relative r-f phase, the power 
in each, and their locations in space.  THAT is why it is important 
to know the v.p. and lengths of all coax used within antenna 
arrays.  In FM arrays these considerations are part of its design, 
and rather transparent to the FM station.  AM arrays are a different 
story, and virtually everything about them is in the hands of a 
custom designer (a consultant).

RF