[BC] Part 15 radio station

[Richard Fry]

Sorry for the late reply (this is written after an 11-hour road trip since 
my first post here today)

>I suppose the question is, when does the actual ground begin and the 
>ground wire end? <snip> However, if the tower itself is considered a 
>ground and not a ground conductor, then...

As far as physics is concerned, an r-f ground does not radiate.  But this 
is not true for a MW tower of any height above the earth.  With a very 
good,  buried radial ground system it may have an AC resistance of one ohm 
or less at its intersection with the plane of the earth -- but at the top 
of the tower it will have that ground resistance plus the DC resistance 
related to the conductivity of the tower material plus the radiation 
resistance of the tower for its electrical height, and a reactance based on 
its cross-section.

Any time r-f current flows along an exposed conductor it produces 
radiation.  Recall that a grounded mast can be shunt fed on MW, and is just 
as efficient a radiator as a series fed tower, other things equal .  It 
should be clear from this that the top of a grounded mast or any other 
conductor is not at DC or r-f ground.  In fact, no part of it that extends 
above the earth will be so.

>No, it wouldn't.  But that would also assume that the top part of the 
>radiator wasn't the antenna.  Sort of like putting an FM antenna on the
tower.  That's also grounded, but the field is created by the radiating 
element and not the tower per se.

An FM antenna is balanced, and needs no reference to a good r-f ground for 
efficient radiation.  A monopole does.  The top 3 meters of a tower-mounted 
Part 15 setup (considered "the antenna") in reality is only a small part of 
the radiating conductors in this setup.  Most of that radiation is produced 
by the conducting path to a true r-f ground buried in the earth, and by the 
audio and power wires connected to the tx.  A true r-f ground does not 
exist at the top of a grounded tower (see above).

>Somewhere I remember reading if a radiator is over a quarter wave above 
>the ground, coverage begins to increase.  If a Part 15 is bolted to the 
>top of a tall building (>1/4 wave) or tower, shouldn't coverage increase?

Only if it has a conducting path to a true r-f ground (buried in the 
earth); maybe the building frame in this case.  But that wouldn't meet Part 
15.219 (b).  Coverage increases with antenna height for systems using the 
space wave (e.g., FM and TV), but Part 15 AM uses the surface wave.

RF