[BC] Part 15 radio station, elevated radiators

[Craig Healy]

> >How about if the thing is simply bolted to the top of a tower with no
> >ground wire at all?
>
> If the tower is grounded at its base, then the tower _is_ the ground
> lead/wire.  If it isn't, the tower still is one arm of a dipole.  Either
> way the tower itself will radiate, unless the Pt 15 tx is insulated from
> the tower.  Even then, the program and power wires leading away from the
tx
> will radiate unless they are decoupled for r-f.

I suppose the question is, when does the actual ground begin and the ground
wire end?  If the FCC considers everything connected to the ground side as a
conductor, then the whole earth could be a factor.  However, if the tower
itself is considered a ground and not a ground conductor, then...

> >I can picture one of those things on top of a 2,000' tower.  Coverage,
> >I'll bet!
>
> The elevation pattern of a vertical monopole of more than 5/8 wavelength
> wouldn't be optimum for producing groundwaves.

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.

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?

For that matter, what if a 2,000 foot tower is erected with all the guy
wires except one set insulated.  A set that's a half wave or slightly more
from the top is bonded to the tower and has an insulator 1/4 wave out.  Then
the ATU is put at that point and the transmission line run to it.  From
there a normal skirt feed is run up about 1/4 wave.  In effect, it creates
an 1/2 wave or so elevated ground plane.  If that were at the top end of the
AM band, wouldn't it provide good coverage?

Not that a Part 15 could ever legally use that, mind you.  But it might work
very well for an X-Band station.  Any of you EZNEC gurus care to take a peek
at that?

Craig Healy
Providence, RI