[Towertalk] Re: "Real" Height of antennas on Buildings

[mdg]

I think the answer to this question is tough but may be simplified as
follows: as far as the antenna's electrical characteristics,  the height
above the roof will affect antenna's resonance, impedance, coupling
between elements etc therefore affecting it's gain, directivity etc. One
can verify this if the antenna is partially over the electrical
(elevator) room and partially over the actual roof. Turning the ant
changes the SWR. 
 The distance between the roof and the actual ground level will affect
the takeoff angle, lobes etc. This is of course assuming that the roof
does not extend beyond 100 lambda or so which may sometimes be the case
on UHF etc. 
My antennas are atop a 10 storey building and I've come to the above
conclusions time and time again.... .  When modelling with EZNEC I
always use two ground levels. 

73 Art VE3PND