Ed wrote in
. 192.196:

but I am at a loss as to what
additional
information one would need to model a center fed 75 foot long half
wave antenna, resosnant at 6.2 MHz, positioned 15 feet above a flat
metal roof, fed with 100 feet of 50 ohm coax (RG8), and operated on
any one of three frequencies.... 3.95MHz, 5.4MHz, and 7.2 MHz.



Small correction.... the feedline is also not important since all I
am
asking for is essentially the feedpoint impedance of this antenna at
those three frequencies of operation.

If the metal roof was infinite (or very large compared to the dipole),
the feedpoint Z looks to be around 3-j711 at 3.95MHz. (This is probably
not a good model of your scenario though.)

Is that all you really need to assess the outcome?

100' of RG8 with such a load will lose 20.6dB.

The ATU will see around 4-j55, so you may well be losing 3dB

Radiator loss (with with you seemed concerned) depends on the wire (which
you haven't told us about), and it might be a dB with such a low
feedpoint R.

The coax loss dominates the problem.

There is a lot more to quantifying the problem, and designing a solution
than knowing feedpoint Z at three spot frequencies for a dipole over an
inadequately specified metal roof.

However, we do know that a coax fed centre fed dipole is not an efficient
multi-frequency antenna if the coax is of significant length (meaning
basically more than nearly zero).


Owen