[TowerTalk] Flagpoles/ Thanks

[VR2BrettGraham]

N5ZC contemplated:

>I think I may be changing directions.  My roof, which is currently cedar
>shake, is going to be replaced within the next few weeks with a metal
>roof.  I have a chimney in the back that I can mount a full size quarter
>wave vertical on.  I think I can get away with 23' of aluminum which
>will be good for 30m and up.  I'll use the aluminum to support quarter
>wave wires for the other bands 10 thru 20, and maybe an inverted L type
>antenna for 40m.  Then a couple of radials and a strap from the ground
>side of the vertical to my metal roof.  That should provide for a nice
>ground plane.

My station in 9M6 has an HF2 mounted above a metallic roof on top of
a small block of flats (perhaps 25m square).

As with any elevated vertical I have ever used, I have found that the
counterpoise for the vertical needs to be resonant to work & decoupled
from anything near it (like the rebar in a reinforced concrete building
such as how even detached houses are constructed in VR).  The radials
in 9M6 took a significant amount of pruning from "standard" lengths due
to the detuning effect of that metal roof.

And if those radials were close to the roof (like a few mm), despite
being made of insulated wire, the slightest amount of rain would cause
everything to detune even more due to increased coupling, not to
mention how towards the ends of the radials it would arc straight through
the insulation with our relatively low power limit (400 watts).

None of the metal sheets that comprise the roof are bonded together.  I
would not like to try feeding the antenna against nothing but such a roof.
I would strongly suggest, after many hours of mucking about up there that
you get the base of your vertical up a good meter or two & an isolated
radial system under it which comes no closer than several dozens of cm
to that metal roof.

73, VR2BrettGraham


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