Topband: [Bulk] Ground screen ????

Grant Saviers grants2 at pacbell.net
Wed Mar 2 00:43:22 EST 2016


I've had two verticals on galvanized metal roofs.  An 80m top loaded 40' 
3" irrigation pipe on a two pitch steep barn roof about 30 x 60' and a 
full size 1/4wl 40m on a essentially flat roof shed 26 x 26'. The 80m 
worked super even though the roof was in bad shape.  I did add a number 
of sheet metal screws at the galvanized lap joints. The 40m was a bit of 
a disappointment, perhaps because the pole barn roof had an 2" vertical 
gap where an extension was added which I bridged every 24" with a wire.  
Both buildings were at 1500' above Silicon Valley with 50 mile views, so 
that helped a bit.

A metal roof is an elevated ground plane so it does want to be insulated 
from earth, but that is in conflict with good lightning protection.  So 
an uncertainty is how the edge of the roof appears at RF to be insulated 
or connected to ground.

Give it a try.  You can add wires off the edges as elevated radials and 
see if they make any difference in the resonance R.  If R goes down then 
you are reducing the ground losses.

I was strongly warned that passive intermod would be a disaster, but how 
would I know running 1 transmitter?  The site was isolated so no other 
hams to hear it if there was PIM (or to make PIM to bother me).  The 
verticals were also was direct clear line of site to a 50kw 1MHz BCB 
station 4 miles away down in the valley which put 1+ volt @ 1MHz on my 
power lines but there was no significant PIM I could hear (a bullet 
proof BCB filter was on the rig).

Grant KZ1W



On 3/1/2016 19:09 PM, Douglas Ruz (CO8DM) wrote:
> Hi,
>
> Need some ideas from your side...
>
> I am trying to find a good solution for my 160/80m antenna.
>
> I am thinking about some kind of Inv L with an 80m trap as a radiator for both bands.
>
> But, what about the counterpoise...Unfortunately in my small city lot I can´t install radials and I can´t get parts for the FCP yet...what about a Ground screen ???...I have a galvanized roof...it is about 50 ft long and 16 ft wide...galvanized sheets are 3 by 8 ft long and all are bolted together. The roof is about 15 ft high.
>
> Can I use this roof as a ground screen for the 160/80m antenna ???
>
> Thoughts ?,
>
> 73....Douglas, CO8DM
> P.S:1- I finish my 50 ft tower installation.
>         2- Thanks for all QSO´s on 160m before ARRL CW contest with me and with our contest               station T48K.
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>



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