RF current causing TVI in tin roofs is particularly possible when the roof is
used AS the current sink or is tightly coupled with high current radials
immediately above it.
The amount of current/voltage in the tin roof is drastically reduced if you
have something like an R8 above it, with NO direct or tightly coupled current,
and only oblique induction (roof is mostly or entirely horizontal, induction is
entirely vertically polarized).
Another argument for the vertical dipole sort of vertical.
>
> From: Mel Farrer <mfarrer@tality.com>
To: <towertalk@contesting.com>
> Date: 2001/06/12 Tue AM 08:03:13 EDT
> To: "'Guy Olinger, K2AV'" <k2av@contesting.com>,
> Barry Kirkwood
> <bjk@ihug.co.nz>, Tower Talk <towertalk@contesting.com>
> Subject: RE: [TowerTalk] Tin roof blues
>
> Tin or metal roofs have a bad habit of also generating TVI/RFI due to the
> poor connections between panels. I have no cure other than bonding the
> panels together, had work and sometimes impossible. On new install, just
> solder or braze them together.
>
> -----Original Message-----
> From: Guy Olinger, K2AV [mailto:k2av@contesting.com]
> Sent: Tuesday, June 12, 2001 7:44 AM
> To: Barry Kirkwood; Tower Talk
> Subject: Re: [TowerTalk] Tin roof blues
>
>
> To the original poster, one of the ground-independent multiband verticals
> like the R8, etc, would be quite good above a tin roof.
>
> The difficulties with ground for a vertical are
>
> 1) insufficient radial density and subsequent power loss at the ground
> connection used as a current sink. The R8 and its kind are a type of
> vertical dipole, so this loss is not present.
>
> 2) E-plane losses in the ground directly underneath the vertical, the reason
> a dense ground screen will help an R8 or the like. Doesn't have to actually
> connect to an R8, it just puts a shield over the ground.
>
> 3) vertical polarization reflection loss in the first several hundred
> meters. Only thing that fixes this is an obscene investment in radials,
> salt-water marsh or ocean shore location.
>
> Your tin room nicely takes care of 2) above. It's about as good a dense
> ground screen under a vertical dipole as you will get. Plus the downward
> radiation will be reflected at low angles in two directions if the roof is
> peaked.
>
> When I lived in an apartment in Washington DC, I had a tar covered, barely
> sloped tin roof, and the vertical I had up there played like gang-busters. I
> just didn't know why. Now I do.
>
> 73 & good luck.
>
> >
> > From: "Barry Kirkwood" <bjk@ihug.co.nz>
To: <towertalk@contesting.com>
> > Date: 2001/06/12 Tue PM 09:11:34 EDT
> > To: "Tower Talk" <towertalk@contesting.com>
> > Subject: [TowerTalk] Tin roof blues
> >
> > Some suggestions on using metal roof as groundplane.
> > When I lived back in the city I had a small two story town house with a
> > galvanised corrugated steel roof.
> > I used various quarter wave verticals with their base on a stub mast
> working
> > against 3-4 slightly sloping quarter wave radials.
> > For 80m I used a quarter wave inverted L (more like half an inverted v).
> > This was alwys used with at least one quarter wave radial. These longer
> > radials would run to a corner of the roof and then along the fence tops at
> > the sides of the lot.
> > I bonded the roof iron by spot soldering thin tinned copper wires at the
> > corners of the metal sheets.
> > Using a simple home made clip on rf current meter showed no current in a
> > wire from the junction of the high band radials to the roof. i.e. the fan
> of
> > radials effectively isolated the system from the metal below.
> > Current flowed in this wire when I worked the 80m inverted L. I measured
> the
> > drop in feedpoint impedance as extra 1/4 wave radials were attached to the
> > 80m feedpoint.
> > I came to the conclusion that the relatively small metal roof was
> equivalent
> > to at least 5 radials so far as currents were concerned.
> > I never tried the higher bands against the roof alone.
> > The 80m L worked very well, better than might be reasonably expected.
> > The little link wires corroded out very rapidly, so use a better bonding
> > technique if going this road.
> > I would certainly advise giving the metal roof a try as a groundplane.
> > 73
> > end
> > Barry Kirkwood PhD ZL1DD
> > Signal Hill Homestay
> > 66 Cory Road
> > Palm Beach
> > Waiheke Island 1240
> > NEW ZEALAND
> > www.waiheke.co.nz/signal.htm
> >
> >
> >
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> >
>
>
>
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