Using a metal roof as a ground plane
 

Hello:

I've just received a 5BTV, 5 band trap vertical (75-10M), and as with
most of them, I would suppose the instructions suggest that ground
mounted is ok, but elevated it would be better to use radials.

I am planning on mounting it on the peak, near the center of a rather
large metal roof. The roof is approximately 58x38 feet and has a pitch
of 4" per foot down from the peak. The roof is composed of 39" strips
of metal roofing, and isn't quite solid metal, but has three sections
three feet wide that use fiberglas panels for the top 35 feet of each
section on each side. (These perform the function of skylights). The
only "bonding" of the roof panels to one another are the stainless steel
screws that hold the panels to the roof substructure, which are placed
about every two feet along each edge, and in three or four places at the
top of each strip of metal roofing where they connect to the ridge.
Each run (except for the fiberglas panels) is a single section
approximately 3ft wide and 19+ feet long. There is a connector panel
about 3 feet long at the bottom of each of the fiberglass strips, so the
bottom edge of the roof is continuous along the 58 foot length. (Think
of it as a nearly solid roof with three strips missing on each side,
making a 3 foot by 16 foot slot in each of the three skylight locations
on each half of the roof.)

After this description, if you're not completely confused, what I'm
wondering is whether the roof will provide a decent ground plane for the
vertical antenna, or whether I should add specific ground radials....

Of course, I'll have to figure out a decent mount so it actually stays
on the roof with our winds (which are typically not that bad, even here
in Hawaii where I live.) It's a rare occurrence when we have 50mph
winds, but 20-30mph "trades" are more normal.

Anything I should be looking out for?

Thanks
--Rick

"Rick Frazier" wrote in message
...
Hello:

I've just received a 5BTV, 5 band trap vertical (75-10M), and as with
most of them, I would suppose the instructions suggest that ground
mounted is ok, but elevated it would be better to use radials.

I am planning on mounting it on the peak, near the center of a rather
large metal roof. The roof is approximately 58x38 feet and has a pitch
of 4" per foot down from the peak. The roof is composed of 39" strips
of metal roofing, and isn't quite solid metal, but has three sections
three feet wide that use fiberglas panels for the top 35 feet of each
section on each side. (These perform the function of skylights). The
only "bonding" of the roof panels to one another are the stainless steel
screws that hold the panels to the roof substructure, which are placed
about every two feet along each edge, and in three or four places at the
top of each strip of metal roofing where they connect to the ridge.
Each run (except for the fiberglas panels) is a single section
approximately 3ft wide and 19+ feet long. There is a connector panel
about 3 feet long at the bottom of each of the fiberglass strips, so the
bottom edge of the roof is continuous along the 58 foot length. (Think
of it as a nearly solid roof with three strips missing on each side,
making a 3 foot by 16 foot slot in each of the three skylight locations
on each half of the roof.)

After this description, if you're not completely confused, what I'm
wondering is whether the roof will provide a decent ground plane for the
vertical antenna, or whether I should add specific ground radials....

Of course, I'll have to figure out a decent mount so it actually stays
on the roof with our winds (which are typically not that bad, even here
in Hawaii where I live.) It's a rare occurrence when we have 50mph
winds, but 20-30mph "trades" are more normal.

Anything I should be looking out for?

Thanks
--Rick



Hi Rick
One advantage of a vertical on a peaked metal roof as opposed to one on a
flat ground plane is that the feed point impedance at resonance is very
close to 50 ohms, depending on the pitch of the roof..
I have a Fluidmotion BiggIR vertical on the peak of a 3500 square foot
aluminum roof and I must say it's the best vertical installation I've ever
used.
Some scheme to limit common mode current on the feed line is in order;
In my case I just looped the coax and control cable seven turns. Other
schemes work too.

How close are you to the water? Or are you up high?
You didn't say if you're in the Hawaiian islands generally or on the Big
Island.
The reason I asked is that I once (1970) operated from St Johns in the
Virgin islands. Some of the operation was from a habitat under water
(Tektite II). We used a HiGain 14AVQ trap vertical on a barge above the
habitat with a big piece of cable splayed out in the salt water. That
antenna worked quite well as I recall.

73
H.
NQ5H

On Wed, 14 Apr 2004 06:41:32 -0500, H. Adam Stevens, NQ5H wrote:

"Rick Frazier" wrote in message
...
Hello:

I've just received a 5BTV, 5 band trap vertical (75-10M), and as with
most of them, I would suppose the instructions suggest that ground
mounted is ok, but elevated it would be better to use radials.

I am planning on mounting it on the peak, near the center of a rather
large metal roof. The roof is approximately 58x38 feet and has a pitch
of 4" per foot down from the peak. The roof is composed of 39" strips
of metal roofing, and isn't quite solid metal, but has three sections
three feet wide that use fiberglas panels for the top 35 feet of each
section on each side. (These perform the function of skylights). The
only "bonding" of the roof panels to one another are the stainless steel
screws that hold the panels to the roof substructure, which are placed
about every two feet along each edge, and in three or four places at the
top of each strip of metal roofing where they connect to the ridge.
Each run (except for the fiberglas panels) is a single section
approximately 3ft wide and 19+ feet long. There is a connector panel
about 3 feet long at the bottom of each of the fiberglass strips, so the
bottom edge of the roof is continuous along the 58 foot length. (Think
of it as a nearly solid roof with three strips missing on each side,
making a 3 foot by 16 foot slot in each of the three skylight locations
on each half of the roof.)

After this description, if you're not completely confused, what I'm
wondering is whether the roof will provide a decent ground plane for the
vertical antenna, or whether I should add specific ground radials....

Yes, it should, even with the fact that it is not continuous sheet metal
it will probably still have a LOT more surface area than a few radials.
I would ground the vertical well to the roof, test the sheet panels are
electrically connected and go for it. If the antenna doesn't seem to
perform well you can always add radials later. I used a metal roof once
with a vertical and it worked very well indeed.

Larry VE7EA


--
********************************
to reply via email remove "fake"

Rick,
It ought to 'work fine 'n last a long time'! Well,
maybe not a real long time, but a couple of years or
so (don't tempt Moma Nature LOL).
Your metal roof should make as good a ground as any
dirt/radial system, especially where you live, with the
volcanic 'stuff'. Just how 'good' will depend on the
mounting location, the pitch of the roof, and so on.
There's no way of predicting exactly what your results
will be as far as the input impedance since it will
depend a lot on how well you tune the antenna.
'Doc

PS - If it doesn't work well, send me a bus ticket and
I'll help you 'fix' it???

In article ,
Rick Frazier wrote:

Hello:

I've just received a 5BTV, 5 band trap vertical (75-10M), and as with
most of them, I would suppose the instructions suggest that ground
mounted is ok, but elevated it would be better to use radials.


snipped for brevity


Anything I should be looking out for?

Thanks
--Rick



Hi Rick,

I live far out in the alaskan bush, and use a very similar antenna
system. I have a 5BTV mounted at the center of my peak ridge, and
grounded to the Galvinized sheet Metal Roofing. The only thing I did
different than you have suggested, is to run a Cooper Braid along the
ridgecap that connected to each of the sheets of Roofing. This vertical
Antenna System loads really well, and has provided good serviceablity
for me.

Bruce in alaska AL7AQ
--
add a 2 before @

"Rick Frazier" wrote in message
...
Hello:

I've just received a 5BTV, 5 band trap vertical (75-10M), and as with
most of them, I would suppose the instructions suggest that ground
mounted is ok, but elevated it would be better to use radials.

I am planning on mounting it on the peak, near the center of a rather
large metal roof.

As others have mentioned, check the continuity
between the panels... some metal roofing comes
coated with plastic or paint (even if it looks galvanized).

If the skylights you mentioned interfere with
the ground connection along the peak of the
roof, I'd go ahead and run a strap down the
center of the roof peak line, bonding it to all
metal panels along the way.

Metal roofs can help, or hurt! If the connections
are bad between panels then you'll get loss and
noise. Otherwise, it's the best of both worlds, a
long-lasting roof that makes an excellent ground
plane.
__
Steve
KI5YG
..

"Larry Gagnon" wrote in message
...
On Wed, 14 Apr 2004 06:41:32 -0500, H. Adam Stevens, NQ5H
wrote:

"Rick Frazier" wrote in message
...
Hello:

I've just received a 5BTV, 5 band trap vertical (75-10M), and as with
most of them, I would suppose the instructions suggest that ground
mounted is ok, but elevated it would be better to use radials.

I am planning on mounting it on the peak, near the center of a rather
large metal roof. The roof is approximately 58x38 feet and has a
pitch
of 4" per foot down from the peak. The roof is composed of 39" strips
of metal roofing, and isn't quite solid metal, but has three sections
three feet wide that use fiberglas panels for the top 35 feet of each
section on each side. (These perform the function of skylights). The
only "bonding" of the roof panels to one another are the stainless
steel
screws that hold the panels to the roof substructure, which are placed
about every two feet along each edge, and in three or four places at
the
top of each strip of metal roofing where they connect to the ridge.
Each run (except for the fiberglas panels) is a single section
approximately 3ft wide and 19+ feet long. There is a connector panel
about 3 feet long at the bottom of each of the fiberglass strips, so
the
bottom edge of the roof is continuous along the 58 foot length.
(Think
of it as a nearly solid roof with three strips missing on each side,
making a 3 foot by 16 foot slot in each of the three skylight locations
on each half of the roof.)

After this description, if you're not completely confused, what I'm
wondering is whether the roof will provide a decent ground plane for
the
vertical antenna, or whether I should add specific ground radials....

Yes, it should, even with the fact that it is not continuous sheet metal
it will probably still have a LOT more surface area than a few radials.
I would ground the vertical well to the roof, test the sheet panels are
electrically connected and go for it. If the antenna doesn't seem to
perform well you can always add radials later. I used a metal roof once
with a vertical and it worked very well indeed.

Larry VE7EA

Considering the GP that is normally used with this antenna for elevated
mounting it should work quite well with what you have. I would probably want
to mount it in the center of the roof and run 4 wires from the antenna to
the corners of the roof bonding the wire to each metal panel it crosses. One
problem with doing this is that bad connections with adjacent panels could
cause noise on recption and intermitent changes in tunning during transmit.
Just be aware of what you may have to do to fix some strange intermittent
problems.

"Jimmy" wrote in message
m...
"Larry Gagnon" wrote in message
...
On Wed, 14 Apr 2004 06:41:32 -0500, H. Adam Stevens, NQ5H

wrote:
"Rick Frazier" wrote in message
...
I've just received a 5BTV, 5 band trap vertical (75-10M...

I am planning on mounting it on the peak, near the center of a rather
large metal roof. ...

.... mount it in the center of the roof and run 4 wires from the antenna
to
the corners of the roof bonding the wire to each metal panel it crosses.
One
problem with doing this is that bad connections with adjacent panels could
cause noise on recption and intermitent changes in tunning during
transmit.
Just be aware of what you may have to do to fix some strange intermittent
problems.



I agree that any intermittent contacts in the roof metal CAN make noise.
Even if the wire is routed as suggested. Receive is covered above... During
transmit there are two effects to watch for.



Any noise caused by bad connections in the wind WILL *modulate*
your transmitter with the same noise. Most likely, this won't cause a
problem for you except to other radios close by. However, the frequency
range can be extensive. On Apollo communication ships the HF transmitters
caused interference to the on-board *radars* because of the junk caused by
the deck railing chain links. Whatever the noise spectrum looks like, this
can cause an AM on your signal with the same shape sidebands. Because your
transmitter is right there and part of the antenna current is flowing in the
roof, the effect can be rather strong. On sideband I think this could sound
like splatter. Bottom line, I think is increased possibility of TVI or BCI
for the nearest neighbors.





The other is increased ability to make Intermodulation (IM). This
should only be a problem if you have two transmitters operating. This is
the sum / difference and 2A-B type of IM caused by partly rectifying
junctions in the corroded metal.



--
Steve N, K,9;d, c. i My email has no u's.