"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