Vertical On Concrete Roof

[KD6VXI]

Got a question maybe someone can answer.

How do you model a concrete structure?  In escrow on a house here in the Caribbean and want to start modelling the house for antenna placement, etc.

But what do you use for ground conductivity?

I want to (at least temporarily) put a vertical multiband up (DX Commander, 1/4 wave 5 band antenna).  Cool.  But, do I need to look at this as an elevated quarter wave needing 4 x cut to length radials or do I think of it as needing 32 + radials all 1/4 wave long at 80 meters?

The antenna feedpoint will be a foot or so off the concrete roof.  It is a flat roof. 

From eve to eve in the widest parts of the roof (it's a rectangular house) I have about 78 feet, so maybe 39 foot radials.......

I have 2 towers here to go up but am eyeing the lot behind mine.  It has a culvert going through it so is considered unbuildable (at least if you want a loan) but would be great area for tower growing. 

Anyone have experience with verticals on a parking garage or concrete building?  It's single story.

--Shane
WP2ASS / ex KD6VXI

[W1ITT]

Shane...  Probably it's not easy to find out, but I'd want to know what's in the concrete.  Is there rebar or remesh structural stuff?  Or is there any of that smaller mesh material that is commonly used under stucco in the tropics?  And is it bonded to the other material or just "in there".
This sounds like a temporary situation until you get the real-man antennas up.  I think I'd just go with some welded wire fence material, 3 or 4 feet wide and as long as it takes, in a cross formation with the antenna at the junction.  I've done this at a few locations to quickly establish an image plane, however imperfect, and get on with things.  AT that point it's an antenna on a raised plateau, easier to model.  The Rob Sherwood work, detailed on a white paper on his web site, has some good measurements to confirm that it's not just radio witchcraft.
73 de Norm W1ITT

[Steve - K4HX]

According to the Concrete Society (whatever that is), moist concrete behaves as an electrolyte with resistivity of up to 100 ohm-m.  Air dried concrete has a resistivity in the order of 10 000 ohm-m, whilst oven-dry concrete has a resistivity in the order of 100 000 000 ohm-m.

Soils have these approximate resistivities Ohm-meter):

Clay               1-100
Loam             4-40
Top Soil         40-200
Sandy Soil     400-4000
Loose Sands  1000-100000

So, it would seem that cured concrete is a pretty poor conductor compared to most soil, except for sand.

You could probably run your simulation/model using low/poor soil conductivity.

I did not look up the permittivity numbers for concrete. Also, assuming the RF penetrated the concrete (and it will at HF freqs), the steel rebar may have some positive impact.

[KD6VXI]

Norm,

The roof is poured concrete , I'm not sure the thickness (it's been added to since initial construction).  It has the webbed wire they use in place of rebar in it.  There is a few pieces of rebar that extend from the roof for a second story.

The walls are cinder block poured concrete inside.  Not sure of any rebar in it.

As Steve said I am figuring it's going to pretty much be crappy soil as far as models go.

Temporary has a way of becoming 'just another antenna' here although I do plan on the 2nd story addition in the next year .....

I like the wire mesh fence idea.   Have some already for the dog fence!

Thanks!

--Shane
WP2ASS / ex KD6VXI

[W3SLK]

I've noticed not only there in the Caribbean but down in Colombia where my wife is from, they use masonry and concrete construction for most of the houses! I guess we use wood here in the NE because of its insulating properties in the winter.

[KD6VXI]

Here masonry is used for insulation.

Insulation against a hurricane ripping your roof or walls off!

It's next to impossible to get insurance on stick built here.

5 to 6 percent of your cost to rebuild, per year, for homeowners insurance with a steel roof.

That drops to 2.5 to almost 4 percent total rebuild cost per year for all concrete.

It does help to regulate heat excursions but with the normal daytime being 88 and 78 being night time (summer) and drop 10 degrees off both for our winter, insulation from heat isn't that bad.  Humidity is the killer.

--Shane
WP2ASS / ex KD6VXI