On 6-Aug-2009, Michael Coslo wrote:

I'd say with this setup, you could do DXCC. DXCC and other awards have
been achieved on more modest setups than what you are looking at
building. You won't be the loudest signal on the band, but there can
only be one of those anyhow.

How much attenuation is a wood roof with asphalt shingles going to introduce? All of the duct work
in my house is in the crawl space, and the phone and cable TV lines are also in the crawl space. So
there is no metal other than the roofing nails, and there's nothing between the antenna and the sky
other than the roof decking and shingles. I don't expect them to add a lot of attenuation, but I
could be wrong.

KJ4NTS wrote:
On 6-Aug-2009, Michael Coslo wrote:

I'd say with this setup, you could do DXCC. DXCC and other awards have
been achieved on more modest setups than what you are looking at
building. You won't be the loudest signal on the band, but there can
only be one of those anyhow.

How much attenuation is a wood roof with asphalt shingles going to introduce? All of the duct work
in my house is in the crawl space, and the phone and cable TV lines are also in the crawl space. So
there is no metal other than the roofing nails, and there's nothing between the antenna and the sky
other than the roof decking and shingles. I don't expect them to add a lot of attenuation, but I
could be wrong.

There will be a slight effect of the other wires. Nails? probably not
measurable. The roof itself at HF is not attenuating much, it increases
as the frequency goes up. The biggest issue of this sort of antenna is
the height above ground, and the possibility of RF interference with
things in the house.

- 73 de Mike N3LI -

there's nothing between the antenna and the sky
other than the roof decking and shingles. I don't expect them to add a
lot of attenuation, but I
could be wrong.

Maybe 3' makes a big difference. I have always seen a difference between
antennas in the attic or stretched out on the roof, and antennas even a few
feed above the roof. I don't know if it was shingles or what. It would
really be interesting to see what kind of shingles are transparent. I have
seen antenna farms hidden behind fake gables.

there's nothing between the antenna and the sky other than the roof
decking and shingles. I don't expect them to add a lot of attenuation,
but I could be wrong.

Maybe 3' makes a big difference. I have always seen a difference between
antennas in the attic or stretched out on the roof, and antennas even a few
feed above the roof. I don't know if it was shingles or what. It would
really be interesting to see what kind of shingles are transparent. I have
seen antenna farms hidden behind fake gables.

I've never "played" with it at HF, but I do know that the dielectric
constant of nearby materials affects antennas at VHF.

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
NRA Life Member & Certified Instructor for Rifle, Pistol, & Home Firearm Safety
Also Certified Instructor for the Kansas Concealed-Carry Handgun (CCH) license

JB wrote:
there's nothing between the antenna and the sky
other than the roof decking and shingles. I don't expect them to add a
lot of attenuation, but I
could be wrong.

Maybe 3' makes a big difference. I have always seen a difference between
antennas in the attic or stretched out on the roof, and antennas even a few
feed above the roof. I don't know if it was shingles or what. It would
really be interesting to see what kind of shingles are transparent. I have
seen antenna farms hidden behind fake gables.

Well, there's nothing like ground truth, JB, so I'll defer to your
experience vs my conjecture. Possibilities for further RF attenuation
might be the materials in use. Those little rocks used for color and
grip. Fiberglass is used a lot for the base, but there may be other
fabrics too. It's a composite material, who knows, there might be carbon
in it as well.

- 73 de Mike N3LI -

KJ4NTS wrote:
On 6-Aug-2009, Michael Coslo wrote:

I'd say with this setup, you could do DXCC. DXCC and other awards have
been achieved on more modest setups than what you are looking at
building. You won't be the loudest signal on the band, but there can
only be one of those anyhow.

How much attenuation is a wood roof with asphalt shingles going to introduce? All of the duct work
in my house is in the crawl space, and the phone and cable TV lines are also in the crawl space. So
there is no metal other than the roofing nails, and there's nothing between the antenna and the sky
other than the roof decking and shingles. I don't expect them to add a lot of attenuation, but I
could be wrong.


Attenuation isn't a real concern, except when it's raining or the roof
is wet.

What should be a concern is the following:


1) weird interactions between antenna and conductive stuff in the attic.
Don't expect the same feedpoint impedance vs frequency you get with
the antenna out in the open.

2) Fi The voltage gets pretty high at the ends of a dipole when
you're radiating even 100W. Or, more to the point, the voltage gets
high in nearby wires that happen to be the "wrong" length. A small
ember smoldering in your attic is a huge problem. This is something you
need to figure out how to check.. keeping the active ends of the dipole
clear of flammable stuff is straightforward, but the accidental
excitation of something else, isn't. (wiring isn't a problem.. it's
usually connected to something else that shunts the current.. it's
isolated conductors, like plumbing, HVAC ducts, etc. In your situation,
yours is in the crawl space, so you're in good shape.)

3) RF exposure. Do your calculations and make sure you're not cooking
things you shouldn't be.