... but asphalt shingles are much more lossy than the old
wooden ones.
Can you elaborate a bit on asphalt shingles being lossy when the wire is layed down directly on the top of it? Is there a typical amount of lossiness (measurable how?) compared to the wire being 1" or 1' or 5 feet above the shingles?...
I don't have accurate numbers.
There are two effects: the dielectric loading, where the antenna
resonates at a lower frequency when close to the roof, and
the losses in the material itself. The first one is easier to measure.
I've installed a 40m loop around the roof at several houses, often
finding the resulting antenna resonated significantly lower in
frequency than I had expected. The loop at this house was about
5.5 MHz, with most of the wire laying on the shingles. Then I
tried propping it up on a dozen plastic pint refrigerator containers,
perhaps 4" or so (although the wire still touched the roof in places
in between them). That raised the frequency to 6 MHz or so.
It still didn't seem work very well, but I was plagued with noise
so I didn't get to try it out much. Now my noise has (mostly)
disappeared, and I have a 40m doublet fed with 300 ohm twinlead,
about 2 - 3 feet above the roof. Still doesn't work very well, but
part of that problem is due to losses in the tuner (the impedance
at the end of the feedline on 20m is about 5 ohms, which is not
good for tuner efficiency).
By contrast, when I've done that with wood shingles, they were
relatively transparent to RF (at least when dry), and I've had
good results just tucking the wires under the corners of the shingles.
It would, however, be an interesting study. I think erecting a loop or
dipole on the roof and measuring the impedance over frequency,
possibly at different heights above the roof, might give us some
more details to work with.
Some day...
Of course, there are a lot of details that will affect the results, like
the use of foil-backed insulation beneath the shingles, the actual
composition of the roofing, other wiring in the attic, etc.
My guess is that a foot or so off the roof would make a significant
difference.