My house has a corrugated metallic roof. 2500 sq. ft.
The roof is grounded via couple cables.
What's your comment on using the roof as the ground for a short center-loaded vertical 80M antenna?
Thanks David Hart VE6AQF, V21C
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$\begingroup$ Be aware that the two cables between the earth and your roof do not automagically place the roof at earth potential, no matter how good of an RF ground you might have at the bottom of those cables. How long are they? $\endgroup$ – Mike Waters♦ Jun 26 '17 at 22:56
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$\begingroup$ SDsolar, Mike, I have used this technique and in my case it seemed to work great. From what I have seen the bigger problem would be did any of the metal on metal joints develop any corrosion to the point where the user would be generating a 3rd harmonic energy to interfere with other frequencies. This used to be a big problem on Navy Ships and ECAC had a big problem getting people to start looking at this when I worked there. (these were the government people that looked at electromagnetic compatibility of equipment etc.) $\endgroup$ – Poe Aug 18 '17 at 23:44
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basically: large metal plates work great as ground plane, even if not connected to earth (if large enough).
Now "large" in "large metal plate" depends on the wavelength – and from the geometry of the problem stems the fact that a ground plane should be at least about one quarter of the wavelength in radius.
In your case, that's 80/4 m = 20m radius, or 40m across. 2500 ft² is, best case, 50 ft × 50 ft, and that's smaller.
Now, not all is lost, because of course you still have the actual ground as ground plane, but modeling that gets much harder. For example, you'll often find that in Northern Germany, radio broadcast antennas are preferrably placed on slightly moist ground – because that increases the conductivity.
Luckily, field density is highest close to the antenna, so your roof will have a strong positive effect. I'm afraid that's all I can say, lacking explicit experience with such antenna systems.
Regarding corrugation: Shouldn't matter, structure size is much much smaller than wavelength, but the altered path the currents in the metal take might make for an interesting delay – might electrically look a bit as if the roof was curved downwards orthogonal to the corrugation, but ever so slightly.
It's probably much more interesting how well the connections/overlap between the sheet metal elements of your roof are electrically connected.
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$\begingroup$ As usual, you hit it right on the head, @Marcus. That very last part was my first question. Like using the structure of a mobile home as a ground, a roof like this is probably held together with screws or nails that can rust. So I would question whether that roof would present as a single piece of metal. But using a short loaded antenna is already a compromise, and any metal is better than none. So I wouldn't see the need for another counterpoise on top of this roof. My other question is why anybody would want a vertical on 80M at home. That's just begging for QRN. $\endgroup$ – SDsolar Jun 26 '17 at 16:05
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1$\begingroup$ QRN - QRNature - static - since most hams use horizontally-polarized dipoles on 75/80, more impulse noise than signals would be received with a vertically polarized antenna. On this band you would be working mostly with ground waves which tend to retain their polarization as opposed to the rotations you get from ionospheric bending or bouncing. on the higher bands like 20m. In other words, you get a lower SNR by using a vertical on 80. I can understand that on a vehicle it may be the only choice, but at home I always use inverted Vs, sometimes sloped to favor a particular direction. $\endgroup$ – SDsolar Jun 26 '17 at 21:55
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$\begingroup$ ah, yeah, makes sense :) also, 80m implies that things get high, and thus complicated to build. (which probably is the reason why at these $\lambda$, horizontal is preferred. A horizontal thing has a harder time trying to topple over.) $\endgroup$ – Marcus Müller Jun 26 '17 at 21:57
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$\begingroup$ That is a good point, @Marcus. Every single AM broadcast station I've worked at/for, had a vertical, and you better believe the guy wires were always maintained properly under my watch. ;-) $\endgroup$ – SDsolar Jun 26 '17 at 21:59
