Thank you Rich, RFB, and other contributors for this information, which I think is an important part of the total knowledge of understanding the question of putting RF up on buildings, up off of the ground.
I now have my own picture of what I would do on top of a building that I would consider legal under part 15. If radials travel on a horizontal plane, with equal opposites laid on both sides of a vertical monopole, they would not add radiation and thus be legal under part 15. Also, at such low power levels there would be no RF hazard, only the safety matter of "tripping" someone walking on the roof.
However, that only answers one question (in my mind), that of working on a flat rooftop. An "A"-frame house needs a different discussion, since radials could not be laid horizontally.
Also still unanswered is what to do about a middle-floor, with no access to the rooftop, but only windows and perhaps a porch.
Of this I am confident... a part 15 AM transmitter with 100mW input, operating from an upper floor without benefit of ground radials, is in little danger of exceeding 15.209 (24000/F@30meters).
Needed - Upper story apartment and condo dwellers for part 15 R&D (research and development).
First a reminder that the "legality" of the horizontal radials as discussed here is not established...but the installation could be "defensible" based on the theory that the equal and opposite radial currents produce fields which will cancel.
Consider the A frame situation Carl mentioned. Radials along the ridge would be at all points equal in elevation and the fields should cancel. However, if installed so they slope along the pitch of the roof cemtered one the ridge this is not the case. If we resolve the currents into horizontal and vertical vector components the horizontal components are still equal and opposite but the vertical components are not so these will not cancel.
Here's why. In the horizontal plane the currents at an instant will be going toward the center where the transmitter return (ground) is attached with cancellation but at this instant the current in each vertical plane will be moving in the same direction (up toward the transmitter ground) and the fields will add and there will be net radiation. At the same instant the current in the vertical antenna radiator is moving upward also so the vertical fields will add.
That's my thought experiment on this and a simulation would be helpful in confirming this.
Neil
Neil wrote: At the same instant the current in the vertical antenna radiator is moving upward also so the vertical fields will add. That's my thought experiment on this and a simulation would be helpful in confirming this.
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Yes, NEC shows more free space gain in the horizontal plane for a monopole system with 2 x 6-meter symmetric radials when "drooping" 3-m below the base of a 3-m vertical radiator than when the radials are horizontal.
In advertising back in the 60s if you ran an idea through your head, you were "blue skying" the idea. Let me blue sky what Neil said about the top ridge of the roof on an A-frame house.
In essence a vertical mast in the center of the ridge, with horizontal ground plane wires of equal length going equally on both sides as far as the ridge allows, would be a possibly legal (defensible) ground plane.
Of course having only the two radials, pointing let's say, north and south, would tend to make the overall signal directional with two lobes and two nulls.
If you were lucky enough to have two symmetrical dormers, windows with roof tops on the same height as the main roof, you could have short horizontals out to the limit of the dormers, slightly filling in the nulls.
OR, out at both edges of the side of the house poles could be mounted allowing additional radials on the horizontal. That's it! That's the way to do it!
I will be shaking hands as you file out.
Carl Blare wrote: "...In essence a vertical mast in the center of the ridge, with horizontal ground plane wires of equal length going equally on both sides as far as the ridge allows, would be a possibly legal (defensible) ground plane. ... Of course having only the two radials, pointing let's say, north and south, would tend to make the overall signal directional with two lobes and two nulls."
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This may seem to be true, but in reality the only conductor producing useful far-field radiation from this configuration is the 3-meter vertical mast -- and its radiation pattern in the horizontal plane is omnidirectional (no lobes or nulls).
Then, if it radiates uniformly, do the two horizontal ground wires on either side of the vertical mast give the desired "ground return", which is the reason for a ground plane?
To put it another way, does having those ground wires give a better signal than not having them?
Carl, can you explain the ridge -
I seem to have missed that. Are
you talking about the hill in your
back yard?
Bruce, DRS2
The "ridge" in this particular example is the top most point of an A-frame house, where the only horizontal plane exists along the top ridge of the house. All other surface area of the roof slopes downward toward the rain guttering.
Thanks Carl. I'll have to think
about this for a while.
Best Wishes,
Bruce, DRS2
do the two horizontal ground wires on either side of the vertical mast give the desired "ground return", which is the reason for a ground plane?
Yes, two wires are better than none. The objective is to lower the equivalent series resistance seen by the transmitter at its output. More radials are better but two are better than none.
Neil