Adjacent (within 10 feet) of where we intend on placing our whip antenna (for AM broadcasting) there is a decent sized metal roof.
The roof slopes slightly down towards ground to flow water and snow off. Approximately, 15ft x 10ft of roof made of aluminum. This roof is approximately 20ft above grade and about 5-8ft below height of spot where antenna will go.
Will this aluminum roof create any positive or negative effect that others can think of? See the mention of ground plane effect with FM broadcasting.
I'd put that antenna at the center of that aluminum roof and use the aluminum roof as the counterpoise.
RFB
It's a consideration @RFB. I could mount the antenna there, but would require something creative as the aluminum isn't that sturdy. Was thinking about a wooden protrusion from the roofing eaves and supported from beneath as well. Basically a big upside down L made out of lumber. That would put the antenna within 2 feet of the aluminum roof.
To accomplish the counterpoise nothing would need attached from the antenna to the aluminum roof? It would just be an effective of the proximity, correct? The recent counterpoise example on here involved a vertical rod/wire up high... (i.e. well above the roof surface).
This would likely eliminate the need/benefit of radials also, correct?
Only thing missing there is a ground 🙂
I am so glad we have folks like you RFB to point the obvious and stupid to folks like me that aren't in the know yet.
"I am so glad we have folks like you RFB to point the obvious and stupid to folks like me that aren't in the know yet."
I prefer to reference it as "unaware" or "needing to know" rather than use the word stupid. No one is stupid.
Einstein said "The beginning of wisdom is knowing you do not know".
The support idea sounds good. I would also use a spring base on the whip so that it can sway a little bit in high wind. The spring base won't affect the antenna ability to resonate or perform for the AM band. It will help prevent the antenna from putting too much wind load stress on the aluminum roof and added support. The spring base will also eliminate the need to string up guy wiring which would have to be of non-conductive material. If the winds are strong as you noted in the other thread, and no spring base is used, you will have to consider anchoring that antenna with guy lines. Use fishing line or light nylon rope.
"To accomplish the counterpoise nothing would need attached from the antenna to the aluminum roof? It would just be an effective of the proximity, correct?"
Your describing a capacitive coupled counterpoise which can work quite effectively, and eliminate the "wahh wahh" of long ground leads etc. But any elevated antenna does need a direct path for static build up and lightning to dump somewhere besides down the audio/power wiring. In either case, you can use a choke to shunt off any RF from emitting from the grounding line that provides the DC path for static and lightning. Though it is not 100 percent effective in preventing a long ground line from radiating, it will knock it down a bunch but keep that DC path intact with a very very tiny resistance in that DC path from the choke.
RFB
RFB is quite right that you should have a safety ground. But you can mount your transmitter/antenna setup a few inches above the metal roof and achieve the same counterpoise benefit as if you had a wire running (and without the radiating ground wire effect).
I've never done it with AM, but noticed that effect (quite by accident) when I was broadcasting on FM. I moved my transmitter and in the process eliminated the tripod it was mounted on - the signal strength (and range) increased dramatically, and since Part 15 FM (even in Canada) is based on signal strength, I was forced to put the transmitter back on the tripod (about 6 feet above the roof).
Perhaps make a large X out of wood or angle iron. Probably just large enough to span a couple rafters.
Mount the antenna in the middle of the X with maybe a brace or two to stabilize the antenna mount. Attach the X to the roof in a couple places and put a dab of sealer where the protrusions would be.
It would look a little like those sleds they use on flat roofs for small Dish antennas. They usually place a couple cement blocks on them to hold 'em down but in this case you'd be securing it to the rafters.
I would think a simple whip would not have very much wind load.
I once had my TV mast pipe go through the roof and attach to the side of hte rafter and ceiling joist. I used a plumbing vent boot to make it weather tight. But then my roof had shingles. Why? I didn't want to buy a tripod. Moot point!
If the roof has a peak, you could make a saddle that would go over the peak with legs on either side of the peak, antenna at the peak.
My main roof is shingled with the old common flat non dimensional asphalt shingles. It's a peaked roof with a pretty good angle incline (old snow shed crazy pitch). Due to get up there and handle windblown damage and replacements (happens multiple times per year). Told you we get lots of wind here 🙂
Metal roof (aluminum is immediately below the main roof and attached at the eaves and serves as a cover for the back porch. Nice location perhaps for transmitter since have coverage from rain there and electric line already plus available area to drill from inside for any cable runs that might be necessary (audio input).
Technically, the house is probably best described as a Cape Cod architecturally, with the second story jutting out of windows being on the rear of the building (above that metal porch).
Interesting topic. I placed my antenna at the top of my aluminum roof and I the effect is so normal. You should try it.