Ground Plane antenna near metal roof?
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| Jul 6th 2013, 15:28 | |
KO0YTotal Topics: 0 Total Posts: 0 |
I have an Arrow 2 meter/70 cm ground plane antenna (model GP 146/440) that I have been using as a portable, but now want to mount on a mast at one end of the peak of a 3:1 pitched roof. It is a metal roof, and is not grounded. How far above the roof should the antenna extend? (Lower would be better from a stealth perspective.) Would the metal roof add to or adversely affect the ground plane? Does it matter, to the antenna, whether the nearby roof is grounded? We live in a Colorado park, that is, a small circular valley surrounded by peaks, including one directly north and towering 300 feet above us. Hence, I have been more concerned about surge currents than a direct strike. The antenna will be connected to an Alpha-Delta transi-trap, which is grounded via earth rod. I have been using a Yagi placed in the trees above the house (but well below the peak) and pointed southwest. But I've noticed that the ground-plane, when placed on a pole on my south-facing second-story deck, gives much the same coverage, without the 150 foot feed of the Yagi. This location is 20 feet below, and 10 feet laterally, from the roof. Thanks for any guidance in this. |
| Jul 8th 2013, 15:04 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
You should install it high enough that the roof does not de-tune the radials--if the SWR is high you should try raising the antenna to reduce the interaction. For lightning protection, you need to be aware that lightning coming in via the power lines will often seek out the shack ground--it is best if it can do so external to the house--hence the NEC requirement to bond everything together. I'd make the feedline as long as necessary to keep the ground wires straight. Zack Lau W1VT ARRL Senior Lab Engineer |
| Jul 9th 2013, 15:49 | |
KO0YTotal Topics: 0 Total Posts: 0 |
I put the antenna on a painter's pole and stuck it out the upstairs bedroom, about 2 feet above the roof, and the SWR was around 1.1-1.3 on both VHF and UHF-better than on the deck! So I guess it will work. About grounding: I plan to run a #8 bare copper conductor from the antenna's ground-plane side to the 8 foot copper rod that also serves to ground the HF antenna and shack (each on a separate conductor). Should I use plastic for the antenna mast or would it matter? Should I leave the roof ungrounded or should I run another #8 from the roof in a different location (straight down to the grounding rod)? The antenna ground will have two bends to reach ground: 30 then 60 degrees. Is this acceptable or should I connect the antenna to the roof and depend on the roof's straight grounding conductor? Thanks again for guidance. I want to get everything straight before I rent the 32-foot ladder needed to reach the roof peak from the surface! |
| Jul 9th 2013, 18:49 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
A metal mast typically makes it easier to ground the antenna installation. There should be a ground wire directly between the mast/antenna and the ground rod. The bends in the ground wire should be a gentle as possible--lightning doesn't like to go around bends. If you have concerns about the roof grounding, you may want to consult someone locally who can inspect the situation and give a recommendation. http://www.arrl.org/lightning-protection An excellent reference is the NEC 810 material by Mike Holt Zack Lau W1VT ARRL Senior Lab Engineer |
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