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R-7 and metal roof

Sep 25th 2011, 21:02

KA0LDG

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Will my Cushcraft R-7 vertical de-tune or will I have other issues if I mount it a few feet over a metal storage shed (roof is metal too)? I would rather mount it ~ 7-8 feet above the ground, so no one comes in contact with the ground counterpoise wires or any other part of the antenna. TNX!
Sep 26th 2011, 05:53

KE8DO

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Any metal is going to de-tune the antenna, the question is by how much. I would not think that it would be that much. You should mount it at the new location and then check it. If it goes lower in frequency, as I would guess, then you would want to shorten it some and check it again. You should be able to use the manual for the antenna as a guide as to how much to shorten or lengthen it.
73 Don KE8DO
Sep 30th 2011, 10:42

W1RFIAdmin

Joined: Jul 25th 2011, 14:25
Total Topics: 0
Total Posts: 0
If the antenna is mounted on the roof, with the metal roof underneath it, this really should not "detune" the antena, although the resonance of an antenna/ground system does vary a bit with the conductivity of the ground.

If anything, the metal roof will act as a ground plane, which could improve the efficiency of the vertical somewhat.

This is not without its caveats, however. See my response on the "metal roof and longwire antenna" thread for some important thngs to consider.

73.
Ed Hare, W1RFI
ARRL Lab
Technical forums moderator
Oct 11th 2011, 20:40

W4PG

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I had an R-7 mounted a foot above the top of a chimney that had a metal cap. The antenna was seriously detuned, especially on 40 meters, where resonance was about 6.8 mHz. I raised it up several feet above the top, and that improved but did not completely resolve the issue. It's not the same as a metal roof, but an example of what might happen.

.............Bob

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