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metal roofs and long wire antennas

Sep 22nd 2011, 19:25

HAGL290

Joined: Feb 18th 2001, 00:00
Total Topics: 0
Total Posts: 0
has anyone had experience dealing with longwire antennas in the attic that has a METAL roof. my current antenna setup with an asphalt roof works quite well, however i'm concerened that as we switch to a metal roof my reception will drop considerably. any sugestions would be quite helpfull. thanks joe KJ4QNR
Sep 25th 2011, 23:56

KM3F

Joined: Mar 6th 2008, 13:50
Total Topics: 0
Total Posts: 0
Interesting question Joe.
Normally on a wire antenna, a ground is supplied at some point as the 'other half' of the antenna system.
What do you have now?
It may be a suprise if it happens but suppose your ground were to be the metal roof as far as the antenna goes but just on top instead of being below hoping the roof metal is floating and not grounded enough to act like a shield.
Code may make it manditory to ground the roof, though.
It may just still work ok but would expect a difference in impedence matching from what you now have.
Be sure to let us know how it works out.
Sep 26th 2011, 13:58

HAGL290

Joined: Feb 18th 2001, 00:00
Total Topics: 0
Total Posts: 0
my existing attic antenna is not grounded at all, just wraped arounded the attic several times, down through the ceiling to my 2nd floor bedroom and straight to my LISTENING radio ( tecsun pl660 with synchronous detection ). it works great ! even australia is usually fairly easy to pick up here in N.C. but what did you mean by "but just on top instead of being below". i'm not that educated in electronics like alot of you guys are. my contractor says the metal roof will be "floating" not grounded as you refered to. are you sugesting that i connect the loose end of the long wire ( 200 ft +-) antenna to the peak of the metal roof ? i'll try it; however, what about lightning strikes or induced current from a nearby strike. what do you think. thanks joe kj4qnr ( hagl290 )
Sep 30th 2011, 10:32

W1RFIAdmin

Joined: Jul 25th 2011, 14:25
Total Topics: 0
Total Posts: 0
Indoor antennas are, by necessity, comrpromises. The transmitted and received signals are subject to losses propagating though the building and by the interaction with the antenna with nearby conductors such as electrical or other wiring in the walls, plumbing, etc. If a ham has done the best he or she can, then the inefficiencies are just a necessary part of the station design.

Look at it this way -- if an antenne were only 5% efficient, that sounds pretty horrible, but if one is running 100 watts into a 5% efficient antenna, that's like running 5 watts into a perfect antenna, and QRPers do it all the time. Hams should generally strive to make their antennas as good as possible, but if cicumstance requires iniefficiency, this will not stop a ham from building an antenna and making contacts with it.

The metal in the roof may of may not be grounded. Even if grounded, at radio frequencies, a ground is not always a ground. The wire used to connect the "grounded" roof to earth has fininte length, so it is not a ground, it is a single-wire transmission line transformer. Like your antenna, that wire will have a standing wave that will transform the 0 ohms that would be present at a perfect ground to be some other value, with inductive or capacitive reactance, at the metal roof. The metal also has fininte size, so it, too, will have a standing save on it, as your transmitted antenna parasitically couples into it.

The net effect is that the metal roof is not as much a shield as it is an active part of your antenna system. All of the electrcial wiring and metal plumbing in the house will also be interacting with the antenna, much the same way that a director or reflector in a Yagi array interacts with the signal radiated by the driven element to change the radiated pattern, giving gain.

That interaction will take place with an indoor antenna. Whether is is "gain" will depend on the amount of signal lost in those other conductors. More than likely, the process will be somewhat lossy.

The principles of coupling between antennas is discussed in detail on an article on the W8JI.com site: http://www.w8ji.com/antenna_coupling.htm.

The subject of grounding has been given a thorough and accurate treatment at http://www.w8ji.com/ground_systems.htm.

As to lightning strikes, if your roof recieves a direct hit, there is no doublt that damage to your home and station will result. Nearby lightning strikes will also couple energy into the metal in your roof and home wiring and plumbing. The risk of direct or indirect strikes is not significantly different with or without that antenna wire in the roof. However, that nearby strike can couple a lot of energy into your station, possibily damaging equipment that is connected to it, To the extent possible, you should take the same precautions with respect to an indoor antenna as an outside antenna, including a correctly implemented station ground (see the URLs above), devices such as lightning protectors in your feed lines and disconnecting your antenna from your station equipment when it is not in use.

You should also NOT use any "grounded" conductor within your home as the ground wire for your station, other than the third-wire ground that may exist in an electrical plug used with some of your station equipment. Pipes or, worse, "grounded" roofs may have those loose connections, too, and if you are putting a lot of RF onto that "ground" wire, sparking can be rather severe.

One may be tempted to make up for the inefficiencies with power. This can work, but when you transmit, there will be actual RF current flowing through the metal roof and other nearby conductors. Those conductors were not designed to carry RF and if there are any loose connections in them (corrision between sheet sections on the lap joints on a metal roof, for example), sparking could occur at those loose connections. The effect could range from the creation of noise when you transmit -- a probable RFI problem -- to actual sparking, forming a potential fire hazard.

I've seen sparking in a metal window frame when I had part of an indoor antenna passing wtihin inches of it, at about a 200-watt transmitter power. Personally, for this reason, and for reasons of RF safety, I would not use an amplifier with an indoor antenna.

73.
Ed Hare, W1RFI
ARRL Lab
Technical forums moderator

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