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Author Topic: Steel Roof and Siding  (Read 653 times)
AC0KT
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Posts: 8




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« on: March 21, 2008, 03:10:50 AM »

Alright, I'm finally done playing around with this on my own, and would like some gut-feeling responses from those with more experience than I....

I've got steel roofing and steel siding on the house (welcome to iron country, MN)  and while talking with another ham about all the problems it was causing, he suggested that there has to be *some* way (unknown to us) that I could actually use this to my advantage, rather than having it cause random trouble.

It's a single story, extremely small (~35'x~25') house, with a fair amount of trees nearby.  Right now, I'm just running a simple 40m dipole @ 35ft.  I've tried providing decent electrical connections between all the roof and siding junctions, and in turn, provided them with a good RF ground.  It made a huge difference, to the point where  I was switching the ground in and out, and depending on time and band, it was making a 60db difference in the received signal.  What frustrated me was that half the time, it was better with the solid ground, half the time better with the disconnected ground.

So what are your gut-feelings on this?  Is providing an RF ground up that high making the antenna operate as if it was lower than it actually is?  Could using the house as a makeshift ground radial system be sufficient for a vertical?  If an antenna was mounted to one side of the house, could I lay out ground radials only on the other side of the antenna?

Not having much in the way of field strength measurement, I really find myself lacking in the ability to determine the performance of various antenna "experiments" I'm trying.  With the short summer up here, and a limited amount of operating time available, I'm afraid that this whole experiment is just playing with fire, and I may never *know* if I'm degrading my stations performance with all these tweaks.
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WA3SKN
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Posts: 6526




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« Reply #1 on: March 21, 2008, 04:24:50 AM »

"Is providing an RF ground up that high making the antenna operate as if it was lower than it actually is?"...Yes!
"Could using the house as a makeshift ground radial system be sufficient for a vertical?"...Depends on band.  20 meters and up...yes, 40 meters and down...no, 30 meters...maybe!
"If an antenna was mounted to one side of the house, could I lay out ground radials only on the other side of the antenna?"... Yes, but it could become directional.
Just make sure the metal is bonded together and there is no corrosion, this could cause a semi-condutor that could generate harmonics when you transmit!
73s.

-Mike.
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AA4PB
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Posts: 14361




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« Reply #2 on: March 21, 2008, 05:26:15 AM »

How close is your dipole to the house? I expect it must be *very* close if switching a ground connection to the siding makes a 60dB difference in received signal strength. Even Yagis generally don't have a 60dB front to back ratio. How did you take these measurements? Are you sure that it wasn't propagation that was making the difference rather than grounding the siding?

For the metal roof to be effective on any band, the vertical will have to be mounted in the center of the roof. It won't work well to use the house as the counterpoise on one side and ground radials on the other since radials need to be at the same level as the base of the antenna.

Give us some more description of the antenna and feed line in relationship to the house location.
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Bob  AA4PB
Garrisonville, VA
AC0KT
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Posts: 8




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« Reply #3 on: March 21, 2008, 05:39:40 AM »

WOW!

Sounds like I'm "playing with fire" here, and I don't want my dipole, which has been more than sufficient for my casual operating purposes, suffering from my twiddling around.

Judging by the 20 and up being effective comment, I think this is something I should revisit at the top of the cycle, and hopefully by then I'll be off of this postage stamp lot, and can actually satisfy my desire for an efficient homebrew antenna compilation.

For he who asked, the 60db ratio was taken from a few signal reports, which as we all know, are unfortunately only moderately accurate.  Basically, just getting a report, changing configuration, and getting another report.  They weren't close enough to be ground wave, but they were all "continental" so I may have just found a better cloud to warm for those particular stations.  That's why the 50/50 ratio was particularly disturbing.

Thanks for the suggestions, I'm a little smarter now than I was before, which is the whole point... Smiley
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W5FYI
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Posts: 1054




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« Reply #4 on: March 21, 2008, 06:07:17 AM »

Are you sure you're switching the ground and not the antenna? 60dB is equivalent to switching from 1000 watts to 1 milliwatt! I have no idea of what would cause that much attenuation. What more information can you provide?
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WA3SKN
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Posts: 6526




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« Reply #5 on: March 21, 2008, 07:49:09 AM »

You said you had trees, any room for a loop antenna?  They are great multiband fed with open wire and a tuner!

-Mike.
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AA4PB
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Posts: 14361




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« Reply #6 on: March 21, 2008, 11:58:35 AM »

Here's an idea to get a better idea of the difference: Tune in a AM broadcast station on 40M at night. Try to pick one that gives a somewhat stable average reading. Go switch the ground and as quickly as possible check the signal strength again. If the attenuation is really 60dB, an S9 signal would drop well below the noise level based on 6dB per S-unit.

Antenna gain and directivity are reciprocal so the "difference" you see in receive will be the same on transmit to that same location.

My "guess" is that the big difference you saw had more to do with propagation/station differences than with interference from the metal building. For reference, I've driven my 40M mobile station out in the clear, under a tree canopy, and up on top of the Chesapeake Bay Bridge without seeing anywhere near a 60dB difference is signal strength.
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Bob  AA4PB
Garrisonville, VA
AA4PB
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Posts: 14361




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« Reply #7 on: March 21, 2008, 12:01:07 PM »

fed with open wire
-------------------
This could present some problems with getting it around and into a metal building.
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Bob  AA4PB
Garrisonville, VA
KF6IIU
Member

Posts: 293




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« Reply #8 on: March 21, 2008, 01:21:52 PM »

As others have pointed out, you roof may be acting less like a ground and more like a directional element for your antenna. The bonding probably won't change that much.

The bonding will get rid of any electrical noise or weird rectification effects caused by rusty metal on metal contacts, though.

I'd leave the switch in place and call it a "switchable array"!
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