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Author Topic: Metal roof ground plane  (Read 8499 times)
K2OWK
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« on: September 07, 2011, 06:21:34 PM »

I have a question. Why do the metal roof shingles need to be electrically bonded together to form a good ground plane? I would guess that the capacitive coupling between shingles would adequate for RF coupling. These shingles are normally in very close proximity if not actual touching. Just curious about the why of the electrically bonding.

73s

K2OWK
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AC5UP
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« Reply #1 on: September 07, 2011, 06:33:45 PM »

Quote
In the presence of strong radio waves, corroded metal contacts or connections can act as detectors and generate unwanted signals that affect receivers in the surrounding area. This is more precisely referred to as external rectification. If the interference affects a broad range of frequencies, the source is often located very close to the most powerful transmitter, on the antenna itself, on the guy wires or very close to the broadcast station. The corroded contact must be identified so it can be cleaned or insulated. A word of caution: there may be more than one source of interference at a single location. The level of interference will decrease as sources are eliminated. Generally, this type of interference disappears when it rains. Involved radio stations will help you identify and eliminate this type of problem.

http://www.selfhelpandmore.com/interference/am-fm-radio-interference.php
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N4CR
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« Reply #2 on: September 07, 2011, 08:40:53 PM »

I have a question. Why do the metal roof shingles need to be electrically bonded together to form a good ground plane? I would guess that the capacitive coupling between shingles would adequate for RF coupling. These shingles are normally in very close proximity if not actual touching. Just curious about the why of the electrically bonding.

The entire idea of a ground plane is to reduce return losses. A bonded set of conductors does that the best. Everything else is worse. More conductors covering more area is better, less conductors covering less area is not as good.

Think of the return circuit as being a bunch of resistors radiating out in a circle like radials. Dirt is high resistance. Air is high resistance. Unbonded metal is high resistance. Every conducting radial shorts out a resistor and reduces the return resistance.

Think of the feed point like two conductors entering a node where Kirchoff's law says the current into a node must equal the current out of a node.

Well, if one of those two conductors has a resistor in series with it, that limits the current into the node. Which also means it limits the current out of the node. You are trying to short out that resistor.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
K2OWK
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« Reply #3 on: September 08, 2011, 01:54:27 PM »

Thank you for the answers. I knew there was a reason for it, but did not know the why till now.

Thanks again,

73s

K2OWK
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AB3NK
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« Reply #4 on: September 08, 2011, 05:56:40 PM »

I hope I can make some comments here without some dumbazz quoting it and trying to use it against me.

You are just kidding yourself if you think that you can use a metal roof as a ground plane.

The problem with this idea is that the roof would need to be inches below the antenna, not feet.   If you put some type of high power signal into the antenna - then the roof is going to cause more problems then the gain you will experience.

The actual ground plane is the earth under the antenna and not the structure.
If the roof was your ground plane - your antenna would radiate most in the direction of the superstructure.
Hence if you put your antenna at the north side of the house and the roof went directly to the south for 50' - your best signal - both transmitting and receiving would come from the south.
The problem is - how do you turn the house if it is aimed in the wrong direction?

The one thing missing here is a description of what type of antenna you are using and the height AGL of the antenna and the amount of power you are using.

As a example, a Uncle of mine had a Duo 3 Beam antenna ( Hy Gain) on a tower, 50' AGL in my back yard when I was a kid.
With his puny 12 watts (PEP) side band signal, his voice could be heard in the neighbors toaster when he talked, bad connection in the toaster, and also wiped out the other neighbors television when he tried to watch the Pittsburgh Pirates on KDKA television - ch 2 or 3 back in the day.

Just think what a couple o hundred watts would do!
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W3LK
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« Reply #5 on: September 08, 2011, 06:19:05 PM »


You are just kidding yourself if you think that you can use a metal roof as a ground plane.

The problem with this idea is that the roof would need to be inches below the antenna, not feet.   If you put some type of high power signal into the antenna - then the roof is going to cause more problems then the gain you will experience.

The actual ground plane is the earth under the antenna and not the structure.
If the roof was your ground plane - your antenna would radiate most in the direction of the superstructure.
Hence if you put your antenna at the north side of the house and the roof went directly to the south for 50' - your best signal - both transmitting and receiving would come from the south.
The problem is - how do you turn the house if it is aimed in the wrong direction?


Well, for several years I have my Butternut HF6V mounted in the middle of a corrugated steel roof - all 23,000 square feet of it. The feed point was five inches above the roof. No problems with the installation and the pattern was omni-directional. BTW, I often ran a KW and never had a problem with RF getting into anything in the building - a two-story office building. Properly done, a large metal roof can and does make an excellent ground plane. The engineers at Bencher/Butternut seemed to have no problem with my installation.
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AB3NK
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« Reply #6 on: September 08, 2011, 07:52:32 PM »

In that for instance - that would work, how ever, it would not be omni directional and it would not be a ground plane - it would be a counterpoise.

All antenna's exhibit some gain in one or more directions, that is just the nature of the beast.
The down side is the only antenna that radiates omni directional is a Isotropic, which is only a antenna in theory.

As a example, a unfolded paper clip is a isotropic on 160 meters.
It would not radiate your signal very well, nor would it actually work as a transmit or receive antenna.

Like I said, some dumbazz will come along and quote what I said
( per verbatim).
If you wanted to make your point - you didn't have to quote what I said - all you had to do was state your opinion.

Unless your steel structure was 3 stories or more in height and unless your antenna was for 10 meters or higher, you were giving up a lot of your main lobe because your antenna is too close to the ground.   So using the roof did nothing for your situation in my opinion.

A ground plane - as you describe it, can be created by 4 or more radials equally spaced 1/4 of a wavelength apart, or more then 4 radials spaced equally apart - as long as they were less then 1/4 of a wavelength apart and as long as they were 1/4 of a wavelength or longer - in length.

This is all basic antenna 101 - which was taught to most any engineering student 30 or 40 or 50 years ago at most any technical school or university.
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W5LZ
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« Reply #7 on: September 09, 2011, 09:18:40 PM »

Odd...  I can think of a lot of elevated radial 'groundplane' antennas that work just dandy.  What's the difference between a metal roof being used as that "groundplane" and with a few radials?
You're probably right, there'z a 'dumbazz' here.  But I don't think it's us...
 Paul


BYW - I don't remember that being taught 30 - 40 years ago when I was in school.  Guess I'm just not 'most' engineering students?  Ya'think?  Ever??
« Last Edit: September 09, 2011, 09:24:34 PM by W5LZ » Logged
WB6BYU
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« Reply #8 on: September 09, 2011, 10:43:03 PM »

I think the best description is "not even wrong".
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AB3NK
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Posts: 27




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« Reply #9 on: September 10, 2011, 07:04:17 AM »

Odd...  I can think of a lot of elevated radial 'groundplane' antennas that work just dandy.  What's the difference between a metal roof being used as that "groundplane" and with a few radials?
You're probably right, there'z a 'dumbazz' here.  But I don't think it's us...
 Paul


BYW - I don't remember that being taught 30 - 40 years ago when I was in school.  Guess I'm just not 'most' engineering students?  Ya'think?  Ever??

Read what you just wrote - elevated ground plane antenna.
Lets use a Hy Gain 117 Super Magnum as a example.
The Hy Gain Super Magnum - right out of the box was designed and cut for channel 12 - or there abouts, because when it was manufactured there was only 23 channels in the chicken band.
It was designed to be installed 33' or more AGL and there was 4 radials - evenly spaced around the bottom of the base of the antenna.  Each radial was probably 12' long.
The base plate was only about 3 inches square.
The end result was there was a slight droop of the - radials - actually called counterpoise - not called ground planes - that is a misnomer.
The purpose of the radials is that it would in theory increase local reception.
It's range - in the mountains of Pennsytuckey was about 37 miles in any direction with a 3 watt AM signal and good propagation.

If you could see the strong points of the signal - what we call a lobe - and if you wanted to talk more then a couple of miles - you would want a increase in height - which would make the main lobe at the bottom reach out further.

At times when propagation is good, a mobile can talk thousands of miles with a simple antenna.
When propagation is bad - you can barely talk to the next county.
All signals reduce at the square of the distance away - hence all reliable communications is line of sight.
The higher you put the antenna, the more you increase your horizon.
Simple 101 stuff her guys...

The downfall of the Super Magnum was not its design, it was that the ground planes were too long to droop 45* and it had no wind resistance when you redesigned it to 45* and no durability and it was expensive to manufacturer and the cell phones and two meters / 440 repeaters took away 90% of the CB radio traffic, once the truckers took over the 11 meters and the non truckers became ham radio operators - most of which did so around about 1982 in my area of the country.

In the 1970's there was 2 meter radios, but they were real expensive and did not usually have PL's built into them and the repeaters were more or less unreliable because a repeater frequency VHF 100 - 1000 miles away could trigger your local repeater and fill it up with traffic from other areas of the country and because before 1972 there was not any good radios made because phase lock loop wasn't perfected for FM before that - and that was the reason why automobiles only came with AM radios before 1972.

The signal would drift up and down the band and two people trying to have a conversation together would chase each other up and down the band trying to zero beat each others signals.
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AB3NK
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« Reply #10 on: September 10, 2011, 07:32:04 AM »

http://en.wikipedia.org/wiki/Phase-locked_loop

When Signetics introduced a line of monolithic integrated circuits that were complete phase-locked loop systems on a chip in 1969,[7] applications for the technique multiplied. A few years later RCA introduced the "CD4046" CMOS Micropower Phase-Locked Loop, which became a popular integrated circuit.

Although the technique was around since at least 1930 and was used in televisions for years in their sweep circuit - they were not small enough to be put into a automotive radio and work.  It all relies on a dependable time base that could be manufactured cheaply.

http://www.radioshackcatalogs.com/catalogs/1972-b/

http://www.radioshackcatalogs.com/html/

Look at the 1972 Radio Shack catalog - there isn't a single automobile FM radio listed in it.

http://www.radioshackcatalogs.com/html/

Look at the 1973 catalog - and you will see the first generation of under the dash FM converters - with cassette, page 133 - item number 12 - 1825 ($109.95 - financing available.)

The minimum wage in 1973 was $1.60 a hour.
http://www.infoplease.com/ipa/A0774473.html
Or $5.65 cents in 1996 dollars - compared to probably as little as $3.75 in 1996 dollars compared to today's dollar in our economy.

That is the reason why the ICOM 746 Pro costs $1699 today at AES and sold for less then $1000.00 10 years ago - with a rebate!
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N2EY
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« Reply #11 on: September 10, 2011, 02:14:38 PM »

Why do the metal roof shingles need to be electrically bonded together to form a good ground plane? I would guess that the capacitive coupling between shingles would adequate for RF coupling.

I think your guess is wrong.

For the ground plane to be effective the capacitance has to present a very low reactance at the operating frequency - which requires a lot of C. For example, a capacitance of 10,000 pF is about 2.5 ohm reactance on 40 meters. Do you think the overlap of the shingles is more than 10,000 pF? I don't.

A metal roof can be an excellent ground plane for a vertical if it is big enough, well bonded and the vertical is placed at least 1/4 wavelength from any edge.

73 de Jim, N2EY
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AC5UP
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« Reply #12 on: September 10, 2011, 03:53:57 PM »

Look at the 1972 Radio Shack catalog - there isn't a single automobile FM radio listed in it.

Better you should look at page 70 of the 1971 catalog...

I can assure you it was very possible to buy a brand new 1970 GM car with a factory AM / FM radio. Stereo, too. Been there, done that, wasn't cheap, but it was possible. The AM / FM car radios of the late 60's weren't particularly drifty although the AFC didn't always behave well in the presence of a strong off-channel signal.
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WB6MEU
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« Reply #13 on: March 08, 2013, 12:06:58 PM »

I contacted Bencher (they make the Butternut HF9V etc) with questions about using a metal roof for a ground plane.  Their suggestion is to run lots of small-diameter (eg, #18 or #16) ground wires underneath the metal roof material (this can be done only for a new roof installation).  The wires will couple RF to the metal roofing material, and it won't matter that the sections of the roof aren't electrically connected to each other.  They also suggest mounting the vertical antenna just a few inches above the roof, in the center, and running a ground strap from the antenna ground connection to a ground rod in the earth.  It should work well, and there's no need to bond the sections of the metal roof together.
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K6AER
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« Reply #14 on: March 08, 2013, 03:01:28 PM »

I have had my SteppIR vertical on the metal roof of the horse barn for about 8 years. Operation is 40-6 meters. The metal roofing material has a 4 inch overlap over an 8 foot length section. Besides being bonded with self-tapping metal screws every 24 inches, I checked the capacitance between two sheets on  saw horses. Much to my surprise the capacitance was over 20,000 pF. More than enough for HF operation even if the sheets were not fastened together.

Wither the roof is sloping or flat has little to do with the real HF beam width.  The vertical lobe (E-Plane) coming off a vertical ground plane is very broad. When you signal is +30 over ā€œSā€ nine who will see the 3 dB of difference.

Hams worry too much over the resistance of ground when even salt water would make the poorest wire radial look 30 dB more efficient.  The only argument for not having the antenna above the house is it will be very close to a lot of digital noise sources.  My horse barn is 200 feet from thehouse.
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