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Author Topic: Vehicle body material as a groundplane.  (Read 4597 times)
N1RND
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Posts: 69




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« on: June 08, 2013, 06:31:59 PM »

I have read in more than one place on the internet that a vehicle with an aluminum body does not make as good as a ground plane as a steel body.
How could this be true?  Many base antennas are made from aluminum.
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KG4RUL
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« Reply #1 on: June 08, 2013, 08:46:49 PM »

Aluminum is good - plastic not good at all!
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K1CJS
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Posts: 6034




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« Reply #2 on: June 09, 2013, 05:39:16 AM »

If I remember correctly, it's something about how the aluminum panels are so soft that flexing of the whip loosens the mounting point after a time.  That causes a weak or intermittant connection, and there's no easy fix--either to strengthen the materials or improve the connection.  Most antenna mounts (NMO type especially) simply aren't made to be attached to aluminum panels.

People who do use backing plates see the body panel deform sooner or later unless the backing plate is large enough, and if that is done, there is the problem of bonding the backing plate to the panel.  All that adds up to what you read.  It isn't that the aluminum panel doesn't make a good ground plane, it's that it's hard to work with successfully.

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M6GOM
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« Reply #3 on: June 09, 2013, 07:00:47 AM »

Go to breakers yard, get them to cut a large bit of metal off the car the same place you plan on drilling the hole about 2ft square and use that as a backing plate. The profile will be exact and hopefully there'll be no distortion.
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N1RND
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« Reply #4 on: June 09, 2013, 08:29:02 AM »

The mechanical issue is not a problem for me.  Its just that a few sources (online retailers of 10 and 11 meter supplies) claim that aluminum does not make a good groundplane and want to sell you a "ground plane kit".
I find it hard to believe that aluminum can't be a good ground plane.  If the truck body (cab mainly) was fiber glass or plastic, then that would be a whole different story.
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M6GOM
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« Reply #5 on: June 09, 2013, 09:08:38 AM »

The mechanical issue is not a problem for me.  Its just that a few sources (online retailers of 10 and 11 meter supplies) claim that aluminum does not make a good groundplane and want to sell you a "ground plane kit".
I find it hard to believe that aluminum can't be a good ground plane.  If the truck body (cab mainly) was fiber glass or plastic, then that would be a whole different story.

Ask them what base station antennas are made from....

Those mobile ground plane kits are so small they're completely useless oh and they actually screw into the "live" side of the antenna connector. Only the stupid buy them.
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KB4QAA
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Posts: 2382




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« Reply #6 on: June 09, 2013, 12:31:40 PM »

The mechanical issue is not a problem for me.  Its just that a few sources (online retailers of 10 and 11 meter supplies) claim that aluminum does not make a good groundplane and want to sell you a "ground plane kit".
I find it hard to believe that aluminum can't be a good ground plane.  If the truck body (cab mainly) was fiber glass or plastic, then that would be a whole different story.
Every airplane for +75 years has used the aluminum fuselage for the ground plane.  Works good, lasts a long time.

Whatever sources you are reading are bogus.
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N1RND
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« Reply #7 on: June 09, 2013, 12:43:46 PM »

All I know is that I get better results with one antenna mounted on the rear top edge of my "semi" truck/tractor, than when I had a co-phased pair mounted on the mirror brackets.  The trucks body, the actual cab, not the hood or doors is one big aluminum shell.  The negative side of the coax at the mounting location is connected to this.
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K1CJS
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« Reply #8 on: June 09, 2013, 03:24:11 PM »

...Its just that a few sources (online retailers of 10 and 11 meter supplies) claim that aluminum does not make a good groundplane and want to sell you a "ground plane kit".
I find it hard to believe that aluminum can't be a good ground plane.  If the truck body (cab mainly) was fiber glass or plastic, then that would be a whole different story.

Those sources are counting on the ignorance of some people so they can make money.  The tipoff here is what I underlined.  You'll find all kind of add-ons and other stuff, (including 'range extenders' for antennas) along with the linear amps and the so called ham transceivers that can be 'opened up' for CB.  Illegal through and through, but still sold.

Stick to recognized ham suppliers and don't pay much attention to those "10 and 11 meter retailers".  All too often they're taken in by their suppliers--who also don't care about the end user, and count on the ignorance of just about everybody involved--except those of us who know better.
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WN2C
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Posts: 454




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« Reply #9 on: June 10, 2013, 02:25:41 PM »

The mechanical issue is not a problem for me.  Its just that a few sources (online retailers of 10 and 11 meter supplies) claim that aluminum does not make a good groundplane and want to sell you a "ground plane kit".
I find it hard to believe that aluminum can't be a good ground plane.  If the truck body (cab mainly) was fiber glass or plastic, then that would be a whole different story.

I think the operative word here is...they want to sell you a "ground plane kit".
Really...you are operating mobil...why do you need a ground plane kit?  Capacitance hat maybe, but not a ground plane kit.
Who are these online retailers?

Rick  wn2c
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K1CJS
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Posts: 6034




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« Reply #10 on: June 11, 2013, 07:53:36 AM »

When I was still driving tractor trailer, I had a General Electric CB worked on at a truck stop.  It needed minor repair, but I didn't have my tools with me, and no place to actually set up to do the repair.  The guy there tried to sell me a so called 'ground plane kit,' three stubs of fiberglass about three inches long with a few windings of wire on them, all screwed into a six sided block which had a 3/8x24 stub on its bottom to put in the antenna mount, and a 3/8x24 threaded hole on its top to screw the antenna into.

He kept claiming that it improved the antenna system to enable two to three times the distance--until I showed him my ham license.  His retort?  "Oh, geez, I can see I won't make any money off of you!"

Kinda says it all, doesn't it?
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AC4RD
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Posts: 1235




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« Reply #11 on: June 11, 2013, 09:23:36 AM »

He kept claiming that it improved the antenna system to enable two to three times the distance--until I showed him my ham license.  His retort?  "Oh, geez, I can see I won't make any money off of you!"

If you had bought the "ground plane," he probably would have tried to sell you some magic beans next.  :-)   "Triples your output power without any more current draw, enables earth-to-moon communications, cures bad breath--buy some MAGIC BEANS today!"  :-)
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W5LZ
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Posts: 477




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« Reply #12 on: June 17, 2013, 06:13:53 AM »

Aluminum/steel can make a mechanical difference, but seldom an electrical one.
 - 'Doc
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M6GOM
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Posts: 915




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« Reply #13 on: June 17, 2013, 06:46:41 AM »

All I know is that I get better results with one antenna mounted on the rear top edge of my "semi" truck/tractor, than when I had a co-phased pair mounted on the mirror brackets.

That is because:

A) You had no groundplane for either of the antennas on the bracket - its the metal underneath that counts, not alongside.

B) To cophase them they need to be an unobstructed quarter wavelength apart which yours weren't.
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