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Author Topic: Metal plate with suction cups for glass roof to mount magmount  (Read 5295 times)
N2LWE
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« on: November 13, 2012, 03:16:41 PM »

Ok, I've been trying to figure out a way to install a 2m/440 mobile antenna to my new car with a glass roof. The gutter or lip mounts won't work, there is no edge to the glass roof to mount to and don't want to crack the glass anyway, so here's what I'm thinking, if I had a square piece of metal with four suction cups I can place this on top of the glass roof and simply put my magmount on top of the metal plate. Does anyone know who might sell such a thing (metal plate with suction cups) that can sit right on top of the roof? I can probaly make one and dip it in a plastic so if it came loose it wouldn't scratch the car, but if I can simply buy one I'd rather do that.
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K1CJS
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« Reply #1 on: November 14, 2012, 04:22:07 AM »

Just one question--If you're going to block out your 'sunroof,' for the want of a better term, with a metal plate, just why did you get a car with a glass top?  The way you want to attach the plate just begs for damage, since the speed of the car going down the road combined with the air pressure will tend to force that metal plate up--and off. 

Far batter that you use another type of mount on another area of the car.  There are 2m/440 antennas on the market that don't require a ground plane.  73!
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WX7G
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« Reply #2 on: November 14, 2012, 08:18:19 AM »

Auto stores sell a type of superglue for attaching rearview mirrors to the windshield. A few drops of this on the suction cups might hold them permanently. When it's time to sell the car the glue can be removed using a razor blade.

The antenna will need a counterpoise having a radius of 19 inches. A way to do this is to use a 6 x 6 inch steel plate for the mag mount to attach to and attach two radial wires to the plate. With the radials opposing the distance from tip-to-tip should be 38 inches. Number 10 solid copper wire might be good for this.
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N6AJR
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« Reply #3 on: November 14, 2012, 12:05:55 PM »

Go under the back end of the car and attach a bar or plate sticking out from under the bumper, bolted or welded to the frame.  mount your antenna on this. 

if you are only using a small 2 meter antenna you could use a license plate mount, which sets in behind the rear plate, or even a light weight trailer hitch and mount your antennas on the stinger.

MFJ sells a small mount you roll up in the window and mount your rubber duck up on it, which is good for  folks who use rental cars,  it is an MFJ 310, about 20 bucks.

any mount like that is an accident waiting to happen.  gluing suction cups to the glass is asking for it to come off and perhaps hit another vehicle.  Take a good look at the under side of the car and think about it.  and having it stick out the drivers side rear at a 45 degree has a couple of benefits, you can see it in the mirror,  the trees are higher in the middle of the street, and little fingers won't twang it as they walk by.  good luck and make a safe mount.
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K5LXP
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« Reply #4 on: November 14, 2012, 02:04:01 PM »

How about metal mesh instead of a solid plate?  Then you could have both a groundplane and still see the sun.

Mark K5LXP
Albuquerque, NM
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N2LWE
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« Reply #5 on: November 16, 2012, 04:23:02 AM »

Just one question--If you're going to block out your 'sunroof,' for the want of a better term, with a metal plate, just why did you get a car with a glass top?  The way you want to attach the plate just begs for damage, since the speed of the car going down the road combined with the air pressure will tend to force that metal plate up--and off. 

Far batter that you use another type of mount on another area of the car.  There are 2m/440 antennas on the market that don't require a ground plane.  73!

I'm not blocking out the sunroof. The window slides back and stops infront of the XM radio antenna near the rear of the roof. I simply wanted to place a small plate near the XM radio antenna approximately five inches by five inches, that does not block the sunroof and has absolutely no affect on the functioning of the window. Then I can simply place the small magmount, nothing fancy or big ontop of the plate. I can't use the gutter mount for the rubber duck because they can only handle 10 watts and I won't be using my HT, I'll be using a Yaesu FT-8800 with up to 50 watts of power. I don't think a small plate with the magmount sitting ontop should blow off. As far as why did I buy the car with a glass roof? Because that was a package in the car and the car gets almost 40MPG. The only reason I was going to use metal was so the magmount would obvious adhere to it. I don't want to the glue the antenna on the roof because I don't want to ruin the roof if I wanted to remove the antenna at some point. I have a Larsen glass mount but not sure if the glass has any metal in it so they wouldn't work then either.
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WI4P
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« Reply #6 on: November 16, 2012, 07:25:42 AM »

I use an mfj-1734 dual band glass mount on my rear window with very good results.

73, John
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K5LXP
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« Reply #7 on: November 16, 2012, 08:26:26 AM »

I simply wanted to place a small plate near the XM radio antenna approximately
five inches by five inches, that does not block the sunroof and has absolutely
no affect on the functioning of the window.

A 5x5 inch plate would be enough to "stick" the antenna, but it would be woefully inadequate as a ground plane.  Expect a lot of common mode with this setup.  There's  no possibility to incorporate a wire or mesh groundplane? 


Quote
I have a Larsen glass mount but not sure if the glass has any metal in it so they wouldn't work then either.

Being a sunroof, you can bet it's passivated.  How about somewhere else on the roof, trunk lid or fenders?


Mark K5LXP
Albuquerque, NM


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WX7G
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« Reply #8 on: November 16, 2012, 08:36:01 AM »


A 5x5 inch plate would be enough to "stick" the antenna, but it would be woefully inadequate as a ground plane.  Expect a lot of common mode with this setup.  There's  no possibility to incorporate a wire or mesh groundplane? 


Two radial wires will provide a sufficient counterpoise.
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N2LWE
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« Reply #9 on: November 16, 2012, 02:52:32 PM »

The Larsen glass mount wouldn't be used on the roof. It would be installed on the side window.
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K1CJS
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« Reply #10 on: November 17, 2012, 03:39:30 AM »

Chances are that the glass of the side window is still passivated.
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WX7G
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« Reply #11 on: November 17, 2012, 10:10:18 AM »

From the Larsen antenna website,

        http://www.pulseelectronics.com/products/antennas/larsen_antenna_products/technical_reference/faqs

"On-Glass Antennas

Q: I moved my on-glass antenna from my old car to a new one. It worked great on the old car, but I have a very high VSWR on my new car? Why?

A: Your new car probably has passivated glass --- glass impregnated with tiny metal particles which render the glass incapable of coupling RF between the inside and outside couplers.

Q: How can I tell if the glass on my vehicle is suitable for one of Larsen´s on-glass antennas?

A: First, look near a corner of a window for words meaning or relating to "sun" or referring to ultraviolet or other forms of radiation. If you find terms like "Soft-Ray", "EZE-Cool", "Solar-Coat", "Solar-Cool", "Insta-Clear" your glass is probably passivated and won´t work with on-glass antennas. If you don't find these helpful words on the glass and want to be 100% certain, ask a 2-way or cellular shop technician to test your glass with a capacitance meter."

Another way to tell is to buy the antenna, mount it, and see how well it works.
« Last Edit: November 17, 2012, 10:14:46 AM by WX7G » Logged
WI4P
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« Reply #12 on: November 20, 2012, 01:38:10 PM »

From the Larsen antenna website,

        http://www.pulseelectronics.com/products/antennas/larsen_antenna_products/technical_reference/faqs

"On-Glass Antennas

Q: I moved my on-glass antenna from my old car to a new one. It worked great on the old car, but I have a very high VSWR on my new car? Why?

A: Your new car probably has passivated glass --- glass impregnated with tiny metal particles which render the glass incapable of coupling RF between the inside and outside couplers.

Q: How can I tell if the glass on my vehicle is suitable for one of Larsen´s on-glass antennas?

A: First, look near a corner of a window for words meaning or relating to "sun" or referring to ultraviolet or other forms of radiation. If you find terms like "Soft-Ray", "EZE-Cool", "Solar-Coat", "Solar-Cool", "Insta-Clear" your glass is probably passivated and won´t work with on-glass antennas. If you don't find these helpful words on the glass and want to be 100% certain, ask a 2-way or cellular shop technician to test your glass with a capacitance meter."

Another way to tell is to buy the antenna, mount it, and see how well it works.

Good info WX7G !  I learn something new everyday.
73, de John WI4P


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N2LWE
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« Reply #13 on: November 24, 2012, 02:04:33 PM »

Sorry, I'm still a little confused. I guess I'm well, nevermind. I don't understand when I read some of you talk about having a ground. If I used a lip or gutter mount the bracket is rubber or plastic coated to prevent scratching and attached to the body with the antenna sitting ontop of the bracket. How is this any different as far as grounding if I attached a metal plate on the glass roof and mounted a magmount on top of that? Please educate me. I'm not being sarcastic, I'm serious, educate me. I want to get an antenna on this car so bad and am running into a brick wall because I need to do this right the first time. I also had a friend bring a lip mount to see if it would work but there is such a tiny gap between the glass roof and the plastic spoiler that protrudes out from the top of the glass on the rear hatch, it's impossible to mount it there. I guess I will have to face the fact that I can't do what I really want to do, having a small antenna onthe glass roof right next to the XM radio antenna.
« Last Edit: November 24, 2012, 02:11:40 PM by N2LWE » Logged
K5LXP
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« Reply #14 on: November 24, 2012, 05:39:38 PM »

Quote
I don't understand when I read some of you talk about having a ground.

Just like with lightning, RF and AC ground discussions, the term "ground" can be used to refer to mobile concepts which can be confusing if you don't already know the difference.  In this context the car body "chassis ground" is more about DC, and "antenna ground" is more about presenting a reference plane for the proper operation of a monopole antenna.  Both can be the same metal "ground" but each has specific properties that makes it optimum for the purpose.  Just "grounding" an antenna to the car body does not automatically satisfy the requirement of a ground plane.  Where and how big it is in relation to the antenna feedpoint matters.  A lot.

Quote
If I used a lip or gutter mount the bracket is rubber or plastic coated to prevent scratching and attached to the body with the antenna sitting ontop of the bracket. How is this any different as far as grounding if I attached a metal plate on the glass roof and mounted a magmount on top of that?

You're extending assumptions here that create an invalid comparison.  Any lip or gutter mount will pierce the paint via set screws to make direct contact to the metal, so there's no magic connection to ground.  When connected to a lip or gutter, you effectively make whatever metal is there that extends beyond that point the antenna ground plane.  The greater this area, the better the ground plane in general.  If the lip or gutter is only part of a hatchback, door frame or other small panel, then the ground plane will not be very good.  So it's not so much about the mount, it's what it's connected to.

Quote
I want to get an antenna on this car so bad and am running into a brick wall because I need to do this right the first time.

The frustration stems from trying to get physics to adapt to your wishes.  A better approach would be to try and accommodate the needs of your chosen antenna, in some clever way that meets your aesthetic requirements.

Quote
I guess I will have to face the fact that I can't do what I really want to do, having a small antenna onthe glass roof right next to the XM radio antenna.

Not without accommodating the physics of monopole antennas.  A "really good" groundplane would be a disc that's a half wave in diameter, with the antenna in the middle.  A metal roof or trunk lid is a pretty good ground plane.  The smaller and less symmetrical you make the ground plane, the more you affect feed Z, feedline isolation and pattern.  Consider a 2M HT - it has a double whammy.  The HT chassis does not present an adequate groundplane nor is a ducky an efficient antenna.  But it "works", if you're willing to accept the tradeoff in size vs performance.  Your situation is no different.  You could make this location "work", if you can reconcile the tradeoff in aesthetics vs performance.   Off the shelf mobile solutions aren't made for an application like this, so you're on your own to figure out how to make it work.  You can probably get there from here, once you stop trying to make antennas work in ways they can't.


Mark K5LXP
Albuquerque, NM
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