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Author Topic: Auto Glass batch formulations and dielectric properties for thru-glass antennas  (Read 2455 times)
AA4N
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Posts: 145




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« on: April 17, 2017, 07:57:21 AM »

Hi all...   Name here is Mike.

I saw some discussion in another thread about whether changes in the formulation of auto glass have changed the efficacy of the capacitive coupling in thru-glass antenna mounts.   I didn't want to hi-jack that thread, so I thought I would start a new one.

As it turns out, I work in a factory that builds auto glass.  So, I am fairly familiar with the subject.  Full disclosure:  The plant I've worked in for the last 25 years doesn't actually melt the sand.  We cut/grind/paint/bend/temper/laminate and such.   This is known in the biz as a "fab" plant, as opposed to a "float" plant (which is where the sand gets melted).  I started out as an engineer, but now I'm just a computer nerd.

However, I know some guys that work with the float lines and other guys that work in antenna development (that is, the screen printed antennas that tend to be at the top of the back glass and sometimes quarter windows).

Anyway, here's some info that I though might be of interest;

There was a major shift in formulation of the glass (in North America) back in the mid-late 90's.   The industry switched from what we called "green" tint to what is now called "solar" tint.    I'm not talking about the dark tinted glass that many vehicles have behind the "B-pillar".  I'm talking about windshields and front door glass.  (they aren't actually clear, they have a slight green tint to them).  When we switched to solar-tint, the look of the glass didn't change much.  It's actually ever-so-slightly darker than the old green-tint, but not so much that you would notice.

Anyway, the question came up about whether this change affects the capacitive coupling with through-glass antenna mounts.   I asked a guy I know at a float plant about this (his degree is in ceramic engineering, and he started his career working with the big float lines).  He told me that the metals in the "batch" (that's the potpourri of stuff that gets melted into glass) are basically there for color, and that they are oxides, not pure metals.   Unfortunately, he didn't know how that would affect the dielectric properties of the glass...

I've sent a query to another guy that might be able to help.  Don (JJ2TUF) might have more info.  He also knows some of the guys that do the antenna work.  So, I hope to have more details in the future.

Just in case you guys needed way too much detail about car windows Smiley   Always happy to oblige...

73    mike
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N8AUC
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Posts: 285




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« Reply #1 on: April 19, 2017, 04:27:47 AM »

Mike,
This is a very interesting topic to me.
I am very interested in any additional information you manage to discover.

73 de N8AUC
Eric
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AA4N
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Posts: 145




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« Reply #2 on: April 24, 2017, 01:26:54 PM »

Sorry, it took me so long to get back.   I took a last week off, so wasn't worrying about glass stuff...

I've heard back from four guys with decades of knowledge about auto-glass, float-glass, and RF radiation.  Unfortunately not all the same guy Smiley

However, the consensus is that the metal oxides that are added to the batch for color tinting are really infinitesimally small compared to the electrically inert materials that make up the glass.   The green vs solar tint products are as hard to distinguish electrically as they are by eye (and they look pretty much identical).   Even the dark gray privacy tint products don't show any significant difference at frequencies below 1GHz.

One of the guys sent me a report showing the test results at frequencies between 50MHz and 900MHz, but the entire thing is in Japanese, so I've got another guy doing a bit of translating for me...   Haven't got that back yet.

Anyway, it's looking like (at least as far as my company's products are concerned) there shouldn't be any problems with thru-glass antennas that weren't there all along.   Of course, you would need to keep it clear of heater grids and printed antennas, and most modern backlites don't have much space that doesn't have some silver conductors printed on them.  You might also need to avoid the black-band printed areas, which would rule out even more of the acreage available for sticking antennas on windows.

On a side note, if you nick one of the silver stripes of the heater grid, it's fairly likely that it will arc across that scratch whenever you turn on the defog heater.   It's not a big enough arc to see (quite impressive under a microscope though), but it makes some wicked radio hash across the spectrum.

73   mike
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K0BT
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Posts: 321




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« Reply #3 on: April 25, 2017, 11:05:13 AM »

Mike,

I can't thank you enough for posting facts on this subject, since there is so much fierce speculation whenever the topic comes up.  Thank you.  Looking forward to whatever else you can tell us.

73,
Bob
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N8AUC
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Posts: 285




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« Reply #4 on: April 27, 2017, 12:47:19 PM »

Mike,
Thanks for the detailed information.
I'd be curious what that Japanese document you mentioned actually says.

I've used on-glass antennas before. But they wouldn't work at all on the last 4 cars I've owned.

73 de N8AUC
Eric
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KK2DOG
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Posts: 34




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« Reply #5 on: May 02, 2017, 10:14:47 PM »

I use a thru-the-glass 2-meter antenna on the rear window of my 2013 Chevy Silverado pick-up and it actually works quite well. I have it spaced between the defrost/heating elements, which I don't use anyway, and typically run about 10 watts which is good for about 5-10 miles on simplex. The glass does have a slight tint to it as well.


              Mike KK2DOG
             Watertown, NY
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KC2MMI
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Posts: 798




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« Reply #6 on: May 05, 2017, 05:39:57 PM »

Mike-
 I've seen articles (no idea where) that point out the through-glass antennas are going to perform differently, pretty much with EVERY variation of the glass. Whether it is made by Ford or BMW or Nissan...which car model it is, whether it is the new "noise absorbing" glass, or another laminate, etc.
 Bottom line?
What is going into the glass is just one more variable. All the end-user or antenna designer can do, really, is measure six dozen windows (G) or, just try to make an antenna that "usually" works. Or has some provision for tuning.

Now, if you really want to lose sleep over glass and antennas, try to dig into electronic toll passes and windshields. The toll authorities all cite some patent relating to using a windshield as a capacitive part of the patch antenna. Or that somehow the glass "focuses" the radio waves. And they all say that once the patch has been glued on (and it MUST be glued on, they say) it couldn't be made re-useable anyway, because it has to be so tightly bonded on.

Uh, no. In fact, you can put a piece of clear packing tape on the toll pass (so the adhesive doesn't bond) and then stick it on with any old tape. And somehow, they are so well engineered that they still work, despite all the toll authorities knowing that it can't be so.

Glass? As an active and critical part of the antenna?
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AA4N
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« Reply #7 on: May 25, 2017, 05:09:16 AM »

Well, I got those study results translated, and I was able to ask a few follow up questions as well.  Here's the short version;

All of the tests were done using calibrated test gear.   The glass samples had all been ground to a uniform 1mm thickness.   The various types of glass in the test were all batch tint types, no coated glass.   Both green and privacy gray tints.

The results were that there isn't much variation to speak of.   The variation was less than the 20% that you might see in common electrolytic capacitors.

Bear in mind that these tests were done in Japan, and there are slight differences in the make up of the batch over there from what we mix up here in the US.

All of that being said...   I also did some google research on this term "passivated".   I had never heard that term before in the industry.   What I found was a good bit of discussion about glass that has had a coating applied to it.  Frequently with regard to bits of glass used in microchip or electronic component manufacturing.

This brought to mind a time, back around the mid 90's when Ford was making a lot cars with mirror like coatings on the glass behind the B-pillars.   I remember seeing a lot of Explorers with that "Smokey and the bandit" mirrored sunglasses look going on.   I also recall a lot of Pontiacs with the metallic purple coatings on the windshields.   I've got a sneaking suspicion that those coated products are what Larsen was talking about when they mentioned "passivated" glass in their literature.  I don't think anybody is doing that anymore.   We certainly never used coatings.   That stuff is expensive, fragile, and kind of goofy looking.

My take away from all of this is the following;    If you would like to try a through-glass antenna, your best chance of success will be had by putting it on tempered glass (not the windshield), that doesn't have any plastic film tint, or any metallic tint coating from the factory.  You also need to stay away from any of the screen printed features like the black band around the edge and the silver heater grid and antennas.   Regular green or gray tempered glass will likely work as advertised.   But, an NMO mount in the middle of the roof will probably work better Smiley

Nuff said...

73  mike
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KK4YDR
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« Reply #8 on: May 25, 2017, 07:47:31 PM »

Def... Passivated
make (a metal or other substance) unreactive by altering the surface layer or coating the surface with a thin inert layer.

In the case of auto glass a thin layer of a metallic substance is applied or doped into the glass/onto the glass. This prevents UltraViolet radiation from penetrating the thin layer of metal that coats the glass. It is far more durable than roll on adhesive window tint that will bubble up after a decade or so, This thin metal film is strong enough to resist radiation of RF through it.

The glass is by all means of definition "Passivated". So don't get wrapped around the axle about never having heard it before anywhere else.

We like to call ourselves Humans but in reality we are Animals even if we never hear ourselves referring to us as such. Passivated glass may not be officially called passivated but none the less it is 100% passivated if it is coated in metal. Humans are still animals hahah especially after watching the news lately about Montana elections haha
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WB4M
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« Reply #9 on: June 22, 2017, 11:00:13 AM »

I checked with Toyota and they said they do not use passivated glass.  I previously use a 2-meter glass mount antenna, cannot remember the brand.  I was well pleased with the results, especially when operating simplex.
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