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Author Topic: Radio Shack dual-band through-glass antenna  (Read 5073 times)
K3UOD
Member

Posts: 145




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« on: November 06, 2000, 12:38:00 PM »

Has anyone ever taken one of these apart? I'm wondering if the coupling between the inside and
                   outside bases is capacitive or inductive. If it's inductive, how are the coils oriented.

                   The reason I ask is that it seems that the 440 MHz counterpoise would work better if it pointed down
                   vice to one side as shown in the directions. A friend thinks that if I mounted the inside part 90 degrees
                   out from the instructions I wouldn't have any coupling between the parts. He believes that the coupling
                   is inductive, with the coils side-by-side vice co-axial. I always thought the coupling was capacitive. If I
                   were going to design an inductively coupled system, I'd make the coils coaxial so it wouldn't matter
                   how the two pieces were oriented.

                   Does anyone know for sure how these antennas work?
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KG4DED
Member

Posts: 17




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« Reply #1 on: November 07, 2000, 06:56:31 PM »

I have one of ther RS on the glass antennas.  I have taken it apart, and even moved it from one location on the vehical to another.  It is indeed capacitive in the way it is coupled.  Inside the box are 3 coils, I think.  One is adjustable, the screw in the cover of the box. The box also contains a capaciator plate.  It looks like a mushroom, and is centered in the face of the box that goes on the glass.

I moved mine from the back glass on a pickup truck to the front glass.  The preformance really improved.

I hope this helps.
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K3UOD
Member

Posts: 145




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« Reply #2 on: November 08, 2000, 08:49:32 AM »

Thanks.

I have the same antenna on my truck.  It works much better than the one on my car.

The one on the car is over one line of the defogger grid.  I scraped away the defogger conductor for 2 in. either side of the ant base and that improved the VSWR from 4:1 to 2.25:1.  I'd like to get it below 2:1 if possible.  The rig's VSWR protection is folding back the power output (rig puts out 47 W into a dummy load but only 30 W into the ant.).  Someone suggested removing the inside base and scraping away the remaining defogger conductor.  I intend to do this.

If that doesn't work, there is a small non-opening window on the  side of the car, I'll try that location but the coax would have to come out the bottom, vice the side.  That was the reason for my original post.

Question- when you moved your antenna, did you use a re-mount kit, some other adhesive, or did you just re-stick it using the same adhesive?
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KG4DED
Member

Posts: 17




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« Reply #3 on: November 09, 2000, 09:38:55 PM »

When I moved my on the glass antenna, I got some new tape.  I used the double sided 3M foam tape available at Office Depot and other places.  I just cut several sections for both inside and outside and trimmed it up neatly.  I also cleaned both the glass and the antenna parts VERY VERY good with alochol and then polished the glass with a dry cloth never touching the cleaned surfaces with my fingers.  I then pressed it in place very tightly and held it for a while.  To fininsh it up, I ran a bead of silicon rtv around the outside "foot" to seal out the water.  It has been on for about a month and is working well.  I try to pull it off once in a while just to see if it is still stuck.

Steven
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K3UOD
Member

Posts: 145




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« Reply #4 on: December 01, 2000, 09:35:36 AM »

I sucessfully removed and re-mounted the inside half of the base by heating the area around the base with a hair drier then carefully prying with a putty knife.  After I scraped the remaining traces of the defogger grid from under the mount, I just stuck the mount back in the same spot using the original adheasive (heating allowed the adheasive to come off without tearing).  The VSWR improved from 1.9:1 to 1.6:1.  This helped some but I still wasn't satisfied with the performance of the antenna.  I replaced the ant with a Larsen 2/70 on a trunk gutter mount.  Great improvement in my signal from the fringes of the repeater area.

I'm going to try the thru-glass on 450 as soon as my new rig arrives.  If not satisfied, I'll junk the ant and patch the defogger grid with a product Locktite sells for that purpose.

Thanks to all.

73, Jerry
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K3UOD
Member

Posts: 145




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« Reply #5 on: December 01, 2000, 09:38:07 AM »

Oh, I forgot.  Another thing I did was to take the swivel joint apart and scrape the paint from the two halves.  This lowered the VSWR a tad also.
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KL0PE
Member

Posts: 24




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« Reply #6 on: December 26, 2000, 12:30:10 PM »

You mounted a glass mount antenna OVER your rear-window defogging wires?  That alone is what increased your SWR since some of your RF was getting coupled into the wires.  You're fortunate to not have smoked your car's electrical system!  I guess you realize that by scraping off a protion of the grid you have effectively "broken" your rear-window defogger, unless of course it didn't work in the first place then never mind.
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K3UOD
Member

Posts: 145




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« Reply #7 on: January 02, 2001, 03:25:49 PM »

You mounted a glass mount antenna OVER your rear-window defogging wires? That alone is what
                   increased your SWR since some of your RF was getting coupled into the wires.
>>>Yes, I knew what I was doing when I did.  The instructions said to avoid the grid if possible.  This is the only location on the car where the antenna fits.  I placed the mount over one line and between two others.


You're fortunate to
                   not have smoked your car's electrical system!
>>>Fortune had nothing to do with it, I cut the grid lines when I installed the antenna to prevent coupling to the electrical system.

 I guess you realize that by scraping off a protion of the
                   grid you have effectively "broken" your rear-window defogger, unless of course it didn't work in the first
                   place then never mind.

>>>I sacrificed the top two lines of the grid to bring the VSWR down to a useable level.  Locktite sells repair kits for defogger grids, they cost about $5 at auto supply stores and are easy to use.  Repairing the grid after my experiment won't be a big problem.  I already have the kit.  The remaining lines in the grid still work just fine.
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W0LPQ
Member

Posts: 31




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« Reply #8 on: January 04, 2001, 09:15:32 PM »

Cleaning the antenna pieces is a great idea.  Like some Cell phone antennas, black to black is paint to paint and NOT conductive.  I've cleaned mine to bare metal and alodined (aircraft use stuff).  My counterpoise is almost straight..I have a 2000 Grand Prix GT 2 Door.  The antenna is mounted on the side window behind the driver.  Oh yes..road noise.  But SWR is no problem.  Tuned up great..not quite 1:1 but darn close.  Have not made it to Columbus to try a 440 machine my friend has, but hope to in a couple weeks.

I have not removed the antenna nor do I intend to unless I sell the car after I retire.  Nice little antenna.  Friend of mine in Iowa has one and advised i try it...not sad that I did...great little antenna and the price was right.

73

Bill, W0LPQ
Greenwood, IN
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