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Author Topic: high swr with on-glass antenna  (Read 355 times)
W4SJV
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« on: April 14, 2005, 08:05:11 PM »

Just purchased a Randall-Larsen 2m/70cm glass-mount.  Installation was easy without any problems.  However, after installing the new antenna, my SWR reading went up to almost 3:1.  Is there any way that anyone knows to help lower the SWR?
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AD5X
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« Reply #1 on: April 15, 2005, 04:45:09 AM »

Assuming that you've adjusted the length of the antenna for the lowest SWR (even though it is still high), you can get a great match by sliding around the inside coupler a little before attaching it to the glass.  This compensates for changes in the match due to different glass widths.  I did this by sliding the inside unit while monitoring 2-meter SWR on my MFJ-259B.  When I found the best position, I marked an outline on the window using one of those erasable markers used for white-boards.  Then I pulled the adhesive covering off the insdie coupler and attached it in place, and wiped off the marks on the glass.  This same position worked well when I went back and checked 440 MHz SWR with an MFJ-219B UHF analyzer.  I'd had this same problem, and contacted Larson.  They passed this info on to me.  Don't know why this isn't in the instruction sheet.

Phil - AD5X
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KG4RUL
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« Reply #2 on: April 15, 2005, 04:45:58 AM »

Are you SURE that the window is not made of 'passivated glass'?  The process of passivating utilizes metal films which can wreak havoc with RF.

Dennis KG4RUL
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KZ1X
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« Reply #3 on: April 15, 2005, 05:35:44 AM »

How are you measuring the SWR?
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K0BG
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« Reply #4 on: April 15, 2005, 06:48:58 AM »

I suspect you won't get the SWR to any reasonable level if the vehicle in question is newer than say 1995. As pointed out above, almost all rear and front glass now utilize metallic material in both the glass and in the coating between them (front glass only). This is yet another good reason to drill a hole and properly mount a good quality antenna like an NMO2/70 Larsen.

Alan, KØBG
www.k0bg.com
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N8EKT
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« Reply #5 on: April 15, 2005, 07:47:56 AM »

Check to see that the arrows are lined up between the inside coupler and the antenna foot.
they must be in the same orientation or it will not match.
73's
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K6TTE
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« Reply #6 on: April 15, 2005, 07:50:02 AM »

Steve --

I have the same antenna and had the same problem.  I too solved it by experimenting with the alignment between the pieces on either side of the glass.  Mine is mounted on a rear side window on my Honda CR-V and works fine.  I used masking tape to hold the yet-to-be-glued side in place while I tested the SWR; this allows you to move it around easily.  You can use a wide putty knife to carefully work between the glass and the antenna base to unstick it from the glass.  HRO sells the remount kits for a little over $5.  Don't try to reuse your existing adhesive.

Good luck!

-- Dale, K6TTE
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K1CJS
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« Reply #7 on: April 15, 2005, 08:38:19 AM »

A couple of other things to check for--I believe these should have been pointed on in the instructions but are sometimes forgotten--sometimes purposefully.

The antenna has to be two to three inches away from the edge of the window framing so the framing or the car body doesn't interact with the capacitive coupling of the antenna to its inside base.  Also, trying to put the antenna on glass which has a defroster grid fused to the inside of the glass will result in a high SWR as well.  A friend of mine got a bottle of grid repair solution and actually made a bypass around the point he mounted his antenna, and he got good results by keeping the grid about 2 1/2 inches away from the antenna base.

It doesn't even make you wonder why the people who manufacture and sell these type antennas don't tell you these things--if some people knew how difficult it was to get these antennas mounted to work properly and how many things you had to watch for to avoid, those antennas wouldn't sell.

Like Alan said, drill a hole and put a permanent, proper mount on the car.  You'll be way better off.
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W4SJV
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« Reply #8 on: April 15, 2005, 07:03:07 PM »

Thanks for the tips & tricks.  I'll experiment around with the suggested locations and see what works.  The last resort will be drilling the hole.  Thanks all!
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