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Author Topic: glass mount antenna and defroster lines  (Read 2842 times)
N6HBJ
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« on: October 24, 2011, 07:33:46 PM »

Does anyone have any experience using a dual band 2 meter/440 glass mount antenna on a window with those defroster lines in the glass?  I'm concerned they may cause SWR issues running between the antenna and the coupler on the glass.

Thanks

Mike
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AD4U
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« Reply #1 on: October 25, 2011, 05:52:49 AM »

I assume you know that even when mounted on clear glass, a glass mount antenna is a rather poor performer when compared to most any permanent mount antenna.  From my personal experience even a trunk lip mount antenna is a much better performer than a glass mount.

I am SURE that the defroster wires embeded in the glass will not make your antenna work any better, and I would bet a steak dinner that they will make the antenna perform even worse than it would otherwise.  SWR may be an issue and the transmitted RF capacitively coupled through the glass to your antenna may (will) couple into the defroster wires and they will radiate and try to be a part of your antenna.  This is not a good situation.

Dick  AD4U
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K2QB
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« Reply #2 on: October 25, 2011, 07:21:26 AM »

I have used this type of antenna on several vehicles and although I admit it is not an ideal antenna, they do work and there is no issue with the defroster lines. Typically where the antenna is mounted, up high on the glass, the defroster lines are few or non existent. I am currently using a Larsen 146/450Mhz on the rear window of my car and have no porblems hitting any of my local repeaters.
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KI4SDY
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« Reply #3 on: October 25, 2011, 07:36:23 AM »

I would try to mount the antenna so that is is not covering any of the heater wires in the glass. On lower frequencies, my experience has been that the best of these antennas work well, but they are secondary in performance to a good magnetic antenna on the roof of the vehicle.  Wink
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WB6BYU
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« Reply #4 on: October 25, 2011, 08:50:48 AM »

There are at least three methods that I've seen used to couple the signal through the
glass, and performance will vary depending on the type.

(You also have to consider passivated glass in addition to the defroster wires.)

The original type with a single set of capacitor plates probably are the worst choice, both
for coupling to the defroster and for common mode current on the feedline.  This is the
type that established a lot of the reputation of glass mount antennas as being poor
performers.  It is particularly bad with passivated glass.

Some versions use two sets of capacitors to couple both sides of the feedline through the
glass to a tuned network outside at the base of the whip.  This reduces some of the
problems.

The current Larsen dual-band, as far as I can tell, uses inductive coupling between the
inside and outside of the window instead of relying on capacitance through the glass.
I've had one of these on my van for several years and compared it against a number of
other antennas - performance is comparable to a quarter wave whip, but more directional
because it is mounted on the side of the vehicle.  In actual tests while driving in weak
signal conditions, I haven't found any antenna that beats it by more than 3dB on 2m,
in spite of the horror stories you hear about glass mount antennas.

With the inductive coupling I'd think that the impact of the heater wires would be less
than for the old capacitive-coupled types, as long as they are so thick that you have
problems mounting the inner unit flat on the glass.
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K1CJS
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« Reply #5 on: October 25, 2011, 11:34:49 AM »

Does anyone have any experience using a dual band 2 meter/440 glass mount antenna on a window with those defroster lines in the glass?  I'm concerned they may cause SWR issues running between the antenna and the coupler on the glass.

It will make a difference.  First, what year is the car, and what make?  If it's a fairly new vehicle, passivated glass will most likely have been used for the window glass.  That alone will make the antenna much less efficient.  The defroster lines affect performance even more.

If you're set on using such an antenna, try to mount it where there are no defroster lines--if it can be done, on a side glass.  Other than that, I would recommend using another type mount altogether.  Trunk lid mounts are better, but the very best way is to drill the hole and permanently mount the antenna!
« Last Edit: October 25, 2011, 11:38:17 AM by K1CJS » Logged
K1CJS
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« Reply #6 on: October 30, 2011, 03:40:04 AM »

If you're hell bent on putting a through the glass antenna on your back window, I just recalled that there is a bottle of defroster grid repair liquid for sale--possibly at JC Whitney.  Another way to do what you're looking to do is to simply scrape off and 'reroute' the defroster grid lines around the area the antenna plate will be attached by use of the paint on repair solution.  Also remember to 'complete the circuit' of the grid lines by painting a grid line from one side of the cut grid lines to the other.

If you're going to do something like this, just remember to apply the repair liquid a little heavier on the band around the attachment point so it won't overheat and cause the grid to fail.
« Last Edit: October 30, 2011, 03:44:59 AM by K1CJS » Logged
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