The dimensions I have given are for around 98MHz, i.e. bang center of band, 20MHz is fairly narrow, so this antenna will work.
As for the metal part, its better to mount it on front windscreen, but if you use a high end car or have a anti Infrared sun film, it may not work. Reason, many modern high end glass as well as films have lot of metal in them. Thats the reason windshield mounted GPS antennas also do not work well in these cars.
But since mostly people going in for such contraptions drive normal cars, you can just use what I listed.
Now comparing OEM, Aftermarket and DIY antennas, well its just a wire. OEM antennas take aesthetics into play, so they are usually worse off. For example a 1 meter long straight wire will work as a good enough antenna too if you are near a FM station.
So just buy that TV antenna wire, make this antenna, stick it on the windshield. In case of cars like indica which already have a windscreen mounted antenna, all you need to do is disconnect that, and connect the 2 wires to this DIY antenna.
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