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if you're using only a 102" whip WITHOUT the heavy duty spring and ball mount then the antenna is too short. (physically and electrically) if it's mounted right behind the cab then coupling capacitance between the cab and the antenna is shortening it (electrically) even further. there should be no body metal that rises above the base of the whip in the near field. the truck cab is in the near field.

a 102" steel whip including the heavy duty spring and ball mount measures approximately 107.5". under these conditions with a mounting location other than described above and a properly terminated image plane (the vehicle body) typical vswr is usually 1.5:1 or better. if none of the above applies then you have no ground plane for the antenna (whip, spring & ball) to image itself against. a quarter wave antenna (assuming the proper length) more often than not will present an input impedance of approximately 16 - 17 ohms at the feedpoint when no image plane is present. that input impedance would also place the voltage standing wave ratio at 3:1.

<blockquote data-quote="freecell" data-source="post: 129670" data-attributes="member: 30">if you&#039;re using only a 102&quot; whip WITHOUT the heavy duty spring and ball mount then the antenna is too short. (physically and electrically) if it&#039;s mounted right behind the cab then coupling capacitance between the cab and the antenna is shortening it (electrically) even further. there should be no body metal that rises above the base of the whip in the near field. the truck cab is in the near field.a 102&quot; steel whip including the heavy duty spring and ball mount measures approximately 107.5&quot;. under these conditions with a mounting location other than described above and a properly terminated image plane (the vehicle body) typical vswr is usually 1.5:1 or better. if none of the above applies then you have no ground plane for the antenna (whip, spring &amp; ball) to image itself against. a quarter wave antenna (assuming the proper length) more often than not will present an input impedance of approximately 16 - 17 ohms at the feedpoint when no image plane is present. that input impedance would also place the voltage standing wave ratio at 3:1.</blockquote>

[QUOTE="freecell, post: 129670, member: 30"] if you're using only a 102" whip WITHOUT the heavy duty spring and ball mount then the antenna is too short. (physically and electrically) if it's mounted right behind the cab then coupling capacitance between the cab and the antenna is shortening it (electrically) even further. there should be no body metal that rises above the base of the whip in the near field. the truck cab is in the near field. a 102" steel whip including the heavy duty spring and ball mount measures approximately 107.5". under these conditions with a mounting location other than described above and a properly terminated image plane (the vehicle body) typical vswr is usually 1.5:1 or better. if none of the above applies then you have no ground plane for the antenna (whip, spring & ball) to image itself against. a quarter wave antenna (assuming the proper length) more often than not will present an input impedance of approximately 16 - 17 ohms at the feedpoint when no image plane is present. that input impedance would also place the voltage standing wave ratio at 3:1. [/QUOTE]

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