The 102" whip is a general 1/4 wavelength antenna designed for placement on top of a ground-plane.
Whenever the antenna is mounted in such a way that this ground-plane or its effect is not perfect in relation to the 102" length of the antenna, the result of being out of resonance start to appear in high SWR and transmit-receive efficiency drops rapidly.
Very rare is the antenna electrically too short for most applications, thus DO NOT CUT IT.
Listed below this guide are some common errors which CREATE VSWR.
Ensure that the common errors do not apply to your installation. Once this is done, you will need to start adjusting the antenna and compensate for the error in its ground-plane surroundings.
You will need two or three small nylon cable ties and a length of clean metal rod about 10 to 15 inches long and the thickness of the whip near the end before the BALL. The BALL is a safety device to prevent the end of the antenna rod from poking holes in things like your eyes, etc.
It has a better purpose. The BALL is called a Corona Ball. It helps prevent the antenna from burning away the tip during high power radiation. This Corona can be seen in dim light as an arc of RF energy from the antenna into the atmosphere.
Overlap this new length of clean metal rod to the whip as if you were extending the length of the antenna.
About 6 or 7 inches over-lap is a good start. Temporarily secure this added length in place using the small nylon ties.
Always use the low power of the radio alone to do your basic adjustments. Take an SWR reading with this new added length to your 102" whip which should look like it is now about 108 or so inches long. Shorten and/lengthen this extended tip under you reach the lowest VSWR you can get with your configuration.
Once the VSWR is determined to be the lowest, measure the ADDED length to the solid 102" whip. This is the length you will need to extend the whip.
This extension needs to be added to the bottom of the whip to act as an extender. The attempt to just add the length of wire to the end of the antenna will prove to be fruitless as these cannot be wrapped with ties, taped, wire-wrapped, welded or bonded. All these will fail.
Instead; have someone make a 1/2" thick of solid rod the length you need and drill/tap both end with a 3/8" fine thread and stud one end to go into the base mount nut and your original 102" whip screwed into the other threaded end. It too is best not to use thread-all or tubing as these will break or collapse because of their weak structure..
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COMMON ERRORS WHICH CREATE SWR PROBLEMS
A. Broken, shorted, pinched, curled, melted, or kinked coax.
B. Low-grade coaxial cable.
C. Poor, or no chassis ground on the antenna mount. Reflective ground too far beneath the antenna. (This holds true with vehicles which have a fiber-glass body).
D. Short or open connections at the mount.
E. Wrong coax type. Single antennas use 50 or 52 ohm impedance coax (RG-58C/U, RG-58(F),RG-8A/U,RG-213/U,RG-8(F),RG-17A/U,RG-218/U). Dual antennas use 72, 73 or 75 ohm impedance coax (co-phased/balanced), (RG-59B/U,RG-59(F),RG-11A/U).
F. Excess cable wound into a small coil (12" or less) can create a choking effect and create a magnetic field which will cause undesirable SWR.
G. Improper cable length. To calculate as close as possible what length should be used, formula 468 divided by the frequency (in MHz) will give 1/2 of a wavelength (in feet); thus as an example, 468 divided by 27.205MHz (chan 20) gives 17.2 feet for a 1/2 wavelength of coax. (Divide again by 2 for 1/4 wave length).
H. Antenna tuned without the tip cover installed and later the cover replaced.
I. Antenna tuned with the tip installed and later the tip removed or lost.
J. Tuning the antenna in a garage, under car-port, touching tree limb, and other areas that confine the antenna's radiated field.
K. Taking measurements with doors open, hood up, trunk lid open and someone standing near the antenna. (clear at least 1/4 wave-length).
L. Faulty equipment for measurements.
M. Loose antenna, mount, mounting stud, and/or connectors/reducing sleeves.
N. Amplifiers installed out of phase.
O. Ropes, strings, chains, tennis-balls, garden-hoses, and other junk holding/touching the antenna in place. (If items like these are needed, make sure they are all in place when the SWR is checked and calculated, otherwise, they will influence the readings.
P. The shorter the antenna is physical, the more sensitive it is to adjustments. [Electrical length compensated by a coil, reflecting rods/arms].
Hopefully, these "HINTS" will make your 102" antenna installation less troublesome.
FTR
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