I wanted to have a resonant multi band HF mobile antenna for my truck without the aid of using an antenna tuner. As I own a Webster Bandspanner, similar to the screwdriver style antenna but without a motor for frequency adjustment. I don't like going out and manually adjust it and checking the SWR every time I change bands.
Because many hams including myself cannot afford one of the better auto tuning antennas, I looked at a commercially made HF multi band antenna and began to do some research. I ended up using an 80 meter Hamstick for the purpose. It was mounted on a 6 inch stainless steel spring for logging road and bush travels. I also use a bungee cord to help it from really swaying and bending during highway speeds.
First, I tuned the antenna properly to the frequency area that I use most in the 80 meter band. You will note that I have a matching coil at the bottom of the antenna. This is needed for a 50 ohm or close impedance match to satisfy the rig for 80 and 40 meters due to low impedance characteristics of low frequency verticals. (See the ARRL Antenna book) I then stripped most of the upper part of the heat shrink off the antenna and scratched off the enamel of some windings at near where I figured the sweet spot was to be for 40 meters.
I made an insulated, flexible #14 flexible wire long enough with alligator clips on each end to wrap several times around the antenna winding section able to have it completely shorted out. The reason for wrapping around the antenna winding is to prevent the wire from blowing off during vehicle movement. This shorting wire must be wound on the antenna at all times for all bands except 80 meters, not tight but not too loose either.
The shorting wire must be completely removed from the antenna when working 80 meters, if not, it will create a coupling effect making the antenna unusable on 80.
With the help of a digital antenna analyzer, the MFJ-259, I found the sweet spot by shorting the winding to the bottom of the antenna (near the 3/8 x 24 threads) after stripping some of the enamel off at some areas of the winding. Thus eliminating part of the winding for 40 meters. On one other antenna I constructed, I also found the 60 meter amateur portion.
I then disconnected the impedance matching coil out of circuit and proceeded to find the individual sweet spots for 20, 17,15,12 and 10 meters. A completely shorted winding ended up as an excellent 10 meter quarter wave antenna. When the sweet spots were found, I carefully tinned the winding at their appropriate places allowing to later solder on some wire tails.
I then carefully soldered a one and one half (1.5) inch length of #16 insulated wire at each spot. Make sure the wires are stripped and tinned on both ends before soldering onto the windings.
I then re-checked the SWR on all bands except 80 meters with the shorting wire. When everything was acceptable, I proceeded to place heat shrink with holes at appropriate places for the added wires to protrude for connection accessibility onto the antenna. To protect the other bare copper areas that had enamel scratched off of the winding; I used nail polish to prevent copper oxidization. I placed heat shrink tubing in separate lengths, starting from the lower part of the 40 meter section and overlapped it with the next piece until the top of the complete winding was covered. But, before heat shrinking each tube, I put RTV silicone inside where the connecting wires protruded to be completely waterproof when the tube was shrunk.
Communication results were excellent, as to be expected. I still carry a few single band mobile antennas with me in my travels, including an 8 foot stainless steel whip, just in case.
I used 1.7:1 bandwidth instead of the regular 2:1 bandwidth as personal preference. I believe solid state transmitters should be used within 1.7:1 maximum bandwidth, but I personally prefer 1.5:1.