The Helical Whip for RV and Mobile Use

When attached to the common spring mount on the top of the motor home, four of the antennas had low VSWR (less than 1.5:1) without retuning.

The center unit (20 meters) was affected slightly, and I needed to shorten the tip ½ inch to obtain the best VSWR for the phone band. The 20 meter antenna also had narrow bandwidth, although it still covered the phone band. For this reason, it might be better to put one of the narrow band antenna (12 or 17 meters) in the center.

While traveling, tilting the assembly about 45 degrees or more to the rear should help you avoid losing some of it to an underpass or bridge. You will encounter another hazard in residential areas, where large trees often overhang the streets. Mobile operation is still quite feasible, even at a 45 degree tilt, if you mount the assembly on a rear luggage rail or ladder.

Testing Procedure

I used the test setup in Figure 2 to check all the antennas during construction. If you do not plan to go into heavy production, the steel top of a car or pick up truc should work quite well as a base for the magnet mount. Take precautions to protect the top of the vehicle. A thin sheet of plastic should work, as capacitance between the magnet mount and car top will serve as an effective RF ground connection.

I consider a grid- dip meter a must for checking antenna resonant frequency, but an RX noise bridge may work if you are proficient in its use. Checking VSWR is useless for initial testing unless the resonant frequency happens to be in or very near the band in question. If you have a BNC fitting in the magnet mount, you can easily attach a small loop for grid- dip meter readings, exchanging it for coax for testing VSWR or on- the- air use.

After you adjust an antenna roughly to the correct frequency with the grid- dip meter, make the adjustments by connecting the antenna to a transceiver and finding the frequency of lowest VSWR with a VSWR meter. An adjustments of as little ¼ inch to the tip of the antenna may be necessary to bring lowest VSWR point to the center of the desired band.

Figure 1

It is usually sufficient to check the VSWR every 100 kHz to find the low point. A dual needle VSWR meter is a real time saver here.

It is better to start with an antenna that is too long; you can find the correct length with careful plunging. If you have not found a low VSWR point after your first check with a grid- dip meter, recheck the meter frequency. Don not be concerned with other higher frequency dips. Some may be harmonic dips and some may not. Depending on the length of the wire up the point, the loading coil may act as a trap or a choke, creating a much higher frequency dip.

Construction of the Modified CB Antenna

The modified CB antenna requires the least work and is very efficient. I recommend using a top- loaded type with short tuning stubs at the very top, just above the loading coil. The one I used had about 3 feet of helical winding spaced evenly on constant diameter fiber glass rod,

www.antentop.org

Page- 45