A Theoretical Approach To A Multi-band Vertical
Multi-band trap vertical antennas on the amateur bands have been used for many years with varying degrees of effectiveness. Most of them suffer from narrow bandwidth, particularly on the lower bands. Many of them use some type of loading so as to keep the overall height to a reasonable level for a residential area. There have been many schemes to increase the bandwidth of the shortened vertical, some that are within the budget of the average amateur and some that are not. For the antenna experimenter, an inexpensive alternative is sometimes not available.
The antenna described here is entirely a theoretical antenna, and no one needs to hesitate if they want to build one. Construction should not be very difficult, but some help will be needed to put the antenna up, in most cases. Materials are for the most part available at a building supply store or plumbing supply.
Technical Aspects
The chart of quarter wave vertical heights and frequency will give an indication of how much antenna length changes with frequency. The 160-meter band shows the greatest length variation, with 80/75 meters next, and ten meters showing the least variation. Band edge to band edge variations are also listed as are the incremental decreases in length as the frequency is changed for the three lowest bands. Notice that 160 meters has a 13 foot change in length from 1.8 to 2.0 MHz, which is equal to a one percent change, while forty meters only changes 1.4 feet for a total of .04 per cent. Ten meters shows the least change, with a change of 1.5 MHz. only changing the length by 4.56 inches for a 1.5 MHz. frequency change.
Ideally if we were able to actually vary the length of the vertical itself by some sort of mechanical means, motors, gear drives, pulleys, etc., the problem would be solved. However in the real world of money and the lack thereof, unsympathetic wives, neighbors and other hostile entities, this is not a practical solution. Automatic antenna couplers can solve the problem but again the problem of expense pops up like mosquitoes after a spring rain. So for those of us that are eating tuna fish and crackers and cheese for lunch, there needs to be a less expensive means to solve the problem.
Marconi had the solution to an antenna that was either too short or too long. For the first he used a coil to electrically lengthen it, and for the antenna that was too long, he used a capacitor to electrically shorten it. There have been many schemes down through the years that used motors to tune antennas in this manner, both with coils and capacitors to accomplish the required tuning. Once again, this adds complication and expense to something that should be accomplished easily and automatically. A fan type dipole with multiple radiator lengths for both elements accomplishes this on other bands, so why not use the same principal on a vertical. By using multiple length elements fed at a common point, the bandwidth of a vertical can be expanded the same way.
The construction of a vertical of this type is made simpler with the availability of pvc tubing. By drilling holes approximately 1/2 in. apart around the diameter of a 3 inch pvc, 18 holes can be drilled just above the swedged end of the pipe. This will allow the use of 18 individual wires of varying lengths to be stretched parallel to each other and soldered together at the bottom for a common feed point. At the other end of each wire, a small pilot hole can be drilled for a self-tapping screw so that a lug can be soldered to the end of the wire and screwed down. If a wire longer than the longest element is included, then that wire can be used to tie another section of pvc wire antenna support together, allowing the antenna to be used on many frequencies.
By stacking pvc sections, each with its own elements, a broad band antenna could be made that would have no traps and still be capable of band switching. As an example, with 7 ten foot lengths of pvc, the bottom 3 sections along with three and a half feet of the forth section would form a 7/10/14/18/21/24/29 MHz. vertical. The next section could be covered with several wires running the full length of the pipe to make a cage that will simulate a large diameter conductor. The section that will be the 50-ft. section could have the bottom 5 ft. covered with wire in a similar fashion, with two elements of 3 1/2 and 4 1/4 ft. respectively. This will give a total length from bottom to element end of 58 1/2 and 59 1/4 ft., which would correspond to 4.0 MHz and 3.950 MHz. The section that forms the 60-ft. section would have the rest of the 75/80 elements on it. Since the pvc is not self supporting, guy ropes will have to be used.
The guying system at the top of the vertical can then form a capacity hat with the elements of the top hat made up of the necessary lengths of wire to bring the entire system into resonance at the frequencies in the 160-meter band. A loading coil could be wound on the top of this section to shorten the top hat elements somewhat, but this might cause the bandwidth to narrow down some. The use of parallel tuned traps at the top of each of the higher frequency sections would also help to eliminate the possibility of sub harmonic radiation.
This system is only a blue-sky project now, but it is hoped some interested experimenter will give this a whirl. True, a lot of the things that influence an antenna’s behavior at multiple frequencies has not been discussed, but this is only a think-about-it antenna. Also, no mention of matching to the feedline has been discussed. If any one wants to try this, feel free to do so, just let us know how it works or does not work
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 5th April 2024