The MFJ-1796 Multiband Vertical Antenna
A lot of antenna in a small package.
by Don Johnson K7VGQ
Vertical antennas have the dubious reputation of being either loved or hated by members of the ham radio community. In simple terms, if the antenna is property designed and installed, the vertical will provide creditable performance and, in certain instances, will out-perform a Hat-topped horizontal wire. Because of its natural low angle of radiation, a vertical can haul in the DX better than a comparable horizontal flat-top antenna. How. ever, for short, local contacts, its low angle of radiation coupled with "cross polarization" (theoretical 90-degree shift from horizontal to vertical), will perform somewhat less than a horizontal wire antenna.
Failure of vertical antennas to perform well is often attributed to two basic problems: poor multiband design and inadequate radial systems.
MFJ has introduced a family of autoband switching multiband vertical antennas that addresses both issues. Eliminating the need for radials through a design that electrically mimics a half-wave antenna and reducing losses common with end-fed trap verticals, the engineers have addressed issues responsible for most amateurs' poor experience when using vertical antennas.
A basic 40 meter quarter-wave vertical antenna is approximately 33 feet tall and requires a very good ground or counterpoise system to perform properly. In fact, the absence of a well-designed ground system is most often the reason horror stories circulate during vertical antenna discussions. The ground system must be electrically the same as the quarter-wave vertical section, on all operating frequencies, if any system efficiency is expected. Most ham urban/ suburban homes do not have enough land, free of obstacles, to connect the several hundred feet of buried or elevated wire necessary for a good RF ground or counterpoise. An acceptable way to overcome the requirement for a cumbersome ground/ counterpoise system is to center feed the antenna. However, a basic 40 meter center-
fed vertical will stand prohibitively tall at approximately 66 feet.
A vertical can be short and still resonate at the designed frequencies by placing matching devices (loading coils and capacity hats) in each of the vertical elements. The location of a matching device can be at the feed point (bottom loading), in the middle (center loading}, or at the end (end loading). While end loading is by far the most efficient of the three, rt is also the most demanding for good physical design. A poorly-designed end-ioad-ed antenna will surely self-destruct (cantilever) during a moderate windstorm.
The MFJ-1796 is a centered or balanced-fed electrical half-wave vertical. In other words, a dipole antenna turned 90 degrees to the vertical. As a result, both half waves of the radiated signal are accounted for. At an overall length of 12 feet, loading coils and capacity hats for each band (40-20-15-10) are placed at the ends of each vertical component. This technique provides an electrical half-wave match while maintaining a small unobtrusive overall height. The antenna provides 6 and 2 meter performance through full-sized half-wave elements.
On the four HF bands, the antenna is designed to handle up to 1,500 watts of power, 750 watts on 6 and 300 watts on 2.
As discussed earlier, an antenna with end loading requires a very good physical design. Engineers at MFJ designed the 1796 with 1.16' diameter 6061-T6 aluminum tubing. The critical center and base insulators are made of reinforced Fiberglas, V in diameter. All coil forms are also made of Fiberglas.
A four-inch air-wound current balun mounted at the base of the antenna chokes off unwanted RF on the coax cable shield.
Various lengths of aluminum rods are used for capacity hats. Installation of the rods couldn t be made any easier. Once installed, they can be pruned or bent to fine-tune the different resonance points of the antenna.
Though somewhat fragile, the design of the capacity hat rods affords a substantial amount of ductility. The rods will not break under normal use. If accidentally hit during in* stallation, they will deflect or bend. Real physical abuse is needed To break them.
The 14-page manual offers step-by-step assembly instructions with several very clear drawings. Additionally, the manual discusses vertical antenna theory and offers hints for choosing a location for the antenna and tuning instructions. I strongly recommend that you follow the assembly manual exactly as written, especially the step that suggests that a temporary mounting or staging area be assembled before constructing the antenna. Once assembly begins, you will need a place to rest the antenna.
As with all MFJ equipment, the warranty explicitly states that the owner is authorized to attempt to repair any defective condition without affecting warranty coverage. A toll-free telephone number is provided for assembly or installation support if required.
Nothing is more discouraging and frustrating than to assemble an antenna, perform frequency adjustment, and raise the antenna to height only to have to tower it for additional adjustments. The 1796 is designed to mini* mize this installation cycle.
As can be expected with a physically short antenna, the lower frequency resonance bandwidths are narrower and more sensitive to adjust than the higher ones. Range of operation (2:1 SWR points) on 40 meters is 40 kHz wide. Before any frequency adjustment is made, you should decide on what segments of the various bands you intend to operate in: CW, phone cr somewhere in between.
If assembled per the instruction manual, the resonant frequency for each band will occur at or just below the lowest frequency for each band. For example, 20 meters should be resonant at approximately 13.9 MHz* To increase the operating frequency to the desired segment of the band, you must trim one spoke in each coil assembly A chart is provided describing the effect of cutting one inch off of the spoke. If I wanted to raise the 20 meter resonant frequency from the assembled 13.9 MHz to 14.2 MHz, the chart says to cut three inches off of one of the 20 meter capacity hat spokes. Since this is a balanced vertical dipole, both sides of the antenna must be adjusted at the same time. I was able to hit my desired frequency, on all bands, the first time!
Extra spokes are provided in case you make you an error when cutting or should you decide to lower the initial frequency.
Antenna comparisons can be like comparing apples and oranges; they are both fruit, but are really different in many other ways. However, comparisons help make decisions, so here goes.
The MFJ-1796 works! How well it works depends upon several factors. Remember, the 1796 is not a multi-element beam, nor should one expect results to be comparable to a beam in any way. When installed 35 feet up and compared to a full-size 60 meter dipole (30 feet up) and a horizontally-mounted magnetic loop (35 feet up), comparable results were obtained... sometimes!
Because of two basic antenna facts, low angle of radiation and phase shift, the 1796 does not perform on local contacts as well as the magnetic loop or the dipoie. However, if the local station is using a vertically-mounted antenna (same polarization), then the 1796 outperforms both of the other antennas. Because of the inherited low angle of radiation you should expect the 1796 to give a very good account of itself on long-haul DX. And it does. Stations 3,000 miles or so away were heard and worked when the dipole couldn't copy them at all.
The 1796 is a noisier antenna than either the dipole or the loop. In fact, if an electrical storm is within several hundred miles, you may not hear much of anything. This is not the fault of the 1796 but a typical characteristic of all verticals
Several other things to consider when comparing antennas to the MFJ-1796 are: the small footprint (2 square feet), no radials only 12 feet tall, lightweight, sound construction, and broad frequency coverage (40 meters through 2 meters).
Remembering that the MFJ-1796 is a short (compromise) antenna, overall performance is less than a full-size antenna. Therefore, if you can install a full-size antenna, the 1796 is not your besl choice. The engineers at MFJ have given serious thought in the areas of antenna design that increase efficiency in an effort to overcome the shortcomings of this type of antenna, ff you are on a limited budget and/or must contend with restricted installation space, I strongly recommend the MFJ-1796. 73. year 1995
MFJ-1796 Product User Manual MFJ-1796.pdf