Theory Behind Sommer (DJ2UT)
Multiband Beam Antennas

Technical data on XP Series XP-403 XP-504 XP-704 XP-804

20 Meters
On this band the DJ2UT multiband beam system is a full-sized beam utilizing 1/2-wave elements without traps. The primary difference is that all elements are driven via a phasing line, in a fashion similar to the log periodic. This technique provides "monoband" gain or better without the narrow bandwidth commonly associated with such antennas. As on all bands, the compromise between forward gain and f/b ratio has been optimized for amateur use through actual on-the-air testing and not random selection.

15/17 Meters
Because the 20-m elements are approximately 5/8-wave long on these bands, the resulting high feedpoint impedance must be compensated for. Instead of resorting to "power-hungry" L/C traps, DJ2UT utilizes the capacitance found in the phasing line, in combination with another unique approach. Clustered about the feedpoint are 3 or 4 elements (depending on antenna model) in close proximity to each other. This combination of parasitic and driven elements simply and effectively brings the system impedance down to the desired 50 ohms. In addition, a special means of feeding the elements reduces the unwanted side-lobe radiation common to 5/8-wave systems.

10/12 Meters
Since the primary elements of this design or 1/2-wavelength on 20 m, they are approximately a full wavelength on 10 and 12 m. In the DJ2UT system, they are fed via the phasing line as "split" half-wavelength elements in a collinear fashion. This configuration also presents a high feedpoint impedance. As on 15/17 m, this high value is reduced to 50 ohms by the influence of 3 or 4 elements in close proximity, along with an auxilliary 12-m element.

30/40 Meters
By ignoring all elements but the longest (found at the rear of the boom), we may consider the DJ2UT antenna a simple dipole with a transmission line attached. Since this element, or dipole, is only 11.6 m (approx.) long, it is too short to be resonant on either 30 or 40 m and presents a capacitive reactance at these frequencies. On 30 m, the problem is solved by the simple addition of an L-C match. Similarly, 40-m compensation is achieved with a coil and/or coaxial capacitor. Since these networks are not in series with the antenna and only serve to cancel the "blind" reactive components, the problems of L/C trap loading are again avoided. It should also be noted that on 30 m, the 3- or 4-element cluster at the feedpoint serves as a director--providing some gain and a cardioid directive pattern in the larger models. On 40 meters, the mounting height of the antenna determines which element serves as radiator. As a result, the major radiation pattern may be in front or in back of the antenna, or even under 90 degrees, depending on antenna height.

NOTE: No other big beams or dipoles, such as 40/30 m systems, should be operated below or above any Sommer beam, as they will have a negative impact on the radiation characteristics of our beams. William S. Orr, W6SAI, in "Stacking Experiments," Beam Antenna Book, 5th edition, p. 83, makes the following comment on stacking 15 and 20 meter beams:

This proved to be impractical as the detuning effect between the various elements was severe. No reasonable adjustment or element spacing or tuning would allow either beam to match its feedline with a low value of SWR.

Sommer Antennas

P.O. Box 710
Geneva, FL 32732
Phone:( 407) 349-9114 | Fax: (407) 349-2485