Pfeiffer Quads
KQ6RH
(C) 1998, 1999, 2000
Ray Jurgens
(Up-Dated 3/07/2000)
Pfeiffer Quads:
The Pfeiffer Quad was described in QST and has the desirable feature that it is smaller than a standard cubic quad but does not compromise performance very much. The Pfeiffer design is rather difficult to construct because of the complicated linear loading networks required. However, these complicated structures are unnecessary and do not add much, if anything, to the performance. The Pfeiffer quad has linear loading stubs at each corner that are pulled into the center toward the hub forming a Maltese Cross. These could just as well be replaced with inductors or transmission line stubs. In fact, the losses on 450 Ohm ladder line are small to insignificant, so Pfeiffer's complicated stub assembly can be replaced with simple transmission line stubs with no loss of performance. The only complication is in the electromagnetic modeling; here the density of pulses must be very high to accurately handle the closely spaced wires. The stubs can be stretched along the spreaders and attached to the antenna wires at each of the tips. In this case, 8' spreaders are convenient, so all that is required is to find the proper length of the ¼ wave shorted stubs that act as inductors to bring the loop into resonance. These stubs can be made slightly longer for the reflector and shorter for the director if one is used. Placing the 450-Ohm ladder line along the spreader slightly lowers its impedance and lengthens its electrical length by the same proportion. The 8' spreaders can be used to cover 20, 17, and 15 meters. In the 15m case, the stubs are only a few inches long and for 20 meters they are about 3/4 of the length of the spreader. If terminal strips are mounted at the end of each tip, the stubs can be changed easily to accommodate various bands. Some experimentation may be required to get the length of the stubs set, but once set, assembly is very quick for this quad. Our Products Page lists the our design document which gives the a complete set of graphs and design data as well as full construction details for our modified Pfeiffer Quad.
The Pfeiffer Quad is normally operated in the X configuration. The impedance of this antenna will be lower than for a full size quad, and some matching network may be required. The version presented in QST used a gamma match on one of the stubs to drive the loop. For example, at 20 m, the stubs must to be about 74" long, and the driving impedance of a single loop is about 47 Ohms. Thus, a single loop is a good match to 50-Ohm line. The addition of the parasitic elements further lowers the impedance, and depending on the spacing of the loops, the impedance could drop to 25 Ohms. For further details on the original Pfeiffer Quad, check the following reference:
Andy Pfeiffer, K1KLO, "The Pfeiffer Quad Antenna System," QST, March 1994, pp. 28-31.