This is quite true although some patterns will be closer to theory than others. In a commercial phasor as is used for broadcast there are several elements that must be taken into account. The first is the matching at each vertical element. It must take the actual drive impedance of the elements and match to the transmission line (usually 50 ohms). These networks all have phase shift which must be taken into account as a part of the total system design. The next important element is the power divider. There are various designs but in the end they all permit the adjustment of power to each element so the currents are correct. The ARRL patterns are idealized examples where the current in both elements are equal but in the real world this is not often the case. A power divider also has phase shift. The final element is the means of making the phases of each tower the desired value while taking into account the fixed phase shift of the matching network and the power divider. There are two common methods used. The first is a lumped constant network which adds or subtracts phase to achieve the correct values after transmission line phase delay is added into the system. The other method is to use the transmission line to achieve the proper phase relationships. Even when transmission lines are used it is necessary to have a small lumped constant network to trim for minor variations in phase. It is apparent that such a system is very sensitive to a change in frequency.
Adjusting a typical phased array requires a means of measuring impedance and typically there is a phase monitor which shows the phase and current ratio of each tower. This is all verified by field measurements that require a calibrated field intensity meter and a lot of paperwork to plot the pattern or a more complex (and expensive) GPS based measurement system. It should be obvious at this point that no amateur phased arrays are built as described above and do not have the same requirements. Commercial arrays are usually designed to provide protection to other stations on the same or adjacent frequencies. Amateur arrays are generally intended for gain in a desired direction and must have at least a small amount frequency agility. While the ARRL patterns are very much theoretical a few can be implemented for practical amateur use. The broadside two tower array spaced 180 degrees is probably the best if space is available. One drawback is the figure 8 pattern has zero db front to back ratio. This could be handled with a parasitic reflector but at the expense of more complexity and space. The bottom line is that practical amateur arrays will not achieve performance close to theoretical. On 11/24/11 10:59 AM, Rik van Riel wrote: > On 11/22/2011 08:06 PM, Gerry Treas, K8GT wrote: >> Hi Dale, >> >> I'm no antenna expert, but certainly read as much of the experts >> publications as I can get my hands on, but having a Teflon brain, it doesn't >> stick very well. >> >> That said, the ARRL Antenna Book has a page that shows the patterns of >> various spacings and phasings of vertical antennas, which I found very >> enlightening. >> > Enlightening, but also somewhat misleading... > > The patterns in the ARRL Antenna Book are correct if the > current in both elements is the same. > > However, if you feed an array of antennas with delay lines, > those delay lines will act as impedance transformers for > the antenna impedance of each element (like all feedlines do). > > This can result in each element getting different currents, > and the pattern no longer being what it was. > > This makes feeding a phased array with delay lines much > trickier than one would imagine at first glance. > _______________________________________________ > UR RST IS ... ... ..9 QSB QSB - hw? BK > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK