I have a system with two gamma matched towers that works pretty well. I did all I could to make them appear to be similar. The gamma dimensions and capacitance are identical - and I used a loading wire on top of one of them - making it longer and longer until the match was similar.
After that - I used current probes to check the current levels in the tower - and they looked fairly close. Then I went driving around a few miles away checking the pattern and found it to be very nice. I see probably 15 or 20 db F/B ratio from the end fire pattern. It has some gain in the broadside pattern as well. For awhile - I had relays setup so I could detune one of the towers and compare signals with just one tower fed. Once I had confidence in the system - I removed that feature. K6SE was the first guy I saw doing this - with much shorter towers than mine. So - it can be tricky - but if your towers are pretty similar in dimensions and you can use identical parameters for the gamma match - then you might be surprised how easy it is to get working. Tree N6TR On Thu, May 30, 2013 at 3:53 PM, Tom W8JI <[email protected]> wrote: > I did try it with two shunt fed towers on 160 meters 1/4 apart using a > Wilkinson power divider and had little success >>> > > Because of mutual coupling, antennas with unidirectional patterns have > grossly dissimilar impedances at each element. The elements, provided they > have reasonable loss, have nowhere near similar impedances and nowhere near > the impedance as a single element. > > Elements in two-element arrays require equal currents to have deep nulls. > > Since the impedances are grossly different, there isn't any divider that > will supply the design phase and current ratios to low loss elements. It > takes a phasing system custom designed for the actual impedances involved. > > > <<and then read in ON4UN’s Lowband book that phasing shunt fed tower was > very difficult if possible at all. >> > > That is not correct. > > I successfully phased a G5RV (100 ft high as a "T") against a 130 foot > shunt fed tower. It had extremely deep nulls with three patterns, and I > had four patterns. I had unidirectional NE, unidirectional SW, > bidirectional NE/SW with deep nulls NW and SE, and broadside (which was > almost omni because of the close spacing). > > Shunt fed towers do not behave like normal tower because the shunt wire > acts like an additional length of transmission line. This alters feedpoint > requirements, and the elements no longer require equal currents at the > feedpoint for deep nulls like a current maxima fed element, or equal > voltages like a voltage maxima fed element. They require something between > a voltage fed element's requirements of equal voltages and a current fed > elements equal currents, the exact requirement dependent on the shunt > characteristics as a transmission line. The shunt also introduces phase > shift that must be allowed for. > > While it is not an easy cookie cutter task, it is not nearly impossible. > It can actually be pretty fast to set up, if the phasing system is > adjustable in delay and ratio. > > Wilkinson's and similar only supply the required ratios and phase when > terminated in the design impedance, and that does not happen with > unidirectional patterns in small endfire arrays (unless significant loss is > added). When the division and phase errors caused by mutual coupling > creating dissimilar impedances is combined with the shunt feed adding phase > shift, and the shunt changing required ratios in voltage and current, it > probably would not work very well. That doesn't mean phasing shunt systems > cannot work, or even that the job is difficult. It is just different. > > 73 Tom > All good topband ops know how to put up a beverage at night. > _________________ > Topband Reflector > All good topband ops know how to put up a beverage at night. _________________ Topband Reflector
