----- Original Message ----- From: "Rik van Riel" <r...@surriel.com> To: <topband@contesting.com> Sent: Thursday, November 24, 2011 10:59 AM Subject: Re: Topband: no response to query: feeding phased verticals at half wave spacing
> 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. For 2 elements coax phasing is sufficient if 15dB or a bit better F/B is acceptable especially when using low noise receiving antennas. I was more than satisfied with a pair of sloping wires on 160 (NE/SW cardiod, NW/SE figure 8) and was happy to not be wasting power in a dummy load. I used a noise bridge and the rig to cut the lines to exact lengths and about 18dB FB was the norm, sometimes a bit better or worse but I could also load it up to about 1950 in a contest without wasting more RF in a tuner. Carl KM1H _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK