Geoff, A half square antenna is also a phased pair of 'upside-down' verticals - easy to get up if you have 2 supports just a bit more than 1/4 wave high and a bit more than a half wavlength apart. Feed at the upper corner directly with coax or with a parallel resonant tank at the lower end of the vertical section.
I have one observation about the 'coax dipole' that you described - the choking impedance must be very high for it to work properly - consider that the impedance at the end of a halfwave dipole is quite high (4000 ohms or so), to effectively de-couple the remaining coax shield at the 1/4 wave point, you would need a choking impedance at least 5 times the impedance at that point - more like 20,000 ohms. The impedance of a few turns of coax is not going to make an effective choke at this high impedance point. I might consider it to be a 3/4 wave dipole fed at the 1/4 wave point with the choke placed at a point 1/2 wavelength down the coax. I guess I have to try one just to see how the feed impedance works out, several folks report success with these coax antennas, but I do not understand how the choke could work at 1/4 wavelength down the coax. 73, Don W3FPR > -----Original Message----- > > If it is not possible to put up a vertical taller than a 1/4 wave in among > small trees and shrubs, in my experience better results can be obtained if > the antenna is turned upside down so that the high current > portion is at the > top, and the bottom end a few feet above ground. This way the > antenna has a > better chance of looking over the vegetation. The T antenna is an > example of > this scheme, and can be voltage fed at the bottom with a parallel > LC 'tank' > with link or tap for the coax feed. ...(snip) > > A neater method can be used with vertical centre fed 1/2 wave dipoles. The > top 1/4 wave section is made from wire (Flexweave is good) with > its 'bottom' > end connected to the centre conductor of the coax feeder, the braid is not > connected to anything. The bottom 1/4 wave part of the dipole uses the > outside of the coax feeder's braid. The RF current flowing on the > inside of > the feeder loops over to the outside of the braid at the open end of the > coax, and continues to flow down the outside of the feeder untill told to > stop.This is done by introducing a high impedance trap, again > using a coiled > length of the feeder as part of a trap, approximately 1/4 wave > down from the > feeder's open end. ...(snip) > 73, > Geoff > GM4ESD > > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
