Topband: Vertical array
k7tjr at msn.com
Thu Dec 13 01:01:17 EST 2007
If you are looking at reducing stateside signals (why?) from the rear of a cardiod antenna such as the 2 element you describe I think you should look at selecting the phasing that will best reduce the high angle back lobe that occurs on almost all receiving antennas. Most all stateside signals arrive at a fairly high angle as they are much closer than DX. I do not think that changing the phasing a few degrees will change forward gain or RDF enough to even notice. Do not expect to see much more than 20 dB front to back in real life on any receiving antenna as there are too many environmental factors in most cases that will prevent it. All the plots in the world of great antennas usually do not take in to account the feed lines, power poles, buildings, street lights, flag poles, phone wires, and who knows what that may be in the vicinity of a given antenna. I have programmed in those sorts of things on my RX antennas and find that most of the mentioned things have very noticeable effects on high performance low band antennas. My experience shows that a 2 element vertical array will most often outperform most loop type antennas in terms of front to back and RDF for DX. There are also too many environmental factors involved with the phasing and you in all probability will not get what you think you are getting within a few degrees. Even the arriving signal is varied in phase by local grounded objects and radials etc. I have measured this. My suggestion is to put this thing up and try it. Then tell us if it works or not. My experience tells me not to worry a great deal about particulars on antennas with less than 12dB RDF or front to back greater than 25dB. You just do not gain anything noticeable on a well made antenna. I also suggest you read and reread "www.w8ji.com" receiving antennas and the 8 element array conclusions page at "www.k7tjr.com" .
Lee K7TJR Oregon
More information about the Topband