On 9/20/2011 6:07 AM, ZR wrote: > With commercial field strength meters being fairly available on the used > market it would be one way to end the arguments with things that are purely > speculative.
You're certainly right that comparative field strength measurements are the only good way to determine how well a given antenna is working, and I agree that one of these AM broadcast FSMs would be a good way to do it. The K3 can be set to read the strength in dB of one signal relative to another, and it can also be calibrated to a signal generator. Last summer, I added a pair of sloping verticals to my 120 ft tower, and wanted to compare it to my omni vertical, an 86 ft tee, which, by the way, is top loaded to the point where a series cap matches it to 50 ohms. The omni has 70 irregularly spaced radials varying in length from about 70 ft to about 130 ft. The slopers have eight elevated radials, and the tower has eight radials on the ground. EZNEC predicts that the slopers should beat the omni Tee by about 2 dB in their favored direction, and that their vertical patterns are the same. So far, in about a dozen ground wave tests with K3s on the other end, I've only seen that twice.During several contests, I did a lot of switching between the antennas and noting relative signal strength. The slopers are about 6dB quieter on RX, but they rarely had greater signal strength than the omni. I've also tried watching signal strength readings from Skimmer spots while QSYing a few kHz when switching antennas. And tried running on the slopers, but always went back to the omni because I had much better rate, even in the favored direction of the slopers. Even carefully done comparative antenna evaluations like this are tough, thanks to QSB. Another point. I've modeled a vertical, center-fed half dipole for 40M, and built it using the coax feedline as the lower half of the dipole and ferrite chokes (two 5Kohm chokes in series) as the lower end insulator. It's hanging from a pulley in a redwood next to my house, with the bottom of the antenna at about 40 ft. I modeled the dipole at various heights above ground, and nowhere does the pattern or relative field strength look very good. On-the-air performance isn't very good either. When I first built it, I did a number of comparisons with horizontal half wave dipoles at about 100 ft, and the horizontal dipoles beat it by several S-units. There are no radials under it. 73, Jim K9YC _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
