I won't try to argue with you Jeff, but those patterns might still be on file at the FCC somewhere. This test was done on a professional antenna range by the same folks who verified all the antennas that we mounted on the airborne reconnaissance aircraft for the military.
We have mounted complete fuselage sections on this test range and done both horizontal and vertical patterns. We only did the horizontal pattern for the DB-224 elements mounted directly to the Rhon 25 sections. The gain figures I quoted are dBd reference the dipole mounted in place of the DB-224 before the test. The Scientific Atlanta turntable was connected to a circular strip chart and the amplitude measurements were recorded directly to the strip chart which was submitted. The turntable was mounted on top of a five story building with the signal source about 500 feet away mounted near the ground. The tilt angle between the source and antenna to be measured is to prevent ground reflections from entering into the results. I did not attempt to do any measurements to make sure it was a circle, but it sure looked like a circle to me - with the 3 dB offset from the center toward the direction the dipoles were pointed. Again, the results were perfectly smooth with a 9 dB gain in the direction the dipoles were pointed, varying very smoothly with no nulls or dips to 3 dB gain in the direction through the tower oposite the dipoles. I don't see your point on where the energy comes from to make the extra 3 dB gain, as it obviously comes from the 3 dB reduction in gain on the back side of the tower compared to an omni antenna. 73 - Jim W5ZIT --- On Wed, 7/1/09, Jeff DePolo <[email protected]> wrote: From: Jeff DePolo <[email protected]> Subject: RE: [Repeater-Builder] DB-224 patterns on side of tower. To: [email protected] Date: Wednesday, July 1, 2009, 7:11 PM > Back when we had to submit an antenna pattern in order to get > a repeater license for the ham bands, I mounted four elements > of a DB-224 directly on one leg of a Rhon 25 tower and > mounted the two tower sections on an antenna test pedestal > and ran the pattern. With the antenna sections directly in > line and pointed away from the tower, we had 9 dB gain in the > favored direction, 6 dB gain at plus and minus 90 deg, and 3 > dB gain off the back side of the tower. > > The plot was perfectly round with the 3 dB offset for the > center point of the plot. I'm not trying to pick a fight here, but this seems to defy the laws of physics. There would have to be nulls that were quite deep elsewhere in the pattern aside from the four azimuths you cited if those values were indeed valid. Think about this, a grossy simplified example that would be difficult to achive in the practical world, but valid in theory nonetheless. Say you have an omnidirectional antenna that has some gain due to elevation pattern compression (for example, a stacked dipole array like we're talking about). Or it could even be an inefficient antenna (rubber duck) that has negative gain relative to a dipole. The omni gain isn't really important, it just serves as reference, but for sake of argument say it's a 4-bay dipole array that has 6 dBd gain omni. So, 6 dBd would be 0 dBr (r=reference) . Now we do something to alter the azimuth pattern which results in there being a 90 degree arc that has 9 dBd (+3 dBr) gain uniformly over that arc, two 90 degree arcs that have 6 dBd (+0 dBr) gain uniformly over those arcs, leaving a fourth 90 degree arc. What's the gain over that remaining 90 degree arc assuming it, too, is unform across its 90 degrees? If you answered 3 dBd (-3 dBr), you're wrong. If you answered 0 dB (-6 dBr), you're wrong. In fact, you'd be infinitely wrong if you answered anything other than negative infinity dB's (i.e. ZERO radiation). Using your examples/numbers above, and assuming a "perfectly round" pattern offset from the center by 3 dB plotted as a typical logarithmic polar plot, you're saying that over an arc of 180 degrees (the "forward" direction) the gain was a minimum of 6 dBd (0 dBr), and a maximum of 9 dBd (+3 dBr), correct? That would be impossible as you couldn't take enough power out of that remaining 180 degree arc on the backside to ever balance out the forward gain you claim to have achieved! There would have to be other deep nulls elsewhere in the pattern for there to ever being a chance of those gain values being being possible. If it helps to understand this better, think of a two-way power divider. A perfect two-way power divider would yield -3dB at each of its output ports relative to the input signal. Let's say the input power is 0 dBm, so at each port we have -3 dBm. Now say we modify the power divider so that one port is 3 dB hotter than what it was originally, resulting in 0 dBm coming out this "hot" port. What's left coming out the weak port? Hint: it's NOT -6 dBm!!! Antennas can't create power, they can only direct it where you want it to go. An antenna that has 100 watts at its input terminals can't radiate more than 100 watts total. It can concentrate more power in certain directions (i.e. gain), but the sum-total of the radiated power can't be more than the input power. --- Jeff WN3A
