For what it's worth, the pattern shown in the Decibel catalog for the DB224 with all elements pointing the same direction, on a top-mounted mast, shows 9 dB to the front, 6 dB to the sides and 3 dB to the back of the mast. Almost a circle, offset.
Chuck WB2EDV ----- Original Message ----- From: "Jeff DePolo" <[email protected]> To: <[email protected]> Sent: Wednesday, July 01, 2009 8:11 PM Subject: RE: [Repeater-Builder] DB-224 patterns on side of tower. >> 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 > > > > > > ------------------------------------ > > > > Yahoo! Groups Links > > > >
