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Re: [TowerTalk] Quads vs. YAGI (gain)

 To: "RICHARD BOYD" ,"towertalk reflector" Re: [TowerTalk] Quads vs. YAGI (gain) "Tom Rauch" Tom Rauch Tue, 31 Aug 2004 22:14:50 -0400
 ``` One or more people have mentioned their "quad" was not as directive as their > SteppIR. > Since they did not mention how many elements their quad was, I assume it was > a 2-element, > in which case it's not surprising it didn't have great directivity. It's pretty easy to analyze a quad. A quad is really just two 1/4 wl long elements stacked 1/4 wl apart. The two non-fed sides simply serve to end-load the 1/4 wl radiators. The shorter width elements reduce "collinear" gain and cause a slight increase in beamwidth for horizontally polarized quads. Thus all things equal, they have wider beamwidth because current is not "spread" over as broad a frontal area as a full size dipole in a Yagi. The stacking gain of two 1/4 spaced elements at absolute optimum conditions is in a graph on my web page at: http://www.w8ji.com/stacking_broadside_collinear.htm It is as approximately 1dB. This gain is from a slight vertical nulling. Now if you mount a quad at a height where the ground forces a null straight up, the 1/4 wl stacking gain of a quad goes to zero!!! If you mount a quad at a height where the ground causes a peak straight up, stacking gain is at a maximum. The same is true of freespace. The reason for this is because in order to have gain, we must have energy to remove from a null area. If the null area has zero field, stacking to force a null in a null can't add gain. If you model a quad and compare it to a dipole, you'll see all of this is true. If you accurately measure one, it will behave the same. Now as the quad is made longer and longer, the multiple elements force a null where the quad "dipole stacking" has a null. The longer the quad, the less of that 1dB potential gain that can be obtained even at a sweet height. This doesn't mean quads don't have advantages. They are less susceptible to corona and are easier to make multi-band. The fact is, no antenna is a magic bullet. They all have pros and cons. The exception is the occasional "new invention" antenna we read about, at least until someone honest measures it. 73 Tom _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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