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Re: [TowerTalk] Funniest thing I've seen in weeks

 To: "Jim Smith" ,"towertalk reflector" Re: [TowerTalk] Funniest thing I've seen in weeks "Jim Lux" Sun, 10 Oct 2004 07:35:57 -0700
 ```----- Original Message ----- From: "Jim Smith" To: "towertalk reflector" Sent: Saturday, October 09, 2004 9:45 PM Subject: Re: [TowerTalk] Funniest thing I've seen in weeks > Sorry to have taken so long to respond to this. > The noise power at the antenna terminals will now be 1 uW X 60 deg X 4 = > 240 uW > The signal power at the antenna terminals will now be 360 uW X 4 = > 1440 uW > SNR is now 10 log 1440/240 = 7.8 dB > > > Interestingly, the antenna gain cancels out. i.e. with the same pattern > a gain of 12 dB will still yield an SNR of 7.8 dB, just like everybody's > been telling me. So how come I can hear things on a yagi that I can't > hear on a dipole at the same height? Directivity vs gain... You can have a directive antenna that doesn't have much gain (i.e. it has loss). A good example would be your first directional antenna, with the same gain, but only 60 degrees beamwidth. > > > If the antenna has 6 dB gain but responds over only a 30 deg arc then: > The noise power at the antenna terminals will now be 1 uW X 30 deg X 4 > = 120 uW > The signal power at the antenna terminals will now be 360 uW X 4 = > 1440 uW > SNR is now 10 log 1440/120 = 10.8 dB > > So, more antenna gain but with the same pattern makes no difference to > received SNR. > > Narrower pattern, regardless of gain, will increase SNR. > > So it would seem that the fact that a yagi at the same height as a > dipole will hear signals that the dipole won't, not because of the > yagi's gain but because it's pattern reduces the total noise power > picked up by it. Precisely... from a receiving standpoint, it's the shape of the pattern, rather than the absolute gain, that makes the difference (assuming you're limited by atmospheric noise) Of course, on transmit, gain in the direction of interest is important, and you don't much care about the pattern. Squirt as much power in the direction of the receiver as you can. If you go to different transmit and receive antennas, it opens up a whole lot of interesting possibilities. On receive, a phased array is very nice, because it can null the interfering signals (both kinds), and you don't much worry about efficiency (i.e. you can use broadband small loaded antenna elements, for instance) but on transmit, given that you are power limited, efficiency is important. It gets even more interesting if you start considering a couple real world aspects of the problem: 1) Ionospheric multipath - turns out that diversity combining of two co-located antennas with different polarization responses is very effective. Fading on skywave paths is largely a manifestation of multipath, and the various paths have different polarizations, so a suitable antenna system can eliminate the effect. Unfortunately, the folks who have tried to do this with two way haven't been too successful because the ionosphere changes too quickly, and, an even bigger problem, it's anisotropic, so the path isn't the same in both directions (which folks doing satellite comms or EME on lower frequencies (<2 GHz) are well aware of). Knowing the optimum receiver in one direction doesn't tell you anything about the optimum transmitter to shoot back with. Just go for the "squirt the max power in the right direction approach" 2) The sky noise is highly, highly non-uniform. There's some really nice work being done by C.J. Coleman VK5AHZ?) in Australia( he's at University of Adelaide, South Australia) who is using a HF OTH radar as an antenna and correlating that with models of noise using global lightning distribution and ionospheric propagation models. Very fascinating data (http://hawk.iszf.irk.ru/URSI2002/GAabstracts/papers/p0738.pdf) > > I seem to be happy with this explanation provided by my highly idealized > (but easy to deal with) model. Don't know if anyone else is, though. > > 73 de Jim Smith VE7FO _______________________________________________ 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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