----- Original Message -----
From: "Jim Smith" <jimsmith@shaw.ca>
To: "towertalk reflector" <towertalk@contesting.com>
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.
<snip>
> 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
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