Owen wrote:
On 01/10/10 07:44, Jim Lux wrote:

The other thing is whether the difference is big enough to "make a
difference" in observed system performance. For a lot of operators, a 1
dB change in performance might not be noticeable. If you're in a "either
propagation is there, or it isn't" situation the difference between good
and bad is 10s of dB. There are relatively few people who work at 0dB
SNR (where tenths count) on a regular and continuing basis, and they're
not necessarily the ones who are interested in doing experiments on
antennas on the scale needed.

Yes, there will be differing view on what is significant difference. I
am not in the school of declaring less than one or two S points is
insignificant in general.

yeah, but there's a big difference between 6-12 dB and 1dB.. I think
most users would care about 6 dB. Many fewer about 1 dB. And even
fewer care about 1 dB AND have the desire and means to perform the
experiment in a controlled way. (well, this latter category probably has
less than 10 people in it, and only 1 has published in the last 50 years)



In the case of a four square in the DX segment, users are looking for
performance... and it seems to me that the elevated three radials, eight
wave vertical with capacity had is very close to quarter wave over
buried radials... depending of course on the soil type.

Hmm.. and there the real question is what kind of performance are we
talking about: the power radiated in a desired direction (Tx) or the
ability to null unwanted signals (Rx). Given the generally high noise
levels on low bands for Rx, a 1 dB change in efficiency of the antenna
might not make any difference for the latter.

A bigger effect on a phased array is the relative phasing. For a 4
element array, you can have pretty big errors in phase on transmit
without changing the forward gain much (30 degree phase error on one
element might give you a 1dB change). But a 30 degree phase error on
receive could turn a -30dB null into a -7dB one..

And for that, the lower loss of your elevated radials might make things
"pickier".. that is, as frequency or surroundings change, the reactive
term for each element changes, which could change the power distribution
and phasing among the elements (depending on the feed system used).
(obviously, one of the "current forcing" drive schemes would be less
sensitive to this)



You mention the modelling tools, I am not so much concerned as to
whether the elevated radials model is good, but whether the NEC4 buried
radials model is good, and likewise for radials on and just above the
ground because those models are setting the benchmark for the
performance of the alternative.

The modeling performance of NEC4 for buried wires and wires just above
the surface is quite good. Where I would be suspicious is for a wire ON
the surface or partly embedded in the surface.

Look for that paper by Burke and Poggio on validating NEC3 and NEC4 (it
was published at some conference in Ankara Turkey)