On 01/10/10 07:44, Jim Lux wrote:
Owen wrote:
On 01/10/10 03:51, Jim Lux wrote:

Thanks Jim.


I would think that the buried radials are more convenient (broad band,
etc.)

Yes, I understand that there are advantages to buried radials, but I
don't understand the preponderance of cases where I see 120 radials
pinned on the top of infertile dirt. They still present a trip hazard,
and less money spent on just a few elevated radials may perform just
as well.

never underestimate the power of tradition. It was written by BL&E that
120 radials work, and the FCC accepts that for broadcast, so by golly,
that's what we do. Why 120? it was at the point of diminishing returns
or practicality back when the study was done (e.g. there was no
detectable change from going to more)

As for laying on ground.. I think that's more the laying on grass, and
eventually, the wire sinks into the grass/turf.

There's also the whole "the radials must be resonant" misconception..



Look at the performance of your ankle biting radials when the dimensions
are changed slightly.. For instance, if you shorten them by 5%, does it
make a big difference? For the buried radials, the length is very, very
non critical.

Yes, of course the feedpoint impedance is more sensitive to change in
length or conversely change in frequency.

While for a buried radial system (probably because of the losses) it's
going to be less frequency sensitive.

I expect so.




Something else to look at is the sensitivity of "efficiency" (and your
definition of radiated power in the hemisphere/power into antenna is
fine) to soil properties.. if the soil conductivity or epsilon changes
(as it will with changing water content) does the efficiency change
rapidly?

Yes, efficiency is sensitive to soil parameters... for both types, but
not very sensitive.

Maybe less sensitive for the buried radials? Or, it was "good enough"
for BL&E, so being so written, so shall it be done.

BL&E were measuring ground wave, I think solely. My efficiency measure
is the hemisphere, so ground losses play a different role.



Because of the impedance change mentioned above, the impedance
transformation needs adjustment for wide range frequency operation.
Not such an issue in the intended application, the DX window on 80m
here is just 50kHz.




If I haven't got something quite wrong in the modelling, it would seem
worthwhile to prototype the shortened version with a view to extending
the system to a four-square if suitable.

The shortened version will, of course, aggravate the tuning sensitivity.

Yes, but the model suggests that the variation in R is very small, and
variation in VSWR (with shunt coil match) is small... in that band segment.





I have still to read Rudy's papers... I am away from home (less
bandwidth) and I will download them later today when I get home. I
suppose that the proposed design challenges the norm of a very large
number of buried radials. In our case, part of the property is quite
rocky, and a configuration with just a few elevated radials offers
deployment opportunities that aren't suited to buried radials.

So, my original question is no so much suggesting everyone else got it
wrong, but why don't I seem more people doing it this way. Could I be
forgive in thinking that the popular, nearly universal, way is to
uplift the BL&E research at MF and apply it to 80m?

Tradition is a powerful force. Look how many years it took for someone
(e.g. Rudy) to put the substantial work into doing a real quantitative
experiment. For most hams, they're only going to do something once, and
if works ok, that's how it stays. Almost none are going to do a well
controlled A/B study, especially if there's a (not necessarily valid)
tradition that says A works better (where better is ill defined and
probably a combination of radiation efficiency and installation
convenience)

Until recently, modeling tools available to most amateurs were not
suitable for making the call, although there have been some people who
did models and published it, but, in the face of decades of "lay down
120 radials" it was a tough sell.

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.

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.

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.

Owen






Owen