Jim wrote:
...snip...
> Interestingly, the gain tends to run around -1.1 dBi for all cases, with
> maybe 0.2 dB difference between the cases.
>
> Also interestingly, just like the authors of NEC claimed in their report,
> adding a ground stake (or, at least, a connection to ground under the
> antenna) makes things worse. This kind of thing is precisely one of the
> things they wanted to model with NEC4 when it was written. I haven't
> compared my modeling approach with theirs, though, so my results (ground
> stake wise) shouldn't be taken seriously yet.
>
> Anyone aware of any similar modeling or testing that has been done,
> specifically to compare subobtimal or marginal grounding systems.
>
> Jim, W6RMK
Jim,
I have done some testing with radials, ground stakes, and the like here. The
literature that discusses the topic at length echos your model's results
regarding gain. Very little difference in gain per se.
I also did an experiment here when I installed my bi-directional beverage. I
needed decent terminations at both ends and doing the 1/4wL radial trick was
not going to work because the location precluded it (not enough space). So I
took a 22' stick of fence post top rail--galvanized--and resonated it on the
various bands using a collection of inductors wound on toroids. I tried 160M,
80M and 40M. The object was to compare the model results over perfect ground
with what I found with the setup I was using.
On topband, a pair of ground stakes in my loamy-over-wet-clay soil, 10' apart
and connected by a #10 bare copper wire, showed about 30 ohms on 160M, ~ 55-60
ohms on 80M, and somewhat higher on 40M. I then compared this to the table on
W8JI's website.
http://www.w8ji.com/ground_resistance_measurements.htm
Comparing Tom's table to my results, I found that the 2 - 8' ground rods
performed about the same as 7 radials and 3 ground rods in Tom's soils. Your
mileage will vary depending on the soils nearby.
Now, stabilizing a ground for a receive antenna is one thing. Getting ground
to conduct current is another.
Last weekend, we installed a 67' high Rohn 20 vertical stinger with 8 - 1/4wL
radials--no ground rod. These were aluminum radials of 12.5 gauge. It was
very irratic to measure impedances, and it resonated at 4MHz! The ground was
not stable at all. We should have supplimented it with a ground stake.
We also installed 3 - 40M verticals in a triangle array. All three ground
mounted. Again, 8 radials were used on each stick. Very irratic ground again.
I'm going to revisit all of this using an elevated version of the verticals,
which should play MUCH better.
Ford-N0FP
ford@cmgate.com
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