Dave,
To determine your worst case ground resistance, you’ll want to get ahold of
a local geotechnical engineer and ask them to come do it for you. You’ll
want it done during the driest time of the year so you get the worst case
scenario. They’ll need to use the Werner 4 pin resistivity method. Some may
not know what you’re talking about so call around. If I lived in Florida
and not Utah I’d come do it for you. I do it for cell towers and commercial
antenna installations and the power company quite frequently, I’ll test for
a ham for free. Even if your local geotechnical engineer isn’t a fellow
ham, it shouldn’t be too expensive, it takes about 30-45 minutes to do the
test, and maybe 30 minutes worth paperwork. Unfortunately there are some
tables that show basic resistivity numbers that some people use, but when
their electronics get fried they wonder why. I’d recommend getting the
actual value for your condition. I’ve seen neighboring properties with
similar soils have different resistivity values, so obviously the tables
are not correct, and aren’t very conservative either. In my opinion most
hams have a severely undersized lightning protection system. I’ve never
seen a commercial installation with as small of a system as the largest ham
system I’ve seen. They do oversize commercial installations by about 1.5 to
2 times because the cost of having the radios go down far outweighs the
cost of a bigger lightening protection system. But I’ve seen commercial
equipment get fried as well.
Personally, I think the best protection is to disconnect the antennas from
the radios and completely ground them during a storm. But all my antennas
are home made so the cost is nothing compared to the radios, and I always
have spares I can string up on my tower fairly quickly. But the grounding
of the antennas is tied into the lightening arrestor system. And a good
lightening protection system is worth its weight in gold.
Brian
KF7OVD
On Thursday, September 9, 2021, Artek Manuals <Manuals@artekmanuals.com>
wrote:
> I 've read a lot of articles over the years about ground systems and how
> to build a good one. Most of them ignore actual soil conditions and few
> talk about the differences between "DC/AC (Low Freq)/Lightning" and RF
> ground vs frequency
>
> But how do I actually measure it ? And once I have this number how good
> is "good enough"
>
> Take my situation I live on ancient sandbar 65' above sea level In Florida
> . Now this is practically a mountain in Florida and the soil is so poor
> that below about 6" nothing but granular quartz exists, mostly not even
> roots, a desert with a lot a rainfall. The soil is so soft that I can push
> the first 5' of a 10' ground rod in by hand. I often wonder why my house
> doesn't sink into it and from a phenomena called "sink holes" a few house
> do.
>
> So to coin a take on the old light bulb joke " How many engineers with 10'
> ground rods does it take to make a good ground (RF ground in this case@
> 1.8 Mhz) and how will they know when they have enough 10' rods"
>
> Dave
> NR1DX
>
>
> --
> Dave Manuals@ArtekManuals.com www.ArtekManuals.com
>
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