[TowerTalk] Bracing, grounding

[Roger (K8RI) on TT]

On 5/26/2014 9:14 AM, Jim Lux wrote:
> On 5/26/14, 5:49 AM, Mike Reublin NF4L wrote:
>> The base for my crank-up tilt-over 70' tower is poured in a 48"
>> diameter 10' deep steel culvert. My ground wires are attached to the
>> rebar and were also supposed to have been attached to the culvert,
>> but were not (long sad story). It would be very very difficult to
>> attach them to the culvert now.
>>
>> I will drive 2 or 3 rods out from each leg. Should the first one be
>> next to the base, or a couple of rod lengths away?
>>
>
I agree with Jim...mostly  You will have a good ground and are fast 
approaching the point of diminishing returns.
My system would be considered overkill by many. I don't, but it is far 
more elaborate than necessary.  I have 32 or 33 8' ground rods, 
CadWelded to  a network of over 600 feet of bare #2.

The tower was taking an average of 3 visually verified, direct hits a 
year for 5 years and 2 hits the last year it was hit. AFAIK the tower 
has not been struck in over 10 years.  There has been no damage to 
equipment since the ground system was finished.  IIRC I lost a 2-meter 
rig and a brand new computer with a single strike the first year the 
tower was up and before the ground system was finished.

My individual ground runs are about 80' long.  Figuring the average rise 
time, that should still be working, but area is far more important than 
length or number of rods.  I agree with Jim in that the effectiveness 
falls off rapidly with length.  OTOH Lightening is anything except 
consistent. Weaker strikes are likely to have slower rise time and the 
longer runs will be more effective, while the monsters have faster rise 
times and area is all important. It's likely that anything we do as hams 
will protect us from a "super strike", but thankfully they are rare.

I put the large system in because I had the copper on hand and it was 
relatively cheap..."Back then".  It'd be very expensive now days, on the 
order of $3.00 a pound for scrap.  I don't remember the price back then, 
other than new, bare #2 was a fraction of today's scrap price. It got to 
a point where they'd steal the pipes and wire out of unfinished homes.  
That's when builders went to only piping and wiring walls they could 
close before the end of the day.


>
> The concrete inside the culvert already gives you a better path to 
> "earth" than a few rods. A rod driven next to the base would make 
> almost no difference.

Compare the surface area of that culvert to the area of a ground rod. 
It's not a direct conversion, but that's about how many ground rods 
(plus wire) it'd take to equal the culvert,

>
>
> You'd have to get pretty far away before it would make a difference 
> (the usual "spacing of the rods" rule of thumb is for rods that are 
> very thin).  By the time you get 20-30 feet away, the series 
> inductance of the wire going to the rod means that it's not going have 
> much effect.

I do think 20 - 30 feet is still pretty effective, but area is still 
more effective in most cases I can think of.
>
> If you want to reduce "step potential" (the voltage gradient on the 
> surface), you'd be better off using the money to bury radials, which 
> has the useful side effect of increasing the apparent soil 
> conductivity for HF frequencies (and reducing "ground losses" if you 
> operate your tower as a vertical).  If you'll never use your tower as 
> a vertical, then radials aren't all that useful either.
>
> You've got a great grounding electrode now.
>
>> My raising scheme is a length of 4" diameter pipe embedded 3.5' in
>> the concrete a yard behind the tower. Two diagonal braces will run
>> from the pipe to the front leg mounts.
>>
>> Will 2" water pipe work? Sched 40 or 80? Angle iron?

It depends on the weight and how far it reaches beyond the lifting point.
Water pipe is not normally considered a good structural material. It's 
relatively soft, easily bent, and heavy for strength.  It would work if 
the load is not great.

Good luck,

73,

Roger (K8RI)

>>
>> 73, Mike NF4L _______________________________________________
>>
>>
>>
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>
> r
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