[TowerTalk] Clamp on ground resistance meter

[Grant Saviers]

Approximately.  The meter is measuring the sampled leg against the 
parallel resistance of the other two legs (if there is no other 
connection).  So if the reference parallel resistance is low then the 
error for the measured leg is low. (but always positive)

Measuring    Against

r1		r2||r3  ie r2*r3/(r2+r3)
r2		r1||r3  etc
r3		r1||r2

where || means parallel.  In my ground field, none of the leg 
resistances are the same, so there isn't a simplification of the 
equations for me.

If a more accurate result is desired, then if you want to mess with the 
algebra substitutions, the 3 simultaneous equations have three variables 
so are solvable.  I've searched for the handy web app to do these calcs, 
but not found one.

As if this is not hard enough, the algebra is very messy with more than 
3 legs.  I had a 630 sq ft Ufer + service entrance connection to my 
tower so think my measurement error for 6 radials is quite small.  So if 
your tower also has a similar connection, the error should be low when 
that connection is made.



Note that if the full field is installed, connected, and the rod is 
accessible below the #2 wire exothermic weld then a clamp meter 
measurement rod by rod will be against the N other rods and there should 
be little error.

Any ground to your shack - coax shield, rotator ground, ground lead, 
surge protection, etc should be a fairly low ground resistance.  Which 
is one of the reasons thought needs to be given to the actual circuit 
being measured, it is really easy to be measuring a copper loop.  eg my 
tower ground wire to the shack against the coax shields bonded to the 
tower.  Any resistance measurement less than 10 ohms is a reason to draw 
out the circuit, IMO.

Grant KZ1W

On 10/5/2019 1:51 PM, John Simmons wrote:
> Can I infer that a 3-legged tower with 3 ground wires, each with 
> multiple rods would have a total ground resistance of each wire's 
> resistance in parallel? I.E. 1/R wire1, etc?
> 
> -de John NI0K
> 
> Grant Saviers wrote on 10/2/2019 3:46 PM:
>> The loop you want to measure is the single rod through the earth to 
>> multiple rods or a very low R ground such as a building Ufer.
>>
>> Start with two rods connected with a wire, put the meter on that wire, 
>> measure R. The assumption is that each rod has a resistance of R/2. 
>> When you install the 10th rod the assumption is that the nine already 
>> placed, connected together, and connected by a single wire to rod #10 
>> have a much lower parallel resistance than the new rod. Of course 
>> there is some residual error, but if you go back and measure them one 
>> at a time, one against nine, you will get close approximate 
>> resistances of each rod.?? Then these values can be computed as 
>> parallel resistances for a close approximation of the net ground field 
>> resistance.
>>
>> This is why a star layout of rods and connecting wires that looks like 
>> a vertical antenna radial field is easy to measure, one rod at a time. 
>> OTOH, if you have a perimeter wire, the meter will measure the wire 
>> resistance of that loop.
>>
>> Plus such a layout is the way to minimize the paralleled inductance.
>>
>> Grant KZ1W
>>
>> On 10/2/2019 11:59 AM, Wes wrote:
>>> Hmmm.. Every other reference I see about using one of these says that 
>>> a loop is required.
>>>
>>>
>>> On 10/2/2019 11:00 AM, Bryan Fields wrote:
>>>> On 9/29/19 2:51 PM, Julio Peralta via TowerTalk wrote:
>>>>> Does anyone have any experience using a clamp on ground resistance 
>>>>> meter?
>>>> Yes, they work well.
>>>>> I bought one to use to help with my grounding project but don't 
>>>>> have any
>>>>> experience using this type of meter. Any advice would be appreciated.
>>>> Ensure it's calibrated every time you use it.?? You should ensure 
>>>> it's not
>>>> measuring a loop, that will give errors.?? Single point or off a 
>>>> master ground
>>>> bar feed is the right place to measure it.
>>>>
>>>> I have this model:https://www.amazon.com/gp/product/B01N7FSF6N
>>>>
>>>> Was a steal for 300 dollars.?? I've used it at my home and numerous 
>>>> sites to
>>>> ensure the building ground is sufficient.
>>>>
>>>> 73's
>>>
>>>
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