Meters & Grounds

A week or so back, someone asked about converting DC meters to
read AC (rf).  SHort answer: connect the meter inside a bridge
rectifier.  Long answer:

            |  + |    |
	----+    M    +------
            |    |    |

Chose didoes to suit.

In a "tesla" enviroment, more thn slight risk of blowing the diodes...

Short version:
Variation of ground resistance with temperature, with references.

Electrical Protection Guide for Land Based Radio Facilities.
Joslyn Electronic Systems

[They mean LIGHTNING....]  68 pp.
Joslyn works in an unforgiving bizness.  Keeping radio stations
and their equipment intact thru lightning storms, including
direct hits.  Bot it just for fun in a second hand book store.[1]

A year or so ago, some observations, indicating coild changing
tune during operation were reported about this time.  I did some
handwaving, proposing frozen grounds, changing characteristics
as current from the base of the coil thawed them out.[2]  More
exerienced opinions were that i was a minor effect. 

Section 5.1, and accompanying figure 15 are of interest.

	Freezing greatly increases the resistivity [3] of soil.
	Consequently it is advisable to have a substantial part of
	any electrode system buried at a depth below the frost line.

	It is desirable that the ground .... not deteriorate
	significantly druing cold weather.    ...freezing of
	the earth around an electrode will increase its resistance
	to remote earth and ... degrade its ... effectiveness.

	... such effects are frequently overlooked.  Grounds are
	usually construicted and measured during temperate weather.
	Typical measurements ... may give a mistaken view of their

Table i extracted from text:
	Temp  Resistivity[3]
	60F	82	32
	33F	135	51
	31F	330	128

Figure15, in graphical form:

	Temp	Resistivity
	68F	72
	32.1F	140
	31.9F	300
	14F	3300

(Its not clear if these are intended to represent the same data...)
They go on to reccomend a minimum depth of 20" or so.
The actual value of the soil resistance/resistivity will vary with
moisture content, with dry soild varying less warm/cold than wet ones.

The bibliography alone makes interesting reading.  The temperature
section cites particularly:

Grounding Electric Circuits Effectively
J R Eaton Bulletin 25T2
James G Biddle Co, Philadelphia PA

Impulse and 60 cycle characteristics of Driven Grounds
P L Bellaschi, P L Armington & A E Snowden, Part II
AIEE Transactions, Vol. LXI, 1942.
(Hmmmm.  I have some of those.  MAYBE....)

As near as i can tell, Lightning's typical spectrum is pretty much
dead center in the usual coilng frequencies, ca 100-200 KHz.  I
believe this data to be applicable to coiling.  It IS copyrighted,
so i won't offer to do copies.  Cost $3 in 1971.  Their details
of ground construction match what i see reccomended here.

OK.  I have a WIERD definition of fun.

I think thinking about this made me conscious of something about
grounded Tesla Coil operation:
	Just as the VOLTAGE multiplies and peaks at the top, the
	CURRENT peaks (and is multiplied) at the base.  hmmmm
(I know.  Old News.)

First approximation: think of it as resistance.  In detail, it is
an electrical engineerng unit, in ohm-meters, that presents the
resistance of a material INDEPENDENT of the size of the sample.
The third column in the table is the actual resistance measured to
a given rod.