[Heathkit] Differntial earth

[Bill Guyger]

Hi All

Wide flat ground busses good, wire jumpers even substantial size bad. At work we use 4" copper strap to buss all transmitters, ATU's and the like to the ground system. At home, I ripped off Poly phaser's entry panel idea and built a "Billy Phaser" in the window of the room I'm in. It's a 4" Cu plate 1/8" thick set into a wooden frame that the window sash closes onto rather than the sill. It's connected to my ground system with 6" Copper strap (since the window is up about 8'). 

The ground system is 3 rods 5' apart bonded together with 3 ga. bare copper below ground level and about 150" of 12 ga. copper radials run out into the yard also bonded to the 3 ga. with all bonds being brazed connections using 15 % Silver rods. The inside ground buss is 2" x 1/4" Copper bar stock mounted on some stand off insulators I had, and connected to the entry panel with more 6" strap. All equipment is bonded to the 2" buss bar with some 1" heavy braid I scored from a friend. I can't under score enough having tight connections. I use stainless hardware everywhere with copper gel between all copper parts (another idea I stole from Poly Phaser).

At the dv/dt or di/dt involved with lightning surface effect is king. Wide flat conductors are the only way to fly since they are almost all surface and no core for eddy currents to come into play. I was lucky to have a friend with 2 rolls of  1" heavy duty braid, but as Steve mentioned, RG-8 / 213 braid should work just peachy.

I haven't had any problems so far, but the best laid plans and all that.

Sorry about your problems Duane, especially after your roof issues.

73's to all!

Bill AD5OL

>>> "Steve Beveridge" <steve.b at internode.on.net> 02/04/06 05:15PM >>>

Greetings folks,
Everything seems to be right, but the problem was probably inside the
"boxes". In effect, poor earth connections between boards within the radio,
etc. can mean that there are small (probably very small) resistances between
the original earth and the "earth" which the board sees. At the sort of
potentials associated with lightning strikes, this means that there is a PD
between true earth and the earth as seen by the board - thus a 'differential
earth'. We know that PCB screws come loose - as may be seen by the myriad
'fixes' which simply involve re-tightening PCB screws.

A colleague (ex RF design and manufacturing engineer) always uses the outer
braid of RG58 to earth boards within his radios. Doesn't rely on the typical
push-on connectors. Sounds like a lot of hard work, but it seems to do the
trick.

Cheers
Steve
VK2XWL


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