Hi Bob,
One of the things that has been difficult for me to understand since
becoming a ham is the difference between micro electronics and (for
lack of a better term) macro electronics. When the leads are 0.25"
thick and feet long, the potential difference at the ends of a wire is
significant, and if there is more than 1 wire the varying impedances
can cause currents where you don't think there should be any.
Because of this, what I used to understand as a single "ground" (e.g.
a layer on a PC board) is really 4 separate things in a ham shack. The
ground is all electrically a single circuit, and all drain current,
but because they have human-scale sizes, different currents will take
different paths due to the differing impedances from source to drain
in the circuit. So while your house ground and your ground rod may
both be buried in the ground, they may have significant differences in
potential.
This difference is how everything in your house can suddenly become
"hot". They are hot because, relative to the different ground
potentials, there is a voltage change. The grounds also both have
huge current carrying capacity, which can kill you.
An excellent description of these different grounds is found in the
2010 ARRL Handbook:
begin quote
28.1.8 Grounds
As hams we are concerned with at least four kinds of things called
"ground," even if they really aren't ground in the sense of connection
to the Earth. These are easily confused because we call each of them
"ground."
1) Electrical safety ground (bonding)
2) RF return (antenna ground)
3) Common reference potential (chassis ground)
4) Lightning and transient dissipation ground
IEEE Std 1100-2005 (also known as the "Emerald Book," see the
Reference listing, section 28.1.13) provides detailed information from
a theoretical and practical standpoint for grounding and powering
electrical equipment, including lightning protection and RF EMI/EMC
concerns. It's expensive to buy but is available through libraries.
end quote.
The National Electric Code requires that all ground circuits be bonded
together at the service panel entrance to prevent electrocution.
Inside the house, the "green wire" is used by ground fault protection
circuit. Your ground rod with it's short connection to your shack will
provide a lower impedance path for RF return and common reference
potential for your equipment (and help keep stray RF currents out of
the rest of the house). Lightning ... well, mother nature is one
tough cookie.
Again, the ARRL Handbook has excellent information on grounding and
shack safety in general. I have also been studying Jim K9YC's papers
on RFI, which includes extensive information on the many strange ways
RF currents can get to where we do not want them.
http://audiosystemsgroup.com/RFI-Ham.pdf
I'm sure someone will correct any egregious errors I have made above,
but I hope I got the salient point across: all your ground circuits
must be bonded to the service entrance panel or you risk serious
injury.
73, Byron N6NUL
----
- Northern California Contest Club
- CU in the 2011 Cal QSO Party 1-2 Oct 2011
- www.cqp.org
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
