[TowerTalk] [RFI] Back of desk grounding buss
Jim Lux
jimlux at earthlink.net
Fri Mar 21 09:06:00 EDT 2014
On 3/20/14 10:34 PM, Jim Brown wrote:
> All of the replies I've seen here are obsessing on the wrong topic. By
> all means DO take care to avoid oxidation and dissimilar metals
> problems. But that ground bar, whatever it is, is NOT an important
> element of what you're doing, which is primarily LIGHTNING SUPPRESSION,
> but also minimizing hum, buzz, and RFI.
>
> What matters is that EVERY CHASSIS in your shack be bonded together by
> short fat copper, and that combination of chassis bonded to everything
> in your house by short fat copper.
I agree on bonding, but not on fat. At RF frequencies, the inductance
(1 uH/meter) will dominate over the AC resistance. At 1 MHz (lightning
frequencies) 1 meter of AWG 16 copper wire has a AC resistance of about
0.07 ohms. It has an inductive impedance of 6.3 ohms.
That includes antenna entry, every
> ground rod, the power system ground, the telco ground, the CATV ground,
> the satellite ground, structrural steel ground (if any), and, if your
> tower is close to the house, the tower ground.
>
> You are also forgetting that bonding between equipment should also be
> short, fat copper. When you run wires from each piece of gear to a bar
> at the back of he desk, that usually makes the connection more than
> twice as long. That means double the resistance and double the
> inductance. NOT a good thing.
>
> The BEST bonding method in the shack is short, fat copper from chassis
> to chassis of every piece of gear in the shack, then a bond from one of
> those chassis (preferably the rig) to those other grounds in your home.
> Chassis to chassis bonding minimizes hum, buzz, and RFI. Bonding all the
> grounds together and to earth provides lightning safety.
I'd rather have someone use smaller easier to manipulate wire that is
short, than big fat stiff bars or wires that wind up being long.
And, of course, avoiding loops which can intercept the magnetic field
from a transient flowing in another conductor.
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