Re: Station With Center-Fed Dipole - Best Grounding Technique?
- From: Jim Lux <james.p.lux@xxxxxxxxxxxx>
- Date: Fri, 01 May 2009 08:48:25 -0700
w5net@xxxxxxxxxx wrote:
Okay, since I'm installing an entirely new setup, what I'm looking for
here is the best way to ground my equipment and a large center-fed
wire dipole.
Lets start with the shack first. The radio and speaker are both
connected to the house ground through their power cords. To provide
better RF grounding, I'm thinking of everything (including radio &
speaker) connected with 1" copper braid to two salted (for increased
conductivity) ground rods just outside. Is that sufficient?
Maybe a few questions might help clarify what you're trying to do, and what your objectives are.
What is connecting the radio and speaker to an earth ground doing? Are you hoping to provide an alternate path if the AC line shorts to the case inside? The green-wire ground does that very nicely.
Are you thinking that RF currents will flow from radio chassis to ground? The RF current should be in the antenna feedline, only.
In general, an earth ground for your equipment isn't needed.
(Consider this.. an airplane has lots of radio gear, external antennas, etc. and gets hit by lightning, and still keeps going, and there's no connection to earth ground)
Next, to reduce the likelihood of lightning reaching the shack, the
only thing I can think of is a lightning arrester inline to two
additional salted ground rods, with the coax disconnected when not
used. I don't believe RF is an issue with a dipole, so is this
sufficient for lighting protection?
Do you care about electrical code compliance?
There are specific rules in Article 800 of the NEC.
There's several issues here. What's the overall goal? protect the house from burning down (but sacrifice the gear if needed) or protect the gear at all costs? Protecting against direct strike on antenna or induced voltages from nearby strikes?
House burning down is handled by using a listed antenna discharge unit (ADU) connected to a suitable lightning dissipation ground. (salted rods are not a suitable lightning dissipation ground, but more anon) The ADU has to be mounted outside, close to the point of entry into the house.
The ADU will make sure that most of the lightning current is safely conducted through something other than the flammable stuff of which your house is built. But, it might not save your gear (for a variety of reasons).
If you want to have a better chance of saving the gear (aside from disconnecting, in which case you need to short the incoming antenna connections to the lightning ground, as well.. don't just leave it open), you want to do what is good practice for EMI/EMC anyway. Have everything referred to a common potential. In a sort of idealized scheme, you'd have your AC receptacle mounted on a conductive plate which also has the antenna feedthrough. The plate is bonded to the electrical service ground (so the breaker trips if there's a short inside your gear) and is also connected to the lightning ground (since that's where the antenna surge is going to go). A listed transient suppressor is connected to the AC line and the grounded plate as well.
Now, all the gear (radio, speaker, etc.) is all connected to this one plate, so if lightning hits, and there's a many kilovolt drop in the grounding wire, everything goes together, and you've minimized the voltage between any two conductors connected to the radio.
It's the airplane thing... the lightning hits the airplane, but everything inside the airplane stays at the same potential relative to itself so there's no problem.
You'll hear a lot about needing a big flat ribbon for lightning ground. Maybe, maybe not. Here's why. If it's any length at all, the lower inductance of the flat ribbon is still high enough that the voltage rise at the panel (L di/dt) from the lightning impulse is still hundreds or thousands of volts. If your system can take 1000V rise, it can probably take 5000V, so the absolute rise isn't as important. And, a higher inductance, higher resistance conductor actually slows down the transient a bit, which reduces the currents induced by the magnetic field. Lightning may be a huge current, but it's short duration, so it actually doesn't take a very big wire to carry it (AWG 10, for instance) without melting.
The key here is to understand what you're trying to do, and how you're doing it. Don't blindly assume that since radiotelegraph stations with beverages and longwires in 1920 used big ground rods that's what you should be doing in your shack today.
---
Now.. about rods, ground, salting, etc.
Your ground should be a concrete encased grounding electrode (aka Ufer ground), which is what the electrical code requires. Rods can be used for supplemental grounds, but you have to worry about making suitable connections (exothermic welds, etc.), the soil drying out, and the generally high resistance of ground rods. Salting eventually washes out, so isn't worthwhile in most cases. There are soil additives (e.g. bentonite) that are used in connection with grounding, but they work by being very hygroscopic and keeping the soil conductivity high by keeping water in the soil. But overall, the Ufer ground is your friend: low resistance, reliable, large area (so current density is low.. no "smoking rods").
The code requires that ALL grounds be bonded together with a suitable conductor.
.
Give me your opinions. Am I missing something? Is there a better way?
stewart / w5net
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