Roy Lewallen wrote:
wrote:
Unfortunately, grounds are sometimes necessary (neighbor, TVI/RFI),
even with dipoles/loops. Wish it weren't, I'm lazy...
'Doc
Hummmm...Maybe, but I'm having a hard time of thinking
of the uses of a ground to cure said problems... Most
of those problems would seem to be better cured using chokes,
etc. If the problem is fundamental overload to their gear, any
grounding on your end won't cure that. MK

If you have an imbalance current trying to find its way to ground,
it'll take the path of least resistance (technically, impedance). If
that path is the mains wiring, you have a lot of potential for RFI. If
you can convince some of that current to go elsewhere by "grounding"
your station, you're likely to cut down the RFI. But a better solution
is to get those feedline currents balanced so you won't have any
imbalance or "ground" current to deal with in the first place. It
brings the added benefit of putting the power into your antenna to be
radiated rather than being radiated from the conductors carrying the
imbalance current.

Sometimes the improvements to the antenna and feedline are not enough to
prevent interference. Also, RF grounding may be ineffective because it's
impedance is too high to successfully shunt the ground current away from
the mains wiring.

However, there is also a third option: use a series RF choke in the
station mains feed. This can have two beneficial effects. It keeps the
ground currents out of the mains where they can cause interference, and
it can also reduce the incoming common-mode current on the feedline.

A suitable mains choke for the whole station can be made by winding
*all* of the mains conductors (live, neutral and safety ground) on a
stack of ferrite rings. It's necessary to choke all the mains wires
because they are capacitively coupled together at RF. The choke is a
high impedance at RF, but the safety ground wire is continuous through
the choke. Alternatively you could thread all the wires through a string
of large ferrite beads. Of course it's no coincidence that these mains
chokes look a lot like the chokes you'd use on a feedline.

An alternative is to use a commercial three-wire mains filter, which has
an RF choke in the ground conductor, as well as the normal pi-filter in
the power conductors. (I wouldn't recommend a homebrew mains filter. The
safety and code compliance issues are better left to full-time
specialists.)

Another essential is to organize the mains wiring of the whole station
so that *all* the mains feeds and mains ground connections pass through
the RF choke. (This goes along with the established safety
recommendation to have one Big Red Switch supplying mains to the whole
shack.) If there is a sneak path to ground that doesn't go through the
station mains choke, then of course the RF current will take that easier
path and the choke will be ineffective.

A clip-on RF current meter is an excellent trouble-shooting tool. There
are constructional details on my 'In Practice' pages, and MFJ sell two
different ready-made models.

When you have installed a station mains choke and eliminated all sneak
ground paths, the RF current meter will show you two things:

1. Without an RF ground, the incoming common-mode currents should be
significantly lower than before. In simple terms, common-mode feedline
currents cannot enter the shack if you have choked off their exit path
to ground.

If you try to look at the situation in more technical detail, there are
more unknowns (such as distributed capacitance to ground) than you can
accurately identify. That's why RFI is so hard to predict and generalize
about. Therefore the best approach is always to *measure* the RF
currents on all entering and exiting conductors, before and after each
modification you try.

2. An RF ground connection may make the common-mode currents *worse* by
providing a path through the station. If that is the case, then don't
use an RF ground - it isn't compulsory to have one. (You still have to
think about lightning protection, but that most certainly does *not*
involve routeing the lightning currents to ground through your shack!)

There are three good articles on RF grounding in the public area of the
ARRL website:
http://www.arrl.org/tis/info/grounding.html



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek