Reg Edwards wrote:
Radiation from the feedline is usually the least of one's problems.

99 % of RFI is due to radiation from the very nearby antenna.


That's rubbish - have you ever *measured* the RF currents in your
station wiring?

....no, I thought not.



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

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

That's an interesting statistic. How did you arrive at it?

Roy Lewallen, W7EL

Reg Edwards wrote:
Radiation from the feedline is usually the least of one's problems.

99 % of RFI is due to radiation from the very nearby antenna.

==========================================

"Ian White GM3SEK" wrote
Reg Edwards wrote:
Radiation from the feedline is usually the least of one's problems.

99 % of RFI is due to radiation from the very nearby antenna.


That's rubbish - have you ever *measured* the RF currents in your
station wiring?

==========================================

No, but I HAVE measured RF current in the antenna - from where most
RFI originates.

Have you ever run 400 watts from an 80 meter 1/2-wave dipole a few
feet above your roof and not far from the TV aerial?
----
Reg.

Roy!

Just a legitimate crude guess - perhaps biassed a little in the
opposite direction to the normal excessive bias.

If I remember correctly, you, your very good self, used Eznec to
demonstrate that even under the most adverse worst-case conditions you
could think of, power radiated from the feedline is only a fraction of
that radiated from the antenna.

.. . . . . and the equipment being interfered with is in the
near-field of the antenna just as it is in the near-field of the
feedline. That, for most people, includes your next-door neighbors.
Especially if your next-door neighbors happen to be within the antenna
beam.
----
Reg.

============================================

"Roy Lewallen" wrote
That's an interesting statistic. How did you arrive at it?

Roy Lewallen, W7EL

Reg Edwards wrote:
Radiation from the feedline is usually the least of one's
problems.

99 % of RFI is due to radiation from the very nearby antenna.

Reg Edwards wrote:

"Ian White GM3SEK" wrote
Reg Edwards wrote:
Radiation from the feedline is usually the least of one's problems.

99 % of RFI is due to radiation from the very nearby antenna.


That's rubbish - have you ever *measured* the RF currents in your
station wiring?

==========================================

No, but I HAVE measured RF current in the antenna - from where most
RFI originates.

Can't you see the circularity of your own 'logic'? If you haven't
measured common-mode RF current in the feedline as well, you don't
actually have a clue where the interference is originating. All you have
is an unsubstantiated opinion.


Have you ever run 400 watts from an 80 meter 1/2-wave dipole a few
feet above your roof and not far from the TV aerial?

I'm not disputing that RFI very often originates from the antenna - of
course it does (and the question you pose is a set-up, designed to make
*sure* it does).

What I object to is your sweeping generalization that "99 % of RFI is
due to radiation from the very nearby antenna." When absolute nonsense
is delivered with absolute confidence, some unfortunate beginner might
actually believe it.

However, since you asked...

In a typical small British back garden, I have been forced to do
something very similar to what you describe. Sure enough, it caused RFI,
but the interference was *not* due to my antenna! It was entirely
mains-borne, and only to the two neighbours (not the closest) who were
connected to the same phase as ourselves. The cause of the interference
was that a substantial fraction of my RF ground current was going into
the mains ground. Once I'd cleaned up my act and stopped injecting RF
current into the mains, the interference stopped too.

The high field strength from my antenna wasn't a problem at all, and the
neighbours continued to watch their TVs until I took the antenna down.

That's how it was in that one particular case.... but unlike you, I'm
not claiming it represents 99% of anything.

With RFI, the only safe generalization is to keep your mind open to
*all* possibilities.


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

Reg Edwards wrote:
Radiation from the feedline is usually the least of one's problems.

99 % of RFI is due to radiation from the very nearby antenna.

In some cases, that nearby antenna is a counterpoise
laid across the floor which some people consider to
be a "ground" of sorts. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Reg Edwards wrote:
Radiation from the feedline is usually the least of one's problems.

99 % of RFI is due to radiation from the very nearby antenna.

==========================================


REALLY??

I thought most RFI issues, in today's world, are related to lack of
selectivity in the device experiencing the interference. RFI is
generally a susceptibility in the interefered with device. Read ALL the
information provided and you should find a manufacturer's disclaimer.

If the transmitted signal is 'clean' [meets applicable standards] then
the cause of RFI is SUSCEPTIBILITY in devices designed for MAXIMUM profit.

I'm not responsible for susceptibility issues. A neighbor complained
that my station was coming through his computer speakers. I invited him
to my station and turned on my computer and speakers. At 1 KW output
there was/is no RFI in my speakers !!! [My computer is co-located with
my station]. I suggested that he contact the manufacturer of the
speakers and obtain information to reduce susceptibility. I showed him
how I solved the susceptibility issue for my computer. The
responsibility is now his.

On Tue, 3 Jan 2006 14:44:01 +0000, Ian White GM3SEK
wrote:

Can't you see the circularity of your own 'logic'? If you haven't
measured common-mode RF current in the feedline as well, you don't
actually have a clue where the interference is originating. All you have
is an unsubstantiated opinion.

Ian, Ian you should know by now you're dealing with the "Prince of
Factoids".

Danny, K6MHE




email: k6mheatarrldotnet
http://www.k6mhe.com/

On Mon, 02 Jan 2006 20:30:21 -0800, Bill Turner
wrote:

Even a good
dipole will beat a longwire worked against ground.


Not always!

Danny, K6MHE



email: k6mheatarrldotnet
http://www.k6mhe.com/