Like MFJ's 931 Artificial Ground ?

[Able2fly]

Is there a kit version of the "artificial ground" available? Even a  
schematic would be nice. I would assume that its just a series tuned L/C with a  meter
to measure RF current somehow, but if anyone has come up with a version of  
their own, or knows of a kit, I'd like to hear about it.
 
Thanks, Bill  K3UJ
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[Mychael Morohovich]

Hi, Bill-

Unless I am mistaken, there is supposedly a schematic available for the
MFJ-931 in Devoldere's "Antenna and Techniques for Low-Band DXing".

I have the 931 and use if with my EFW, both in the shack as well as while
portable. It works well for me, plus one cannot have too many gadgets
containing wiggly meters hanging around- hi. If you cannot get a copy of the
schematic, let me know off list and I'll open up my unit and trace something
out for you. As I recall, it is a simple variable L/C network whose output
passes through a toroidal transformer that provides current info to the
front panel meter. You could of course dispense with the meter and use a lil
light bulb to help tune for maximum current.

73,

Mychael/AA3WF
K2#1025


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[Michael Melland, W9WIS-2]

Ten-Tec has a kit version ($59) that in my experience works even better then
the MFJ....

http://www.tentec.com/Tk1251.htm

Mike, W9WIS


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[George, W5YR]

At the risk of seeming to rain upon the parade, let's keep in mind that the
931 can at most provide a low-impedance path to "ground." Whatever that
"ground" happens to be is what determines the success or failure of using
the 931.

In almost all cases, adding a simple ground connection to a station with "rf
in the shack" problems is not the solution and may, in fact, make matters
worse.

Very few earth grounds constructed by the typical amateur are very
effective, presenting hundreds of ohms of connection resistance to the soil.

The preferred approach is to find out where the rf is coming from and
correct that situation. All coax cables entering the shack should be
provided with baluns at the antenna feedpoint and again at the shack
entrance if needed. Antennas that place the shack equipment at other than
zero rf potential should be avoided.

Finally, the most effective way to place the equipment at zero rf potential
is the "driven ground." This consists of a 1/4 wave length of wire with one
end connected to what is decided to be the station "ground" point and the
other end unconnected. The wire can lay on the shack floor. It will carry
induced current and radiate and thus is a source of RFI. But the open end
will assume a high r-f potential thereby driving the connected end to very
near zero potential. A wire for each band can be used if needed.

73, George W5YR
Fairview, TX
[hidden email]
http://www.w5yr.com

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[Vic K2VCO]

George, W5YR wrote:

> At the risk of seeming to rain upon the parade, let's keep in mind that the
> 931 can at most provide a low-impedance path to "ground." Whatever that
> "ground" happens to be is what determines the success or failure of using
> the 931.

George is correct.  The way I like to look at is that there are at least
four different 'grounds' that you have to worry about, and you need to
provide for them separately.

1) The electrical safety ground.  This is to prevent shocks in case the
chassis of some equipment becomes 'hot' from the AC line.  Electrical
codes should be followed in shack wiring, all gear should have 3-prong
plugs, and other grounds should be tied into this.

2) The lightning protection ground.  This provides a low-impedance path
for static discharges and lightning strikes to antennas.  Requirements
are quite stringent to do this correctly.

3) The RF ground.  If you have an unbalanced antenna such as a long wire
or vertical monopole, there needs to be a ground return for RF.  This is
what the radials of a vertical and the MFJ 931 are for.  If your antenna
is a dipole or beam with a balun, you probably don't need an RF ground.

4) The 'shack common' ground.  All of your equipment should be connected
with low-impedance straps to a common ground bus (which of course will
be tied to the safety ground).  The purpose of this is to insure that
all the gear will be at the same RF ground potential and reduce the
effect of the unavoidable ground loops created by interconnecting cables.

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco



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[Don Wilhelm-2]

Vic,

You are entirely correct - and I have long advocated thinking about the
'ground' terms separately.  It should be common knowledge (but it isn't)
that one 'ground' can be at a different potential than another 'ground'.  I
learned early in my hamming days because we often referred to 'chassis
ground' which was a common wiring point and often had nothing to do with any
earth ground.  (That was before the time of the 3 wire wall outlets!!!)

73,
Don W3FPR

----- Original Message -----
 > ...  The way I like to look at is that there are at least
> four different 'grounds' that you have to worry about, and you need to
> provide for them separately. ...snip
>


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[Ron D'Eau Claire-2]

That's exactly what the "Artificial Ground" does, George.

When you have a ground wire going to a ground stake it makes a lousy RF
ground, but it'll be a much worse one if the wire to the stake is more than
a couple of feet long because the wire is an appreciable fraction of a
wavelength at HF. If the same connecting wire goes to a wonderful multi-wire
counterpoise, the effectiveness of the counterpoise is badly compromised if
the connection to the rig isn't very short and direct. Using very broad
straps for conductors helps, but they are impractical for many situations.

The "Artificial Ground" is simply a network to tune out the reactance in the
connecting wire so the ground can act at maximum efficiency. It brings a
distant ground "to the rig" electrically. As you said, you need a decent
ground just the same, both for avoiding RF on the rig and for decent
efficiency from a short (1/4 or less) end-fed antenna. But, sometimes, even
a lousy ground like a stake in the soil is enough to eliminate the "RF on
the Rig" problem, if the reactance of the connecting wire is tuned out.

The "Artificial Ground" also allows "tuning" a simple counterpoise for
maximum effectiveness on each band. That's how I use one with excellent
results. It eliminates the need for a 1/4 counterpoise wave wire for each
band in use.

Ron AC7AC

-----Original Message-----
At the risk of seeming to rain upon the parade, let's keep in mind that the
931 can at most provide a low-impedance path to "ground." Whatever that
"ground" happens to be is what determines the success or failure of using
the 931....

...the most effective way to place the equipment at zero rf potential is the
"driven ground." This consists of a 1/4 wave length of wire with one end
connected to what is decided to be the station "ground" point and the other
end unconnected. The wire can lay on the shack floor. It will carry induced
current and radiate and thus is a source of RFI. But the open end will
assume a high r-f potential thereby driving the connected end to very near
zero potential. A wire for each band can be used if needed.

73, George W5YR
Fairview, TX
[hidden email]
http://www.w5yr.com


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[Stuart Rohre]

The nice thing about the Ten Tec kit, is that it can be easily converted to
an "L" network tuner, if you switch the point where the pick up circuit
monitors current, and substitute a coax jack for the connection in one
original hole.  That way, it can be converted back to artificial ground
service by use of a shorting plug, if I remember the scheme I used some
years ago.
-Stuart
K5KVH


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[Jim Brown-10]

On Mon, 02 Aug 2004 10:38:52 -0700, Vic Rosenthal wrote:

>The way I like to look at is that there are at least
>four different 'grounds' that you have to worry about, and you need to
>provide for them separately.

It's OK to think of them as fulfilling four different functions. But from a safety
(and building code) perspective, ALL of these functional grounds MUST be
bonded together. And HOW they are bonded together can make a huge
difference when something goes wrong (like lightning).

Jim Brown K9YC



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[Don Wilhelm-2]

Jim,

I will agree with you wholeheartedly, except for the RF Ground part ---

In reality, RF ground has only to do with a zero impedance reference for RF
at the frequency of operation.  It can actually be up in the air (example -
at the midpoint of a dipole, normally the dipole feedpoint), or at the other
extreme it can be close to a physical groundpoint (like in the shack at a
tuner groundpoint).

It has been said that balanced antennas need no RF ground - I am just saying
it differently - that the RF 'ground' is built into a balanced antenna - or
at the feedpoint in the case of a properly fed vertical, etc.

Let this not be construed to imply that a hamshack does not need a good RF
ground - in fact, every hamshack DOES need an effective ground point for RF
ground, AC safety ground, and lightning protection, AND they should all be
connected together at the hamshack common grounding point.  Additionally,
the best station ground type is the single point ground - where each piece
of equipment has a ground wire going to this common point (like a grounded
entry panel through which all wiring to the shack enters).  If a bus-bar
type ground is used, it should be a very low impedance - as short as
possible and physically large conductors.

A really GOOD lightning protection ground at the shack will by its nature
provide a good high current RF ground too, because to be effective it must
shunt all frequencies to ground - but few hamshacks have the luxury of a
protection system that effective.

73,
Don W3FPR


----- Original Message -----
>
> >The way I like to look at is that there are at least
> >four different 'grounds' that you have to worry about, and you need to
> >provide for them separately.
>
> It's OK to think of them as fulfilling four different functions. But from
a safety
> (and building code) perspective, ALL of these functional grounds MUST be
> bonded together. And HOW they are bonded together can make a huge
> difference when something goes wrong (like lightning).
>
> Jim Brown K9YC


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