[Antennas] MFJ Artificial Ground

Mon, 1 Mar 2004 23:14:32 -0600

FALSE COMPARE HAIKU

Shall I compare thee 
To a summer�s day? Or an
Apple, or orange?

Dave          W9LYA
What wrought doG hath?

On Mon, 1 Mar 2004 13:00:49 -0500 "Robert Lay" <[email protected]>
writes:
> > Bob, is it your opinion that a properly installed 1/4 wave 
> counterpoise
> > will be as effective as the tuned artificial ground?
> 
> Dear Joe,
> 
> Not in the configuration that I understand that Bill Marx has.
> 
> Under normal circumstances, the two things do not compare - apples 
> and
> oranges.
> 
> When you need a counterpoise or radials, it's for the benefit of 
> the
> antenna -period.
> 
> When you need an artificial ground, it's because you are trying to 
> ease
> yourself out of a bad situation with respect to choice of antenna 
> type and
> shack location.
> 
> The artificial ground was never intended as a substitute for a 
> proper
> earthing of a Marconi type antenna. It was intended as a means of 
> cooling
> down the chassis of shack equipment that should never have been at 
> elevated
> RF potentials in the first place.
> 
> As I said in my original comments, the artificial ground is nothing 
> more
> than a series tuned circuit designed to bring the equipment to which 
> it is
> attached down to zero potential with respect to earth. A half wave 
> length of
> heavy wire would do as well or better - but only at the one 
> frequency to
> which it is cut. The advantage of the artificial ground is that it 
> is
> tunable over a large frequency range - 80 m through 10 m, for 
> example.
> 
> When you are operating out of a shack on the 2nd floor and you are 
> operating
> a Marconi type antenna, then you are stuck. You have no choice but 
> to employ
> the artificial ground in the interest of cooling down the hot 
> chassis. As a
> result ALL of the antenna current is passing through the artificial 
> ground -
> that's not what the designers had in mind, but it works, up to a 
> point. Try
> running a KW, and see how long it lasts!
> 
> Imagine that the antenna circuit starts at the top, far end of the 
> antenna.
> It then comes down into your shack to a tuner and out the ground 
> side of the
> tuner. From that point the antenna circuit either goes to ground via 
> a long
> ground wire to earth, or it goes through the artificial ground 
> circuit and
> again through that long wire to earth. Either way the antenna 
> current gets
> to ground, but with the artificial ground, there is now an 
> artificial half
> wavelength between your chassis and the earth, so the chassis seems 
> to be at
> zero potential. Without the artificial ground that long length of 
> wire to
> earth might just be 1/4 wavelength and then your chassis could be a 
> thousand
> volts above ground.
> 
> Now, back to your counterpoise question. Where does the counterpoise 
> fit in?
> It's down there at near ground level trying to create a better earth 
> than
> you are getting with your 8' copper rod that you drove into the 
> ground as
> your antenna ground. All well and good, but what has that to do with 
> the
> artificial ground, which is serving an entirely different purpose?
> 
> Faced with such a situation, I would change to a Herzian antenna, so 
> as to
> NOT be working against ground. Then, the only grounding problem I 
> would have
> would be the stray, common mode RF leakage coming down the outer 
> shield of
> my coax. Or, in the preferred design, using a balanced feed line, I 
> would
> have virtually no common mode RF anywhere, and my shack grounding 
> system is
> there only to provide a path for precipitation static and the 
> occasional
> lightning bolt - God forbid!
> 
> I should mention that around 40 years ago, when I was younger but 
> old enough
> to know better, I had a 40 meter rig (using the old Heathkit DX-40) 
> that was
> Pi coupled to a long wire running directly out of a second floor 
> window to
> the telephone pole at the back of the lot. I also had a ground wire 
> that
> went out the same windows to an earth ground near the air 
> conditioner
> unit.(I probably took advantage of someone's existing earth rod at 
> that
> point). Needless to say, I could only operate at 2 am, because I had 
> every
> kind of BCI, TVI, and other kinds of problems that one could ever 
> have -
> including 2nd harmonic Pink Tickets from the Grand Island monitoring 
> post.
> 
> Good Luck,
> and 73,
> 
> Bob Lay (W9DMK), Dahlgren, VA
> [email protected]
> http://www.qsl.net/w9dmk
> 
> 
> - - - 
> 
> Your moderator for this list is:
> Larry Wilson KE1HZ [email protected]
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> 

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