[Elecraft] Artificial RF Ground

[David Gilbert]

Sorry, but I believe your posting to be full of bad advice.

1.  Most bad reviews for companies that cater to hams are due to bad 
customer service.  MFJ is one of the few that consistently gets bad 
marks for quality of construction and durability of use.  If you don't 
believe that, you simply haven't been paying attention.

2.  An "artificial ground" doesn't keep RF out of the shack ... it just 
tunes a low impedance path to something, which may or not be a ground of 
any sort.  In any case, the chances of finding a functioning "RF ground" 
in an otherwise insulated basement room are not great.

3.  Vertical antennas are notorious for putting RF on the coax shield 
when the coax is lying right alongside the radials.  Why would you 
expect such an arrangement NOT to couple RF to the coax, and therefore 
right into the shack?  Check around and you'll find all sorts of 
instances where people had such problems with a vertical antenna and 
used a current choke to fix it ... there are writeups everywhere on it.  
I agree with K9YC that a coil of coax will normally do the job just 
fine, although I'm not sure that the base of the antenna is always the 
best place to put it.  If the coax is lying next to the radials it seems 
to me that there could still be a lot of coupling to the coax on the run 
to the shack.  I'd be more inclined to put the choke closer to the 
house, but I'll defer judgment on that to the experts.

Dave   AB7E




Ron D'Eau Claire wrote:
> Ha, ha! There are "MFJ Bashers" just like there are those who must complain
> about just about every company. As my Grandmother used to say of such
> people, "He'd complain if they hung him with a new rope." 
>
> To your point, yes the MFJ "Artificial Ground" works FB. One the MFJ tuners
> I have around is the MFJ-934 which has the AG built in and I've used it
> quite successfully with a variety of antennas. 
>
> If you have Moxon's "HF Antennas for All Locations" he discusses such
> grounding schemes in detail, but it seems to be something ignored in many
> texts. What you are doing is arranging a 1/4 wave wire connected to your
> rig. At 1/4 wave, it presents a rather low impedance to the rig, which keeps
> it at a low RF voltage. The AG is nothing more than a simple single-wire
> "tuner" to establish that condition with almost any wire across the HF
> spectrum. The tuner circuit is a tapped coil in series with a variable
> capacitor that can be adjusted to compensate for a wide range of reactance
> values present at the end of your "ground" wire attached to the tuner. Since
> you are working for the lowest impedance at that point, it includes an RF
> ammeter in series with the circuit. You adjust the coil and cap for maximum
> reading on the ammeter. At any given power, maximum current equals lowest
> impedance equals lowest voltage.
>
> As Moxon points out, you can do the same thing with any length of wire that
> is < 1/4 wave and a simple loading coil at the rig, just as you might do
> with a short antenna. (The variable capacitor in the AG covers situations
> where the "ground" wire is longer than 1/4 wavelength). A small flashlight
> bulb in series with the wire will indicated the proper amount of inductance.
> Adjust the power for some indication and set the coil for maximum
> brilliance. You can remove the bulb when you find the right setting so it
> won't burn out at higher powers. 
>
> It's not a panacea. There are some situations where it doesn't cure the
> problem.
>
> As others took pains to point out in the other recent thread, there is no
> "perfect" (zero ohm) ground. As frequency increases, dealing with reactance
> becomes more of an issue and, at RF, we have significant conductor
> resistance as skin effect enters into the equation. But that doesn't mean
> the effort is useless or ineffective. 
>
> You raise an interesting issue though, saying you're using a vertical with
> radials on the ground. Those antennas aren't typically RF feedback prone
> antennas. 
>
> What are the "feedback" symptoms.
>
> Does the feeder show a low SWR at the Antenna? 
>
> Is the shield of the coax properly connected to the radials with a good, low
> resistance connection? 
>
> Ron AC7AC
>
>
>
>
>
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of NG3V
> Sent: Thursday, April 02, 2009 2:43 PM
> To: Elecraft at mailman.qth.net
> Subject: [Elecraft] Artificial RF Ground
>
> Good Evening Group,
>
>  
>
> All this talk of RF grounds has re-kindled my desire to run more than 10
> watts without RF feedback in the shack (yeah, I know 10 watts is enough, I
> just like the ability to have more).
>
>  
>
> Anyway, my shack is in an enclosed room in the basement with no direct
> access to the outside world.  My antenna is a ground-mounted vertical about
> 15 feet from the house with buried radials.  The feedline is laying on the
> ground.  Poor arrangement, but best I can do for the time being.
>
>  
>
> I asked DX Engineering if they thought their Feedline Current Choke might
> help and Bob, N8QE, replied with a very nice message suggesting that I look
> at the MFJ-931 artificial RF Ground.
>
>  
>
> Have any of you had experience with this thing?
>
>  
>
> The eham reviews were mostly what we've come to expect about MFJ stuff.  Buy
> it, open it, fix it, use it.  I'm interested in the 'use it' part. 
>
>  
>
> TIA es 72,
>
>  
>
> Tom, ng3v
>
>  
>
>