Artificial RF Ground

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Artificial RF Ground

ng3v
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

 

 

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Re: Artificial RF Ground

AC7AC
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Re: Artificial RF Ground

Jim Brown-10
In reply to this post by ng3v
On Thu, 2 Apr 2009 17:42:43 -0400, NG3V wrote:

>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).

There is a MUCH better (and MUCH LESS EXPENSIVE) solution. Simply wind a
coaxial choke around a 2.4-inch toroid, following the instructions in the
"Choke Cookbook" section of

http://audiosystemsgroup.com/RFI-Ham.pdf

Place this choke at the feedpoint of your antenna. It will do a VERY good
job of isolating the coax from the antenna.

>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

Huh? There's nothing wrong with this arrangement that I can think of. It
is, of course, important that the shield of the coax have a very short
connection to earth for lightning protection (that is, a ground rod), and a
short, beefy connection between that rod and the ground rod(s) for your
electrical system and telephone system and CATV system. These connections
should be outside your building.

There is NO need for an ADDITIONAL connection to the earth inside your
shack, but the connections listed above are VERY important for lightning
safety.

The confusion evidenced by your "poor arrangement" statement is why I so
strongly object to ANY use of the words "RF ground." The many complex
electrical and radio sytems in a modern aircraft have NO connection to the
earth, but they work just fine! And that 400 Hz whine you hear when you
plug your headset in -- that's a pin 1 problem in their audio system, again
the result of fuzzy thinking about "ground."  

73,

Jim Brown K9YC


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Re: Artificial RF Ground

Don Wilhelm-4
In reply to this post by ng3v
Tom,

The only possible troublesome item that you have talked about is the
"feedline laying on the ground".  That could be picking up radiation
from the vertical and bringing it into the shack - that would be from
'common mode' current being induced on the coax shield.  Burying the
coax may help.  Many such problems can be tamed with a balun at the base
of the antenna, plus a current choke about a half wavelength away (or at
the entry to the building).  Normally, only the balun at the antenna is
sufficient, but in your case, it may be that the shield of the feedline
is picking up radiation from the antenna itself and carrying it into the
shack - thus my recommendation for a current choke at the entry point.  
A ferrite choke will produce the best results, but if you have excess
coax length, try coiling the coax into an inductor - 6 to 8 turns about
6 inches in diameter at the shack entry point may help tremendously.

If you must resort to the use of an artificial ground, I recommend that
it be attached to the coax shield at the point where the coax enters the
house - in other words, get rid of the common mode RF before it gets
into the shack.

OTOH, if your feedline is not coax, other means may be necessary.  One
could try various incantations while waving chicken carcasses about - it
may not help, but it can make one feel better about it.

No number of ground rods will fix the problem.  RF feedback is best
handled at the antenna. or at other select places in the antenna
system.  The exact location depends on your installation, there are no
general answers that will work in every situation.  RF moves in ways
that appear to be mysterious until one realizes that *anything* (even a
ground rod) can become a part of the antenna system - and then one can
move forward to control where the RF currents will flow.

73,
Don W3FPR

NG3V wrote:
> Good Evening Group,
>
> 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.
>
>  
>
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Re: Artificial RF Ground

David Gilbert
In reply to this post by AC7AC

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: [hidden email]
> [mailto:[hidden email]] On Behalf Of NG3V
> Sent: Thursday, April 02, 2009 2:43 PM
> To: [hidden email]
> 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
>
>  
>
>  
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Re: Artificial RF Ground

AC7AC
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Re: Artificial RF Ground

AC7AC
In reply to this post by David Gilbert
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Re: Artificial RF Ground

w7aqk
In reply to this post by David Gilbert
David and All,

I'm not jumping into the middle of the discussion on RF
ground, but I'm certainly enjoying the commentary.  It's
very thought provoking.

However, I do want to "take issue", with all due respect, to
your point #1.  I've used a variety of MFJ accessories over
the last 30 years or so, and I find most of them to be quite
competent.  I might agree that there is the occasional
situation where a connection is bad, or something is
rattling around inside the case, but I think that is
certainly the exception rather than the rule.

MFJ products are generally aimed to provide a lower cost
approach to something.  As such, they tend to be somewhat
closer spec'd in order to reduce cost.  Nevertheless, when
used as designed, they usually work, and work well.  Many of
their items, like tuners, are relatively simple with few
components.  There's not much to go wrong, unless the item
is abused, which I maintain is why many users end up being
unhappy.  When MFJ says it's a 200 watt tuner, they usually
mean it!  I don't think I've ever had a problem with an MFJ
tuner, and I've had (have) a bunch of them!  But I've never
run more than 100 watts through them either.  But if you
bang one with power over the spec'd rating, or even close to
the spec'd rating with a horrible load (you should tune at a
reduced power first) for very long, something's probably
going to blow!

A lot of unhappiness with MFJ products is also due to the
fact that they tend to be fairly basic--as in not too many
"bells and whistles".  I can't even count the number of time
I've seen others opt for an MFJ item because it was cheaper
than "brand X", and then discover that it wouldn't do some
of the things brand X would do.  Of course, brand X was
twice as expensive too!  But suddenly, the MFJ item becomes
"junk" in their minds, and they give it a 1 or 2 rating on
Eham.  That's even though the MFJ item did what it said it
would do--just not a lot more.

MFJ isn't innocent when it comes to claiming "great
results", but neither are other manufacturers.  A good
example is their antennas and how "wonderful" they are.
They aren't!  But they work, and they are comparatively
inexpensive.

So, I suggest that most of the whining about MFJ stuff is
self induced.  Either it's excessive expectation, abuse, or
both.  After all, in most cases now they give you a "no
matter what" guarantee.  As a QRPer, I've found many MFJ
items to be extremely beneficial.  I think Martin Jue, who
owns MFJ, is "borderline genius" when it comes to
identifying a niche and filling it.  He's done more to
accomodate the QRP community (and QRO folks too) than just
about anybody.  I remember over 30 years ago, when I lived
in Memphis, seeing him selling his gadgets at the local
hamfest.  Since then he's been incredibly prolific (and
successful) , and it's been to our benefit.  The problem is
some folks want a Cadillac for a Chevrolet price.

As for the MFJ artificial ground accessory, I don't have
one, and have never used one.  But I bet it works--at least
it probably does what it is designed to do.  However, it's a
compromise solution, and not a perfect solution.
Accordingly, some folks are going to be disappointed.

Dave W7AQK

----- Original Message -----
From: "David Gilbert" <[hidden email]>
To: "Ron D'Eau Claire" <[hidden email]>
Cc: <[hidden email]>
Sent: Thursday, April 02, 2009 5:49 PM
Subject: Re: [Elecraft] Artificial RF Ground


>
> 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: [hidden email]
>> [mailto:[hidden email]] On Behalf Of
>> NG3V
>> Sent: Thursday, April 02, 2009 2:43 PM
>> To: [hidden email]
>> 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
>>
>>
>>
>>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list:
> http://www.qsl.net/donate.html
>


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Re: Artificial RF Ground

Jim Brown-10
In reply to this post by David Gilbert
On Thu, 02 Apr 2009 17:49:26 -0700, David Gilbert wrote:

>I agree with K9YC that a coil of coax will normally do the job just
>fine,

Note that I am NOT recommending "a coil of coax," I am recommending
a coil of coax through a suitable ferrite core. BIG difference. A
coil of coax is inductive (and not a big inductance). A feedline
less than a quarter wave looks capacitive (as a radial or common
mode circuit element), so a simple inductor (the coil of coax) can
resonate with the line and INCREASE the current. Not a good thing. A
feedline between 3/4 and 1 1/4 wavelength is also inductive. Also,
velocity factor does NOT apply to the common mode circuit, but Rudy
Severns (N6LF) has shown that elevated radials DO exhibit a velocity
factor as they interact with the earth. Google to find his website
for really excellent work on this.

A ferrite choke is a low Q parallel resonant circuit. When wound to
put its resonance at the transmitting frequency, it looks like a BIG
resistor in series with the common mode circuit, and it blocks the
common mode current, no matter what the length of the coax.

As to WHERE the choke should go -- if you're short of radials and
WANT the coax to be another radial, then put the choke at the shack.
I have a lot of radials and the coax run to my 160 vertical is short
(about 35 ft), so my choke is at the feedpoint. BTW -- think about
it -- I'm running legal power to that vertical, which is only 25 ft
from my operating position, and I've NEVER had even the slightest
symptom of RFI in my shack on 160M.  

I do NOT see coax laying on the ground picking up any more induced
current than if it were up in the air. Remember that the EARTH is a
LOUSY CONDUCTOR, and the object of radials is to put that current in
low loss wire rather than lossy earth.

73,

Jim Brown K9YC


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