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Part 15 AM Artificial Ground experiment

J

JasonW

Guest
Hello All,

Still being in a tinkering mood, tonight I tried using an Artificial Ground (a series-tuned counterpoise wire) with a TalkingSign www.talkingsign.com Part 15 AM transmitter.

I began by connecting a Pomona BNC male/two banana jacks & screw terminals adapter to a BNC female/RCA male adapter, which was then plugged into the RCA antenna jack on the back of the TalkingSign transmitter.

For the Artificial Ground, I used an MFJ-956 www.mfjenterprises.com/products.php?prodid=MFJ-956 Long Wave/Medium Wave/Short Wave Antenna Pre-selector/Tuner. This series-tuned unit utilizes a 330 microhenry inductor (RF choke) for frequencies between 500 kHz and 1700 kHz. The tuning capacitor (variable, of course) is 365 pF. The MFJ-956 is only intended for receiving, but I didn't think running 0.1 watts of RF through it would do any harm.

I used an 8 meter long, #19 gauge insulated counterpoise wire, laid across non-conducting boxes and chairs about 0.6 meters above the floor. For comparison with the Artificial Ground, I first directly connected this wire to the ground post on the Pomona adapter on the back of the transmitter. I connected the 3 meter wire antenna to the other post on the Pomona adapter using a short alligator clip jumper wire.

With the transmitter set to 1690 kHz and the antenna matching network properly tuned (as indicated by maximum needle deflection on the transmitter's built-in RF ammeter), my Micronta Field Strength Meter indicated a relative field strength of "1" (out of a range of 0 - 5) with the meter's sensor antenna ~10 mm from the counterpoise wire.

I then shut off the transmitter and connected the counterpoise wire to the MFJ-956, which was in turn connected to the ground post on the Pomona adapter on the back of the transmitter. I used PL-259 male/BNC female adapters, Pomona BNC male/two banana jacks & screw terminals adapters, and short alligator clip jumper wires to make the connections. I set the MFJ-956's variable capacitor to minimum capacitance (~10 pF) and rotated the unit's inductor selector switch to 500 kHz - 1700 kHz (330 microhenries).

I turned the transmitter back on, and its built-in RF ammeter still read maximum. I moved the Micronta Field Strength Meter's sensor antenna to ~10 mm away from the counterpoise wire, and its needle moved about 20% of the way to "1" on the scale. I then began turning the variable capacitor, and the needle suddenly jumped to over "2" and then fell back. The tuning was quite sharp, but I had little difficulty centering the needle at its maximum deflection of about "2.1" on the scale. While I tuned the Artificial Ground I was also watching the antenna RF ammeter on the transmitter, and its needle barely moved at all. However, the Micronta Field Strength Meter indicated that the antenna was radiating slightly more RF than before I connected the Artificial Ground, so it did make the counterpoise wire a more efficient RF ground for the antenna to "push against."

An Artificial Ground will definitely increase the total RF radiation from the antenna and the tuned counterpoise wire. While I don't advocate using a tuned counterpoise wire with a Part 15 AM transmitter (it and the antenna are obviously well over the 3 meter total antenna + ground lead length permitted by the Part 15 rules), an Artificial Ground *can* legally be used in some Part 15 AM installations. For example, using a ground-mounted RangeMaster transmitter (or an outdoor TalkingSign or Talking House ATU) with the 2.59 meter CB whip antenna, an Artificial Ground could be connected directly between the transmitter (or outdoor ATU) ground post and the ground rod. For indoor installations, an Artificial Ground connected directly between a transmitter and a metal desk or metal filing cabinet would also improve the system's radiating efficiency. -- Jason
 
> ...For indoor installations, an Artificial Ground connected
> directly between a transmitter and a metal desk or metal
> filing cabinet would also improve the system's radiating
> efficiency.
__________________

Your project was an interesting read, but I think it would more accurately be described as experimenting with antenna configurations, rather than with an artificial ground. In your setup the antenna on the Part 15 tx really is comprised of the 3-meter whip and the "artificial ground" wire, together.

A true artificial r-f ground system or counterpoise has no net radiation of its own. It must consist of at least two, linear, elevated radial conductors of equal electrical length, connected together at the base of the vertical, and from there to the "ground" terminal of the tx. The far end of the radials is not connected to anything. If the radials run in opposite directions in the horizontal plane, then the r-f currents in them will flow in opposite directions, which means that their net radiated field is zero. But it does supply an r-f ground plane reference for the vertical to "work against," without needing a conducting path (a ground lead) to the physical earth.

If properly configured, such an artificial ground or counterpoise can be the electrical equivalent of the classic r-f ground consisting of 120 buried wire radials, each at least 1/4-wave long.
 
I use the prevailing, commonly-understood terminology. Hams call single, series-tuned, electrically-short counterpoise wires artificial grounds (they also often call the counterpoise matching network itself an artificial ground).

But your point is well-taken--that's why I suggested using symmetical objects such as ground rods, metal desks, and metal filing cabinets as artificial grounds. I'll try this again with a long, elevated counterpoise wire connected in the middle to the transmitter's ground post, as soon as I can re-arrange the living room furniture to accommodate it. -- Jason

> > ...For indoor installations, an Artificial Ground
> connected
> > directly between a transmitter and a metal desk or metal
> > filing cabinet would also improve the system's radiating
> > efficiency.
> __________________
>
> Your project was an interesting read, but I think it would
> more accurately be described as experimenting with antenna
> configurations, rather than with an artificial ground. In
> your setup the antenna on the Part 15 tx really is comprised
> of the 3-meter whip and the "artificial ground" wire,
> together.
>
> A true artificial r-f ground system or counterpoise has no
> net radiation of its own. It must consist of at least two,
> linear, elevated radial conductors of equal electrical
> length, connected together at the base of the vertical, and
> from there to the "ground" terminal of the tx. The far end
> of the radials is not connected to anything. If the radials
> run in opposite directions in the horizontal plane, then the
> r-f currents in them will flow in opposite directions, which
> means that their net radiated field is zero. But it does
> supply an r-f ground plane reference for the vertical to
> "work against," without needing a conducting path (a ground
> lead) to the physical earth.
>
> If properly configured, such an artificial ground or
> counterpoise can be the electrical equivalent of the classic
> r-f ground consisting of 120 buried wire radials, each at
> least 1/4-wave long.
>
 
Yes, MFJ makes an "artificial ground" tuner. It allows you to resonate whatever poor ground wire/system you have so that it is most efficient at whatever frequency you have tuned up on. I think they only go down to maybe 1800khz, though, as they are for the ham bands..

df



> I use the prevailing, commonly-understood terminology. Hams
> call single, series-tuned, electrically-short counterpoise
> wires artificial grounds (they also often call the
> counterpoise matching network itself an artificial ground).
>
> But your point is well-taken--that's why I suggested using
> symmetical objects such as ground rods, metal desks, and
> metal filing cabinets as artificial grounds. I'll try this
> again with a long, elevated counterpoise wire connected in
> the middle to the transmitter's ground post, as soon as I
> can re-arrange the living room furniture to accommodate it.
> -- Jason
>
> > > ...For indoor installations, an Artificial Ground
> > connected
> > > directly between a transmitter and a metal desk or metal
>
> > > filing cabinet would also improve the system's radiating
>
> > > efficiency.
> > __________________
> >
> > Your project was an interesting read, but I think it would
>
> > more accurately be described as experimenting with antenna
>
> > configurations, rather than with an artificial ground. In
>
> > your setup the antenna on the Part 15 tx really is
> comprised
> > of the 3-meter whip and the "artificial ground" wire,
> > together.
> >
> > A true artificial r-f ground system or counterpoise has no
>
> > net radiation of its own. It must consist of at least
> two,
> > linear, elevated radial conductors of equal electrical
> > length, connected together at the base of the vertical,
> and
> > from there to the "ground" terminal of the tx. The far
> end
> > of the radials is not connected to anything. If the
> radials
> > run in opposite directions in the horizontal plane, then
> the
> > r-f currents in them will flow in opposite directions,
> which
> > means that their net radiated field is zero. But it does
> > supply an r-f ground plane reference for the vertical to
> > "work against," without needing a conducting path (a
> ground
> > lead) to the physical earth.
> >
> > If properly configured, such an artificial ground or
> > counterpoise can be the electrical equivalent of the
> classic
> > r-f ground consisting of 120 buried wire radials, each at
> > least 1/4-wave long.
> >
>
 
Yes, you're right about MFJ's Ham Radio Artificial Ground counterpoise tuner.

Here are links to web pages about home-brewed Artificial Grounds:

http://www.remeeus.nl/english/hamradio/artificial_ground.htm
http://home.att.net/~w4cwg/artgnd.htm

The MFJ-956 receive-only Pre-selector/Tuner worked fine for my experiment, but I suspect that the counterpoise wire would have radiated even more RF if I had used a series-tuned network that had a large inductor instead of the tiny, lossy 330 microhenry RF choke that the MFJ-956 uses for Medium Wave (AM broadcast band) frequencies. I certainly wouldn't attempt to run more than 0.1 watts through the MFJ-956. -- Jason

> Yes, MFJ makes an "artificial ground" tuner. It allows you
> to resonate whatever poor ground wire/system you have so
> that it is most efficient at whatever frequency you have
> tuned up on. I think they only go down to maybe 1800khz,
> though, as they are for the ham bands..
>
> df
>
>
>
> > I use the prevailing, commonly-understood terminology.
> Hams
> > call single, series-tuned, electrically-short counterpoise
>
> > wires artificial grounds (they also often call the
> > counterpoise matching network itself an artificial
> ground).
> >
> > But your point is well-taken--that's why I suggested using
>
> > symmetical objects such as ground rods, metal desks, and
> > metal filing cabinets as artificial grounds. I'll try
> this
> > again with a long, elevated counterpoise wire connected in
>
> > the middle to the transmitter's ground post, as soon as I
> > can re-arrange the living room furniture to accommodate
> it.
> > -- Jason
> >
> > > > ...For indoor installations, an Artificial Ground
> > > connected
> > > > directly between a transmitter and a metal desk or
> metal
> >
> > > > filing cabinet would also improve the system's
> radiating
> >
> > > > efficiency.
> > > __________________
> > >
> > > Your project was an interesting read, but I think it
> would
> >
> > > more accurately be described as experimenting with
> antenna
> >
> > > configurations, rather than with an artificial ground.
> In
> >
> > > your setup the antenna on the Part 15 tx really is
> > comprised
> > > of the 3-meter whip and the "artificial ground" wire,
> > > together.
> > >
> > > A true artificial r-f ground system or counterpoise has
> no
> >
> > > net radiation of its own. It must consist of at least
> > two,
> > > linear, elevated radial conductors of equal electrical
> > > length, connected together at the base of the vertical,
> > and
> > > from there to the "ground" terminal of the tx. The far
> > end
> > > of the radials is not connected to anything. If the
> > radials
> > > run in opposite directions in the horizontal plane, then
>
> > the
> > > r-f currents in them will flow in opposite directions,
> > which
> > > means that their net radiated field is zero. But it
> does
> > > supply an r-f ground plane reference for the vertical to
>
> > > "work against," without needing a conducting path (a
> > ground
> > > lead) to the physical earth.
> > >
> > > If properly configured, such an artificial ground or
> > > counterpoise can be the electrical equivalent of the
> > classic
> > > r-f ground consisting of 120 buried wire radials, each
> at
> > > least 1/4-wave long.
> > >
> >
>
 
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