*Product Review* MFJ-931 Artificial RF Ground

I’m sure 43DA050 Geoff on Queensland’s Sunshine Coast will agree that operating a ham station from the upper floor of an apartment block presents many challenges indeed.  Apart from antenna limitations/restrictions (N.B. Geoff uses an IMAX 2000 by Solarcon erected on his balcony), one challenge is grounding the transceiver and the subsequent high levels of RF which result when/if an effective grounding is not achieved.

In my experience, even if you do have a decent ground though, a long connection wire can ruin its effectiveness by isolating a true RF ground from your rig.  Sometimes this RF is so horrific that it might ‘bite’ you when you transmit, lead to your rig not working on some bands, cause you to get weak signal reports because of extreme ground losses or radiation pattern distortion on others, or create such dreadful TVI/RFI that neighbours in the same apartment block come bashing on your door threatening legal action.  Ouch!

One solution to this impasse is creating an artificial ground with the assistance of an MFJ-931, a top little product I.M.O. manufactured by MFJ Enterprises in Mississippi USA.  Essentially, this product places your transceiver near actual earth ground potential even if it’s on the second floor or higher with no earth ground possible.

In addition to facilitating grounding in apartment blocks, I also suspected that this product would be useful for reducing RF in stations which incorporate hi-fi or studio equipment.  In fact, this is the reason why I purchased the product on eBay USA second hand and had it delivered to my home in Australia – at a cost of just over $100 AUS + postage.  RF was getting into my Heil Heritage microphone and causing distortion and I hoped the MFJ-931 Artificial RF Ground would eradicate it…

Construction

The MFJ-931 Artificial RF Ground measures a compact 7.5W x 3.5H x 7D inches and comes in a sturdy metallic grey case with an attractive black facing.

There are 3 selectors on the front…

1. Sensitivity
2. Inductance
3. Capacitance. 

There is also a signal meter for RF Ground current readings on the front (See right).

How it Works

The MFJ-931 Artificial RF Ground covers 1.8 – 30 MHz and connects between your rig and a random length of wire or a connecting ground wire.  It’s tuned for maximum ground current using the built-in RF meter and works by electrically placing a far away RF ground directly at your rig – no matter how far away it is.  It also reduces the electrical length of the ground connection wire to virtually zero by tuning out its reactance.

Creating Artificial Ground – How it’s done

1. Connect your transmitter of antenna tuner to the binding post labelled “To Transmitter or Antenna Tuner Chassis Ground” on the back of the MFJ-931.

[Note: This wire needs to be as short as possible, preferably with the ground posts as close as possible i.e. the units side by side or one on top of the other].

2. Attach the length of random wire to the red binding post labelled “To Counterpoise Wire or Ground Connection Wire” on back of the MFJ-931.  Be sure not to reverse these wires.  The random wire should be a quarter-wave length or less at the operating frequency.  Be sure to throw the random wire along the floor and to tape up the open end of the wire to avoid RF burn.

3. To obtain maximum RF ground current for a low impedance RF ground, alternately adjust the two controls on the front panel labelled “INDUCTANCE” and “CAPACITANCE”. Turn the INDUCTANCE control first until the highest amount of current can be registered on the built-in AMMETER. Then adjust the CAPACITANCE control to increase the amount of RF current.

4. Try several inductances setting for best results.  What if the needle goes off the scale or doesn’t move at all?  Then the sensitivity of the meter needs to be adjusted.  Turn the SENSITIVITY control until the needle is in the middle of the scale.  Then readjust the INDUCTANCE control for the highest amount of RF current and use the CAPACITANCE control to achieve maximum RF current.  If the needle is still off scale or zero, readjust the SENSITIVITY control and repeat the tuning process until maximum RF current is achieved.  After maximum RF current is reached, retune your antenna tuner for minimum SWR. You may need to retune both the MFJ-931 and your antenna tuner several times to achieve best results.

Electrically Placing Your Far Away RF Ground at Your Radio

1. Connect your transmitter or antenna tuner to the binding post labelled “To Transmitter or Antenna Tuner Chassis Ground” on the back of the MFJ-931 with the shortest possible wire(s). If necessary, place the MFJ-931 and your transmitter or antenna tuner side by side or one on top of the other to insure the ground of each are as close to each other as possible.

2. Connect your existing ground wire to the red binding post labelled “To Counterpoise Wire or Ground Connection Wire” on the back of the MFJ-931. Do not reverse these wires. After making these connections, follow the same procedure for maximizing RF ground current as described in the Creating Artificial Ground section.

3. After maximum RF current is reached, retune your antenna tuner for minimum SWR. You may need to retune both your antenna tuner and the MFJ-931 several times to achieve best results.

Some Findings

One hitch I experienced with the MFJ-931 Artificial RF Ground straight off was with the cold solder joints falling off the connections on the board and producing sporadic problems.  A couple of the wires from the selector to the coil were touching each other also.  From what I’ve read, this has been an issue with some of the older units but after resoldering all joints and manipulating the wires, which only took about 30 minutes, everything worked great!

I also found that one of the selectors exhibits ‘hand capacitance’ meaning that the meter reading changes when I touch the knob.  I recall that this issue was highlighted in an ARRL product review many years ago and at this time, I’ve not been able to find a solution.  There remains a capacitance variation when my hand approaches it and the RF Ground Current reading on the meter changes somewhat.  But is it a major problem?  NO!

Testing

The MFJ-931 Artificial RF Ground is very easy to use and eliminated the RF problems I was having with the Heil Heritage microphone.  For this, I’m very thankful as I’m now able to enjoy the full voice articulation of this wonderful Heil mic.

Also, using an indoor bazooka antenna for testing [See http://www.delta-alfa.com/bazooka-antenna/] I could see the SWR change as I tuned the artificial ground.  Of course with an indoor antenna such as this one you have a lot of RF floating around so the MFJ-931 Artificial RF Ground passed this test with flying colours.

I’ve no doubt it would also alleviate the RF issues experienced by guys such as 43DA050 Geoff who operate a ham station from apartment blocks, many, many meters off the ground.

Summary

To my knowledge, there isn’t another commercial product that does what the MFJ-931 Artificial RF Ground does.  If you do buy one of these products though, used or new, then my advice is to take the lid off and check for bad joints before you start to use it.  1 bad joint or touching wire will render this product useless and it really does have so much to offer any radio comms enthusiast.

In conclusion, I’d recommend trying the MFJ-931  to anyone who can’t get a good ground due to station location, or to someone whom is having RF problems in the shack.  MFJ make some super products and you won’t be disappointed with this one either.

73 de Darren, HQ Team