Discussion in 'General Technical Questions and Answers' started by AI6DX, Oct 10, 2008.
Does that device really work in place of ground radials?
Yeah, it works but you still need a wire. All it does is tune the wire to resonate on whatever band you're working. But yes, it really does work and no, it doesn't work as well as a number of resonant ground radials.
Wire lengths and numbers
The MFJ-931 certainly works. Try putting many wires onto artificial ground output terminal, all the same length and all at the same time, and spread them out. Voila! Plural radials.
Another trick is to connect wires of different lengths, all at the same time on the output terminal. That way you get multi-band tuning. I use approximate quarter waves for all of the bands I use. Exact fine tuning of the ground wire can be achieved using the MFJ-931.
Not just for Verticals. A tuned ground is a must for long wires and particularly random wires. The MFJ-931 can resonate the "other half" of the antenna. Use an ATU for the antenna and the MFJ-931 for the counterpoise, and watch the signal strength soar!
Finally, a ground tuner such as the MFJ-931 is really a cut down ATU. Make, beg or borrow and RF ammeter, put an ATU where you would put the MFJ-931, adjust it the same way for maximum current reading in the ground wire, and see if it works OK for you. Please note that the SWR reading of the ATU is to be totally disregarded for this exercise. Only the RF ammeter reading counts. Watch you do not overload any meters.
It does scoot the current distribution around, and can solve certain "R.F. in the shack" problems when you have no other options. But it is NOT a replacement for a ground radial system. Is it worth having? Certainly.
It's a band-aid.
Spend the $$ instead to erect a antenna that does not require the artificial "Ground".
Rege, I do agree with you 100% about the good antenna. BUT, when one doesn't have the ability to erect a proper antenna, the artificial ground may be the only viable (and last resort) option.
They can also be put to good use for portable operating setups where running a large ground system is not possible.
1.) It cannot move current around in the ground. That is electrically impossible.
2.) It cannot transform or decrease ground resistance.
The only thing it can do is cancel reactance in the ground lead. If you have a ground lead that has 629 -j1100 ohms impedance with a current maxima 10 feet from the radio and you tune for maximum ground current, the very best result will be some resistance over 629 ohms j0 with maximum current 10 feet from the radio.
It is electrically impossible for it to do anything other than this, because the only thing it can do is add a series reactance (along with some loss resistance) to the connection point.
629 Ohms? Sounds way too high to me. Any measurements I have made indicate more like 40 or 50 ohms, the radiation resistance of the counterpoise wire. Do not be put off!
What no one has said is that the MFJ unit is a solution for people in apartments who are too high above actual ground. It that situation, a piece of wire to the real earth may be a DC gound but is no RF ground. If you are at or near 'real' earth, use it.